Journal of Magnetism and Magnetic Materials 324 (2012) 3986–3990

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Journal of Magnetism and Magnetic Materials

0304-88

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Temperature and composition dependence of magnetic propertiesof cobalt–chromium co-substituted magnesium ferrite nanomaterials

Muhammad Javed Iqbal a,n, Zahoor Ahmad a, Turgut Meydan b, Yevgen Melikhov b

a Surface and Solid State Chemistry Laboratory, Department of Chemistry, Quaid-i-Azam University, Islamabad 45320, Pakistanb Wolfson Center for Magnetics, School of Engineering, Cardiff University, Cardiff CF24 3AA, UK

a r t i c l e i n f o

Article history:

Received 29 November 2011

Received in revised form

11 June 2012Available online 1 July 2012

Keywords:

Ferrites

Microemulsion method

Mossbauer analysis

Law of approach

Magnetic anisotropy

53/$ - see front matter & 2012 Elsevier B.V. A

x.doi.org/10.1016/j.jmmm.2012.06.031

esponding author. Tel.: þ92 51 90642143; fa

ail address: mjiqauchem@yahoo.com (M.J. Iqb

a b s t r a c t

The temperature and composition dependence of magnetic properties of Co–Cr co-substituted

magnesium ferrite, Mg1�xCoxCrxFe2�xO4 (x¼0.0–0.5), prepared by novel polyethylene glycol assisted

microemulsion method, are studied. The synthesized materials are characterized by the Mossbauer

spectrometer and standard magnetic measurements. Major hysteresis loops are measured up to the

magnetic field of 50 kOe at 300, 200 and 100 K. The high field regimes of these loops are modeled using

the Law of Approach to saturation to determine the first-order cubic anisotropy coefficient and

saturation magnetization. Both the saturation magnetization and the anisotropy coefficient are

observed to increase with the decrease in temperature for all Co–Cr co-substitution levels. Also, both

the saturation magnetization and the anisotropy coefficient achieved maximum value at x¼0.3 and

x¼0.2, respectively. Explanation of the observed behavior is proposed in terms of the site occupancy of

the co-substituent, Co2þ and Cr3þ in the cubic spinel lattice.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

Magnetic spinel ferrites are of a great interest in fundamentalscience, especially for addressing the fundamental relationshipsbetween magnetic properties and their lattice structure. This isdue to the fact that the ability to substitute different cations into thetetrahedral/octahedral sites of spinel lattice offers the opportunity toselectively enhance properties to match intended applications.Recently, the synthesis of magnetic materials on the nanoscale hasbeen a field of intense study due to the novel mesoscopic propertiesshown by particles of quantum dimensions located in the transitionregion between atoms and bulk solids [1].

Spinel ferrites have been investigated in recent years for theiruseful magnetic properties and applications in information sto-rage systems, magnetic bulk cores, sensors/actuators, magneticfluids, microwave absorbers and medical diagnostics. The synth-esis and magnetic structure characterization of spinel nanofer-rites have been investigated with much interest and a lot ofattention has been focused on the preparation and characteriza-tion of metal oxide nanoparticles of spinel ferrites, MeFe2O4

(Me¼Co, Mg, Mn, etc.) [2–4]. Nanoparticles of magnesium ferrite(MgFe2O4) are the potential candidates for various applications inmagnetic and microwave devices [5,6].

As many important properties of the spinel ferrites dependcrucially on the exact nature of the cation distribution over the

ll rights reserved.

x: þ92 51 90642241.

al).

octahedral and tetrahedral sites in the spinel cubic lattice,chemical substitution can tune the properties of magnesiumferrite by altering this cation distribution. This influences themagneto-elastic properties of these materials, which can beexplained in terms of the change in important magnetic proper-ties such as the magnetization characteristics and magnetocrys-talline anisotropy. For the above-said purpose, structural andmagnetic characteristics of magnesium ferrite with a non-mag-netic substitution such as Zn1þ [7], Cd2þ [8], In3þ [9] and Ti4þ

[10] for either individual Mg2þ or individual Fe3þ site only, havebeen investigated. However, doping of magnesium ferrite with ametallic binary mixture of magnetic ions to substitute iron andmagnesium simultaneously has not been extensively studied.

