Journal of Molecular Structure 1006 (2011) 447–452

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Journal of Molecular Structure

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Substitutional effect on structural and magnetic propertiesof AxCo1�xFe2O4 (A = Zn, Mg and x = 0.0, 0.5) ferrites

Dinesh Varshney ⇑, Kavita Verma, Ashwini KumarSchool of Physics, Vigyan Bhawan, Devi Ahilya University, Khandwa Road Campus, Indore 452 001, India

a r t i c l e i n f o

Article history:Received 11 August 2011Received in revised form 22 September 2011Accepted 23 September 2011Available online 1 October 2011

Keywords:FerriteX-ray diffractionRietveld refinementRamanMagnetization

0022-2860/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.molstruc.2011.09.047

⇑ Corresponding author. Tel.: +91 731 2467028; faxE-mail addresses: vdinesh33@rediffmail.com, vdines

a b s t r a c t

The influence of the Zn and Mg content on the structural and magnetic properties of cubic cobalt ferrites(CoFe2O4) synthesized by chemical co-precipitation method was investigated using X-ray powder diffrac-tion (XRD), Raman spectroscopy and vibrating sample magnetometer (VSM). Rietveld – refined X-raypowder diffraction patterns at room temperature confirmed the formation of single-phase cubic (FCC)structure with Fd3m space group for all prepared samples. Slight variation in the lattice parameter ofMg doped CoFe2O4 has been observed. Raman analysis reveals the doublet like nature of A1g mode forall synthesized samples. Small shift in Raman modes and increment in the linewidth has been observedwith the doping ions. The magnetic measurement explored that the saturation value (Ms) is maximum forCoFe2O4 as compared to Zn and Mg doped cobalt ferrites samples.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Spinel ferrites are magnetic materials with a general formula(MxFe1�x)[M1�xFe1+x]O4, where x is the cation distribution factor,the round and square brackets denote the tetrahedral (A) and octa-hedral (B) interstitial sites. M represents the divalent metal cation(Mn2+, Fe2+, Co2+, Ni2+, Zn2+, etc.) and Fe is the trivalent (Fe3+) metalcation occupying the FCC lattice formed by O2� anions. For normalspinel ferrites x = 1, with all the divalent (M2+) cations on the tet-rahedral (A) sites and the trivalent (Fe3+) cations on the octahedral(B) sites, and is represented by the formula (M2+)[Fe3+Fe3+]O4.However, x = 0 represents the ‘‘inverse’’ spinel ferrites, with theformula (Fe3+)[M2+Fe3+]O4, in which the divalent cations occupythe B-site and the trivalent cations are equally divided amongthe A-site and remaining B-site. The magnetic properties of a spinelare sensitive to the types of cation and their distribution amongstthe two interstitial sites of spinel lattice [1]. The cation distributionbetween A and B site depends on the ionic radii, the type of bond-ing and the preparation method. Changing the temperature, pres-sure, magnetocrystalline anisotropy, and composition of metalions can change the distribution. The ferrites with spinel structurehave attracted considerable interest due to their remarkable opti-cal, electronic, mechanical, thermal, and magnetic properties.These properties are exploited in technological applications like

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: +91 731 2465689.h33@gmail.com(D.Varshney).

ferrofluid, biomedicine, targeted drug delivery, magnetic reso-nance imaging, and recording media [2–5].

ZnFe2O4 is a normal spinel, where Zn2+ preferably occupies thetetrahedral sites due to their affinity for sp3 bonding with oxygen an-ions leaving all the ferric ions on the octahedral sites. The normalspinel structured ZnFe2O4 is antiferromagnetic in nature due tolow Néel temperature and is paramagnetic at room temperaturedue to weak super exchange interaction attributed to 90� angle inFe3+–O–Fe3+ [6]. Thus, ZnFe2O4 exhibits lower magnetic moment(�5 emu/g) at room temperature. However, on vacuum thermal an-nealed at 1000 �C and cooled to room temperature it becomes mag-netically ordered with a large magnetic moment (�62 emu/g). Thischange is attributed to the change in the cation distribution from thenormal to mixed spinel type, where Fe3+ and Zn2+ occupies both sitesunder vacuum thermal annealing [7].

