Study of Structural, Magnetic and Dielectric Properties of

Study of Structural, Magnetic and Dielectric Properties

of Ferrite/Chromite Nanoparticles

By:

Muhammad Kamran

(22-FBAS/PHDPHY/S-13)

Supervisor:

Dr. Kashif Nadeem

Assistant Professor

Department of Physics, FBAS, IIUI

Co-Supervisor:

Dr. Muhammad Mumtaz Associate Professor

Department of Physics, FBAS, IIUI

Department of Physics

Faculty of Basic and Applied Sciences

International Islamic University, Islamabad

(2018)

i i

Study of Structural, Magnetic and Dielectric Properties of

Ferrite/Chromite Nanoparticles

By:

Muhammad Kamran (22-FBAS/PHDPHY/S13)

A thesis is submitted to


for the award of the degree of

Doctor of Philosophy in Physics

Signature_________________________________________

Chairman, Department of Physics


Signature__________________________________________

Dean Faculty of Basic and Applied Science



Faculty of Basic and Applied Sciences


(2018)

ii ii

Final Approval

It is certified that the work printed in this thesis entitled “Study of Structural, Magnetic

and Dielectric Properties of Ferrite/Chromite Nanoparticles” by Muhammad

Kamran, registration No. 22-FBAS/PHDPHY/S-13 is of sufficient standard in scope and

quality for award of degree of PhD Physics from Department of Physics, International

Islamic University, Islamabad, Pakistan.

Viva Voce Committee

Dean (FBAS) ________________________________________

Chairman (Physics) ___________________________________

Supervisor ___________________________________________

Co-Supervisor ________________________________________

External Examiner 1 ___________________________________

External Examiner 2 ___________________________________

Internal Examiner _____________________________________

iii

iv

DEDICATED

To

My elder brother

Muhammad Usman

v

Declaration

It is hereby declared that the work presented in this thesis has not been copied out from any

source, neither as a whole nor a part. Furthermore, work presented in this dissertation has

not been submitted in support of any publication other than those included in this thesis,

any other degree or qualification to any other university or institute and is considerable

under the plagiarism rules of Higher Education Commission (HEC) Pakistan.

Muhammad Kamran

(22-FBAS/PHDPHY/S-13)

Date

vi

Certificate

The thesis entitled “Study of Structural, Magnetic and Dielectric Properties of

Ferrite/Chromite Nanoparticles” submitted by Muhammad Kamran in partial

fulfilment of PhD degree in Physics has been completed under my guidance and

supervision. I am satisfied with the quality of student’s research work and allow him to

submit this thesis for further process to graduate with Doctor of Philosophy degree from

Department of Physics, as per IIU rules and regulations.

Dated:_____________________

Co-Supervisor Supervisor

Dr. Muhammad Mumtaz Dr. Kashif Nadeem

Associate Professor (TTS) Assistant Professor (TTS)

Department of Physics, Department of Physics,

International Islamic University, International Islamic University,

Islamabad. Islamabad.

.

vii

ACKNOWLEDGMENTS

First, I owe my deepest gratitude to Almighty Allah for all of his countless

blessings. I offer my humblest words of thanks to his most noble messenger Hazrat

Muhammad (P.B.U.H), who is forever, a torch of guidance and knowledge for all

humanity. By virtue of his blessings today I am able to carry out our research work and

present it.

I would like to acknowledge the worth mentioning supervision of Dr. Kashif

Nadeem and co-supervision of Dr. Muhammad Mumtaz who guided me and supported

me during my whole research work. Frankly speaking without effort of Dr. Kashif

Nadeem, it was impossible to complete this hard task of my life. Almighty Allah blessed

him in every part of life. Moreover, I would like to express my sincere thanks to all the

faculty members of Department of Physics IIU Islamabad especially to Dr. Mushtaq

Ahmed (Chairman, Physics). I express my thanks to all staff of Physics Department, IIUI,

for their various services. It is a matter of great pleasure and honor to express my gratitude

to Prof. Dr. Heinz Krenn, Prof. Xianggang Qiu, Dr. Dorothee-Vinga Szabó, Dr.

Iftikhar Gul for valuable discussion and measurements. I shall express my heartiest thanks

to all my research colleagues Faisal Zeb, Asmat Ullah, Yasir Mehmood and Aaqib

Javed for being very supportive and co-operative all throughout my research work.

I would like to acknowledge the efforts of my sweet wife Irram Kamran, without

her support and everlasting love; I would not have been the person I am. My dana always

motivates and supports me during my whole study. I especially want to acknowledge my

brothers, sisters and bhabhies for their indescribable encouragement during my whole

studies especially to my younger brother Muhammad Furqan for his financial support. I

would like to thank my nephew Muhammad Talha Qamar for his moral sport. Finally, I

am thankful to my parents for their love, care and support in my life, which has been

directly encouraging me for my study. My parents’ prayers have always been a big support

in solving my problems.

Muhammad Kamran

viii

CONTENTS

1 Chapter 1: Introduction……...……………………………….…………..…..……...1

1.1 Nanoparticles...……………………………….....………………………………...1

1.2 Magnetism ………………………………………………....……………………...1

1.3 Classifications of magnetism……………………………………………..……….2

1.3.1 Diamagnetism………….………………………..…………………….......2

1.3.2 Paramagnetism…………………………………………………………….2

1.3.3 Ferromagnetism…………………………………………………...............3

1.3.4 Antiferromagnetism……………………………………………………….3

1.3.5 Ferrimagnetism…………………………………………………………....3

1.4 Anisotropy………………………………………..………………………………..4

1.4.1 Magneto crystalline anisotropy………….…………………………...........4

1.4.2 Magneto static anisotropy……………………………………...………….4

1.4.3 Surface anisotropy…………………………………………………………5

1.4.4 Exchange and dipolar anisotropy………………………………………….5

1.5 Spin glass-state…………………………………………………..………………...6

1.5.1 Super spin-glass state……………………………………………...............6

1.5.2 Surface spin-glass state……………………………………………………6

1.6 Dielectrics…………………………………………………………………………7

1.6.1 Polarization mechanism……………………………………………..…….7

1.6.2 Electronics polarization……………………………………………………8

1.6.3 Ionic polarization…………………………………………………………..8

1.6.4 Dipolar polarization………………………………………………………..8

1.6.5 Interfacial polarization…………………………………………………….9

1.7 Spinel crystal structure..……………………………………………..…………….9

1.7.1 Normal spinel…………………………………………………………….11

1.7.2 Inverse spinel……………………………………………………………..11

1.7.3 Mixed spinel……………………………………………………………...11

1.8 Spinel chromites..………………………………………………………………...11

1.9 Cobalt chromite………………………………………………………………….12

ix

1.10 Spinel ferrites…………………………………………………………………...13

1.11 Maghemite……………………………………………………………………...14

1.12 Nickel ferrite……………………………………………………………………15

1.13 Statement of problem…………………………………………………………...15

1.14 Aim and objectives……………………………………………………………...17

2 Chapter 2: Literature Review of Chromite and Ferrite Nanoparticles……...….18

3 Chapter 3: Characterization and Synthesis Techniques…………...…………….23

3.1 Characterization techniques……………………………………………………...23

3.2 X-ray diffraction…………………………….………………………....…………23

3.3 Fourier transform infrared spectroscopy………………………………………... 25

3.4 Superconducting quantum interference device……………………..…................26

3.5 Transmission electronic microscopy……………………………………………..28

3.6 LRC meter ..……………………………………………………………………..29

3.7 Synthesis of nanoparticles………………………………………………………..31

3.7.1 Top down approach………………………………………………………31

3.7.2 Bottom up approach……………………………………………………...31

3.8 Synthesis of chromite and ferrite nanoparticles………………………………….32

3.8.1 Sol-gel method…………………………………………………………...32

3.8.2 Microwave plasma synthesis……………………………………………..35

4 Chapter 4: Structural, Dielectric and Magnetic Properties of Chromite

Nanoparticles………………………………………………………………………..37

4.1 Introduction……………………………………………………………………..37

4.2 Results and discussion of low temperature magnetic response of CoCr2O4

nanoparticles……………………………………………………………………40

4.2.1 X-Ray diffraction ………………………………………………………..40

4.2.2 Transmission electron microscopy ……….……………………………...41

4.2.3 Fourier transform infrared spectroscopy …..……………………………..42

4.2.4 Magnetic properties…………………......………………………………..43

4.3 Results and discussion of effect of Mg doping on structural, magnetic and

dielectric properties of CoCr2O4………………………………………………..49

x

4.3.1. X-Ray diffraction …… ……………………………….…………………49

4.3.2. Transmission electron microscopy …………...………………………….51

4.3.3. Raman spectroscopy……………………………………………………...53

4.3.4. Fourier transform infrared spectroscopy ……………………....…………55

4.3.5. Magnetic properties……………………...……………………………….57

4.3.6. Dielectric properties…...………………………………………………....60

4.4 Results and discussion of effect of SiO2 coating on structural and magnetic

properties of CoCr2O4 nanoparticles…………………………………………....65

4.4.1 X-Ray diffraction ………………………………………..……………….65

4.4.2 Transmission electron microscopy …………...………………………….67

4.4.3 Magnetic measurements………………...………………………………..68

4.5 Conclusion……………………………………………………………………...74

5 Chapter 5: Structural, Dielectric and Magnetic Properties of Ferrite

Nanoparticles……………………………………………………………………….75

5.1 Introduction………………………………………………………………………75

5.2 Results and discussion of effect of surface spins on magnetization of Cr2O3 coated

γ-Fe2O3 nanoparticles………………………………………………………………...77

5.2.1 X-Ray diffraction…………………………………………..…………….77

5.2.2 Transmission Electron Microscopy……………………....………………78

5.2.3 Magnetic properties………………………………………………………79

5.3 Results and discussion of study of Cr doping on structural, dielectric and magnetic

properties NiFe2O4 nanoparticles…………………………………………………….88

5.3.1 X-Ray diffraction…………………………………………..…………….88

5.3.2 Transmission Electron Microscopy……………………....………………90

5.3.3 Magnetic properties………………………………………………………91

5.3.4 Dielectric properties………………...……………………………………93

5.4 Conclusion………………………………………………………………………..98

6 General conclusion..………………………………..……….……............................99

References…………………………………………………………………….………..102

xi

List of Figures

Fig. 1.1: Super spin glass. ...................................................................................................6

Fig. 1.2: Surface spin glass. ................................................................................................7

Fig. 1.3: Schematic diagram of electronic, ionic, dipolar and interfacial polarization. .....9

Fig. 1.4: Schematic diagram of tetrahedral site ................................................................10

Fig. 1.5: Schematic diagram of octahedral site ................................................................10

Fig. 1.6: Crystal structure of CoCr2O4 ..............................................................................13

Fig. 1.7: Crystal structure of maghemite ..........................................................................14

Fig. 1.8: Crystal structure of NiFe2O4 ..............................................................................15

Fig. 3.1: Bragg’s law representation .................................................................................24

Fig. 3.2: Experimental arrangement of Michelson interferometer ...................................26

Fig. 3.3: Superconducting coil in the SQUID magnetometer ..........................................27

Fig. 3.4: SQUID-magnetometer facility at the Institute of Physics, Karl-Franzens

University, Graz, Austria ..................................................................................................28

Fig. 3.5: Working principle of TEM ................................................................................29

Fig. 3.6: Working principle of LRC meter .......................................................................30

Fig. 3.7: Flow chart for synthesis process of ferrite and chromite nanoparticles .............33

Fig. 3.8: Schematic diagram of microwave plasma synthesis ..........................................36

Fig. 4.1: XRD pattern of CoCr2O4 nanoparticles .............................................................40

Fig. 4.2: TEM image of CoCr2O4 nanoparticles at 100 nm scale .....................................41

Fig. 4.3: FTIR spectrum of CoCr2O4 nanoparticles .........................................................42

Fig. 4.4: ZFC/FC curves of CoCr2O4 nanoparticles at 50, 500, and 1000 Oe ..................44

Fig. 4.5: (a) M-H loops at 5, 25, 50, 75, and 100 K, (b) Variation of MS with temperature

(solid line just showed the trend) and (c) Variation of HC with temperature (black solid

line) of CoCr2O4 nanoparticles fitted with modified Kneller’s law (dashed red line). ......47

xii

Fig. 4.6: (a) Zero field cooled (FC) relaxation curve of CoCr2O4 nanoparticles under field

H = 100 Oe at temperature T = 5 K, orange solid line shows the best fit of stretched

exponential law, (b) Field cooled (FC) relaxation curve of CoCr2O4 nanoparticles under

field H = 100 Oe at temperature T = 5 K, red solid line shows the best fit of stretched

exponential law. ................................................................................................................49

Fig. 4.7: (a-g) Rietveld refinement fitting results of the XRD of Co1-xMgxCr2O4

nanoparticles at 300 K, showing the observed pattern (diamonds in red colour), reflection

markers (vertical bars), the best fit Rietveld profiles (black solid line) and difference plot

(blue solid line at the bottom), (h) the variation of lattice constant and (i) average crystallite

size plotted as a function of Mg concentration (x). ...........................................................51

Fig. 4.8: TEM images at (a) 110 nm and (b) 70 nm scales for Co0.2Mg0.8Cr2O4

nanoparticles .....................................................................................................................52

Fig. 4.9: Raman spectra of Co1-xMgxCr2O4 nanoparticles. ................................................54

Fig. 4.10: Fourier transform infrared spectroscopy of Co1-xMgxCr2O4 nanoparticles. ......56

Fig. 4.11: (a-e) ZFC/FC curves of Co1-xMgxCr2O4 nanoparticles under field H = 50 Oe 58

Fig. 4.12: (a-d) FC curves of Co1-xMgxCr2O4 nanoparticles with applied field 5 T ........59

Fig. 4.13: Variation in dielectric constants; (a) real and (b) imaginary part with frequency

for Co1-xMgxCr2O4 nanoparticles ......................................................................................62

Fig. 4.14: (a) Tangent loss and (b) ac conductivity of Co1-xMgxCr2O4 nanoparticles .....64

Fig. 4.15: (a) XRD patterns of CoCr2O4/(SiO2)y nanoparticles, (b) variation of average

crystallite size and (c) lattice parameter with SiO2 concentration. Dashed lines just show

the trends. ...........................................................................................................................67

Fig. 4.16: TEM image of CoCr2O4/(SiO2)y, y = 0 % nanoparticles at 50 nm scale .......68

Fig. 4.17: (a) ZFC and FC of CoCr2O4/(SiO2)y nanoparticles (b) variation in TF, TS and TC

value with SiO2 concentration. Dashed lines just show the trends. ...................................70

Fig. 4.18: (a) M-H loops of CoCr2O4/(SiO2)y nanoparticles at T = 25 K and (b) variation

of MS and HC with SiO2 concentration. Dashed lines just show the trends. ......................72

Fig. 4.19: ZFC AC susceptibility (in-phase part) of CoCr2O4/(SiO2) nanoparticles. ........73

xiii

Fig. 5.1: X-ray diffraction patterns for Cr2O3 coated γ-Fe2O3 nanoparticles.....................77

Fig. 5.2: (a) TEM image at 10 nm scale (b) STEM-image at 50 nm scale (inset shows the

results of red marked area by STEM-EELS) of Cr2O3 coated γ-Fe2O3 nanoparticles and (c)

STEM-EELS spectra of γ-Fe2O3 core (red color)-Cr2O3 shell (green color) nanoparticles.

............................................................................................................................................79

Fig. 5.3: ZFC/FC experimental (blue solid triangles) and simulated (red open squares) dc

susceptibility curves of Cr2O3 coated γ-Fe2O3 nanoparticles under 50 Oe .......................81

Fig.5.4: (a) M-H loop at 5 K, (b) MS at different temperatures (Bloch’s law fitting is in

form of red dashed line) and (c) HC at different temperatures (Kneller’s law fitting is in

form of red dashed line) for Cr2O3 coated maghemite nanoparticles. ..............................85

Fig. 5.5: (a) In-phase ac susceptibility of Cr2O3 coated γ-Fe2O3 nanoparticles. The f-

dependent TB is fitted with (b) Arrhenius law (c) Vogel-Fulcher law and (d) dynamic

scaling law. .......................................................................................................................88

Fig 5.6: (a) XRD patterns and (b) lattice constant and average crystallite size of NiCrxFe2-

xO4 nanoparticles ...............................................................................................................89

Fig. 5.7: TEM images of NiCr2O4 nanoparticles at (a) 20 nm and (b)100 nm scale .......91

Fig. 5.8: (a) M-H loops at T = 5 K, (b) MS variation and (c) HC variation for NiCrxFe2-xO4

nanoparticles with Cr concentration (x). solid lines just reveal the trend. .........................93

Fig. 5.9: (a) Real and (b) Imaginary part of NiCrxFe2-xO4 nanoparticles. ........................95

Fig. 5.10: Tangent loss of NiCr2Fe2-xO4 nanoparticles. ....................................................96

Fig. 5.11: AC conductivity of NiCrxFe2-xO4 nanoparticles. ..............................................97

xiv

List of Tables

Table 4.1: Vibrational bands in Raman spectra of Co1-xMgxCr2O4 nanoparticles. ..........55

Table 4.1: Vibrational bands in infrared spectra of Co1-xMgxCr2O4 nanoparticles. .........56

xv

List of Publications

[1]. “Structural, magnetic, and dielectric properties of multiferroic Co1-xMgxCr2O4

nanoparticles”

M. Kamran, A. Ullah, S. Rahman, A. Tahir, K. Nadeem, M. Anis ur Rehman, and S. Hussain

Journal of Magnetism and Magnetic Materials 433, 178-186 (2017). Impact Factor: 2.630

[2]. “Negative and anomalous T-dependent magnetization trend in CoCr2O4 nanoparticles”

M. Kamran, K. Nadeem and M. Mumtaz

Solid State Sciences 72, 21-27 (2017). Impact Factor: 1.811

[3]. “Role of SiO2 coating in multiferroic CoCr2O4 nanoparticles”

M. Kamran, Asmat Ullah, Y. Mehmood, K. Nadeem, and H. Krenn

AIP Advances 7, 025011 (2017). Impact Factor: 1.568

[4]. “Role of surface spins on magnetization of Cr2O3 coated γ-Fe2O3 nanoparticles”

K. Nadeem, M. Kamran, A. Javed, F. Zeb, S.S. Hussain, H. Krenn, D. V. Szabo, and U. Brossmann

Solid State Sciences 83, 43-48 (2018). Impact Factor: 1.811

[5]. “Surface spins disorder in uncoated and SiO2 coated maghemite nanoparticles”

F. Zeb, K. Nadeem, S. K. A. Shah, M. Kamran, I. H. Gul, and L. Ali

Journal of Magnetism and Magnetic Materials 429, 270-275 (2017). Impact Factor: 2.630

[6]. “Effect of air annealing on structural and magnetic properties of Ni/NiO nanoparticles”

K. Nadeem, Asmat Ullah, M. Mushtaq, M. Kamran, S.S. Hussain, and M. Mumtaz

Journal of Magnetism and Magnetic Materials 417 (2016) 6-10. Impact Factor: 2.630

[7]. “Surface spin-glass in cobalt ferrite nanoparticles dispersed in silica matrix”

F. Zeb, W. Sarwer, K. Nadeem, M. Kamran, M. Mumtaz, H. Krenn, and I. Letofsky-Papst

Journal of Magnetism and Magnetic Materials 407 (2016) 241–246. Impact Factor: 2.630

[8]. “Dielectric properties of (CuO, CaO2, and BaO)y/CuTl-1223 composites”

M. Mumtaz, M. Kamran, K. Nadeem, Abdul Jabbar, Nawazish A. Khan, Abida Saleem, S. Tajammul

Hussain, and M. Kamran

Low Temperature Physics, 39, 622-629 (2013). Impact Factor = 0.881

xvi

Abstract

This thesis is schematically based on synthesis and characterization of cobalt

chromite (CoCr2O4), maghemite (γ-Fe2O3) and nickel ferrite (NiFe2O4) nanoparticles, as

well as selective coating and doping in host compounds in order to tune its structural,

dielectric and magnetic properties. CoCr2O4 and NiFe2O4 nanoparticles were synthesized

by sol-gel method, while γ-Fe2O3 nanoparticles were synthesised by microwave plasma

technique. For chromite nanoparticles, the low temperature magnetic response of CoCr2O4

nanoparticles, magnetic and dielectric properties of Mg doped CoCr2O4 nanoparticles and

magnetic properties of SiO2 coated CoCr2O4 nanoparticles have been studied in detail. X-

ray diffraction revealed the cubic spinel structure of the nanoparticles. Zero field cooled

and field cooled (ZFC/FC) curves revealed a paramagnetic (PM) to ferromagnetic (FiM)

transition at TC = 97-100 K with conical spiral state at TS = 27 K and lock-in state at TL =

13 K. Negative magnetization is observed in the ZFC curve under 50 Oe applied field,

which gets suppressed upon the application of higher field due to reorientation of the

nanoparticles magnetization in the direction of applied field. The TC was shifted towards

higher temperature with the application of higher field, while TS and TL remain unaffected

which was attributed to strong B-B interactions which act as a frozen spins or canted spins

at surface. M-H loops showed an abnormal decrease in MS which may be due to presence

of stiffed/strong conical spin spiral and lock in states at low temperatures. Modified

Kneller’s law showed a good fit for temperature dependent HC at higher temperature and

deviated at low temperature (< 25 K) which was attributed to frozen disordered surface

spins. Nanoparticles showed slow spin relaxation in both ZFC and FC protocols at 5 K,

which signifies the presence of spin-glass like behavior at low temperatures. Mg doped

CoCr2O4 nanoparticles showed non-monotonous trend in the average crystallite size and

showed a peak behaviour with maxima at x = 0.6. The members CoCr2O4 (x = 0) and

MgCr2O4 (x = 1) are FiM and antiferromagnetic (AFM), respectively. TC and TS showed

decreasing trend with increasing x, followed by an additional AFM transition at TN = 15 K

for x = 0.6. The system finally stabilized and changed to highly frustrated AFM structure

at x = 1 due to formation of pure MgCr2O4. Dielectric parameters showed a non-

monotonous behaviour with Mg concentration and were explained with the help of

Maxwell-Wagner model and Koop’s theory. Dielectric properties were improved for

xvii

nanoparticles with x = 0.6 and is attributed to their larger average crystallite size. SiO2

coated CoCr2O4 nanoparticles showed decreasing trend of the average crystallite size and

cell parameter with increasing SiO2 concentration. The decrease in average crystallite size

is due to SiO2 coating which limits the growth of nanoparticles by generating more

nucleation sites. All the magnetic transitions of CoCr2O4 nanoparticles shifted towards low

temperatures which is due to decrease in average crystallite size. SiO2 concentration also

decreased saturation magnetization (MS), which was enhanced surface disorder in smaller

nanoparticles.