In the present study, we have investigated co-substitution ofCo2þ–Cr3þ in place of some of Mg2þ and Fe3þ in magnesiumferrite and we have studied its properties at various temperaturesin addition to the room temperature properties only reportedbefore [11]. The octahedral site preference of Co2þ in the cubicspinel lattice of magnesium ferrite and the additional substitutionof Cr3þ produced a more drastic change in properties in compar-ison to previously tried substitutions.

2. Experimental details

Nanosized Mg1�xCoxCrxFe2�xO4 (where x¼0.0–0.5) are pre-pared by the polyethylene glycol (PEG) assisted microemulsionmethod. Aqueous solutions of proper compositions are mixedwith aqueous solution of polyethylene glycol (PEG) in well

Rel

ativ

e In

tens

ity

0.5

0.4

0.3

0.2

0.1

M.J. Iqbal et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 3986–3990 3987

defined proportions. After the formation of the precipitates, theyare heated at low temperature for calcination at 573 K for 2 h.Finally, the powdered material is annealed at 1123 K for 8 h toobtain the pure spinel phase (for detailed description see [11]).

Structural characterizations of the samples performed byX-ray diffraction (PANalytical 3040/60 X Pert PRO diffractometer),SEM analysis (Hitachi S-3400 N) and Energy Dispersive X-RayFluorescence (ED-XRF) spectrometer (Horiba, MESA-500) havebeen reported in our earlier publication [11].

Mossbauer spectroscopic studies have been undertaken usingSEECo MSCI Mossbauer spectrometer running in constant accel-eration mode with a source of 50 mCi 57Co in Rh matrix. Themodel of Lorentzian multiplet analysis is used to analyze the dataobtained. The variation in technical saturation of magnetizationwith temperature and Co–Cr content is measured using a super-conducting quantum interference device (SQUID) magnetometer(Quantum Design Magnetic Properties Measurement System).Symmetric magnetic hysteresis loops are measured at 300, 200and 100 K, using a maximum applied field of 50 kOe (m0H¼5 T).For determination of cubic anisotropy coefficients (K1) of thesamples, the high field regions of the magnetic hysteresis loopsare modeled using Law of Approach to saturation (LoA) [12]. As itis accepted that the Law of Approach (LoA) is valid in the regionabove 97% of maximum magnetization [13], therefore this corre-sponding region has been fitted to the LoA to determine aniso-tropy coefficient, K1.

-20 10-10 20Velocity (mm/s)

0.0

0

Fig. 1. Mossbauer spectra of Mg1�xCoxCrxFe2�xO4 (x¼0.0–0.5).

3. Results and discussion

3.1. Structural analyses

XRD patterns of powdered samples Mg1�xCoxCrxFe2�xO4

(x¼0.0–0.5) assured the formation of single phase materials.Crystallite size, D, calculated by the well-known Debye–Scherrerequation is found to be in the range of 23–47 nm. The differentXRD parameters, i.e. lattice constant, a, and X-ray density, dx,which were calculated from XRD data show significantvariation with increase in Co–Cr contents. SEM images ofMg1�xCoxCrxFe2�xO4 for compositions x¼0.0 and 0.4 depict thatthe surface is almost smooth and is a mixture of individualparticles. For detailed description of structural analyses see [11].