CoFe2O4 is typically an inverse spinel ferrite in which Fe3+ ions areoccupied equally, with their spin in the opposite direction, in the tet-rahedral (A) and octahedral (B) sites. The magnetic property is mainlydependent on the Co2+ (3d7) ions that have an orbital moment of3.7 lB per formula unit [1]. It has a coercivity of�0.75–0.98 kOe alongwith 80 emu/g magnetization at room temperature and is a good can-didate for the magnetic recording application [8]. Magnetic coercivityof any material depends on several factors like magnetocrystallineanisotropy, shape anisotropy, strains, defects, size of the particles,doping, nature of the surface, and interface [9–13]. A number of ef-forts have been made to increase the coercivity in cobalt ferrite usingdifferent strategies. However, the maximum coercivity reported sofar in the powders of CoFe2O4 is 5.3 kOe for the samples prepared

10 20 30 40 50 60 70 80

Inte

nsity

(arb

. uni

t)

2θ (degree)

(533

)(5

00)

(440

)

(511

)(4

22)

(400

)

(222

)(3

11)

(220

)

(111

)

CoFe2O

4

Co0.5

Mg0.5

Fe2O

4

Co0.5

Zn0.5

Fe2O

4

Fig. 1. X-ray diffraction pattern for AxCo1�xFe2O4 (A = Zn, Mg and x = 0.0, 0.5) ferritesamples.

448 D. Varshney et al. / Journal of Molecular Structure 1006 (2011) 447–452

by chemical route. Such a large coercivity is attributed to smallparticle size and large residual strain [14]. The maximum room tem-perature coercivity reported in CoFe2O4 particles synthesized bymechanical milling process is 5.1 kOe. This large coercivity is attrib-uted to the high strain and defects produced in CoFe2O4 powders bymilling [10].

On the other hand, MgFe2O4 is a soft magnetic n-type semicon-ducting material, and is very important member of ferrite family.The cation distribution in MgFe2O4, upon which many physicaland chemical properties depend, is a complex function of process-ing parameters and mostly depends on the preparation method ofthe material. The earlier reported cation distribution reveals thatMg2+ ions exist in both sites but have a strong preference for theoctahedral site [15]. The magnetic moment of Mg2+ is zero, sothe magnetic couplings in MgFe2O4 purely originate from the mag-netic moment of Fe cations and may be relatively weaker. Thus, themagnetic anisotropy in MgFe2O4 could be lower than that of otherspinel ferrites. From the past few years, several researchers haveincorporated various substitutions and studied the structural,vibrational and magnetic and properties of mixed ferrites. Padaliaand coworkers have earlier estimated the valance state of Co asCo2+ and Fe as Fe3+ in Zn doped Cobalt ferrites using X-ray finestructure method. Additionally the bond length estimated fromfine structure method was close to those obtained from crystallo-graphic data [16]. Furthermore, Amulevicius et al. explore thedependence of average hyperfine field and temperature dependentmagnetic anisotropy of mixed ferrites using multilevel relaxationmodel [17].

Apart from magnetization, Raman spectroscopy is also a power-ful probe to reveal the structural and vibrational properties of spi-nel ferrites. CoFe2O4 as reported earlier belongs to cubic inversespinel structure with Fd3m space group similar to that of Fe3O4.Although the X-ray powder diffraction pattern of CoFe2O4 andFe3O4 are quite similar, their Raman spectra are quite different.The full unit cell of cubic symmetry contains 56 atoms but thesmallest Bravais cell contains only 14 atoms. Therefore, 42 vibra-tional modes are possible. Group theory predicts the followingoptical phonon distribution: 5T1u + A1g + Eg + 3T2g; the 5T1u modesare IR active [18], whereas other five (A1g + Eg + 3T2g) are Ramanactive modes composed to the motion of O ions and both the A –site and B – site ions [19,20].