In study of structural, magnetic and dielectric properties of ferrite nanoparticles,

chromium oxide (Cr2O3) coated γ-Fe2O3 nanoparticles and NiCrxFe2-xO4 ferrite

nanoparticles have been studied in detail. Simulated ZFC/FC curves exhibited large value

of effective anisotropy of Cr2O3 coated γ-Fe2O3 nanoparticles as compared to bulk γ-Fe2O3

but less than bare γ-Fe2O3 nanoparticles which is may be due to weak interface anisotropy

between ferrimagnetic γ-Fe2O3 core and antiferromagnetic Cr2O3 shell. Bloch’s law was

fitted on T-dependent MS data and revealed the higher value of Bloch’s constant and lower

value of Bloch’s exponent as compared to bulk γ-Fe2O3. Spin glass behaviour was

investigated by using different physical laws for f-dependent ac susceptibility and they

confirmed the presence of spin glass behaviour which is due to disordered frozen surface

spins. XRD analysis of Cr doping at B site in NiFe2O4 nanoparticles confirmed the cubic

spinel structure for all samples with x = 0, 0.2, 0.4, 0.8, 2.0 concentration. Saturation

magnetization depicts decreasing trend with addition of Cr3+ concentration which is

attributed to replacement of large magnetic moment of Fe3+ by smaller magnetic moment

of Cr3+. HC reveals minimum value for NiFe2O4 nanoparticles and showed increasing trend

with addition of Cr3+. This increase in HC may be attributed to change in magneto

crystalline anisotropy. Dielectric constant showed increasing trend with the Cr+3

concentration due to less conductive nature of Cr as compared to Fe. In summary, a detail

study of structural, dielectric and magnetic properties of chromite and ferrite nanoparticles

have explored with tremendous results that will open a new insight in device applications

such as automatic switching, magnetic memory and targeted nanotherapeutic.

Chapter No.1 Introduction

1

Chapter No. 1

Introduction

1.1 Nanoparticles

Nano is a Greek word which means very small. A nanoparticle is a quasi-zero-

dimensional nano-object in which all dimensions are of the same order of magnitude (not

more than 100 nm). The nanoparticles behave differently to their bulk counterpart due to

increase of surface-to-volume ratio and occurrence of quantum mechanical effects at

nanoscale [1]. For example, the gold is very stable in the bulk state while it is reactive at

nanoscale. In bulk materials, the energy band gap is continuous while it is discrete at

nanoscle. Electrical conductivity in the bulk material is also continuous and in

nanoparticles, charge transfers through tunnelling process. Nanoparticles have amazing

and useful properties with many structural and non-structural applications. For example,

Carbon nanoparticles add strength, flexibility, heat protection to metals, ceramics and

plastics [2]. There are different applications of nanoparticles which are used in various

fields of life such as aerospace, automotive, consumers, environmental to control pollution,

industrial coating, power transformers, solar panels, microbial fuel cells, and information

storage devices [3].

1.2 Magnetism

Repulsion and attraction force of magnetic material by prearrangement of atoms is

called magnetism. The magnetism phenomenon is closely related to response of material

with application of applied external field. The magnetism of material is due to spin and

orbital motion of electrons within atoms of materials. Magnetization can be defined as

‘‘orientation of magnetic dipole moments along or versus the direction of applied external

field’’. Mathematically magnetization can be written as [4]

M =𝜇𝑡𝑜𝑡𝑎𝑙

V (1.1)


2

Where, µtotal, V and M represent total magnetic dipole moments, volume and magnetization

of material, respectively.

1.3 Classifications of Magnetism

Classifications of magnetism in different materials are:

• Diamagnetism

• Paramagnetism

• Ferromagnetism

• Antiferromagnetism

• Ferrimagnetism

1.3.1 Diamagnetism

Diamagnetism is present in materials which have completely filled shells having no

unpaired electron. It is present in all materials but very weak. These materials usually

induce the magnetization due to flux change in the current loops in presence of external

magnetic field. Lens’s law uses to describe the induced magnetization which explains that

magnetic moments oppose the applied magnetic field which reduces the magnetic flux

density. Diamagnetic material can be described by negative magnetic susceptibility due to

the opposite direction of magnetization as compared to applied field. No permanent

magnetic dipole moments are present in the diamagnetic materials. The induced magnetic

moments disappear upon the removal of applied field. The examples of the diamagnetic

materials are: Nitrogen, Helium, Neon, Sulphur, Hydrogen etc. [5].

1.3.2 Paramagnetism

The paramagnetism arises due to the spinning of unpaired electrons. Atomic

moments are random in alignment in paramagnetic materials and these materials show zero

net magnetization [6]. In presence of magnetic field, magnetic moments of these materials

get align along field direction and exhibit net magnetization. When the applied field is

removed, the magnetic moments again randomly distributed and net magnetization

becomes vanishes. Paramagnetic materials have permanent magnetic dipole moment and


3

exhibit magnetic susceptibility greater than zero. The paramagnetic materials are alkali,

alkaline earth metals, potassium, platinum, manganese etc.

1.3.3 Ferromagnetism

In the term of domains having spontaneous magnetization is responsible for

ferromagnetism. In the ferromagnetic material, each domain has about 1015 or 1016 of atoms

and magnetic moments are align parallel with each other. Ferromagnetism produces due to

the spinning of unpaired electron even without existence of applied field. Quantum

mechanically, the magnetic dipole moments have strong coupling force and overlapping of

wave functions of electrons in ferromagnetic materials create an exchange interaction

called direct exchange. The adjustment between the domain is about 100 atoms in the

transition region are called domain walls. The direction of domain is randomly oriented in

the absence of applied magnetic field and aligned themselves within the wall of the

magnetic domain when we applied the external magnetic field [7]. In domains spontaneous

magnetization maintain at certain temperature and after that temperature ferromagnetic

transferred to paramagnetic materials is called Curie temperature. The iron and nickel have

Curie temperature 770 and 1135 0C respectively.

1.3.4 Anti-ferromagnetism

The material in which magnetic moments are equal in magnitude and opposite

aligned without applied field which results zero magnetization called antiferromagnetic

materials. In these materials, two sub lattices occurred with oppositely aligned magnetic

moments. At specific temperature, these materials turn into paramagnetic material is

known as “Neel temperature”. Magnetic susceptibility of anti-ferromagnetic materials

increases inversely with temperature above Neel temperature and decreases inversely

below this temperature [8].

1.3.5 Ferrimagnetism

Ferrimagnetic materials reveal almost same behaviour as a ferromagnetic material.

In ferrimagnetism, magnetic moments are oppositely aligned with unequal magnitude and

partially cancelled magnetic moment results in a net magnetization. In these magnetic


4

materials, two sublattices A and B with unequal and opposite spins occur and are

responsible for net magnetization. These materials also contain spontaneous magnetization

due to unequal and opposite spins at two sub lattices. The magnetic moments in a

ferrimagnetic material modify their orientation in direction of applied field and increase

the net magnetization. Usually, this type of magnetization occurs in ionic compounds [9,

10].

1.4 Anisotropy

The direction of single crystal indicates the difference between its physical and

mechanical properties. A material is said to be anisotropic if its properties changes at

different crystallographic orientations. Magnetic anisotropy determines specific spatial

directions in which magnetization of the sample is different. Therefore, magnetic

anisotropy is an important ingredient to keep the magnetization vector in a preferred

direction. There are easy and hard axes to magnetize the magnetic material. Different types

of magnetic anisotropy are given as:

1.4.1 Magneto crystalline anisotropy

It is also known as crystal anisotropy and intrinsic property of magnetic material.

Its origin lies in spin-orbit coupling. In this anisotropy, the magnetization is coupled to

certain crystallographic directions. This anisotropy affects very effectively on magnetic

properties of materials, such as the magnitude of coercive force, the shape of hysteresis

loops, the domain structure, magnetization processes and permeability [11].

1.4.2 Magneto static anisotropy

This anisotropy is due to inside magnetic field of the system. It arises from the

magnetic poles which are present on the surface of magnetized material. This magnetic

field is also called demagnetizing field. Magneto static anisotropy strongly depends on the

shape of particle. For example, a non-spherical shape particle with finite magnetization has

large magneto static energy for orientation of the magnetic moments as compared to

spherical. Thus, the shape has strongly impact for determining the magnitude of magneto

static energy as a function of magnetization orientation. Therefore, magneto static


5

anisotropy is also named shape anisotropy [12]. In bulk systems, the magneto crystalline

anisotropy is important while at nanoscale shape and surface anisotropy pay an additional

contribution very well in the magnetic properties.

1.4.3 Surface anisotropy

The surface of nanoparticles is very important in determining magnetic properties

due to a possible change of state and disorder of surface spins. The surface atoms have

broken symmetry which is responsible for surface anisotropy. The magnitude of this

anisotropy increases with the decrease of particle size. This effect is attributed to large

surface to volume ratio of nanoparticles [11]. The surface of a nanoparticle contains atoms

with bond deficiencies creating frustration and disorder on the surface. This frustration and

disorder on the surface of nanoparticles causes a disorientation of magnetization vectors

for surface spins unlike those in the core. Neel has shown that the surface contribution

becomes relevant only for particle smaller than ~ 10 nm. If the surface anisotropy is

different from the core anisotropy, then the core spin vector prefers a different

magnetization direction with respect to the surface spins.

1.4.4 Exchange and dipolar anisotropy

Two magnetic particles in a close proximity have a magnetic interaction. The

orientation for relative two interacting magnetic moments gives information about the easy

direction. When the magnetic spins come very close to each other, their wave functions

overlap and the dominant interactions are direct exchange interactions. If the magnetic

moments are coupled via electron hopping across an intermediate oxygen ion, exchange

interactions are super exchange interactions. Exchange interactions are of short range and

much stronger than dipolar interactions. Dipolar interactions are weak and long range.

These interactions lead to an additional anisotropy energy. In most cases, it is assumed that

the sum of all contributions to the magnetic anisotropy energy results in an effective

uniaxial anisotropy [13].


6

1.5 Spin Glass-State

Surface functionalization tuning is very significant in nanoparticles which can alter

their physical properties. In term of magnetic nanoparticle, surface spins become very

significant in controlling magnetism of individual nanoparticle [14]. Spin-glass state was

first determined by V. Cannela and J. A. Mydosht in 1970, while observing AC-

susceptibility of gold iron alloy [15]. Magnetic frustration and disorder are the main causes

for spin glass state. [16]. In spin-glass system, there is a distribution and randomness of the

exchange constant. Distribution of exchange constants and competing interactions among

spins cause disorder and frustration in the system. There are two types for spin glass states

in case of nanoparticles as given below.

1.5.1 Super spin-glass state

Super spin-glass system can be defined due to random freezing of giant nanoparticle

spins embedded in magnetic or non-magnetic material due to dipolar interactions at low

temperature. When we cool down the sample, the nanoparticles become correlated below

a certain freezing temperature and get frozen in a spin-glass like state as shown in Fig. 1.1.

Fig. 1.1: Super spin glass.

1.5.2 Surface spin-glass state

The broken bonds at individual nanoparticle’s surface reveals a certain degree of

disorder and frustration at the surface. In ferrite and chromite nanoparticles, this kind of

surface disorder and frustration is dominant because of competing exchange interactions


7

among coupled spins on the nanoparticle’s surface. It is called surface spin glass state or

surface spin glass system as shown in Fig. 1.2.

Fig. 1.2: Surface spin glass.

1.6 Dielectrics

Generally dielectric is a non-conducting or insulating material. The dielectric

phenomenon arises due to electric force which occurs due to the attraction and repulsion

of the electric charges. If the strength of the dielectric material is higher than it is important

for different applications such as in parallel plate capacitor. These dielectrics become

polarized in presence of applied electric field. These materials are widely used in electrical

circuits due to high resistance. The ferrite and chromite compounds have very large

resistance and behave just like as an insulator. Due to this proper, ferrite and chromite

compounds are very useful for electrical circuits [17].

1.6.1 Polarization mechanism

The dielectric materials redistribute charges with application of applied electric

field. As a result, dipoles formation occurs. Consider a dipole having dipole moment “µ”

which is given as

Surface spins

Ferrimagnetic core


8

qd (1.2)

Where “q” is the magnitude of the charge and “d” be the separation between the

charges. When the electric field is applied on materials, they polarized due to alignment of

an induced and the permanent dipoles along with applied field. Then the polarization will

become

P Nqd (1.3)

Where “N” represents number of dipoles. In dielectric materials four types of polarizations

occur. The essential requirements of all these polarization mechanisms are the time i.e. the

time variation of the electric field. There are four types of polarization [18].

1.6.2 Electronic polarization

In electronic polarization, the electron displaces relative to nucleus when material

is inserted in an applied electric field. In this polarization the atoms behave as a

momentarily induced dipole. It is the important phenomenon for the pure materials because

in the pure material there will be no formation of the covalent bonds.

1.6.3 Ionic polarization

When an ionic material is inserted in an applied electric field then ionic polarization

will occur. These bonds are elastically deformed. This type of polarization occurs mainly

in the ceramic materials. The cation and the anions are moving either closer together or

move apart from each other with the applied field direction. This mechanism contains

usually very small dipole moment. NaCl and KCl are best examples of ionic polarization.

1.6.4 Dipolar polarization

This polarization occurs in those materials which have permanent dipoles. These

dipoles are in random direction and give net polarization zero in absence of applied field.

When we apply electric field, the electric dipoles arrange themselves in field direction and

result a polarization. The example of dipolar polarization is water.


9

1.6.5 Interfacial polarization

There will be impurities occurs in the crystal structure. Due to the impurities the

charge will be developed at the interfaces of the material. The charges move on the surface

of the material by placing it an external magnetic field. This type of polarization usually

occurs in ferrites, chromites and semiconductors.

The total polarization of materials is sum of these four polarizations. The schematic

diagram of these four types of polarization is given in Fig. 1.3.

Fig. 1.3: Schematic diagram of electronic, ionic, dipolar and interfacial polarization [18].

1.7 Spinel crystal structure

Spinel crystal structure is the most diverse, useful and common type of cubic system

with space group Fd3m. Spinel compounds generally follow AB2O4 formula. In this

formula, A is a metallic divalent ion i.e. Ni2+, Fe2+, Mg2+ and Co2+etc and B is trivalent ion

i.e. Fe3+, Cr3+ and Al3+ etc. These compounds have FCC structure and 32 ions of oxygen

forming close packed structure unit cell. There are two lattice sites in these compounds:


10

• Tetrahedral lattice sites

• Octahedral lattice sites.

Tetrahedral lattice site contains of five atoms, with four oxygen atoms and one metal ion.

Three atoms of oxygen are joined with each other in same line while forth atoms are on top

of symmetric position of metal ion. A whole unit cell of these compounds consists of 64

sites, where 8 sites are occupied only. The tetrahedral lattice site of face centered cubic,

hexagonal closed packed and body centered cubic is shown in Fig. 1.4 in which oxygen

ions are presented by purple colour and metal ions are presented by green colour.

Fig. 1.4: Schematic diagram of tetrahedral site [19].

Octahedral lattice site contains of seven atoms, with six oxygen atoms and one metal ion.

Four atoms of oxygen are joined with each other in same line while two atoms are on top

and bottom of symmetric position of metal ion. A whole unit cell of these compounds

consists of 32 sites, where 16 sites are occupied only. The octahedral lattice site of face

centered cubic, hexagonal closed packed and body centered cubic is shown in Fig. 1.5 in

which oxygen ions are presented by purple colour and metal ions are presented by green

colour.

Fig. 1.5: Schematic diagram of octahedral site [19].


11

The spinel structures have mainly three types given as

• Normal spinel

• Inverse spinel

• Mixed spinel

Let tetrahedral lattice sites as A-sites and octahedral lattice sites as B-sites

1.7.1 Normal spinel

General formula for Normal spinel structure is [D2+]A[T3+]BO4. All the [D2+] ions

are divalent which present at A-sites and all the [T3+] ions are trivalent which present at B-

sites. Normal spinel ferrite unit cell contains of 16 octahedral sites and 8 tetrahedral sites.

Zinc ferrite (ZnFe2O4) shows normal spinel structure.

1.7.2 Inverse spinel

General formula of inverse spinel structures is [D3+]A[T2+D3+]BO4. Both A and B

sites are engaged by trivalent cations in equal part and divalent cations are engaged by B-

sites. Cobalt ferrite (CoFe2O4) is the best example for it, in which divalent Co2+cations are

at B-sites and trivalent Fe3+cations are at A and B-sites equally.

1.7.3 Mixed spinel

When Inverse and normal spinel structure are mixed is called as mixed or

transitional spinel structure. General formula of mixed spinel structure is [T2+δD

3+1-

δ]A[T2+1- δ D

3+1+ δ]BO4, where sigma (δ) is inversion factor. For inverse spinel structure δ =

0, for normal spinel structure δ = 1 and for mixed spinel structure δ fluctuated from 0 to 1.

The divalent and trivalent cations engaged by B-sites are equal in mixed spinel ferrites

[19]. MnFe2O4 is example of mixed spinel structure.