3.2. Mossbauer analysis

Mossbauer spectra of Mg1�xCoxCrxFe2�xO4 (x¼0.0–0.5) recordedat room temperature (�300 K) are shown in Fig. 1. The magneticcomponents of the spectra have been fitted by using Lorentzian linefitting to assess the number of interactions with a minimumw2�1.0 and solid lines in Fig. 1 show the obtained computer fitted

Lorentzian curves. All samples exhibit well resolved and magne-tically normal Zeeman split sextets attributed to the presence ofiron ions at both tetrahedral (A-sites) and octahedral (B-sites)sites which confirms the fact that the synthesized materialspossess ferrimagnetic nature with mixed spinel structure. How-ever, a slight broadness is observed in Zeeman lines as it isevident from Fig. 1. The broadening of lines could be attributed toslight change in the magnetic environment surrounding Fe3þ ionsin the same sublattice; such changes would lead to a change inthe magnetic field which consequently results in an appreciablebroadening of the Zeeman lines. In addition, the changes in themagnetic environment would affect center shift values slightly,displacing sextets with respect to one another, and would causegeneral broadening of these lines. The presence of relaxation effectalso conceivably results in broadening of the lines. The increasedbroadening in the lines with respect to increasing amount of

substituents might be attributed to large number of probabledistribution of Co2þ and Cr3þ ions surrounding Fe3þ ions at B-sites.

A summary of the hyperfine interaction parameters is pre-sented in Table 1. The obtained results are in close agreementwith the studies reported earlier for spinel ferrites [14,15]. Thecenter shift results from the electrostatic interaction between thecharge distribution of the nucleus and s-electrons with finiteprobability being found in the region of the nucleus. It is evidentfrom Table 1 that the center shift CS (A) and CS (B) values haverandom trend, indicating that s-electron charge distribution ofFe3þ is not much influenced by Co–Cr co-substitution. As expected,the outer sextet shows a larger center shift (Dd�0.06–0.11 mm/s)because of the difference in the Fe3þ–O2- inter-nuclear separationnormally larger for B-site ions as compared to that for A-site ions.Consequently, smaller overlapping of the orbital of Fe3þ and O2- atB-site occurs, resulting in smaller covalency and hence largercenter shift, CS, for B-site Fe3þ ions [16]. The sextets belonging tooctahedral (B) and tetrahedral (A) sites have been assigned basedon the values of hyperfine magnetic field at nucleus and centershift. In most of the ferrites, generally the higher value of hyperfinemagnetic field at nucleus and center shift corresponds to B-sitesextet and the lower values of the same correspond to A-sitesextet [16].

The values of quadrupole splitting (QS) for hyperfine spectra ofall the samples investigated here are found to be negligibly small(see Table 1). This can be attributed to the fact that overall cubicsymmetry is not much altered between Fe3þ ions and theirsurroundings by substitution with Co–Cr ions in magnesiumferrites.

The variation in hyperfine magnetic field, (HA and HB), at thetwo sublattices, A- and B-sites with Co–Cr contents is shown in

Table 1Center shift (CS), quadrupole splitting (QS), hyperfine magnetic field (H) and relative area (RA) of Mg1�xCoxCrxFe2�xO4 (x¼0.0–0.5).

Parameters Bond area x¼0.0 x¼0.1 x¼0.2 x¼0.3 x¼0.4 x¼0.5

CS (mm/s) 70.001 A-site 0.286 0.263 0.309 0.318 0.299 0.300

B-site 0.383 0.370 0.394 0.381 0.384 0.395

QS (mm/s) 70.001 A-site �0.002 0.008 0.005 �0.046 0.007 0.015

B-site �0.121 �0.120 �0.141 �0.031 �0.114 �0.017

H (kOe) 72 A-site 536 528 514 512 505 498

B-site 558 549 547 537 519 518

RA (%) 70.2 A-site 30.6 40.1 47.0 51.5 55.1 64.2

B-site 69.4 59.9 53.0 48.5 44.9 35.8

-400

-300

-200

-100

0

100

200

300

400

-50M (k

A/m

)

H (kOe)

0.0 0.10.2 0.30.4 0.5

50403020100-10-20-30-40

Fig. 2. Magnetic hysteresis loops of Mg1�xCoxCrxFe2�xO4 (x¼0.0–05) at 200 K.

