Effect of Cr on structural and magnetic properties of cobalt ferrite R.K. Panda and D. Behera*

National Institute of Technology, Rourkela, Odisha-769008.

Conclusion: Cr substituted cobalt ferrite nanoparticles was prepared by the auto combustion method. The phase was

confirmed by the XRD analysis and the lattice constant decreased with increasing Cr addition. The particle size in nano order

was estimated from the surface morphology study of FESEM image. Particle size, saturation magnetization and coercivity

were decreased with increasing percentage of Cr to parent material .

Fig.2.Surface morphology of a)CoFe2O4 b)CoFe1.9Cr0.1 O4 c) CoFe1.8Cr0.2O4

Fig.4.Field dependent magnetic properties of

CoFe1-xCrxO4 at 300 K.

Introduction Magnetic systems with spinel structure have generated interest for its role in

wide range of applications due to their large spin polarization and high

magnetic critical temperatures, typically well above the room temperature.

Ferrimagnetic and electrical properties of the spinel ferrites are strongly

influenced by the distribution of cations along with Fe3+-Fe2+ between the

tetrahedral and octahedral sites. Particularly, cobalt ferrite, inverse spinel at

bulk and partial inversion in nano order. The inter play of cations in site

occupation tune the electric and magnetic properties of the cobalt ferrite. In the

present study we tried to tune the magnetic properties of the cobalt ferrite by

substituting the Cr3+ in place of Fe.

Fig.1.Crystal structure and spin directions

at tetrahedral and octahedral sites.

[Co1-x Fex]A [Cox Fe2-x]BO4

A and B represents tetragonal

and octahedral sites.

If x = 0 Normal spinal

x =1 Inverse spinel

0< x <1 Partial inversion

The net magnetization is MA~MB

Magnetic Properties

Surface morphology& phase analysis

Fig.5. Variation of magnetic properties with Cr.

Fig.3. XRD pattern of CoFe1-xCrx O4

Effect of Cr on structural and

magnetic properties cobalt ferrite

D. Behera

Department of Physics and Astronomy

NIT Rourklea

Plan of Presentation

Introduction on Ferrites

About cobalt ferrite

Materials and methods

Characterization

Magnetic properties

Conclusion

Introduction

Ferrites belongs to ferrimagnetic

group which exhibit net magnetization

at room temperature.

Magnetite (Fe3O4) is the naturally

occurring ferrimagnetic material.

Para Dia Ferro Antiferro

Ferri magnetic

Magnetic materials

Ferri magnetic material

Classification of Ferrites

M1113Fe5O12 M11Fe12O19

Ferrites

Spinel Hexagonal

Garnets

M11Fe2O4

Depending on the site occupancy, spinels are classified into : -normal

-inverse

Fig. Unit cell of normal spinel

Some times inversability is not

complete, is known as partial

inversability

In partial inversability site occupation

[M 1-x Fex]A [Mx Fe2-x]B O4where

A and B represents tetragonal and

octahedral sites and x represents

the inversability.

If x=0 Normal spinal

x=1 Inverse spinel

0<x<1 partial inversion

The net magnetization is MA~MB

Materials and methods

Cr substituted cobalt ferrite nanoparticles was prepared by the auto combustion

method

CoFe1-xCrxO4 with varying x (x = 0, 0.05, 0.1, 0.15, 0.2)

CFO nanoparticles were prepared by the auto combustion method. Analytical

grades cobalt nitrate Co(NO3)26H2O, ferric nitrate Fe(NO3)39H2O were taken as

oxidants while glycine C2H5NO2 was employed as fuel to drive the combustion

reaction.

Phase analysis

XRD of CoFe1-xCrxO4 with varying x

Lattice constant decreasing because lower size Cr3+ (0.615 A) substituting the heavier atom

Fe3+ (0.645 A).

Particle size decreases with increasing addition of Cr3+

Particle size decrease may be due to strain induced by the decrease in lattice constant

Surface morphology of a)CoFe2O4 b)CoFe1.9Cr0.1O4 c) CoFe1.8Cr0.2O4

Magnetic properties

Field dependent magnetic properties of CoFe1-xCrxO4 with varying x at 300 K.

Compositional dependence of magnetic properties

The tetrahedral and octahedral sub-

lattices of ferrite may be subdivided

in such a way that the vector

resultants of the magnetic moments

of the sub-lattices are aligned in such

a direction that will influence the

effective magnetization. Thus the

decrease in magnetization could be

explained on the basis of non-

collinear spin arrangement that arose

due to the substitution of Cr ions.

Ms and Hc for CoFe1-xCrxO4 at 300 K.

In CFO system, the structure transforms from partial inverse to

inverse spinel as particle size changes from nano to bulk range.

Therefore, the magnetic properties must change accordingly with the

growth of particle size. Cobalt occupies the octahedral site in inverse

spinel whereas it occupies both the sites in partial inverse spinel form.

As Chromium substitution brings decrease in particle size therefore,

there may be the migration of cations from one site another site

which leads to decrease in saturation magnetization.

The coercivity of fine particles has striking properties on their size.

As the grain size decreases, the coercivity increases, goes through a

maximum then decreases and become zero for a very small size

known as superparamagnetism.

The change in coercivity is due to the change of the multidomain

nature to single domain. In the multidomainregion, the size

dependence of coercivity is expressed as �� = + �

Where D is diameter of the particle and a, b are constants.

Mössbauer spectroscopy probes tiny changes in the energy levels of an atomic

nucleus in response to its environment.

Three types of nuclear interactions may be observed:

Isomeric shift, also known as a chemical shift

Quadrupole splitting; and

Magnetic or hyperfine splitting, also known as the Zeeman effect.

Due to the high energy and extremely narrow line widths of gamma rays,

Mössbauer spectroscopy is a very sensitive technique in terms of energy (and

hence frequency) resolution, capable of detecting change in just a few parts

per 1011

Mossbauer Studies

Mossbauer Studies for the Cr substituted CFO

Separate contributions due

to the A tetrahedral and B

octahedral sites can be

identified in the Mossbauer

spectra.

In this study, Mossbauer

spectroscopy was used to

investigate the local

environments of the Fe atoms

in these materials. A series of five

powder samples with

compositions CoCrx Fe2−xO4

It would appear that Cr substitutes into the B sites and has the effect

of displacing Co ions onto the A sites.

A typical range of velocities for 57Fe, for example, may be ±11 mm/s (1 mm/s

= 48.075 neV)

Conclusions

Cr substituted cobalt ferrite nanoparticles prepared by the auto combustion

method.

The phase was confirmed by the XRD analysis and the lattice constant is

decreased with increasing Cr addition.

The particle size of Nano order was estimated from the surface morphology

study of FESEM image.

Particle size was decreased with increasing percentage of Cr and decrease in

saturation magnetization as well as coercivity.

Further studies on Mossbauer to show several aspects on magnetic properties

Thank you for attention