1.8 Spinel chromites

A spinel prototype system ACr2O4, in which Chromium (Cr) is essential element at

octahedral site is known as chromite. More recently, multi-ferroicity has been found in

these types of materials. The magneto-electric effect was discovered in spinel chromites in

19th century. The magneto-electric effect is: magnetization tuning with help of an applied

electric field and polarization tuning with help of applied magnetic field [20]. A significant


12

interest for magneto-electrics has boosted recently for various technological potential

applications [21]. The field of magneto-electrics is closely relating to multiferroics:

combining ferroelectric and ferromagnetic properties, although not limited to them.

Chromites with cubic normal spinel type are very attracting materials due to their

multiferroic properties [22]. Multiferroic cobalt chromite and nickel chromite

nanoparticles belong to normal spinel structure and study of magneto-structural coupling

in these compounds are very interesting. Mohanty et al. [23] studied the magneto-structural

coupling in (Ni1xCox)Cr2O4 nanoparticles synthesized by co-precipitation method. They

observed the high value of transition temperature for nickel chromite and cobalt chromite

nanoparticles as compared to reported value with replacement of Ni by Co.

(Ni0.5Co0.5)Cr2O4 showed high coercivity and M-H loop shifted under field cooling

condition.

1.9 Cobalt Chromite

Cobalt chromite (CoCr2O4) is one of the very important ferrimagnetic material in

nature. It has normal spinel structure in which A site contains Co2+ ions in form of yellow

colour and B site contains magnetic Cr3+ ions in form of blue colour as shown in Fig. 1.6

[24]. The magnetic order is controlled by the strong AB and BB interactions mediated by

non-magnetic oxygen [25]. In this compound, spins lie on the conical surfaces and named

as ferrimagnetic spiral. The magnetic order of CoCr2O4 is mostly studied in bulk and single.

Menyuk et al. [26] studied the magnetic ordering of bulk CoCr2O4 through neutron

diffraction and observed short range order of spiral and ferrimagnetic component below

transition temperature TC. At nanoscale, CoCr2O4 usually shows a paramagnetic to

ferrimagnetic transitions at Curie temperature at 100 K along with two other magnetic

orders at low temperatures such as spiral spin state (TS) and lock in state (TL) at 31 K and

8 K respectively [27].


13

Fig. 1.6: Crystal structure of CoCr2O4 [24].

1.10 Spinel ferrites

Ferrites especially ferri/ferromagnetic oxides with iron as their vital metallic

component at B site in general formula of spinel structure. It gives numerous and mostly

interesting new applications of magnetic materials in electrical appliances. Ferrites are used

in electromagnetic material due to very high performance at very low cost [28]. Their

properties can be enhanced by addition of certain divalent elements. These divalent

elements are Co, Ni, Mn, Zn etc. The research of ferrites in conventional bulk preparation

is getting to their limits because of high electrical conductivity and resonance of domain

walls. Due to above reason, ferrite research changed its direction to nano-metric scale to

investigate their properties at nano-scale [29, 30]. According to formula (Aδ B1−δ) [A1−δ Bδ]2

O4 with δ = 1, Zinc ferrite nanoparticles (ZnFe2O4) belong to a normal spinel. Zinc ferrite

have very weak B–B interactions. The Zinc ferrite shows antiferromagnetic long-range

ordering at Neel temperature (TN) = 9 –11 K [31]. Due to reduction of grain size, the

magnetization increases in this compound. This feature is usually related with the reduction

of the size of the grains and with the change of the cation inversion [32]. Nickel–Zinc

ferrites shows low loss due to eddy current losses and high value of electrical resistivity.

Due to these properties, Ni–Zn ferrite are used in the electromagnetic fields at very high

frequencies. With low dielectric losses and high mechanically strength, these magnetic

materials show very high magnetization and magnetic permeability. It is quite interesting


14

to synthesis nano-sized Nickel–Zinc ferrites to minimize energy losses in the bulk powder.

The preparation of Ni–Zn ferrites at nano-scale is achieved successfully at room

temperature [33].

1.11 Maghemite

Maghemite (γ-Fe2O3) is a spinel structure which belongs to ferrites and contains iron

vacancies at B sites. The formula of maghemite is (FeIII8)A [FeIII

40/3 “Θ”8/3]B O32, where

“Θ” represents the vacancy at octahedral site [34]. In spinel structure, two third of the

octahedral sites are normally occupied by divalent metal ions but in maghemite these are

occupied by Fe3+ ions and the remainder is vacant. This produces an equal charge like by

divalent metal ions but creates an imbalance among Fe3+ ions at A and B lattice sites.

Crystal structure of maghemite with tetrahedral and octahedral coordination along with

vacancies are shown in Fig. 1.7. The net magnetic moment assigned to maghemite (γ-

Fe2O3) is 2.5 μB. Due to octahedral vacancies, fine maghemite nanoparticles can also

exhibit disorder in the core magnetization in addition to surface spin disorder. Maghemite

nanoparticles have also many applications in industry, e.g. magnetic data storage, ferro-

fluids and contrast agents [35].

Fig. 1.7: Crystal structure of maghemite [36, 37].


15

1.12 Nickel ferrite

Nickel ferrite (NiFe2O4) have inverse spinel structure with space group Fd3m [34]

and follows AB2O4 general formula of structures. It contains a soft magnetic nature with

low coercivity. This type of spinel structure attained great attraction due to fabulous

application in different fields such as ferro-fluids, gas sensors, transformers, high

frequency devices, telecommunication, contrast agents, drug delivery and radar absorbing

paints. Nickel ions prefer octahedral sites and displace iron ions from octahedral to

tetrahedral sites. In a unit cell, the net magnetic moment is entirely due to Ni2+ ions. The

net magnetic moment of Ni2+ ion is 2μB and hence the formula magnetic moment of nickel

ferrite is also 2μB [38]. Schematic representation of the inverse spinel lattice of NiFe2O4 is

shown in Fig. 1.8. Fe3+ cations (red) are distributed equally across A and B lattice sites,

while Ni2+ cations (green) occupy A sites.

Fig. 1.8: Crystal structure of NiFe2O4 [39].

1.13 Statement of problem

Cobalt chromite (CoCr2O4) nanoparticles attain great attraction from multiferroic

compound family which exhibit ferroelectric and ferromagnetic ordering simultaneously.


16

Multiferroic CoCr2O4 nanoparticles due to coupling between electric and magnetic order

parameters show unprecedented physical properties. From ferrite family, Maghemite (γ-

Fe2O3) and Nickel ferrite (NiFe2O4) nanoparticles are promising candidate for different

applications such as in biomedical therapy and diagnostic, ferro-fluids, magnetic tunneling

barrier for spin filter devices and magnetic data recording. The main problem in utilizing

of ferrites and chromite nanoparticles efficiently in functional devices are their

agglomeration. It decreases the surface energy, reduces the superficial surface area and

interfaces with neighboring particles. Therefore, proper surface coating or developing

effective protection is used to minimize surface energy and to prepare stable nanoparticles

for potential applications. In-situ coating controls surface effects, particle size and

interparticle interactions. Coating not only stabilizes nanoparticles but can also lead to

surface functionalization. Different approaches for coating have been used so far which

include coating with polymer, biomolecules, surfactants, magnetic and non-magnetic etc.

I have preferred non-magnetic coating for chromite nanoparticles and antiferromagnetic

coating for ferrite nanoparticles. In this thesis, I have focused on effect of non-magnetic

Silica coating on CoCr2O4 nanoparticles and Chromium oxide coating on γ-Fe2O3

nanoparticles.

The ferrite and chromite are ferrimagnetic materials having opposite magnetic

moments at tetrahedral and octahedral lattice sites. Their magnetic and dielectric properties

strongly depend upon the cationic distribution between two sites. Doping mechanism is

very interesting tool to tune dielectric and magnetic properties of these nanoparticles by

altering the cationic distribution. I have preferred the doping of non-magnetic Mg2+ ions at

A site for chromite nanoparticles and Cr+3 doping at B site for ferrite nanoparticles. These

suitable doping can reduce magnetic anisotropy and controls the structural stability, which

can result in enhanced physical properties of these nanoparticles. The motivation of Mg

doping comes from our previous work on Mg doped zinc ferrite [40] in which increase in

Mg content increases magnetization. It was explained on the basis of preference of Mg ions

and they distributed in such a way that overall magnetization is increased. The doping of

non-magnetic Mg2+ ions in cobalt chromite controls the magnetic anisotropy and the

structural stability of nanoparticles. Therefore, I have also emphasized on the structural,

magnetic and dielectric properties of Mg doped CoCr2O4 nanoparticles and Chromium


17

doped NiFe2O4 nanoparticles in this thesis. There is some significant interest in magnetism

of CoCr2O4 nanoparticles at low temperature due to surface effects, inter particle

interactions and finite size effects.

1.14 Aim and Objectives

Followings are the aims and objectives of this proposed thesis research work;

❖ Chromites have TC below room temperature with several magnetic transitions.

These transitions are not well understood at nanoscale.

❖ To fabricate and study the temperature dependent magnetic transitions of

CoCr2O4 nanoparticles,

❖ Doping in these materials can control their physical properties,

❖ To fabricate and study the structural, dielectric and magnetic properties of Mg

doped CoCr2O4 nanoparticles,

❖ Insitu coating restricts the growth of nanoparticles,

❖ To study the role of SiO2 coating on magnetic transitions of CoCr2O4

nanoparticles,

❖ Ferrites have high TC above room temperature, therefore magnetic blocking and

surface effects in ferrite nanoparticles are interesting to study for application

point of view,

❖ Surface coating of the nanoparticles plays important role in controlling their

physical properties, also important to study the properties of individual

nanoparticles,

❖ To fabricate and study the role of antiferromagnetic Cr2O3 surface coating on

ac and dc magnetic properties of γ-Fe2O3 nanoparticles,

❖ To fabricate and study the structural, dielectric and magnetic properties of Cr

doped NiFe2O4 nanoparticles.

Chapter No. 2 Literature Review of Chromite and Ferrite Nanoparticles

18

Chapter No. 2

Literature Review of Chromite and Ferrite Nanoparticles

Akyol et al. [41] prepared spinel multiferroic CoCr2O4 nanoparticles using sol-gel

method. They investigated briefly structural and magnetic experimental results along with

their modeling. Their reported temperature dependent magnetic transitions of CoCr2O4

nanoparticles are: ferrimagnetic transition at 96 K, spiral magnetic transition at 27 K and

lock-in transition at 16 K. Exchange bias phenomenon was observed in these nanoparticles

at 5 K with 350 Oe due to exchange interactions in randomly distributed structure and

showed decreasing trend as temperature increases from 5 to 50 K and vanished after 50 K.

The decrease of exchange bias effect is due to decrease in exchange coupling at higher

temperatures. The magnetic entropy was also performed for these nanoparticles around

the transition temperature and found maximum change-0.87 J/kg.K in entropy under 6 T

field.

Chandana et al. [42] synthesized the pure CoCr2O4 nanoparticles by co-

precipitation method having particle size between 8-12 nm. Temperature dependent

magnetization plot shows transition from paramagnetic state to superparamagnetic state.

Usually, CoCr2O4 nanoparticles reveal paramagnetic to ferrimagnetic transition at Curie

temperature. Blocking temperature of these superparamagnetic nanoparticles was 50-60 K.

These nanoparticles also showed loop shifting and an enhancement in coercivity at 10 K

on cooling the sample under 10 kOe field. The disordered surface spins configuration and

distribution of nanoparticle sizes are responsible for that effect. Exchange bias

phenomenon vanished at 50 K which confirms the blocking temperature of

superparamagnetic phase.

Gingasu et al. [43] synthesized CoCr2O4 nanoparticles by tartarate and gluconate

precursor routes. X-ray diffraction (XRD) pattern revealed cubic phase CoCr2O4

nanoparticles and average crystallite size was 14 and 21 nm. CoCr2O4 nanoparticles shows

paramagnetic to ferrimagnetic transition below the curie temperature (TC) 97 K and a phase

transition spiral spin ordering (TS) at ~26 K which is due to long-range spiral magnetic


19

order. They also observed the best catalytic activity of CoCr2O4 nanoparticles as prepared

by gluconate precursor route by using the total methane oxidation.

Mindru et al. [44] prepared CoCr2O4 nanoparticles using precursor compound

oxalate through thermal decomposition. The structural characteristics had been performed

with help of X-ray diffraction, Raman and infrared spectroscopy and scanning electron

microscopy. Average crystallite size calculated by using Scherer’s equation and was found

between 38 and 58 nm. Both CoCr2O4 samples show ferrimagnetic ordering below TC at

97 K and a phase transition at TS ~20 K which is attributed to the onset of long range spiral

magnetic order.

Choudhary et al. [45] studied the effects of Zn, Mg and Cu doping on structural,

magnetic and dielectric properties of CoCr2O4 as prepared using auto-combustion sol-gel

technique. Structural information was obtained with help of XRD which showed single-

phase crystalline nature. Crystal structure was transformed from cubic to tetragonal with

addition of Cu. Dielectric measurements were explained with help of hoping phenomenon.

The maximum value for dielectric constant was observed in case of Zn doping and

attributed to enhanced space charge polarization. Grains and Grain boundaries both were

active at low frequency in this chromite which was confirmed by impedance analysis.

Kumar et al. [46] studied the doping effect of Fe doping on B site in CoCr2O4

nanoparticles prepared by using co-precipitation method. Particle size was found in 16-20

nm range and for x = 0.1 to x = to 0.2 particle size was 6-10 nm. Magnetic measurements

revealed that TC increases with the increasing Fe concentration. Specific heat versus

temperature shows a sharp transition TS in both x = 0.1 and x = 0.2 samples. By adding Fe

in the sample, interaction between Cr-Cr becomes unbalanced and hence it causes an

increase in TC and TS.

Afzal et al. [47] synthesized MnCr2O4 and Cr2O3 by using sol-gel technique. Crystal

structure was identified by XRD. Phase transformation was observed with Mn

incorporation in Cr2O3, from rhombohedral symmetry of Cr2O3 to spinel cubic symmetry

of MnCr2O4. Scanning electron microscope revealed uniformly distributed and well-

shaped nanoparticles in range of 30–70-nm. Magnetic behaviour of these nanoparticles was


20

investigated as a function of temperature and applied field. Paramagnetic behaviour was

observed at both 5 K and room temperature for Cr2O3 nanoparticles, while a

ferromagnetism to paramagnetism magnetic phase transition was observed for

MnCr2O4 nanoparticles at a ~ 50 K Curie temperature TC.

Zakutna et al. [48] studied CoCr2O4 nanoparticles prepared at different

temperatures using hydrothermal treatment of chromium and cobalt oleates. CoCr2O4

nanoparticles annealed at 300o to 500oC temperature range. XRD, high-resolution TEM,

SEM, thermogravimetric and magnetic measurements performed to analyze these

nanoparticles. The observed particle size was 4.4 to 11.5 nm range as calculated from XRD

and TEM. Tendency of aggregation in these nanoparticles found to increase as annealing

temperature increases. Magnetic measurements of CoCr2O4 nanoparticle revealed

suppressed typical behavior of long range magnetic order.

Nadeem et al. [49] prepared maghemite nanoparticles having size 6 nm by using

microwave plasma synthesis method. SEM and XRD were used for structural

characterization of these nanoparticles. Zero field cooled and field cooled (ZFC/FC)

magnetization curves as function of temperature showed maximum magnetization at 75 K

for these nanoparticles. Experimental ZFC and FC data was simulated with help of uniaxial

anisotropy Neel-Brown relaxation model to figure out anisotropy constant of these

nanoparticles. The value of anisotropy constant of these nanoparticles was lager as

compared to bulk maghemite. The ac susceptibility data as function of frequency was fitted

with physical laws and dynamic scaling law confirmed the spin glass behavior.

Thermoremanent magnetization and memory effect were also performed which also

confirm the existence of spin glass behavior. Exchange bias and temperature dependent

coercivity revealed sharp increase due to frozen surface spins at low temperature.

Koseoglu et al. [50] synthesized Mn doped cobalt ferrite nanoparticles with the

formula MnxCo1-xFe2O4 where value of x was varied from x = 0.0 to x = 1.0. Debye-

Scherer’s formula used to estimate the average crystallite size and it was found between 14

to 22 nm. SEM was used to study the morphological information of nanoparticles.

Magnetic measurement shows that sample possess both ferromagnetic and

superparamagnetic phases distinguished by blocking temperature. Blocking temperature of


21

sample starts decreasing by increasing the Mn ratio in the sample. Other magnetic

parameters such as coercivity and ramanent magnetization decreases as concentration of

Mn goes on increasing in the cobalt ferrite nanoparticles.

Wei et al. [51] studied the silica-coated manganese ferrite nanoparticles prepared

by chemical solution phase method which have capability of Radio-frequency-heating.

Transmission electron microscopy showed well dispersed in an aqueous solution coated

nanoparticle having size 7 nm, 12 nm and 18 nm. Energy dispersive X-ray analysis

confirmed the stoichiometry composition of MnFe2O4. They analysed the magnetic

properties of these coated nanoparticles with the help of VSM. Saturation magnetization

showed decreasing trend due to silica coating because it creates spin disordered surface.

They also observed the capability of the heat production in these nanoparticles which

directly depends upon the particle size and radio frequency field strength. In conclusion,

they suggested that silica coated manganese ferrite nanoparticles have great potential for

controlled drug releases and cancer treatments.

Zeb et al. [52] studied the temperature and time dependent magnetization of

uncoated and silica-coated maghemite nanoparticles. Surface spin disorder in these sol gel

prepared nanoparticles were analysed briefly. The average crystallite size was found 29 nm

for uncoated and 12 nm for coated maghemite nanoparticles. The decrease in average

crystallite size was due to silica coating which produce hinder in growth of nanoparticles.

The silica coated nanoparticles showed low value of average blocking temperature and

saturation magnetization and as compared to uncoated nanoparticles which attributed to

smaller average crystallite size. The Bloch's law fitting on experimental temperature

dependent saturation magnetization of coated nanoparticles revealed lower value of b and

higher value of B as compared to uncoated maghemite nanoparticles which was attributed

to finite size effects and weaker exchange coupling due to enhanced surface disorder. The

stretched exponential law fit on experimental FC data of coated nanoparticles showed slow

relaxation which suggest the surface spin glass behaviour in coated nanoparticles.

Yadav et al. [53] investigated the structural, dielectric, electrical and magnetic

properties of NiFe2O4 nanoparticles. The nanoparticles were synthesized using sol gel

honey-mediated combustion technique and annealed at different temperatures (800C and


22

1100C). They reported increased MS and decreased HC with increasing average crystallite

size. The dielectric constant of these nanoparticles revealed maximum value at low

frequency. Frequency independent behaviour was showed by these nanoparticles at higher

frequency. This was due to active grain boundaries at low frequency and active grains at

higher frequency according to Wagner model. At constant frequency, dielectric constant

revealed increasing trend with increasing average crystallite size. AC conductivity of these

nanoparticles were explained with help of hoping mechanism in ferrite and ac conductivity

showed maximum value at higher frequency. The variation in electrical properties and

dielectric constant were due to variation in grain size which were revealed in cole-cole plot.

Hirthna et al. [54] reported the fabrication of magnetically separable and highly

conductive Ni1−xMgxFe2O4 nanoparticles with different concentration of (x) via co-

precipitation technique. The structural, dielectric and magnetic properties of these

nanoparticles were analysed using XRD, FTIR, SEM and VSM. The crystal structure of

Ni1−xMgxFe2O4 nanoparticle was face centred cubic as found by XRD and the average

crystallite size was in range 10 – 28 nm. The average crystallite size revealed decreasing

trend and lattice constant revealed increasing trend with increasing Mg concentration

which was due to replacement of small ionic radii of Ni2+ by large ionic radii of Mg2+. MS

found to increase with Mg doping up to x = 0.6 which was due to misbalance of Fe+3 ions

among lattice sites because of Mg content, non-collinear nature and super-exchange

interactions in magnetic moments which lead to enhanced magnetization.