-300

-200

-100

0

100

200

300

-50

M (k

A/m

)

H (kOe)

300 K200 K100 K

50403020100-10-20-30-40

M.J. Iqbal et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 3986–39903988

Table 1. The hyperfine interaction at A-site is found to decreasefrom 53672 to 49472 kOe, whereas at B-site, it decreases from55872 to 51872 kOe with the increase in Co–Cr co-substitutionlevel, which indicate that the magnetic environment of iron ionsat both sites has been changed significantly with dopant contents.The smaller hyperfine field assigned to the A-site is attributed tothe dipolar field resultant of covalent nature of the tetrahedralbonds [17]. The center shift (CS) and a larger hyperfine field (H)for the same site helped in assigning the outer sextet to theoctahedral sites and the inner sextet to the tetrahedral sites. TheA-site hyperfine fields are typically 4–5% less than the B-sitehyperfine fields and the difference is usually attributed to thelarger covalency at the A-site. The variation in hyperfine fields isdue to the change in A–B and B–B super-exchange hyperfineinteractions as the cation neighbors around Fe3þ ions are changed[18].

The relative area under the resonance curve of the sub-spectradeduced from the measurements is helpful to conceive the Co, Cr andFe site occupancy in sub-lattice sites. The relative peak-area of B-sitedecreased with the incorporation of Co–Cr and subsequentlyobserved to increase for that of A-site (see Table 1). This variationin relative peak-area is supported by the Mossbauer spectra illu-strated in Fig. 1 that the intensity of the outer sextet decreases whilethat of inner sextet increases with the incorporation of the Co–Crions. Together with the conclusions from the Mossbauer spectrashown in Fig.1 and also recalling that pure magnesium ferrite is apartially inverted spinel ferrite, i.e. (Mg1�lFel)

A[MglFe2�l]BO4 [19],we can conclude that Co2þ ions occupy the octahedral sites withCr3þ ions [11,20,21].

Fig. 3. Magnetic hysteresis loops of Mg0.8Co0.2Cr0.2Fe1.8O4 at 300, 200 and 100 K.

3.3. Squid measurements

The basic magnetic parameters, such as, saturation magnetiza-tion, MS, remanence, Mr, coercive field, HC, and the magnetocrys-talline anisotropy coefficient, K1, are obtained from the magnetichysteresis measurements performed at 300, 200 and 100 K. Asan example, Fig. 2 shows the symmetrical magnetic hysteresisloops of Co–Cr substituted magnesium ferrite for x¼0.0–0.5 atspecific temperature 200 K and Fig. 3 shows symmetrical mag-netic hysteresis loops of Mg0.8Co0.2Cr0.2Fe1.8O4 sample measuredat 300, 200 and 100 K. For calculation of saturation magnetization(MS) and first-order magnetocrystalline anisotropy coefficient(K1), the high field regions of these magnetic hysteresis loops aremodeled using Law of Approach to saturation (LoA) [12]. According tothe LoA, the high field region (H*HC) of magnetic hysteresis loopsrepresents the process of reversible rotation of magnetization againstanisotropy and forced magnetization, which can be described by thefollowing relationship:

M¼MS 1�8

105

K1

moMSH

� �2" #

þkH ð1Þ

where M is the magnetization, MS is the saturation magnetization, H

is the applied field, k is the forced magnetization coefficient thatdescribes the linear increase in spontaneous magnetization at highfields, and K1 is the first order cubic anisotropy coefficient. Theconstant 8/105 is specific to cubic anisotropy of randomly orientedpolycrystalline materials [22].

For temperatures 300 and 200 K, data from the region above97% of maximum magnetization are fitted to the LoA (Eq. (1)) todetermine the values of magnetic parameters MS, K1, and k.Whereas at 100 K, MS and K1 are the only fitting parameters asdetailed examination of the curves revealed that the forcedmagnetization is negligible in this regime at this temperature,as might be expected, and, therefore, k¼0 is chosen at 100 K.

Fig. 4a shows that the saturation magnetization increasedmonotonically with decreasing temperature from 300 to 100 Kfor all the samples. The values of MS, computed by fitting Eq. (1) tothe experimental data, are found to be approximately the same asthat measured from maximum magnetization values at 50 kOe for300 and 200 K, whereas at 100 K, the maximum magnetization isslightly larger than MS due to the maximum applied field nolonger being able to saturate these samples at this temperature.

Fig. 4. Variation of (a) saturation magnetization, MS and (b) remanence, Mr with

Co–Cr contents for Mg1�xCoxCrxFe2�xO4 (x¼0.0–0.5) at 300, 200 and 100 K.

Fig. 5. Variation of (a) coercivity, HC and (b) magnetocrystalline anisotropy

coefficient, K1 with Co–Cr contents for Mg1�xCoxCrxFe2�xO4 (x¼0.0–0.5) at 300,

200 and 100 K.

M.J. Iqbal et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 3986–3990 3989

As can be seen from Fig. 4a, on addition of Co–Cr contents intomagnesium ferrite, the saturation magnetization, MS, appears toincrease initially (for xr0.3) and then it decreases for highconcentration of the dopants (x40.3) at all the temperatures.This increase in MS can be understood by considering thestructure of – ferrimagnetic cubic spinel: there are twice as manyoctahedral sites (B-sites) as tetrahedral sites (A-sites) in a spinelstructure, and the magnetic moments on the octahedral sites andtetrahedral sites couple antiparallel leading to the net magneticmoment, M¼Moct�Mtet. Mg2þ is a non-magnetic ion and has nocontribution in the magnetic moment of mixed spinel ferrite(MgFe2O4), which is thus entirely due to the uncompensatedspins of the un-evenly distributed iron ions at both A- and B-sites.

In the present study, simultaneous substitution of Co2þ and Cr3þ

to replace Mg2þ and Fe3þ , respectively, has brought in quitesignificant variation in the magnetic properties. In this quaternarysystem, Mg and Fe ions are partially distributed over both A and Bsites while Co and Cr ions substitute predominantly into the B-sitesof Mg1�xCoxCrxFe2�xO4 [23–25]. Replacement by magnetic ion(Co2þ) for non-magnetic Mg2þ ion would help to enhance themagnetic moment of B-site leading to an increase in the netmagnetic moment. On the other hand, substitution of Fe3þ (5 mB)by less magnetic cation Cr3þ (3 mB) would result in dilution ofmagnetization on the same site. Hence, this might cause an inter-esting competition between two cases at the same site, leading to anincrease in the value of MS up to certain level of substitution and to adecrease afterwards. However, the overall effect of Co–Cr substitu-tion is to increase MS value to considerable level for differentmagnetic applications. Remanence (Mr) value varies with Co–Crconcentration and temperature in way similar to MS value (Fig. 4b).

By increasing the Co–Cr level into MgFe2O4, the HC valueincreases initially to the dopant content, x¼0.2, and then itdecreases for higher dopant concentration as shown in Fig. 5a.The value of HC also increases with the decrease in temperature.This increase in HC is more prominent at lower temperature,100 K. It is well known that as the measurement temperature gobeyond the Curie temperature of magnetic materials, the materi-als become more and more anisotropic and cause an enhance-ment in coercive field of the subjected materials.