Chapter No. 3 Characterization and Synthesis Techniques

23

Chapter No. 3

Characterization and Synthesis Techniques

3.1 Characterization techniques

There are many methods and process which can be used for the characterization of

nanoparticles. We have used X-ray diffraction for the study and determination of crystal

structure, parameters of the unit cell and average crystallite size of nanoparticles. Fourier

transform infrared and Raman spectroscopy were selected for getting information about

vibrational bands. Transmission electron microscopy was selected for getting information

about shape and particle size of the nanoparticles. LCR meter was used for dielectric

measurements. Super conducting quantum interference device magnetometer was used for

ac and dc magnetic characterization. These techniques are discussed as coming sections.

3.2 X-Ray diffraction

The electromagnetic radiations with around 10 nm wavelength are known as X-

rays. The interatomic distance between the molecules of different materials is also of the

order of nanometer range, so a comparable wavelength can be used to detect or observe

diffraction patterns. The benefits by using X-rays diffraction (XRD) is that, it is non-

destructive and helpful in determining the wide range properties of the materials such as

lattice constants, geometry of planes, orientation of atoms, identification of unknown

materials, defects, grain size and stresses in the crystalline structures. The radiation emitted

from transition of electrons from L to K shell or M to K shell are called soft X-rays whereas

the radiations emitted from all other transitions are known as hard X-rays [55]. Bragg’s

law is used to study the diffraction pattern and analyze materials properties.

Bragg’s diffraction depends on the wavelength of incident X-rays and the space

between two atoms. A comparable distance and the wavelength produce diffraction pattern

by the constructive interference of diffracted X-rays. The method of constructive

interference and destructive interference can be explained through Bragg’s law. Bragg’s

law gives a mathematical interpretation for analyzing orientations of different planes of a


24

material. Radiation strikes on a parallel series planes which are equally spaced. The

reflected radiation rays will have a path difference [56]:

nλ = 2dsinθ (3.1)

where “λ” is notation for wavelength, “n” is the integer number and “d” is the inter planer

distance of the crystal lattice.

Fig. 3.1: Bragg’s law representation [57].

A beam of X-ray is incident on a surface of crystal with parallel planes as shown in

Fig. 3.1. On the upper plane radiation beam strikes with atom, while the second beam of

radiation strikes with another atom on the lower plane. On reflection, the second beam

travels extra distance than the first beam. It can be judge that an extra or more distance is

traveled by the second ray. This is done due to the reflection at suitable angles which allows

the reflection of electromagnetic radiation from two consecutive planes [57]. The reflected

beams interfere constructively as the path difference of two beams reflected from two

successive planes is the integral multiple of the wavelength “λ’. By using this law in X-ray

diffraction, we can find the crystalline phase, crystalline orientation, grain size, phase

composition and strain [55].


25

3.3 Fourier transform infrared spectroscopy

A fourier transform infrared spectroscopy (FTIR) is a very basic tool in

characterizing the chemical species of an unknown material. Everybody with finite

temperature emits infrared radiation, and a molecule selectively absorbs radiation

according to its optical active modes of vibrations and electronic transitions. FTIR-

spectroscopy deals with the interaction of radiation of proper wavelength (like infrared)

with matter. It detects in the mid-infrared the vibrational modes of groups of chemical

elements. From the specific absorbed wavelengths, the various modes of the molecular

structure are investigated. FTIR operation is based on the principle of a Michelson

interferometer in which the movable mirror creates a phase delay for all wavelengths of a

splitted infrared beam passing through the specimen as shown in Fig. 3.2. The scattered

infrared radiation is detected by sensitive infrared detectors. Direct detection of the signal

as a function of the mirror displacement gives raw data in the form of an “interferogram”.

FTIR spectrum is usually plotted as absorbance or transmittance spectrum as a function of

wave number. First the background spectrum of the infrared broadband source without

sample is recorded and then the same spectrum with the sample is recorded [58]. Infrared

spectroscopy is useful for following investigations.

i) Recognize anonymous materials

ii) Chemical bonding

iii) Find out the quality sample

iv) Find out the number of components in a mixture.


26

Fig. 3.2: Experimental arrangement of Michelson interferometer [59].

3.4 Superconducting quantum interference device magnetometer

A superconducting quantum interference device (SQUID) is used the most sensitive

device to measure the magnetic moment of small quantity of samples. These devices

consist of mainly two parts one is superconducting material and other is insulator. SQUID

operates in the range of about 4.2- 400 K temperature [60]. There are two modes to set the

desired temperature, one is the settle mode and the other is the sweep mode. When a settle

mode is selected, the system will stay at the desired temperature and one can record

magnetic moment. While in sweep mode the system will pass slowly through the desired

temperature and magnetic moment can be recorded during the slow ramping of the system

through the desired temperature [61]. There are also three different control modes to set

the magnetic field: (1) oscillating mode, (2) no-overshoot mode and (3) hysteresis mode.

In oscillating mode, the magnetic field quickly approaches the desired magnetic field value

but oscillates back and forth with decreasing amplitude around the desired magnetic field.

In no-overshoot mode, the field is ramped quickly up to the desired magnetic field and then

it slowly approaches the desired magnetic field without any overshoot. The no-overshoot

mode is highly recommended for samples which are sensitive to any history of external

magnetic field excitation. In hysteresis mode, the magnet works in the non-persistent mode


27

and takes current directly from the external power supply to set the desired magnet field in

the fastest way, but with less accuracy. This mode is recommended for fast hysteresis loop

measurements on samples having large magnetic moments. In its construction

superconducting ring consists of one or two small or tiny insulating layers which have been

inserted in the device.

Fig. 3.3: Superconducting coil in the SQUID magnetometer [62].

The SQUID-magnetometer detects indirectly the magnetic moment by picking up

the emerging magnetic flux from the magnetized sample. The change of magnetic flux is

monitored during the movement of the sample. The sample passes the several coils in a

sequence from the bottom to the top coil meanwhile generating negative and positive

screening currents which are fed by a superconducting flux transformer into the SQUID as

shown in Fig. 3.3. The screening current fed by the flux transformer into the SQUID is

further amplified and converted by electronic means into a voltage which directly

corresponds to the magnetic moment of the sample [63]. Squid’s are also the well-known

organized devices to measure magnetic fields having very great accuracy even for the

weaker ones. For some example, squids are very sensitive enough for the measurement of


28

the magnetic activities such as human brain. The SQUID-magnetometer which was used

for magnetic measurements of our samples is shown in Fig. 3.4.

Fig. 3.4: SQUID-magnetometer facility at the Institute of Physics, Karl-Franzens University, Graz, Austria.

3.5 Transmission electronic microscopy

Transmission electron microscopy (TEM) is a versatile tool in nanoscience research

for determining shape, crystalline quality and the lattice parameter of small objects. For

nanoparticles, TEM additionally provides information about the size distribution and the

average particle size which are mandatory for investigating their integral physical

properties. A TEM schematic diagram is shown in Fig. 3.5 [64]. High energy electrons of

60 – 400 keV are used for electron diffraction while they traverse the thinned specimen.

The main part of a transmission electron microscope is the electron gun. Electrons from

the cathode are accelerated by at least two anodes. Behind the anodes, there is a condenser

lens system to focus the electron beam on the specimen. The specimen should be thin

enough so that the incoming high energy electrons can pass through it. After passing

through the specimen, the diffracted electrons are imaged on a photographic film or on a

fluorescent screen by a sophisticated arrangement of projection lenses [65]. We can


29

observe specimen to the angstrom level. For example, we can observe small structural

details in the cell or other specimens to the atomic levels.

Fig. 3.5: Working principle of TEM [64].

3.6 LCR Meter

An inductance, capacitance and resistance (LCR) meter is a very useful instrument

to measure rapid and accurate frequency dependent measurements of samples. LCR

contains a power supply to generate the ac voltage. As according to the requirement, the

frequency and the voltage amplitude can be adjusted. Two operational amplifiers worked


30

in it. The operational amplifier provides a signal at its output which is proportional to the

current. Four Kelvin wires for connections use in LCR meter to measure voltage across the

sample and current through the sample. Multiplying the current and voltage signals from

sample with in phase voltage reference signal of power supply, we can obtain real signals,

and those power supply voltage signals which shift 90⁰ give out of phase signals. A

microprocessor is used to read the signals. Finally, the microprocessor converts the

receiving signals into proper data points.

Using LCR meter, we can calculate the dielectric constant with the help of given

formulas i.e.

ℇ′ =cd

Aℰ˳ (3.2)

ℇ′′ = ℇ′ 𝑡𝑎𝑛𝛿 (3.3)

Here ‘C’, ‘d’ ‘A’ and ‘ℰ˳’correspond for the capacitance in farad, the thickness in meter,

cross-sectional area of pellet and permittivity of free space, respectively. Dielectric loss

(𝑡𝑎𝑛 𝛿) is calculated by following formula,

𝑡𝑎𝑛 𝛿 = 1

2𝜋ƒ𝐶𝑃𝑅𝑃 (3.4)

where ‘f’, ‘δ’, ‘Cp’ and ‘Rp’ are notations for frequency, the loss angle, equivalent parallel

capacitance and equivalent parallel resistance respectively [66]. The LCR meter from

National University of Science and Technology, Islamabad which was used for dielectric

measurements. The working principle of LCR meter is shown in Fig. 3.6.

Fig. 3.6: Working principle of LRC meter.


31

3.7 Synthesis of nanoparticles

In nanostructured materials, controlled synthesis and processing are the key factors

to obtain the material at the nanometer scale. Synthesis technique plays an important role

to get nanoparticles of desired properties. Properties of material strongly depend on the

synthesis route followed for the preparation of nanoparticle. There are two methods used

for the synthesis and creation of nanostructured materials which are top down and bottom

up approach.

3.7.1 Top down approach

A physical approach, in which bulk materials are broken down to give small features

at nano scale is called top down approach. This technique is not favorable for the synthesis

of nanoparticles because the product formed in this case have structural defects also non-

homogenous nanoparticles are obtained through this synthesis approach. Synthesis routes

that are used in top down approach are as follows:

➢ Ball Milling

➢ Lithography

➢ Laser ablation method

3.7.2 Bottom up approach

Bottom up is a chemical approach used for the synthesis of nanoparticles. In this

method, chemical routes are used to produce nanoparticles. Small building blocks are

joined together to form a large structure [67]. Nanoparticles obtained through this process

are homogenous and have less structural defects. Synthesis routes that are used in bottom

up approach are as follows:

➢ Sol-gel

➢ Co-precipitation

➢ Hydrothermal

➢ Thin film deposition


32

➢ Self-assembly

➢ Microwave Plasma synthesis method

➢ Colloidal aggregation

3.8 Synthesis of ferrite and chromite nanoparticles

There are different experimental methods that can be utilized for the synthesis of

nanoparticles, but I have chosen sol-gel method and microwave plasma synthesis method

for the synthesis of Co1-xMnxCr2O4 nanoparticles.

i) Sol-gel method

ii) Microwave plasma synthesis method

3.8.1 Sol-gel method

Sol-gel synthesis of ferrite and chromite nanoparticles is a chemical method and

appropriate for producing coated/uncoated metal oxide nanoparticles. It gives a rather

mono-disperse and narrow particle size distribution as compared to other chemical methods

e.g. by the co-precipitation technique. In sol-gel method, first precursors are mixed with

their respective molar ratios in a solvent. Then a specific amount of citric acid is added as

gelatin and firing agent. Ammonia can be used to stabilize the pH value. After stirring for

some time, a gel will be formed with a network of metal ions. Afterwards, the gel is dried

at 80°C to remove the solvent. The dried gel is fired at some temperature for some specific

time to get the required crystalline quality of the nanoparticles. A flow chart of Sol-gel

process to synthesize ferrite and chromite nanoparticles is shown in Fig. 3.7.


33

Fig. 3.7: Flow chart for synthesis process of ferrite and chromite nanoparticles.

Calcinate


34

The chemicals which used for the preparation of chromite and ferrite nanoparticles are:

a) Cobalt Nitrate (Co(NO3)2.6H2O)

b) Magnesium Nitrate (Mg(NO3)2.6H2O)

c) Chromium Nitrate (Cr(NO3)3.9H2O)

d) Nickel Nitrate (Ni(NO3)2.6H2O)

e) Iron Nitrate (Fe(NO3)3.9H2O),

f) Tetraethyl orthosilicate (SiC8H20O4)

g) Ethanol (C2H6O),

h) Ammonium Hydroxide (NH4OH),

i) Citric Acid (C6H8O7.H2O),

j) Distilled Water.

The CoCr2O4, Mg doped CoCr2O4, SiO2 coated CoCr2O4 and Cr doped NiFe2O4

nanoparticles synthesized by using sol-gel method. Co(NO3)2.6H2O, NH4OH,

Cr(NO3)3.9H2O, C6H8O7.H2O and C2H6O are taken for preparation of CoCr2O4

nanoparticles. Mg(NO3)2.6H2O, Co(NO3)2.6H2O, C2H6O, Cr(NO3)3.9H2O, C6H8O7.H2O

and NH4OH are taken for preparation of Mg doped CoCr2O4 nanoparticles.

Co(NO3)2.6H2O, NH4OH, Cr(NO3)3.9H2O, C6H8O7.H2O, C2H6O and SiC8H20O4 (as a

precursor for SiO2) are taken for preparation of SiO2 coated CoCr2O4 nanoparticles. The

chemical reagents like Ni(NO3)2.6H2O, Fe(NO3)3.9H2O, Cr(NO3)3.9H2O, C6H8O7.H2O and

NH4OH are taken for preparation of Cr doped NiFe2O4 nanoparticles. All the chemical

reagents were purchased from Sigma-Aldrich and used in their stoichiometric ratios.

For chromite nanoparticles, the first homogeneous solution is obtained by putting

the mixture of Mg(NO3)2.6H2O/Co(NO3)2.6H2O, Cr(NO3)3.9H2O) and 30 ml ethanol in

beaker and stirred with constant rate. Second solution is made by adding citric acid into

distilled water. Tetraethyl orthosilicate is also mixed in second solution just for coating of

nanoparticles. Citric acid and nitrates are mixed with 1:1 molar ratio. Afterwards, second

solution is dropped in first solution and obtained combined solution.

For Cr doped NiFe2O4 nanoparticles, the first homogeneous solution is obtained by

putting the mixture of Ni(NO3)2.6H2O), Cr(NO3)3.9H2O, Fe(NO3)3.9H2O and 30 ml


35

ethanol in beaker and stirred with constant rate. Second solution is made by adding citric

acid into distilled water. Citric acid and nitrates are mixed with 1:1 molar ratio. Afterwards,

second solution is dropped in first solution and obtained combined solution.

The next procedure is same after getting the combined solution for chromite and

ferrite nanoparticles preparation. Ammonia is dropped into the combined solution to set a

pH value to 5. After that, solution is heated at 70oC till the gel formation. The formed gel

is dried at 100oC in an oven for 12 h. To acquire the powder form, dried gel is grinded. The

powder is calcined at 900oC for 2 h to obtain the required nanoparticles.

3.8.2 Microwave plasma synthesis

Microwave plasma synthesis is a gas-phase synthesis for preparing nanoparticles.

As compared to liquid-phase synthesis e.g. sol-gel method and co-precipitation, microwave

plasma synthesis produces nanoparticles with a highly mono-dispersed and very narrow

particle size distribution. This method involves a gas-phase reaction of the evaporated

precursor materials. It is also a low temperature process because a large amount of energy

for nucleation and growth of nanoparticles provided by the plasma itself. A plasma is

believed to be a degenerate state of (gaseous) matter being ionized and charged. The total

charge of the plasma is zero. A plasma is also conductive due to mobile charged ions and

it responds strongly to electromagnetic radiation. Nanoparticles produced by this method

repel each other which shows very less tendency of agglomeration due to the residual

surface charge on them.

Cr2O3 and poly methyl methacrylate (PMMA) coated γ-Fe2O3 core-shell

nanoparticles synthesised using microwave plasma synthesis method under a 2.45 GHz

Magnetron with 2 consecutively arranged plasma zones, as shown in Fig. 3.8 by one of our

collaborators Dr. Dorothée Vinga Szabó [68] at the Institute for Materials Research III,

Karlsruhe Institute of Technology, Karlsruhe, Germany. Fe(CO)5 has been used as the

precursor for Fe2O3 formation with a feeding rate of 7.5 ml/h, and Cr(CO)6 as the precursor

for Cr2O3 formation. The respective amount of precursor was selected to yield a volume

ratio Fe2O3:Cr2O3 equal 1:1. Methylmethacrylic acid was used for the organic coating,

yielding a monolayer of PMMA on top of each particle. The PMMA coating was used just

for protection of core-shell nanoparticles. Microwave power was set to 1500 W, the


36

Ar/20vol% O2 reaction gas flow was adjusted to 10 l/min, yielding a system pressure of

10 mbar. The -Fe2O3 cores are formed in the first plasma zone, acting as nuclei for the

crystallization of the Cr2O3 shell in the second plasma zone. Polymer coating is performed

immediately behind the second plasma zone. The principles of this synthesis process are

reported elsewhere [69].

Fig. 3.8: Schematic diagram of microwave plasma synthesis [68].

Chapter No.4 Structural, Dielectric and Magnetic Properties of Chromite Nanoparticles

37

Chapter No. 4

Structural, Dielectric and Magnetic Properties of Chromite

Nanoparticles

4.1 Introduction

Recently, spinel type magnetic nanoparticles signify a great attraction due to their

better magnetic, electrical, opto-magnetic and catalytic properties than their bulk

counterpart [70-72]. A small size and high surface area of magnetic nanoparticles reveal

novel chemical and physical properties which lead to various potential applications [73-

75]. The spinel materials generally follow the AB2O4 formula, where A and B are notations

for divalent and trivalent metallic cation, respectively. The oxygen surrounds the A site as

tetrahedral and B site as octahedral [76]. If Fe+3 resides at B site, the material will be called

ferrite and if Cr+3 is at B site, it will be chromite. The chromite spinels are useful for

biomedicine, data storage, electronics, dye, catalyst and magneto-capacitive devices. In

chromite spinel unit cell, there are 16 trivalent Cr cations, 8 divalent metallic cations and

32 Oxygen anions. In chromite spinel, there is antiparallel arrangement of magnetic

moments between two lattice sites [77, 78].

Among such magnetic chromite nanoparticles, Cobalt chromite (CoCr2O4) has

recently attracted a great interest as a multiferroic material due to its potential applications

in modern technology [79]. Bulk CoCr2O4 has normal cubic spinel type crystal structure in

which Co2+ and Cr3+ ions reside at A and B sites, respectively. Bulk CoCr2O4 exhibit

paramagnetic (PM) to ferromagnetic (FiM) transition (TC) at 94 K, conical long range spiral

ordering (TS) at 27 K and lock-in transition (TL) at 15 K [80]. Menyuk et al. [26] reported

that magnetic order of bulk CoCr2O4 comprises of FiM and spin spiral state below TC by

using by neutron diffraction. FiM component shows long range order at all temperature

below TC while spiral component shows short range order at 86 K which finally transforms

into long range order at 31 K (TS). They calculated cone angles for FiM spiral long range

order in term of parameter u, where u is defined as [3],


38

𝑢 = 4𝑆𝐵 𝐽𝐵𝐵

3𝑆𝐴 𝐽𝐴𝐵 ……………………………………(4.1)

Here, JBB is nearest exchange integral spins between B- B sites, JAB is nearest exchange

integral spins between lattice sites, SB is magnitude of spin at B site and SA is magnitude

of spin at A site. They found cone angles u = 2 experimentally for FiM long range spiral

order. JBB interactions (between two chromium ions) are very strong interactions and play

a significant role in cone angles of long range spiral state (TS) and usually controls the

magnetic order in bulk CoCr2O4 [77]. At nano-scale, the low temperature magnetic

response of CoCr2O4 nanoparticles is not well understood in the literature. Galivarapu et

al. [27] examined colossal and unusual magnetic transitions in CoCr2O4 nanoparticles and

reported TC =100 K, TS = 31 K and TL = 8 K. It is noticed that TS remained same in bulk

as well as in CoCr2O4 nanoparticles which was attributed to strong B-B interactions. Long

range spiral state indicates a dominancy of B-B interactions over A-B interactions in

CoCr2O4.