Similarly, the temperature and composition dependence of thecalculated anisotropy coefficient, K1 for different Co–Cr contentsis shown in Fig. 5b. K1 is observed to increase in magnitudemonotonically with decrease in temperature. While, the composi-tion dependence of the calculated anisotropy coefficient, K1

shows that it increases substantially with increasing Co–Crcontents that is of the order of magnitude 105 J/m3, for thespecimen with xr0.3 at 300 K and xr0.2 for 200, 100 K anddecreases for higher level of Co–Cr contents. Such simultaneousincrease in magnetization and magnetic anisotropy up to certainlevel of Co–Cr co-substitution would be very useful for magneticand microwave applications [26].

The analysis of temperature dependence of cubic anisotropy ofCo–Cr co-substituted magnesium ferrite can be divided into twotemperature zones, (i) relatively high temperature (300 K and200 K) and (ii) low temperature (100 K). For the first zone, themaximum applied field is sufficiently large compared to theanisotropy field, which can be estimated as moHK ¼ 2K1=MS [27],and this allows us to apply the LoA successfully. As the tempera-ture decreases, the ratio of exchange interaction to thermalenergy increases, which contributes to the increase in the aniso-tropy for all the samples. However, after some low temperaturereached (the second zone in our case), the anisotropy is so highthat it prevents complete approach to saturation even at thehighest applied field of moH¼5T (H¼50 kOe) (see Fig. 3). At suchtemperatures, the usage of the LoA is questionable and, therefore,the anisotropy coefficient K1 might be suspected even if calcu-lated with the force magnetization coefficient set to zero, i.e.,k¼0 [28]. The results from Shenker [29] for the cubic anisotropyof CoFe2O4 determined using single crystals and torque measure-ments near their easy axes support our hypothesis of highanisotropy fields.

The composition dependence of anisotropy of magnesiumferrite substituted with Co–Cr content (Mg1�xCxoCrxFe2�xO4)can be interpreted in terms of the effects of co-substituents onsite occupancies of the cations. The results of our current studiesof the Co–Cr substituted magnesium ferrites suggest that, insubstituting Co2þ for magnesium and Cr3þ for iron, Co2þ andCr3þ both go into the octahedral sites. According to the one-ion

M.J. Iqbal et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 3986–39903990

model, the presence of Co2þ ions on the octahedral sites of thespinel structure enhances the anisotropy of ferrites [30], whichsupports the observed increase in anisotropy coefficient withincreasing Co2þ content up to a certain level of substitution.However, in case of higher level of substitution, the effect of Cr3þ

substitution on site occupancies of the cations became moreeffective resulting in reduction of the magneto-crystalline aniso-tropy as Cr3þ has stronger preference for the octahedral sites andmay even displace some of Co2þ ions to tetrahedral sites [24,25].

4. Conclusions

A simple and economic PEG assisted microemulsion methodhas been adopted for the synthesis of single-phase, nanocrystal-line magnesium ferrite and its Co–Cr co-doped derivatives, withthe average crystallite size in the range of 23–47 nm. Mossbauerspectra revealed that both Co and Cr substituents have a strongoctahedral site preference. Co–Cr co-substitution brought signifi-cant changes in the temperature dependence of the magnetization,coercivity and magnetic anisotropy of polycrystalline Mg1�xCoxCrx-

Fe2�xO4 (x¼0.0–0.5). It is found that saturation magnetization, MS,remanence, Mr, coercive field, HC, and the magnetocrystallineanisotropy coefficient, K1, increased with the decrease in tem-perature for all levels of co-substituents. Also, it is found that MS

and Mr reach maximum values at level x¼0.3, while HC and K1

attained the highest values at level x¼0.2 of co-substituents. Theobserved variation of all these magnetic properties can be inter-preted in terms of the effects of co-substituents on site occupan-cies of the cations. At the small level of co-substitution the Co2þ

has stronger effect (leading to an increase in MS and K1), and athigher levels of co-substitution the Cr3þ became more influential(leading to a decrease in MS and K1).

Acknowledgment

This research work was supported by Higher Education Com-mission of Pakistan through IRSIP and Indigenous PhD 5000Fellowship Schemes.

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