In fine chromite nanoparticles, disordered and frustrated magnetization such as

spin-glass behaviour is also reported which can also change the physical properties of these

nanoparticles. Tian et al. [81] observed the dynamic behaviour of cluster spin-glass in

CoCr2O4 nanoparticles (D ≤ 5.4 nm) and found glassy transition temperature Tg at 16.3 K

which decreases with reducing the particle size of nanoparticles. Below Tg, exchange bias

effect in CoCr2O4 nanoparticles is also reported which was attributed to pinning force from

some frozen spins to rotatable spins of spin-glass phase [76]. At nano-scale, surface spins

have dominant role in controlling magnetic properties of chromite nanoparticles which

causes reduction in saturation magnetization. Kodama et al. [82] reported that surface spins

in ferrite nanoparticles which do not follow the core anisotropy direction due to presence

of exchange and broken bonds at nanoparticle’s surface. Such particles are known as core-

shell nanoparticles which consist of FM/FiM core and disordered shell. Therefore, the

magnetic properties of core-shell chromite nanoparticles also depend upon surface spins

anisotropy in addition to aligned core spins.

In addition to surface properties of chromite nanoparticles, doping can also

significantly change the physical properties of these nanoparticles. Researchers are doping

suitable cations at A or B sites in chromite nanoparticles to obtain nanoparticles with


39

desired properties for the different practical applications. Bush et al. [83] studied the

dielectric properties of Co1– xNixCr2O4 (0 ≤ x ≤ 1) samples. With increasing concentration

of Ni ions in CoCr2O4, all samples show an increase in magnitudes of tanδ and 1/ρ. Above

literature proved that doping has very interesting role in controlling the dielectric properties

of CoCr2O4 nanoparticles. The type of doped cations and along their cationic distribution

between A and B sites strongly influenced on dielectric and magnetic properties of nano-

sized CoCr2O4. The doping of non-magnetic Mg2+ ions in CoCr2O4 can reduce the magnetic

anisotropy and controls the structural stability resulting in enhanced properties of

nanoparticles [84].

Nanoparticle’s surface spins play important role in controlling its magnetic

properties for various applications and surface functionalization [85, 86]. Due to magnetic

nature of chromites, they have high tendency to agglomerate and one can get their

collective magnetic response [87]. To avoid agglomeration, nanoparticles can be coated or

disperse in non-magnetic matrix to get separate individual nanoparticles. Tsoukatos et al.

[88] reported that SiO2 acts as the best serving non-magnetic material in sputtering

deposition method than any other such type of materials as Al2O3 and TiO2. There are

various advantages of SiO2 such as its excellent stability, controlling particle size,

controlling surface effects, and control of interparticle interactions through its shell

thickness [89]. SiO2 can be used to prepare nanoparticles with smaller size of single phase

because it provides large number of nucleation sites during synthesis process, which finally

restrict the growth of nanoparticles [90-92]. Consequently, it can be used to control the

magnetic properties of multiferroic nanoparticles which will finally affects the magneto-

electric coupling [93]. Therefore, it is interesting to study the effect of non-magnetic SiO2

coating on the structural and magnetic properties CoCr2O4 nanoparticles.

In this chapter, I have focused on the study of magnetic response of CoCr2O4

nanoparticles at low temperatures, effects of Mg+2 doping on and SiO2 coating on the

physical properties of CoCr2O4 nanoparticles.


40

4.2 Results and discussion of low temperature magnetic response of

CoCr2O4 nanoparticles

4.2.1 X-ray diffraction

X-ray diffraction (XRD) is basic characterization tool which is used for getting

structural information of CoCr2O4 nanoparticles such as phase, average crystallite size and

lattice parameter. XRD pattern of CoCr2O4 nanoparticles is shown in Fig. 4.1. This

obtained XRD data was analysed by software the Philips X´pert High Score, which

completely followed (PDF#22-1084) standard patterns [94]. The diffraction peaks at 2θo =

18.4, 30.3, 35.7, 43.4, 57.5, and 63.1 indexed with (111), (220), (311), (400), (511) and

(440) crystal planes which shows cubic normal spinel crystal structure having space group

Fd3m (227). The intense and sharp XRD peaks point out well crystallized structure.

Fig. 4.1: XRD pattern of CoCr2O4 nanoparticles.

Lattice parameter of CoCr2O4 nanoparticles was calculated by using relation,

a = d √ℎ2 + 𝑘2 + 𝑙2…………..………………………… (4.2)


41

where a is the simple cubic lattice constant, d is distance between two planes and h, k, l are

just notations for miller indices of the crystal plane. The lattice parameter of CoCr2O4

nanoparticles is found 8.33Ao which is in agreement with the literature [26]. The average

crystallite size of these nanoparticles is also calculated with help of Debye-Scherrer’s

formula which is given below,

Average crystallite size (t) = 𝐾 𝜆

𝛽 𝐶𝑜𝑠𝛉 …………..……….……... (4.3)

where ‘θ’ represents diffraction angle, ‘𝛽’ represents the full width at half maximum, ‘λ’

is notation for wavelength (0.1541 nm) and ‘K’ is a constant equal to 0.9 for FWHM of

spherical crystals with cubic symmetry. The value of average crystallite size is found 42

nm for our CoCr2O4 nanoparticles.

4.2.2 Transmission electron microscopy

Transmission electron microscopy (TEM) is basic characterization technique for

getting an information about morphology, size and agglomeration of nanoparticles. Fig.

4.2 reveals CoCr2O4 nanoparticles TEM image at 100 nm scale. The image shows quite

well dispersed and non-spherical nanoparticles. There is also low tendency of

agglomeration among these nanoparticles which is due to the magnetic nature and

interaction [95, 96].

Fig. 4.2: TEM image of CoCr2O4 nanoparticles at 100 nm scale.


42

4.2.3 Fourier transform infrared spectroscopy

The vibrational properties of CoCr2O4 nanoparticles were studied at ambient

conditions using Fourier transform infrared (FTIR) spectroscopy. CoCr2O4 belongs to the

normal cubic spinel having space group O7h(Fd3m). The atoms of CoCr2O4 reside at the

8a, 16d, and 32e (oxygen) Wyckoff sites. FTIR spectroscopy is basic tool to explore the

vibrational modes of chemical bonds in CoCr2O4 nanoparticles. Fig. 4.3 shows FTIR

absorption spectrum of CoCr2O4 nanoparticles which confirms the corresponding

vibrational modes of normal spinel structure of these nanoparticles. The IR spectra reveals

two absorption bands at 537 cm−1 assigned to v2 and 446 cm−1 assigned to v3 vibrations.

Torgashev et al. [97] studied IR spectrum of bulk CoCr2O4 and discussed briefly about the

assignment of IR vibrational modes in bulk CoCr2O4. According to their study, Cr–O bonds

at 537 cm-1, on the basis of character and strength assigned primarily to the bands v2. The

IR band at 446 cm-1, formed due to complex translation of both divalent and trivalent ions,

assigned to v3 [98].

Fig. 4.3: FTIR spectrum of CoCr2O4 nanoparticles.


43

4.2.4 Magnetic Properties

To find out the low temperature magnetic response of these nanoparticles, we have

done zero field cooled/field cooled (ZFC/FC) magnetization curves, temperature

dependent M-H loops and ZFC/FC magnetic relaxation. Fig. 4.4 shows the ZFC/FC curves

of CoCr2O4 nanoparticles under 50, 500 and 1000 Oe applied field. At 50 Oe, CoCr2O4

nanoparticles showed PM to FiM transition at 98 K, along with two other low temperature

magnetic transitions; the long range conical spiral state (TS) at 27 K and the lock-in

transition (TL) at 13 K, which are in accordance to the literature [99]. The ZFC curve

exhibits negative magnetization which persisted up to 87 K under applied field of 50 Oe.

This negative magnetization has been reported by Lawes et al.[80] in bulk CoCr2O4

polycrystalline samples which was attributed to uncompensated spins at the grain

boundaries. Kumar et al. [100] studied the negative magnetization in bulk CoCr2O4 and

attributed to a small trapped field in magnetometers and a superconducting magnet under

ZFC condition. Negative magnetization in CoCr2O4 nanoparticles was also reported by

Dutta et al.[101] and they attributed it to the presence of uncompensated spins at the

nanoparticle’s surface. However, Galivarapu et al. [27] did not observe the phenomenon

of negative magnetization in CoCr2O4 nanoparticles. Ohkoshi et al.[102] also observed

negative magnetization in molecular based ferrimagnets which was attributed to

magnetization reversal in ferromagnetic component at compensation temperature, which

describes the change of sign of net magnetization in spinel compound due to different

temperature dependent magnetization of A and B sites [103]. Choi et al. [104] studied the

CoCr2O4 single crystal and did not observe negative magnetization. In our case, the

negative magnetization is only observed at 50 Oe which may be due to uncompensated

spins at the nanoparticle’s surface and/or small trapped field in magnetometers. The

negative magnetization gets vanished at higher fields such as 500 and 1000 Oe due to

reorientation of the nanoparticles magnetization in applied field direction. The peak

associated with TC was found at 98 K at 50 Oe which is shifted towards higher temperature

with increasing applied field such as 500 and 1000 Oe [105]. The dip associated with the

TS was found at 27 K at 50 Oe belongs to a structural transition in which short range spiral

component of spins produced conical magnetic structure [26]. No changes were seen in TS

at 500 and 1000 Oe which was attributed to strong B-B interactions which act as a frozen


44

spins or canted spins at surface [27]. A dip associated to lock-in state was observed at TL

= 13 K, describes a state at which period of spiral spins becomes commensurate to lattice

parameter [106, 107]. No changes were seen in TL at 500 and 1000 Oe which was attributed

to strong spin lattice coupling [108, 109].

Fig. 4.4: ZFC/FC curves of CoCr2O4 nanoparticles at 50, 500, and 1000 Oe.

Temperature dependent M-H loops have been taken to study the magnetic

transitions and surface effects on saturation magnetization (MS) and coercivity (HC) [110,

111]. Fig. 4.5 (a) reveals M-H loops of CoCr2O4 nanoparticles at temperature 5, 25, 50, 75

and 100 K under ± 5T maximum field and inset shows the HC region. M-H loops show

typical ferrimagnetic behaviour at 5, 25, 50 and 75 K but paramagnetic behaviour at 100 K

which is consistent with our ZFC/FC data. The loops are not saturated even at ± 5T due to

random surface spins of nanoparticles which need rather high field to get saturated [38].

Fig. 4.5 (b) exhibits the variation of MS with temperature from 100 - 5 K. The maximum

value of MS = 8 emu/g is found at 75 K. Below 75 K, the MS shows decreasing trend with

decreasing temperature down to 5 K. This abnormal trend deviates from Bloch’s law


45

prediction for ferrimagnetic system [112] and is attributed to presence of stiffed/strong

conical spin spiral and lock-in states at low temperatures in these nanoparticles. Fig. 4.5

(c) reveals the HC variation with temperature. The HC shows an increasing trend with

decrease in temperature. This effect is due to decreasing thermal fluctuations. A sharp

increase in HC below 25 K is observed which is attributed to frozen disordered spins at

surface of these nanoparticles at low temperatures [113]. These disordered surface spins

give addition contribution to effective anisotropy as surface anisotropy. Due to surface

defects in nanoparticles, surface anisotropy is different from the core anisotropy of the

nanoparticles. The surface spins get blocked/frozen in random directions at low

temperatures and provide hinderers to the magnetization reversal of the core spins which

results in sharp increase in HC at low temperatures. The temperature dependence HC data

of CoCr2O4 nanoparticles can be fitted by using Kneller’s law [114],

𝐻𝑐(T) = 𝐻˳[1 − (𝑇

𝑇𝐶)𝛼] ………………………(4.4)

Here Ho is coercivity at 0 K and can be estimated by extrapolating the HC (T) curve, and

TC is the curie temperature of the nanoparticles. Value of 𝛼 is considered 0.5 for non-

interacting single domain and uniaxial ferromagnetic nanoparticles. The less value of 𝛼 in

Eq. 4.4 is named as modified Kneller’s law. The red dashed line in Fig. 4.5 (c) shows the

best fit of modified Kneller’s law to HC vs. T data. The TC and 𝛼 are set as fitting

parameters. The fit is reasonable in higher temperature regime (25 to 100 K) but deviates

below 25 K. The fitting parameters came out as TC = 100 K and modified 𝛼 = 0.3. The

decrease in 𝛼 = 0.3 is attributed to interparticle interactions, finite size effects and

disordered surface spins [115]. As obvious from the Fig. 4.5(c), HC data of CoCr2O4

nanoparticles deviated from thermal activation model (Kneller’s law) at low temperature.

In literature, a deviation of Kneller’s law fit at low temperature for cobalt ferrite (CoFe2O4)

nanoparticles is reported and attributed to frozen surface spins in their random states which

prevents further alignment of spins along applied field and saturates the HC at low

temperatures [113]. Thus in our case, sharp increase of HC is due to additional surface

anisotropy at low temperature.


46


47

Fig. 4.5: (a) M-H loops at 5, 25, 50, 75, and 100 K, (b) Variation of MS with temperature (solid line just

showed the trend) and (c) Variation of HC with temperature (black solid line) of CoCr2O4 nanoparticles fitted

with modified Kneller’s law (dashed red line).

Magnetic relaxation gives information about magnetic dynamics of the system.

Usually magnetic relaxation can be done in two different protocols such as under ZFC and

FC conditions. Both ZFC and FC protocols show slow magnetic relaxation for spin-glass

systems [116]. Here magnetic relaxation has been performed also in both protocols to

investigate the magnetic dynamics of the nanoparticles. Fig. 4.6 (a) and (b) with black solid

line show time dependence ZFC and FC magnetic relaxation curves at temperature T = 5

K under field H = 100 Oe for CoCr2O4 nanoparticles, respectively. It is evident that

CoCr2O4 nanoparticles exhibit slow spin relaxation in both ZFC and FC protocols at 5 K,

which signifies the presence of spin-glass behaviour at low temperatures. Widely used

stretched exponential law is used to confirm spin-glass behaviour [117],

𝑀(𝑡) = 𝑀2 + (𝑀1 − 𝑀2)exp [− ( 𝑡

𝜏 )𝛽]……..……………….(4.5)


48

where 𝜏 is characteristic relaxation time, M1 is initial magnetization, M2 is final

magnetization, 𝛽 is stretching parameter and (M1 – M2) is glassy component or exponential

component of magnetization. The value of 𝛽 relies on energy barrier of magnetic

relaxation and, shows no relaxation at 𝛽 = 0 and fast relaxation at 𝛽 = 1. Range 0< 𝛽 >1

indicates spin-glass behaviour [118]. Stretched exponential law fitting on experimental

data confirms the presence of spin-glass behaviour. Fig. 4.6 (a) with orange line and (b)

with red line shows the best fit of stretched exponential law at Zero field cooled (ZFC) and

field cooled (FC) magnetic relaxation curve of CoCr2O4 nanoparticles respectively. The

value of fitted parameters i.e. shape parameter β = 0.43 and relaxation time τ= 852 s for

ZFC relaxation curves and shape parameter β = 0.36 and relaxation time τ= 564 s for FC

relaxation curves. The value of β lies in the spin-glass regime in ZFC and FC relaxation

curve that confirms spin-glass behaviour at low temperature in CoCr2O4 nanoparticles.


49

Fig. 4.6: (a) Zero field cooled (FC) relaxation curve of CoCr2O4 nanoparticles under field H = 100 Oe at

temperature T = 5 K, orange solid line shows the best fit of stretched exponential law, (b) Field cooled (FC)

relaxation curve of CoCr2O4 nanoparticles under field H = 100 Oe at temperature T = 5 K, red solid line

shows the best fit of stretched exponential law.

4.3 Results and discussion of effect of Mg doping on structural, magnetic,

and dielectric properties of CoCr2O4 nanoparticles


The inspection of the structural and pure phase formation information of the

nanoparticles was obtained by the Rietveld analysis of X-ray diffraction data. The Rietveld

method has made it possible today to routinely make accurate refinements of structure from

powder diffraction data. In this study, the GSAS software suite and its graphical interface

EXPGUI was used for structure refinement. The Bragg peaks have been modelled with

pseudo-Voigt function and the background was modelled by a shifted Chebyschev function

with nine terms [119, 120]. Panels (a-g) in Fig. 4.7 show the Rietveld refinement fitting

results of XRD scans of CoCr2O4 nanoparticles with Mg concentration (x) = 0, 0.2, 0.4,


50

0.5, 0.6, 0.8, and 1. Intensive and sharp peaks of XRD indicate their well crystallized

structure. Both MgCr2O4 and CoCr2O4 nanoparticles show simple cubic-type crystal

structure. All the samples are checked carefully for other impurity phases e.g., Cr2O3, MgO,

and Co3O4 etc. using quantitative Rietveld refinements technique and found no impurity

phases which signifies the formation of single phase Co1-xMgxCr2O4 nanoparticles. The fit

parameter (χ2) for refinement of the structural parameters is around 1.0, which implies that

the fitting is very good. Both MgCr2O4 and CoCr2O4 nanoparticles belong to normal spinel

crystal structure. In this crystal structure, Co2+ and Mg2+ cations reside at tetrahedral sites

and Cr3+ cation at octahedral sites. The diffraction peaks at 2θo = 18.4, 30.3, 35.7, 43.4,

57.5, and 63.1 indexed with (111), (220), (311), (400), (511) and (440) crystal planes which

shows cubic normal spinel crystal structure having space group Fd3m (227) for CoCr2O4

or MgCr2O4 nanoparticles. The peaks at (622) and (331) are observed in XRD scan of

MgCr2O4 nanoparticles, which get prominent with increasing Mg concentration [83].

Fig. 4.7 (h) reveals the variation of lattice parameter (a) as a function of Mg

concentration (x). The lattice parameter shows a non-monotonic behaviour with increasing

x. It first decreases from 8.3310 Ǻ to 8.3231 Ǻ for x = 0 to 0.6 and after that shows

anomalously increasing trend for x = 1 sample. This decreasing trend up to x = 0.6 can be

attributed to smaller ionic radius of Mg2+ (0.65 Å) ion as compared to Co2+ (0.72 Å) ion

[121] but the subsequent increase may be attributed to various uncontrolled parameters (pH

value, dopant’s lattice preference and accumulation to an interstitial sites etc.) in chemical

synthesis techniques like sol-gel method as employed in this study. A convenient method

for calculating average crystallite size of nanoparticles is the broadening of the XRD peaks.

The calculated average crystallite size of these nanoparticles after Rietveld refinement of

samples are shown in Fig. 4.7 (i) as a function of Mg concentration (x). The average

crystallite size shows an increasing trend up to x = 0.6 and then decreases down to x = 1.


51

Fig. 4.7: (a-g) Rietveld refinement fitting results of the XRD of Co1-xMgxCr2O4 nanoparticles at 300 K,

showing the observed pattern (diamonds in red colour), reflection markers (vertical bars), the best fit Rietveld

profiles (black solid line) and difference plot (blue solid line at the bottom), (h) the variation of lattice constant

and (i) average crystallite size plotted as a function of Mg concentration (x).


Transmission electron microscopy (TEM) is basic characterization technique to get

the information about morphology, agglomeration and crystal size. TEM images of

Co0.8Mg0.2Cr2O4 nanoparticles are shown in Fig. 4.8 (a) and (b) at 110 and 70 nm scales,


52

respectively. The shape of quite well dispersed nanoparticles is spherical/non-spherical.

Some degree of agglomeration is also present in these nanoparticles due to their magnetic

nature. The estimated crystallite size as obtained by XRD results is consistent with the

TEM images.

Fig. 4.8: TEM images at (a) 110 nm and (b) 70 nm scales for Co0.2Mg0.8Cr2O4 nanoparticles.


53

4.3.3 Raman spectroscopy

The Co1-xMgxCr2O4 series provide a good opportunity to examine by Raman

spectroscopy that how the vibrational modes of a normal spinel can be affected as only the

divalent cation such as magnetic Co is exchanged with a non-magnetic Mg, while the

octahedral sites are occupied by the trivalent Cr cations for all values of x in the series.

Near-IR laser with wavelength 785 nm is used for Raman spectroscopy. The atoms of

CoCr2O4 and MgCr2O4 occupy the 8a, 16d, and 32e (oxygen) Wyckoff sites. The modes

which predicted by factor group analysis for CoCr2O4 and MgCr2O4 are given as:

A1g(R) + Eg(R) + F1g + 3F2g(R) + 2A2u + 2Eu + 4F1u(IR) + F1u + 2F2u

where R and IR are notations for Raman and infrared active modes respectively, and the

remaining modes (F1g + 2A2u + 2Eu + F1u) are silent or acoustic modes. Selection rules

indicate that in an ideal normal spinel, only five modes (A1g + Eg + 3F2g) are active Raman

modes and 4F1u are four infrared active modes. Fig. 4.9 shows the Raman spectra of the

Co1-xMgxCr2O4 nanoparticles in the range of 100 to 1200 cm-1 at ambient conditions. The

different Raman modes for various Mg contents have been listed in Table 4.1. The F2g(3)

mode shows an overall linear increase in wavenumber with increasing Mg concentration.

However, the Eg, F2g(2) and F2g(1) modes exhibit nonlinear behaviour with a maximum

around 0.5 and 0.6. The largest overall change in Raman phonon frequencies is found in

the lower frequency modes, F2g (3), Eg, and F2g (2). The lowest frequency F2g(3) mode

significantly depends on the tetrahedral cation. It is one of the sharpest mode in case of

CoCr2O4 which is at 192 cm-1 [43, 86] but it’s very weak in MgCr2O4 [122]. Evidence of

this feature is supported by examining the entire Co1-xMgxCr2O4 series and noticing that

the lowest F2g(3) mode increases from 191.6 cm−1 at x = 0 to 203 cm−1 at x = 0.8. This

F2g(3) mode shift towards higher wavenumber with increasing Mg concentration and

attributed to smaller ionic radii and atomic weight of Mg2+ (0.65 Å and 24.312 amu) as

compared to Co2+ (0.72 Å and 58.933 amu), respectively. Therefore, from a simple mass

on a spring model, replacement of the Co atoms with the lighter Mg atoms should lead to

higher vibrational frequencies that confirm the successful doping of Mg at tetrahedral site.

Most literature suggests that the F2g(3) mode is a translational motion of the entire

tetrahedral AO4 unit within the lattice [123-125]. A very small change is observed in A1g


54

mode, which mainly belongs to vibration at octahedral site. This supports the claim put

forth by Preudhomme et al. [126] that the higher and lower frequency vibrations depend

more strongly upon the octahedral and tetrahedral cations, respectively. Malezieux et al.

[127] shows an increase of 11% in the wavenumber of A1g mode by replacing the

octahedral cation in the MgCrxAl2−xO4 series. The slight change in A1g mode signify that

the vibration at octahedral site is not affected by Mg doping due to preference of Mg cations

at tetrahedral site. The F2g(2) mode, located in the 500-550 cm−1 range, has a slight

dependence on the tetrahedral cation [128]. This shift from the peak position in MgCr2O4,

again showing a greater dependence on the octahedral cation. The fifth mode was predicted

by the theoretical calculations at 597 cm-1 or 523 cm-1 [129]. Our results show that observed

F2g mode is very clear and broad band at 590-593 cm-1. In addition, two extra broad bands

are observed only for MgCr2O4 nanoparticles at 351 and 851 cm-1. The bands at 351 cm-1

and 851 cm-1 can be reasonably assigned to an overtone mode [122].

Fig. 4.9: Raman spectra of Co1-xMgxCr2O4 nanoparticles.


55

Table 4.1: Vibrational bands in Raman spectra of Co1-xMgxCr2O4 nanoparticles.

x = 0 x = 0.2 x = 0.4 x = 0.5 x = 0.6 x = 0.8 x = 1 Vibration

mode Symmetry

191.6 190 190 191.25 192 203 203 F2g (3) δ (O – Co

/Mg - O)

447 447 449 478.5 459 473 430 Eg Vs (Cr - O),

Vs (Co/Mg - O)

509 516.25 526 522 535 527 528 F2g(2) V (Cr - O)

593 615 621 629 633 590 590 F2g(1) Vs (Cr - O)

677 667 667.7 672.6 677 672 671 A1g Vs (Cr - O)

4.3.4 Fourier transform infrared spectroscopy

Fourier transform infrared (FTIR) technique is used for getting information about

the vibrational modes of chemical bond present in the Co1-xMgxCr2O4 nanoparticles. Fig.

4.10 shows FTIR spectra of these nanoparticles, which confirms the spinel crystal structure

of these nanoparticles. There are three main absorption bands which observed in mid-

infrared (IR) spectra. The ranges of these bands are 688-680 cm−1 which belong to v1,

544–537 cm−1 which belong to the v2 and 455–446 cm−1 which belong to v3 vibrations.

The all vibrational bands in infrared spectra of Co1-xMgxCr2O4 nanoparticles are arranged

in Table 4.2. Torgashev et al. [97] governed primary bands v1 and v2 for CoCr2O4 by the

character and strength of the Cr–O bonds [98, 126, 130]. The band at 379 cm−1 of these

nanoparticles shifted towards higher wavenumber which contains the complex translation

of Cr+3 and Co+2 ions. Co1-xMgxCr2O4 nanoparticles have not so sharp IR bands spectra

which is probably due to the nano-sized particles. Shifting of bands of the spectra towards

higher wavenumber occurs until Mg content x = 0.6, which is consistent with our Raman

studies.


56

Fig. 4.10: Fourier transform infrared spectroscopy of Co1-xMgxCr2O4 nanoparticles.

Table 4.2: Vibrational bands in infrared spectra of Co1-xMgxCr2O4 nanoparticles.

x = 0 x = 0.2 x = 0.4 x = 0.5 x = 0.6 x = 0.8 x = 1 Assignment Symmetry

446 450 451 452 453 452 430 F1u (V3) (Co-O) &

(Cr-O)

537 540 540 538 544 542 534 F1u (V2) V(Cr-O)

680 683 687 681 687 686 676 F1u (V1) Vas(Cr-O)


57

4.3.5 Magnetic properties

Fig. 4.11 (a-e) shows the zero field cooled/field cooled (ZFC/FC) curves under 50

Oe applied field of selected samples. Paramagnetic (PM) behaviour for all concentration

of Mg is observed at higher temperatures. Co1-xMgxCr2O4 nanoparticles show initially

negative magnetization (in ZFC curve) just for with x = 0 concentration which persisted

till 80 K and x = 0.2 concentration which persisted till 20 K. This negative magnetization

is attributed to uncompensated spin which presents at grain boundaries [80, 104]. ZFC/FC

of CoCr2O4 nanoparticles revealed a transition from paramagnetic (PM) to ferrimagnetic

(FiM) state at Curie temperature (TC) = 97 K and a spiral state (TS) at 30 K, which is

consistent with literature [131, 132]. On the other hand, MgCr2O4 is a highly geometric

frustration AFM which contains Cr3+ ions at tetrahedron lattice. It reveals a complex

magnetic order below a Neel temperature (TN), which is evident by an antiferromagnetic

(AFM) transition at TN = 15 K for nanoparticles with x = 1. Nanoparticles with x = 0.2

show decrease in TC from 97 to 65 K and TS from 30 to 25 K. With further increase in Mg

concentration up to x = 0.4, nanoparticles show more decreasing magnetization and

depicted TC at 45 K and TS at 20 K. The shifting towards lower temperatures of TC and TS

with increasing x are attributed to change of low frustrated magnetic state to highly

frustrated magnetic sate. Ratclif et al. [133] reported that the highly frustrated magnetic

structure instead of general magnetization contains individual magnetized macroscopic

zones. It usually forms cluster of ions which are responsible for decreasing magnetization.

Anomalous behaviour of these nanoparticles is observed in ZFC/FC curves with x = 0.6. It

shows a change of magnetic state from FiM to Neel type antiferromagnetic (AFM) at low

temperature (TN = 15 K) in addition to high temperature FiM state (as evidence by open

ZFC/FC curves up to high temperatures).


58

Fig. 4.11: (a-e) ZFC/FC curves of Co1-xMgxCr2O4 nanoparticles under field H = 50 Oe.


59

The nanoparticles with x = 0.6 contain competing Co2+ and Mg2+ contents at this

composition but still we get clear AFM transition. The early dominance of AFM state (TN)

over FiM state (TC) in nanoparticles with x = 0.6 as compared to other lower x values is

attributed to large crystallite size as related to other values of x. Here at x = 0.6, crystallite

size effect is dominant than the competing FiM and AFM magnetic order. Therefore,

nanoparticles with x = 0.6 composition exhibit both FiM and AFM due to CoCr2O4 and

MgCr2O4, respectively. Melot et al. [134] studied Zn1-xCoxCr2O4 system and also observed

the switching magnetic phase from weak FiM structure to highly frustrated AFM at x =

0.6. For x > 0.6 (60% of Mg), Néel-type AFM is stabilized. Nanoparticles with x = 1

(MgCr2O4) show clear AFM structural transition at TN = 15 K as evidence in both ZFC and

FC curves, which is agreement with the literature [135].

Fig. 4.12: (a-d) FC curves of Co1-xMgxCr2O4 nanoparticles with applied field 5 T.

K

K


60

Fig. 4.12 (a-d) shows high field FC curves of selected samples (x = 0, 0.2, 0.6 and

1). Magnetization of FC curve for all the samples has been increased with application of

field which is due to magnetic moments alignment in applied field direction. High field

does not effect on spiral state of x = 0, and 0.2 concentration at 30 K and 25 K, respectively

which is due to strong exchange interactions of JB-B as compared to JA-B [108]. Tsurkan et

al. [109] also observed the field independent spiral state at 27 K in CoCr2O4 with reasoning

strong spin lattice coupling. Ferrimagnetic transition at TC gets broadened, which is typical

FiM system’s behaviour under high field. Nanoparticles with x = 1 show AFM transition

below the TN under applied field of 5 T. Spiral state temperature is decreased only with

increasing Mg concentration and even strong 5 T applied field does not shows any effect

on it and just enhanced net magnetization of Co1-xMgxCr2O4 nanoparticles is observed.

4.3.6 Dielectric properties

The dielectric properties of materials depend on temperature, crystal structure,

cation substitution, density, grain size, frequency of applied field etc. [136]. Fig. 4.13 (a)

and (b) reveal real (Ɛ′) and imaginary (Ɛ′′) part of dielectric constant of Co1-xMgxCr2O4

nanoparticles where x = 0, 0.2, 0.4, 0.5, 0.6, 0.8 and 1, respectively. Dielectric response of

these nanoparticles is measured at room temperature under 20 Hz - 3 M Hz frequency

range. In range of low frequency region, Ɛ′ and Ɛ′′ reveal maximum value and gradually

decrease to low value increasing frequency of applied field. This behaviour in chromite

can be justified by polarization effect. There are basically four types of polarization:

dipolar, ionic, electronic and interfacial polarization depending upon frequency of external

field. Dipolar and interfacial polarizations are dominant in frequency range of 20 Hz to 3

M Hz and contribute in our dielectric properties of chromites nanoparticles. In chromite

spinel structure, the polarization mechanism is explained on basis of electrical conduction

process. The electrical conduction process in spinel structure is explained by Heikes and

Johnston with help of electron hopping model which explain transferring of electrons

within adjacent sites in spinel structures [137]. The applied field displaces electrons and

these displacements in chromite nanoparticles provide information about polarization. The

Ɛ′ and Ɛ′′ of dielectric constant are calculated by the Eqs. 3.2 and 3.3, respectively. The

observed behaviour of Ɛ′ and Ɛ′′ are explained using Maxwell Wagner model. This model


61

describes that dielectric material is composed of well conducting grains and their poorly

conducting grain boundaries. Electrons pile up at grain boundaries at low frequency due

highly resistive nature of grain boundaries which cause to produce polarization [136].

Therefore, dielectric constant show maximum value at low frequency. The real and

imaginary part show decreasing trend as frequency increases. This effect is attributed to

space charge carriers which are lagging to frequency and do not role play in accumulate

polarization. Hence, dielectric constant show decreasing trend gradually with continuous

increase in frequency [138]. Ɛ′ and Ɛ′′ values at lower frequencies show non-monotonous

behaviour as Mg concentration increases and peaks at x = 0.6 as shown in inset of Fig. 4.13

(a) and (b). This effect is attributed to more formation of resistive grain boundaries with

optimum doping of Mg which shows more polarizability and results in a higher value of

dielectric constant [139].


62

Fig. 4.13: Variation in dielectric constants (a) real and (b) imaginary part with frequency for Co1-

xMgxCr2O4 nanoparticles.

Tangent loss or dielectric loss factor (tan δ) is ratio of dielectric constants

(imaginary to real part). Fig. 4.14 (a) shows the variation of tan δ as function of frequency

for Co1-xMgxCr2O4 nanoparticles. At lower frequency, tangent loss shows maximum value

where the storing charge ability is minimum. Tan δ also exhibits non-monotonous

behaviour with Mg doping and maximum is found at x = 0.6 as shown in inset of Fig. 4.14

(a). This variation in frequency dependent tangent loss is also described by using Maxwell

Wagner model [140-142]. Ac conductivity (σac) of chromite nanoparticles was calculated

by following formula,

𝜎𝑎𝑐 = ℇ′ ℰ˳ 𝜔 𝑡𝑎𝑛 𝛿………………….……………. ..(4.6)

where ℇ′represents dielectric constant (real component), ℰ˳ represents permittivity of free

space, tan δ is tangent loss and ω is angular frequency. Fig. 4.14 (b) exhibits σac variation

for Co1-xMgxCr2O4 nanoparticles as function of frequency. σac shows nearly frequency

independent response in low frequency region while increases sharply in high frequency

region. The variation in frequency dependent σac can be also explained using Koop’s


63

theory. This theory suggested that dielectric medium composes of multilayer capacitor

containing grains along with their grain boundaries [143]. The grain boundaries role plays

actively in conduction mechanism at low frequency. At this stage, grain boundaries provide

obstacle due to resistive nature and hinders the electrons which results decrease in mobility

of space charge carriers. Hence, chromite nanoparticles show minimum value of

conductivity at lower frequencies due to decrease in electron hopping mechanism. At

higher frequencies, only grains role plays actively in conduction mechanism. The nature of

grains is less resistive as compared to grains boundaries. As a result, ac conductivity reveals

increasing trend sharply at higher frequencies due to increase in electron hopping

mechanism. Maximum σac is observed at x = 0.6 Mg concentration as shown in Fig. 4.14

(b) inset which is due to large crystallite size.


64

Fig. 4.14: (a) Tangent loss and (b) ac conductivity of Co1-xMgxCr2O4 nanoparticles.

From this dielectric study, it is concluded that the dielectric properties are enhanced

for nanoparticles with x = 0.6 Mg concentration. This effect is not due to Mg concentration,

but it is related to the average crystallite size (as the variation of dielectric parameters with

x nearly follow the average crystallite size trend with x as evident in Fig. 4.7 (i)). Therefore,

the dielectric properties of these Mg doped CoCr2O4 nanoparticles are more effected by the

average crystallite size than Mg concentration.

The above study demonstrate that the Mg doping and average crystallite size on

structural, magnetic and dielectric properties of Co1-xMgxCr2O4 nanoparticles have

significant effects . The Mg concentration significantly affects the structural and magnetic

properties of these nanoparticles, however average crystallite size also competes with it.

On the other hand, dielectric properties are mainly determined by average crystallite size

than the Mg concentration.

Chapter No. 4 Structural, Dielectric and Magnetic Properties of Chromite Nanoparticles

65

4.4 Results and discussion of effect of SiO2 coating on structural and

magnetic properties of CoCr2O4 nanoparticles


XRD is a powerful technique to determine the crystal structure of the material. Fig.

4.15 (a) shows the XRD diffraction patterns of CoCr2O4/(SiO2)y nanoparticles with

different SiO2 concentration (y) = 0, 45 and 80 wt.%. The data was analysed by using

PANalytical X´pert High Score software, which was in good agreement with the standard

pattern JCPDS No. 22-1084. All the diffraction peaks are well indexed and correspond to

single phase spinel CoCr2O4 structure. No other impurity phases are found. The Debye-

Scherrer’s formula using Eq. 4.3 is used to calculate average crystallite size from the most

intense diffraction peak (311) for all the samples. Fig. 4.15 (b) shows the variation of

average crystalline size with increasing SiO2 concentration. The average crystallite size of

samples with y = 0, 45, and 80 wt.% come out to be 28, 22, and 19 nm, respectively. The

crystallite size reduces with increasing the SiO2 concentration. The decreasing crystallite

size with increasing SiO2 concentration is due to the formation of large number of

nucleation sites during synthesis process which restrict the further growth of nanoparticles

[144]. The lattice parameter “a” was calculated by using Bragg's equation which came out

to be 8.325 Å, 8.314 Å, 8.299 Å for samples with y = 0, 45 and 80 wt.%, respectively. Fig.

4.15 (c) demonstrate the variation of lattice parameter (a) with SiO2 concentration, which

proves the lattice contraction of nanoparticles with increasing SiO2 concentration or

decreasing crystallite size.


66


67

Fig. 4.15: (a) XRD patterns of CoCr2O4/(SiO2)y nanoparticles, (b) variation of average crystallite size and

(c) lattice parameter with SiO2 concentration. Dashed lines just show the trends.


Transmission electron microscopy (TEM) was used to analyse the shape and size

of nanoparticles. Fig. 4.16 shows the TEM image of uncoated CoCr2O4/(SiO2)y

nanoparticles (y = 0) at 50 nm scale. It is observed that the nanoparticles are non-spherical

and quite well dispersed. However, nanoparticles show some degree of agglomeration due

to magnetic interactions. The nanoparticles look larger than their average crystallite size as

obtained by XRD analysis.


68

Fig. 4.16: TEM image of CoCr2O4/(SiO2)y, y = 0 % nanoparticles at 50 nm scale.


Fig. 4.17 shows the zero field cooled (ZFC) and field cooled (FC) curves of

CoCr2O4/(SiO2)y nanoparticles with y = 0, 45 and 80 wt.% under magnetic field of 50 Oe.

The ZFC curve of uncoated nanoparticles exhibits negative magnetization which is very

similar to the data obtained by Lawes et al. [80] and Kahn et al. [145] for polycrystalline

material. The ZFC negative magnetization decreases for 45 and 80% SiO2 coated

nanoparticles. In nanoparticles, broken surface bonds alter exchange interaction and form

different surface spins structure as compared to bulk material. The negative magnetization

also refers to the existence of uncompensated spin at the grain boundaries. Molecular based

ferrimagnets also exhibits negative magnetization due to direction reversal of ferrimagnet

components at a certain temperature known as compensation temperature [102].


69


70

Fig. 4.17: (a) ZFC and FC of CoCr2O4/(SiO2)y nanoparticles (b) variation in TF, TS and TC value with SiO2

concentration. Dashed lines just show the trends.

Panel 4.17 (b) shows the variation of TF, TS and TC value with increasing

SiO2 concentration or decreasing crystallite size. The transition temperatures were obtained

from FC curves. The uncoated (y = 0%) nanoparticles exhibit a transition from

paramagnetic to ferrimagnetic and conical spin state at TC = 101 K and TS = 27 K,

respectively, which are very near to TC = 99 K and TS = 26 K values as obtained by Plocek

et al. [146]. The CoCr2O4 nanoparticles with y = 45% SiO2 coating concentration have TC


71

= 97 K and TS at 25.2 K [107]. The CoCr2O4 nanoparticles with y = 80% SiO2 coating

concentration have TC = 95 K and TS at 20 K. The TC and TS show decreasing trend with

increasing SiO2 concentration. The TS and TF transitions are rather weak in coated

nanoparticles which is due to the fact that SiO2 highly disturbs the surface of nanoparticles

and may create distortions on the surface. The shift of TC and TS towards lower temperature

with increasing SiO2 concentration can be attributed to decreasing crystallite size with SiO2

concentration. In addition to these transitions, a lock-in transition (TF) which usually occurs

in pure CoCr2O4 at 14 K as observed by Choi et al. [104] and Yamasaki et al. [22], is also

observed in our samples but at TF = 12 K (for uncoated nanoparticles). The TF value slightly

decreases with decreasing crystallite size which is also due to finite size effects [106].

Fig. 4.18 (a) shows the M-H loops of CoCr2O4/(SiO2)y nanoparticles with y

= 0, 45 and 80 wt.% under maximum applied field of ± 5T at T = 25 K. M-H loops show

typical ferrimagnetic behaviour for all the samples. The loops are not saturated even at ± 5

T which is typical for nanoparticles due to disordered surface spins. The non-saturation

behaviour of the loops increases with decreasing crystallite size or increasing SiO2

concentration. Fig. 4.18 (b) shows the variation of saturation magnetization (MS) and

coercivity (HC) with increasing SiO2 concentration. We ascribe variation of MS to the non-

magnetic amorphous behaviour of SiO2 which enhances surface spins disorder by creating

a surrounding layer around nanoparticle and leads to decrease in MS value. The surface to

volume ratio becomes also high with the decrease in particle size [147, 148]. It is also

observed that the MS value of bare (y = 0) nanoparticles is greater than coated nanoparticles

which is due to their large average crystallite size of uncoated nanoparticles as compared

to coated nanoparticles [149]. Similar phenomenon was also reported for NiFe2O4 and

CoFe2O4 ferrite nanoparticles [150, 151]. The variation of coercivity (HC) with SiO2

concentration depends upon the anisotropy of nanoparticles. The coercivity shows

maximum value for nanoparticles with y = 45 %. The HC value is related with the

magnetization reversal of the nanoparticles which may be affected by surrounding SiO2

coating material. This anomalous behaviour was also reported by Georgea et al. [152].


72

Fig. 4.18: (a) M-H loops of CoCr2O4/(SiO2)y nanoparticles at T = 25 K and (b) variation of MS and HC with

SiO2 concentration. Dashed lines just show the trends.


73

Fig. 4.19 shows the in-phase part of AC susceptibility (ZFC) as a function of

temperature for CoCr2O4/(SiO2)y nanoparticles with y = 0, 45 and 80 wt.% under applied

AC field of amplitude (HAC) = 5 Oe and frequency (f) = 1 Hz.

Fig. 4.19: ZFC AC susceptibility (in-phase part) of CoCr2O4/(SiO2) nanoparticles.

A sharp peak in χ' is associated with the ferrimagnetic transition temperature at 94 K for

uncoated CoCr2O4 nanoparticles (y = 0%) which is nearly the same as observed by Lawes

et al. [80] and Mantlikova et al. [153]. However, Zakutna et al. [154] and Rath et al. [155]

reported lower T values in the vicinity of 76 K. It can be seen clearly that transition peak

decreases with increasing SiO2 concentration and gets broadened and these findings are

consistent with the dc ZFC/FC magnetization curves. The obvious reason for decreasing

transition peak temperature and its broadening is the presence of amorphous SiO2 which

enhances surface spins disorder and decreases crystallite size. Anomalies associated with

TS and TF are also observed in ZFC AC susceptibility but very weak due to very low applied

AC field.


74

4.5 Conclusion

Magnetic response of CoCr2O4 nanoparticles at low temperatures have been studied

in detail. XRD and FTIR spectroscopy confirm the spinel structure of CoCr2O4

nanoparticles. ZFC/FC curves revealed a PM to FiM transition at TC =97-101 K with a

pronounced spin spiral state TS at 27 K and lock-in state TL at 13 K. The TC is shifted

towards higher temperature with applying higher fields while TS and TL remain unaffected.

A negative magnetization is observed in ZFC curve at 50 Oe which is suppressed at higher

fields. An abnormal decreasing MS trend with decreasing temperature is observed and

attributed to presence of disordered stiffed/strong conical spin spiral and lock in states at

low temperatures. Modified Kneller’s law for HC showed a good fit but deviated from

experimental data at 5 K due to sharp increase in HC at low temperatures which is ascribed

to increased surface anisotropy at low temperatures. These nanoparticles showed also slow

spin relaxation at 5 K in ZFC and FC curves, which signifies the presence of spin-glass

behaviour at low temperatures. XRD of Mg doped CoCr2O4 nanoparticles showed no

impurity phases which confirmed single phase Co1-xMgxCr2O4 nanoparticles. The average

crystallite size exhibited a non-monotonic behaviour with peak value of 109 nm for

nanoparticles with Mg concentration x = 0.6. Both TC and TS decrease as Mg concentration

increases and finally system undergoes a frustrated AFM transition at TN for nanoparticles

with Mg concentration x = 1 (pure MgCr2O4 phase). The dielectric measurements of these

nanoparticles were improved with Mg concentration x = 0.6 and is attributed to their larger

average crystallite size. The SiO2 coating on CoCr2O4 nanoparticles reveals decreasing

trend of average crystallite size and lattice parameter. All magnetic transitions tend to lower

temperatures as SiO2 coating concentration increases. This was due to finite size effects

caused by SiO2 coating. M-H loops reveal decreasing trend in MS value as coating

concentration of SiO2 increases due to small size of coated nanoparticles. The ZFC ac

susceptibility with temperature also indicated that TC deceases from 94 K to 85 K as coating

concentration of SiO2 increases.

Chapter No. 5 Structural, Dielectric and Magnetic Properties of Ferrite Nanoparticles

75

Chapter No 5

Structural, Dielectric and Magnetic Properties of Ferrite

Nanoparticles

5.1 Introduction

In ferrite nanoparticles, surface effects arises due to high surface to volume ratio

which can directly influenced on the magnetization reversal and relaxation of magnetic

nanoparticles [156]. Spin disorder, surface anisotropy and weak exchange coupling near

and at the surface can modify the physical properties of ferrite nanoparticles. The enhanced

surface anisotropy, high field irreversibility and high coercivity, non-saturation

magnetization at low temperature provide clear evidence of spin glass behaviour at low

temperature [38, 157-160]. Among such ferrite nanoparticles, Nickel ferrite (NiFe2O4) and

maghemite (γ-Fe2O3) nanoparticles are very promising due to practical applications in

different fields such as data storage, biomedical therapy and diagnostic, ferro-fluids, and

transformers cores.

γ-Fe2O3 nanoparticles are ferrimagnetic in nature and exhibit spinel structure along

with cation vacancies present at octahedral (B) sites. These vacancies along with competing

surface interactions can lead to surface spins disorder and spin glass behavior in γ-Fe2O3

nanoparticles [161-163]. Fiorani et al. [164] studied the dynamical and static magnetic

response of γ-Fe2O3 nanoparticles which are governed by surface effects and interparticle

interactions. These effects also decrease overall magnetization of nanoparticles as

compared to bulk. Herlitschke et al. [165] observed 44% less magnetization in γ-Fe2O3

nanospheres and 58% less magnetization in γ-Fe2O3 nanocubes than bulk, which is due to

spin disorder as analysed by using nuclear resonant scattering and polarized neutrons.

γ-Fe2O3 nanoparticles are highly reactive which leads to non-functionalized surface

and easily losses their magnetic properties. Therefore, proper surface coating or developing

effective protection is used to minimize surface energy and to prepare stable nanoparticles

for potential applications [116]. Coating not only stabilizes nanoparticles but can also lead

to surface functionalization. Different approaches for coating have been used so far which


76

include coating with polymer, biomolecules, surfactants, magnetic and non-magnetic etc.

Prado et al. [166] reported an improvement in the magnetic anisotropy of γ-Fe2O3

nanoparticles via surface coordination of molecular complexes. Azhdarzadeh et al. [167]

prepared gold coated iron oxide nanoparticles which are useful for photo thermal therapy

of colan cancer and magnetic resonance imaging.

In this thesis, we prepared core-shell Cr2O3 coated γ-Fe2O3 nanoparticles are

prepared by microwave plasma technique. Selection of antiferromagnetic Cr2O3 coating is

due to high hardness, mechanical strength, chemical inertness and low friction coefficient

[168]. These properties make Cr2O3 coating very useful in the field of corrosion protection,

wear resistance and surface modification [169]. Sahan et al. [170] observed surface

modification of spinel LiMn2O4 by Cr2O3 coating and reported enhanced electrochemical

properties for potential application. This study showed that Cr2O3 coating can be used for

surface modification. Therefore, it is interesting to study the effects of Cr2O3 surface

coating on magnetic response of γ-Fe2O3 nanoparticles.

Doping in ferrite nanoparticles have very interesting role in tuning the physical

properties and structure stability of ferrite nanoparticles. Doping at B site may cause the

cationic distribution between two lattice sites of spinel ferrites nanoparticles which can

alter the physical properties of these nanoparticles [171]. Doping of antiferromagnetic Cr+3

at B site in NiFe2O4 nanoparticles can control structure stability which can enhance

physical properties of NiFe2O4 nanoparticles. So, the aim of this interesting research is to

better understand the effect of Cr doping on structural, magnetic and dielectric properties

of NiFe2O4 nanoparticles. We have synthesized these nanoparticles by sol-gel method.

In this chapter, I have emphasized on the surface effects in Cr2O3 coated γ-Fe2O3

nanoparticles and effect of Cr doping at B site on structural, magnetic and dielectric

properties of NiFe2O4 nanoparticles.


77

5.2 Results and discussion of effect of surface spins on magnetization of

Cr2O3 coated γ-Fe2O3 nanoparticles


Powder X-ray diffraction (XRD) is a unique characterization technique for getting

structural information of samples. Fig. 5.1 reveals the XRD scans of Cr2O3 coated γ-Fe2O3

nanoparticles. Broadened peaks in this XRD exhibit the well crystalline nature of γ-Fe2O3

nanoparticles. The indexed peaks (220), (311), (400), (422), (511) and (440) at angles 2θ

= 30˚, 36˚, 43˚, 54˚, 57˚ and 63˚, respectively verify the inverse spinel structure of γ-Fe2O3.

While, the indexed peaks (012), (104), (110), (113), (024), (116), (214) and (1010) at

angles 2θ = 24˚, 33˚, 36˚, 41˚, 50˚, 54˚, 63˚ and 72˚, respectively correspond to the Cr2O3

phase [172, 173]. High intensity diffracted peaks in XRD scans are for aluminium substrate.

The absence of impurity peaks in XRD scan confirms that the synthesized materials have

very high purity.

Fig. 5.1: X-ray diffraction patterns for Cr2O3 coated γ-Fe2O3 nanoparticles.


78

The average crystallite size of these nanoparticles is also calculated with help of

Debye-Scherrer’s formula [174] using Eq. 4.3. The average crystallite size of γ-Fe2O3 core-

nanoparticles is 13 nm. The intensive peaks of Cr2O3 in XRD indicate the higher

concentration of Cr2O3 phase in the sample.


Transmission electron microscopy (TEM) is used for getting information about

nanoparticle’s morphology. Fig. 5.2 (a) shows a TEM image of the nanoparticles at 10 nm

scale. Most of the nanoparticles are spherical with moderate degree of agglomeration [175].

Their size ranges from 5 to 20 nm. A core-shell structure is hardly to detect in these

nanoparticles, due to the weak difference in mass contrast of the phases Fe2O3 and Cr2O3,

respectively. Fig. 5.2 (b) shows a STEM-image. Also here, the contrast difference of the

two phases of interest is very weak. The red marked area was analyzed by STEM-EELS,

and the results are shown in the insets. In detail, the insets show the color-coded

composition map (red: Fe, green: Cr), the detailed STEM image, and the STEM-EELS

images from the Cr-L edge, the Fe-L edge. Fig. 5.2 (c) shows the corresponding STEM-

EELS spectra from the core-region and the shell-region, marked by arrows. This analysis

demonstrates that core-shell nanoparticles are present. Nevertheless, the presence of bare

Fe2O3 and bare Cr2O3 nanoparticles cannot be excluded.


79

Fig. 5.2: (a) TEM image at 10 nm scale (b) STEM-image at 50 nm scale (inset shows the results of red

marked area by STEM-EELS) of Cr2O3 coated γ-Fe2O3 nanoparticles and (c) STEM-EELS spectra of γ-Fe2O3

core (red color)-Cr2O3 shell (green color) nanoparticles.


Zero-field cooled and field cooled (ZFC/FC) protocols were taken to study the

magnetization with temperature of these nanoparticles. Fig. 5.3 shows experimental (solid

triangles) and simulated (open squares) ZFC/FC dc susceptibility curves of Cr2O3 coated

γ-Fe2O3 nanoparticles at 50 Oe. The maximum magnetization is observed at 75 K in ZFC

experimental curve. This temperature is named as the average blocking temperature (TB).

Just below the TB, experimental FC curve turns out to be flat because magnetic moments

of nanoparticles get frozen and could not aligned themselves in a direction of applied field


80

[116] which is clue for presence of interparticle interactions or surface disorder in γ-Fe2O3

nanoparticles [159].

To get information of structural parameter and intrinsic magnetic properties of

nanoparticles, ZFC/FC curves are simulated according to the model of non-interacting

particles. For simulation, we have used Neel-Brown expression for relaxation time ( )

assuming uniaxial anisotropy [176, 177]. The temperature at which = m for a system of

particles with average volume V, is known as blocking temperature TB. Log-normal

distribution function for TB is extracted from log-normal distribution function for particle

size and given as,

B

T

B

B

BT

BB dTT

T

TdTTf

BB

2

2

2 2

ln

exp1

2

1)(

(5.1)

Since, average TB is related to average volume V and given as,

V

k

Tm

B

eff

B

K

0

ln

(5.2)

Where Keff is effective anisotropic constant and is atomic precession time. The ZFC dc

susceptibility according to the model of non-interacting particles consists of two

contributions (i) and (ii) given as [178],

)()(

00

2

)()(ln3

)(

ii

TB

TTB

BB

i

TTB

TB

BBBms

ZFC dTTfdTTfT

T

K

MT

(5.3)

Where contribution (i) is due to superparamagnetic particles and contribution (ii) is for

blocked particles. FC dc susceptibility is given below using the same model [178],


81

)()(

00

2

)()(1

ln3

)(

ii

TB

TTB

BB

i

TTB

TB

BBBms

FC dTTfdTTfTTK

MT

(5.4)

Using Eq. 5.3 and 5.4, we have fitted ZFC/FC susceptibility curve with Keff = 1.4 x 105

erg/cc. Fitted Keff value is greater than bulk γ-Fe2O3 (4.7 x 104 erg/cc) which is due to

additional surface anisotropy offered by frozen disordered surface spins [164]. The Keff of

Cr2O3 coated γ-Fe2O3 nanoparticles is less than bare nanoparticles of γ-Fe2O3 (9.8 x 105

erg/cc) [49] which is due to relatively big particle size of coated nanoparticles. Moreover,

big particle size with antiferromagnetic surface layer may produce small value of interface

anisotropy between core-shell nanoparticles which results decrease in fitted Keff. Simulated

curves also infer about moderate particle size distribution (σD = 0.23). A difference occurs

between experimental FC and simulated curves at low temperatures. This is because real

nanoparticles system contains interparticle interactions and model considers only non-

interacting nanoparticles. FC simulated curve does not flat immediately just below the TB

and flattens at very low temperature.

Fig. 5.3: ZFC/FC experimental (blue solid triangles) and simulated (red open squares) dc susceptibility

curves of Cr2O3 coated γ-Fe2O3 nanoparticles under 50 Oe.


82

Fig. 5.4 (a) shows M-H loop at temperature 5 K for Cr2O3 coated γ-Fe2O3

nanoparticles whereas inset reveals expanded region of coercivity under field of ± 5 T.

Measured value of HC and MS is 214 Oe and 19.7 emu/g, respectively which are nearly

equal to reported value of HC and MS by Tzitzios et al. [179] of same size γ-Fe2O3

nanoparticles embedded in a laponite synthesized via one step chemical route. The

measured MS has lower value than bulk γ-Fe2O3 (MS = 80 emu/g) which is attributed to

dangling and broken bonds at the surface. At nano scale, the dangling and broken bonds

produce less coordination neighbours at surface which are responsible for decrease in

exchange interactions and as a result, MS decreases [164]. The MS of Cr2O3 coated γ-Fe2O3

nanoparticles is also less than bare γ-Fe2O3 nanoparticles MS = 51 emu/g at same

temperature [49]. The lower value of MS of Cr2O3 coated nanoparticles is due to presence

of antiferromagnetic Cr2O3. The observed value of HC is low due to soft magnetic nature

of γ-Fe2O3 nanoparticles [180]. The HC of Cr2O3 coated γ-Fe2O3 nanoparticles is less than

bare nanoparticles of γ-Fe2O3 (HC = 546 Oe) [49]. The possible reason is bigger particle

size of coated nanoparticles. The size of γ-Fe2O3 core-nanoparticles (13 nm) produced with

high feeding rate and with additional high concentration of an antiferromagnetic Cr2O3

shell is larger than uncoated nanoparticles size (6 nm). The larger coated nanoparticles have

weak interface anisotropy between γ-Fe2O3 ferrimagnetic core and Cr2O3

antiferromagnetic shell which refers to lower effective anisotropy of coated nanoparticles

than bare nanoparticles. Thus, antiferromagnetic Cr2O3 shell and interfacial interactions

play critical role in controlling the magnetization of γ-Fe2O3 nanoparticles. Size dependent

HC is remarkable and Trohidou et al. [181] observed the same phenomena in Monte Carlo

studies of ferromagnetic core and antiferromagnetic shell nanocomposites.

We have also studied the temperature dependent saturation magnetization and

coercivity for our nanoparticles. I have taken partial M-H loops under field of ± 5 T at

temperatures; 5 K, 25 K, 50 K, 100 K, 150 K and 250 K for these nanoparticles. Loops are

not saturated even at high field of 5 T due to random surface spins. Fig. 5.4(b) exhibits MS

variation with temperature (solid spheres) and fitting of Bloch’s law (red dashed line).

Increasing trend of MS with decreasing temperature is due to decreasing thermal

fluctuations [182] which is according to prediction of Bloch’s law. The Bloch’s law

equation is,


83

b

SS BTMTM 10 (5.5)

Where MS (T) is measured magnetization as function of temperature, MS (0) is extrapolated

magnetization at 0 K, b and B are Bloch’s exponent and Bloch’s constant respectively

which are used as fitting parameters. The value of B strongly depends on structure of

materials and closely relates to exchange integral J as (B ̴ 1/J) [183]. Bloch’s law is valid

for bulk ferromagnetic materials with b = 3/2 but for nanomaterials, the value of b changes

due to surface spins disorder, finite size effects and inter-particles interactions [184]. We

have fitted Bloch’s law on our experimental MS data, which provides the value of b = 1.10

and B = 3.523 x 10-4 K-b. Higher value of B in these nanoparticles than bulk γ-Fe2O3 is due

to decrease of particle size which results in weaker exchange coupling J (B ̴ 1/J) due to

surface disorder. Lower value of b is due to no spin wave excitation at low temperatures in

presence of large energy band gap at nano-scale due to finite size effects [185].

Fig. 5.4 (c) shows the variation of coercivity (HC) (solid spheres) with temperature

for these nanoparticles. The observed HC reveals increasing trend with decreasing

temperature which is due to decreasing thermal fluctuations and increased effective

anisotropy at low temperatures [180]. HC reveals sharp increase below 25 K due to

disordered surface spins which got frozen at lower temperatures and contributes to effective

anisotropy in the form of surface anisotropy [159]. Molina et al. [186] also reported the

sharp increase in HC in (Fe0.69Co0.31)B0.4 nanoparticles which was due to increase in

effective anisotropy. The temperature dependent HC data can be fitted using Kneller’s law

as given in Eq. 5.6,

B

CCT

THTH 10 (5.6)

Where HC (0), TB and α are extrapolated coercivity at 0 K, average blocking temperature

and constant having value 0.5 for bulk ferromagnetic material, respectively. Values of TB

and α were got from the best fit. Fig. 5 shows Kneller’s law fit (red dashed line) on

experimental HC data for the Cr2O3 coated γ-Fe2O3 nanoparticles with fitting parameters

TB = 120 K and α = 0.13. Lower value of α for these nanoparticles is due to finite size

effects, interparticle interactions and surface disorder [115]. The fit is reasonable at high


84

temperatures but deviates at low temperatures due to sharp increase in HC (contribution of

surface anisotropy below 25 K) which is not considered in Kneller’s law. HC vanishes

above 50 K because of superparamagnetic de-blocking nature and it is consistent with

experimental and simulation ZFC/FC results [187]. Above TB, the huge-core spin thermally

de-block and therefore HC vanishes due to onset of superparamagnetic behaviour of the

nanoparticles [188].


85

Fig. 5.4: (a) M-H loop at 5 K, (b) MS at different temperatures (Bloch’s law fitting is in form of red

dashed line) and (c) HC at different temperatures (Kneller’s law fitting is in form of red dashed line) for

Cr2O3 coated maghemite nanoparticles.


86

DC field magnetic measurements of these coated nanoparticles indicate the

presence of surface spins disorder at low temperature and it is considered as fingerprint for

surface spin glass. A system in which randomly alignment of magnetic spins exists with

their frustrated magnetic interactions, is named as spin-glass system. In temperature regime

of spin-glass system, TB shows variation with changing frequency of applied ac magnetic

field. To study the surface spin glass, I measured frequency dependent ac susceptibility at

temperature range 5 to 300 K. Fig. 5.5 (a) depicts the ac-susceptibility (in-phase) of these

coated nanoparticles in 1 to1000 Hz frequency range under Hac = 5 Oe amplitude. The

curves show increasing trend of TB (95 to 115 K) with increasing frequency (1 to 1000 Hz).

A reasonable shift of TB with frequency is observed in these nanoparticles. This f-shift of

TB can be analysed through various types of physical laws. Arrhenius, Vogel-Fulcher and

Dynamics scaling law are used for getting information about interparticle interactions and

spin-glass state. Arrhenius law is used for non-interacting single domain particles [189].

Arrhenius law is,

BB

a

Tk

E

e (5.7)

Where τo, kB and Ea are atomic spin flip time (10-9 to 10-12 s), Boltzmann constant and

activation energy, respectively. The activation energy is defined as Ea = KeffV (V is volume

and Keff is an anisotropic constant of particles) [190]. Fig 5.5 (b) shows fitting of Arrhenius

law on f- dependent ac susceptibility of coated maghemite nanoparticles. In this fit, τo and

Ea /kB are used as fitting parameters and obtained Ea /kB = 3763 K and τo = 6.71 x 10-18 s.

A very small value of τo and high value of Ea/kB for these nanoparticles are unphysical.

With these inadequate fitting parameters, Neel-Arrhenius law failed to analyze our system

which may be due to interparticle interactions. To find out the possible interparticle

interactions strength, we have used Vogel-Fulcher law [191] having general formula,

TTk

E

BB

a

e (5.8)

Where an additional parameter To represents the strength of interaction between particles

[192, 193]. Fig 5.5 (c) exhibit the fitting of Vogel-Fulcher law at same f-dependent ac


87

susceptibility data. Obtained fitting parameters are Ea /kB = 679 K and τo = 9.84 x 10-9 s

with To = 59 K. Now, both fitting parameters Ea /kB and τo show reasonable values. High

value of To confirms the interactions between these nanoparticles. This may be due to

disordered surface spins and interparticle interactions which leads to surface spin glass.

Dynamic scaling law usually preferred to investigate spin-glass system and general formula

is [49],

zv

B TT

T

(5.9)

Where τ* is coherence time, τ is relaxation time, TB is peak value of (T) curve, To is

freezing temperature and zv is critical exponent. Value of zv for typical different spin glass

systems lies between 4 to 12. I have also fitted Dynamic scaling’s law for possible spin

glass system. Fig 5.5 (d) shows dynamic scaling law fit for Cr2O3 coated γ-Fe2O3

nanoparticles with fitting parameters zv = 10.9 and τ* = 1.3 x 10-5 s. Present value of zv

confirms the spin glass system in these nanoparticles. Value of τ* is high and attributed to

frozen surface spins relaxation. The obtained value of To = 70 K by fitting is consistent to

TB obtained by ZFC experimental result [194, 195].


88

Fig. 5.5: (a) In-phase ac susceptibility of Cr2O3 coated γ-Fe2O3 nanoparticles. The f- dependent TB is fitted

with (b) Arrhenius law (c) Vogel-Fulcher law and (d) dynamic scaling law.

5.3 Results and discussion of study of Cr doping on structural, magnetic

and dielectric properties of NiFe2O4 nanoparticles

5.3.1 X-Ray diffraction

Fig. 5.6 (a) shows XRD pattern of NiCrxFe2-xO4; x = 0, 0.2, 0.4, 0.6, 0.8 and 2

nanoparticles. The observed indexed diffracted peaks for NiCrxFe2-xO4 nanoparticles at

angles, 2θ = 18.5˚, 30.4˚, 35.7˚, 37.4˚, 43.4˚, 54˚, 57.5˚, 63.1˚ and 74.6 ˚ are (1 1 1), (2 2

0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1), (4 4 0) and (5 3 3) respectively for all samples.

Cubic spinel crystal structure is observed for all samples [196]. All the major peaks of

NiFe2O4 remain unchanged after the Cr dopant. The absence of impurity peaks in XRD

confirm the high purity of the single phase NiCrxFe2-xO4 nanoparticles.


89

Fig 5.6: (a) XRD patterns and (b) lattice constant and average crystallite size of NiCrxFe2-xO4

nanoparticles.


90

Fig. 5.6 (b) shows value of lattice parameter (red star) and average crystallite size

(blue solid square) for NiCrxFe2-xO4 nanoparticles. The lattice constant of these

nanoparticles was calculated using Eq. 4.2 [197]. The calculated lattice parameter of Cr

doped nanoparticles revealed decreasing trend with increasing concentration of Cr ions at

B site. Decreasing trend of lattice constant is attributed to replacement of smaller ionic

radius of Cr3+ (0.64 Å) with larger ionic radius of Fe3+ (0.67 Å) [198]. Trivalent Cr3+ ions

have strong preference for octahedral site which would replace Fe3+ ions at octahedral site.

This replacement has not impacted on cubic spinal structure of NiCrxFe2-xO4 nanoparticles

for all samples.

The average crystallite size of these nanoparticles is also calculated with help of

Debye-Scherrer’s formula [196] using Eq. 4.3. The calculated average crystallite size of

NiCrxFe2-xO4 nanoparticles are in 28 - 44 nm range for different concentration of x. The

maximum average crystallite size is obtained for x = 0 and depicted the decreasing trend

as Cr concentration increases which is due to reduction in unit cell.


Fig. 5.7 shows TEM images of NiCr2O4 nanoparticles at 20 and 100 nm scale. TEM

scans depicts well dispersed spherical/non-spherical nanoparticles but most of them are

irregular shape. The nanoparticles show less tendency of agglomeration. The average

particle size was also calculated 47 nm from TEM image which supports well our from

XRD calculated size of 44 nm [199].


91

Fig. 5.7: TEM images of NiCr2O4 nanoparticles at (a) 20 nm and (b)100 nm scale.


The shape of M-H loop depends on porosity, chemical composition of compound

and grain size, etc. Here, M-H loops are presented to explore the effect of antiferromagnetic

Cr concentration on magnetic properties of NiFe2O4 nanoparticles. Fig. 5.8 (a) reveals the

M-H loops of all concentration of NiCrxFe2-xO4 nanoparticles at T = 5 K under field of ±

5T and inset reveals the expanded region of coercivity. M-H loops for all samples exhibit

(a)

(b)


92

ferrimagnetic nature. All loops do not saturate even at maximum field (5 T) which may be

disordered surface spins. Fig. 5.8 (b) exhibits saturation magnetization (MS) variation in

NiFe2O4 nanoparticles as Cr concentration increases. MS reveals maximum value for

NiFe2O4 nanoparticles and depicts decreasing trend with addition of Cr3+ ion concentration

in ferrites nanoparticles. This reduction may be attributed to large magnetic moment of

Fe3+ (5 μB) are replaced by smaller magnetic moment of Cr3+ (3 μB) at octahedral sites of

the NiFe2O4 nanoparticles sublattice. Fe3+(B)/Fe2+(A) ratio decreases with addition of

Cr3+ ion concentration, as a result A-B super exchange interaction also decreases [200].

Fig. 5.8 (c) shows the variation of coercivity (HC) in NiFe2O4 nanoparticles with increasing

Cr concentration. HC reveals minimum value for NiFe2O4 nanoparticles and depicts

increasing trend with addition of Cr3+ ion concentration in ferrites nanoparticles. This

increase in HC may be due to change in magneto crystalline anisotropy. The reason in

magneto crystalline anisotropy transformation may be magnetic coupling. Magnetic

coupling enhances with replacement of quenched orbital angular momentum Fe3+ by

unquenched orbital angular momentum Cr3+ which lead to higher magneto crystalline

anisotropy, as a result coercivity increases [201].


93

Fig. 5.8: (a) M-H loops at T = 5 K, (b) MS variation and (c) HC variation for NiCrxFe2-xO4 nanoparticles

with Cr concentration (x). solid lines just reveal the trend.

5.3.4 Dielectric properties

Dielectric properties of nanoparticles depend on different factors mainly chemical

composition, process of preparation, annealing temperature, ratio of dopant particles and


94

sizes of prepared particles etc. I have studied effect of Cr+3 doping at octahedral B-site on

the dielectric properties of NiFe2O4 nanoparticles. The dielectric response of these

nanomaterials was measured using LCR meter. Frequency range of available LCR meter

is 100 Hz to 4 MHz. I have calculated real and complex (imaginary) dielectric constants,

dissipation factor and ac conductivity for these nanoparticles.

Fig. 5.9 (a) and (b) show the real and imaginary part of dielectric constant of Cr

doped NiFe2O4 nanoparticles for all samples. Real and imaginary part of dielectric

constants are calculated using Eqs. 3.2 and 3.3, respectively. In range of low frequency

region, the dielectric constants (real and imaginary part) reveal maximum value and

gradually decrease to low value increasing frequency of applied field. This frequency

dependent real and imaginary part behaviour is typical in ferrite and chromite system. This

behaviour is due to accumulation of polarization. There are basically four types of

polarization: dipolar, ionic, electronic and interfacial polarization depending upon

frequency of external field. These types of polarization active very well at low frequency

and relaxed at high frequency, resulting in decreasing the real ad imaginary part of

dielectric constant at higher frequency [202]. Koop’s theory and Maxwell-Wigner and

model are used which further explains this behaviour [143]. This model suggested that

ferrites and chromites consist of highly conducing region grains and poorly conductive

grain boundaries. The grain boundaries are more active, and role play in producing

polarization at low frequencies because electrons pile up at resistive grain boundary and

enhances polarization. At higher frequencies, the lagging behind of charge carriers of

grains cannot contribute in polarization of dielectric constant. As a result, real part of

dielectric constants decreases at high frequency [203]. In our frequency range, mostly space

charge polarization occur and play role in dielectric constant [204].

The real and imaginary part of Cr doped NiFe2O4 nanoparticles show increasing

trend of dielectric constant with increasing concentration of Cr+3 dopant. The maximum

value of real and imaginary part of dielectric constant are observed at x = 2. This increasing

trend is attributed to less conductive nature of Cr as compare to Fe. The conductivity of Fe

is 1×107 S/m and the conductivity of Cr is 6.7×106 S/m.


95

Fig. 5.9: (a) Real and (b) Imaginary part of NiCrxFe2-xO4 nanoparticles.


96

Fig. 5.10 reveals the dissipation factor of NiCrxFe2-xO4 nanoparticles. Dielectric

loss tangent has maximum value at low frequency region and reveals decreasing trend with

increasing value of frequency. This decreasing trend is described with the help of Koop’s

model [143]. The grain boundaries are resistive at low frequency and needed more energy

to move the electron. The dissipation factor with increasing doping concentration of Cr+3

on octahedral site depicts increasing trend. This increasing trend is attributed to less

conductive nature of chromium as compare to iron. The conductivity of iron is 1*107S/m

and the conductivity of chromium is 6.7*106 S/m [89]. The maximum value of dissipation

factor is at x = 2 Cr concentration.

Fig. 5.10: Tangent loss of NiCr2Fe2-xO4 nanoparticles.

With the help of AC conductivity using Eq. 4.6, we can understand the mechanism

of conduction for these nanoparticles. Fig. 5.11 reveals ac conductivity of NiCrxFe2-xO4

nanoparticles for all values of x. AC conductivity of nanoparticles increases with increasing

frequency. At low frequency grains boundaries are effective, so electrons cannot overcome

the barrier, therefor ac conductivity is minimum. While at high frequency area grains

t


97

become effective, therefore electrons move easily, and ac conductivity show increasing

trend. As a result charge career hopping rises with increasing applied field and ac

conductivity also increases [205]. Ac conductivity NiFe2O4 nanoparticles has decreased

with increasing concentration of Cr. This decreasing trend is attributed to less conductive

nature of chromium as compare to iron. The hopping of electrons taking place in

mechanism of conduction also decreases. That’s way ac conductivity revealed decreasing

trend.

Fig. 5.11: AC conductivity of NiCrxFe2-xO4 nanoparticles.


98

5.4 Conclusion

Surface effects of Cr2O3 coated γ-Fe2O3 nanoparticles have been analysed by using

detailed magnetic measurements. Microwave plasma method is used for synthesis of these

nanoparticles. XRD result revealed the inverse spinel crystal structure for γ-Fe2O3

nanoparticles. Simulated ZFC/FC data exhibited the lower value of effective anisotropy

constant of Cr2O3 coated γ-Fe2O3 nanoparticles than bare nanoparticles of γ-Fe2O3 which

is due to relatively big particle size of coated nanoparticles and weak interface anisotropy

between ferrimagnetic core and antiferromagnetic shell. Saturation magnetization shows

increasing trend with decreasing temperature and fitted with Bloch’s law which provides

the value of b = 1.10 and B = 3.523 x 10-4 K-b. Higher value of B in these nanoparticles

than bulk γ-Fe2O3 is due to decrease of particle size which results in weaker exchange

coupling J (B ̴ 1/J) due to surface disorder. The sharp increase of coercivity is similar to

bare nanoparticles and is attributed to enhanced surface disorder and high surface

anisotropy below 25 K. Frequency dependent ac susceptibility showed shifting of TB which

was due to interparticle interactions and surface spins disorder. Frequency dependent TB

shift was fitted by using theoretical models. Dynamic scaling law fit to the data confirmed

the existence of spin-glass behaviour which is originated by disordered surface spins

disorder system and in accordance with the dc magnetic measurements. Structural analyses

of sol-gel prepared Cr doped NiFe2O4 nanoparticles were confirmed using XRD analysis.

Saturation magnetization (MS) of coated nanoparticles depicted decreasing trend with

addition of Cr3+ concentration which is attributed to replacement of large magnetic moment

of Fe3+ by smaller magnetic moment of Cr3+. HC revealed increasing trend with addition of

Cr3+. This increase in HC may be due to change in magneto crystalline anisotropy. The

frequency dependent dielectric constants show decreasing trend with frequency which is

due to decrease in polarization. The dielectric constant of NiFe2O4 nanoparticles was

enhanced with concentration of Cr.

99

6 General Conclusion

In conclusion, chromite and ferrite ultrafine and homogeneously distributed

nanoparticles have been synthesized by Sol-gel and Microwave plasma method. Their

properties have been investigated in detail by XRD, FTIR, TEM, LCR meter and SQUID-

magnetometry. Temperature dependent magnetic response of CoCr2O4 nanoparticles shifts

TC towards higher temperature with applying higher fields while TS and TL remain

unaffected due to strong B-B interactions which act as a frozen spins or canted spins at

surface. A negative magnetization is observed in ZFC curve due to presence of

uncompensated spins at the nanoparticle’s surface which is suppressed due to higher fields,

Mg doping and SiO2 coating. There is no role of small trapped magnetic field in

magnetometer in this negative magnetization. MS shows an abnormal decreasing trend with

decreasing temperature due to presence of disordered stiffed/strong conical spin spiral and

lock in states at low temperatures. Modified Kneller’s law for HC showed a good fit but

deviated from experimental data at low temperatures due to sharp increase in HC which is

ascribed to increased surface anisotropy. CoCr2O4 nanoparticles showed slow spin

relaxation in both ZFC and FC protocols, which signifies the presence of spin-glass

behaviour at low temperatures. Non-magnetic Mg doping along with variation in average

crystallite size have also significant effects on structural, magnetic, and dielectric

properties of CoCr2O4 nanoparticles. Both TC and TS decreased as Mg concentration

increases and finally system undergoes a frustrated AFM transition. High field FC curves

showed nearly no effect on the TS due to strong B-B magnetic interactions. The dielectric

properties are improved for nanoparticles with Mg concentration due to larger average

crystallite size. The SiO2 coating on these nanoparticles is very effective and useful in

controlling the average crystallite size and tuning the magnetic properties of the CoCr2O4

nanoparticles. The SiO2 coating on CoCr2O4 nanoparticles reveals decreasing trend of

average crystallite size and lattice parameter. All magnetic transitions tend to lower

temperatures as SiO2 coating concentration increases. This was due to finite size effects

caused by SiO2 coating. M-H loops reveal decreasing trend in MS value as coating

concentration of SiO2 increases which was due to small size of coated nanoparticles. The

100

ZFC ac susceptibility with temperature also indicated that TC deceases as coating

concentration of SiO2 increases.

Surface spin-glass freezing in ferrite nanoparticles can be a promising tool to

control the particle core magnetization which is very useful for applications e.g., for

magnetic data storage. The lower value of effective anisotropy constant (Keff) is observed

for Cr2O3 coated γ-Fe2O3 nanoparticles than bare nanoparticles of γ-Fe2O3 which is due to

relatively big average particle size of coated nanoparticles and weak interface anisotropy

between ferrimagnetic core and antiferromagnetic shell. Dc magnetic measurements of

these ferrite nanoparticles indicate the presence of surface spins disorder and interparticle

interactions at the surface. The surface effects in ferrite nanoparticles are very prominent.

Frequency dependent ac susceptibility showed shifting of TB which was due to interparticle

interactions and surface spins disorder. Frequency dependent TB shift was fitted by using

theoretical models which ensures the presence of interparticle interactions and surface

disorder. Cr2O3 coated γ-Fe2O3 nanoparticles revealed surface spin glass with low HC, MS

and Keff as compared to bare γ-Fe2O3 nanoparticles which is probably due to weak core-

shell interface interactions and enhanced surface disorder supported by Cr2O3 surface

coating. Cr doped NiFe2O4 nanoparticles show strong influence of Cr+3concentration on

the structural, dielectric and magnetic properties of these nanoparticles. MS depicted

decreasing trend with addition of Cr3+ concentration which is attributed to replacement of

large magnetic moment of Fe3+ by smaller magnetic moment of Cr3+. HC revealed

increasing trend with addition of Cr3+. This increase in HC may be due to change in magneto

crystalline anisotropy. Dielectric studies of frequency dependent NiCrxFe2-xO4

nanoparticles shows decreasing trend in dielectric constants with increasing frequency.

This effect is attributed to decreasing polarization because the dipoles cannot follow up the

field variation. The dielectric properties are enhanced with Cr doping in these ferrite

nanoparticles due to less conductive nature of Cr. In this thesis, above finding in chromite

and ferrite nanoparticles reveal that doping and coating has tailored the structural, magnetic

and dielectric properties which make it highly potential for future applications e.g. in bio-

magnetism, ferro-fluids, magnetic data storage and targeted drug delivery.

101

In future, preparation side emphasis should be put on a clear separation between

surface effects on individual, core/shell nanoparticles and interparticle interactions keeping

the size distribution as narrow as possible. By refining preparation methods, experimental

conditions should be better controlled for a systematic study of low temperature magnetic

transitions, freezing and interaction phenomena. The solution of these problems partly

addressed in this thesis could be of vivid interest not only for a basic understanding but

also for future applications.

102

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Study of Structural, Magnetic and Dielectric Properties of