The present work aims at the investigation of structural andmagnetic properties of AxCo1�xFe2O4 (A = Zn, Mg and x = 0.0, 0.5)samples. The chemical co-precipitation method has been used tosynthesize Zn and Mg doped cobalt ferrites. Detailed analyses ofthe structural and magnetic properties of as prepared ferrites arediscussed.

2. Experimental details

2.1. 1 Materials and methods

For the synthesis of AxCo1�xFe2O4 (A = Zn, Mg and x = 0.0, 0.5)samples through chemical co-precipitation technique all the re-agents were commercial products with analytical grade without fur-ther purification. The chemical reagents for this experimentFe(NO3)3�9H2O, Co(NO3)2�6H2O, Zn(NO3)2�6H2O, Mg(NO3)2�6H2O,and sodium hydroxide were used. The deionized water was usedduring the experiments. The aqueous solution of Co, Zn, Fe and Mgsalts were freshly prepared by taking Fe(NO3)3�9H2O, Co(N-O3)2�6H2O, Zn(NO3)2�6H2O, and Mg(NO3)2�6H2O in appropriate mo-lar ratio. This mixture was heated until the temperature reached70 �C. On vigorous stirring, the pH of the above solution was raisedto 12 rapidly, by the addition of 6 M NaOH. The particles settled atthe bottom were collected and the top water layer with excess saltswas discarded. The particles have been washed repeatedly with dis-

tilled water to remove salt impurities. Later, the washed particleswere treated with acetone dried at room temperature and furthercalcined at 700 �C for 5 h.

2.2. Characterizations

The crystal structure and type of phases were identified bymeans of X-ray powder diffraction (XRD) at room temperature,using Bruker D8 Advance X-ray diffractometer with CuKa1

(1.5406 Å) radiation. The data was collected with a scanning speedof 2�/min. with a step size of 0.02� over the angular range 2h(10� < 2h < 90�) generating X-ray by 40 kV and 40 mA power set-tings. The Raman measurements on as synthesized samples werecarried out on LABRAM-HR spectrometer with a 488 nm excitationsource equipped with a Peltier cooled charge coupled device detec-tor. Fourier Transform Infrared (FT-IR) spectra were recorded in thefrequency range of 2000–400 cm�1 employing KBr disc techniqueusing Bruker Germany make spectrometer model vertex-70. DCmagnetization measurements on all the samples were performedusing a vibrating sample magnetometer (VSM, Lakeshore 7300model, USA) as a function of magnetic field from 0 to ±10,000 Oe.

3. Results and discussion

3.1. Structural analysis

The of X-ray diffraction pattern of AxCo1�xFe2O4 (A = Zn, Mg andx = 0.0, 0.5) samples are shown in Fig. 1. From the X-ray diffractionpattern, it has been observed that all the reflection peaks of pure aswell as doped compound matches well with Joint Committee forPowder Diffraction Set (JCPDS) Card No. 22-1086 for CoFe2O4 fer-rite. Furthermore, there is no change in peak positions for all thethree samples, which indicate that all the samples crystallize insingle-phase cubic structure with Fd3m space group.

The Rietveld refinement of X-ray powder diffraction pattern forAxCo1�xFe2O4 (A = Zn, Mg and x = 0.0, 0.5) samples at room temper-ature were performed using FullPROOF program and shown inFig. 2. The performed pattern for the structural modal with spacegroup Fd3m (227) reproduce adequately all the observed reflec-tions and gave practically identical reliability factor. There is agood agreement between observed and calculated pattern usingthe Rietveld analysis, which is confirmed by observing the differ-ence pattern. The refined parameters of as synthesized samples

CoFe2O4 Yobs Ycal Yobs - Ycal

Bragg Position

10 20 30 40 50 60 70 802θ (degree)

Co0.5Mg0.5 Fe2O4

Co0.5Zn0.5Fe2O4

Inte

nsity

(arb

. uni

ts)

Fig. 2. Rietveld refined XRD pattern for AxCo1�xFe2O4 (A = Zn, Mg and x = 0.0, 0.5)samples.

D. Varshney et al. / Journal of Molecular Structure 1006 (2011) 447–452 449

are summarized in Table 1. We also identify the residuals for theweighted pattern Rwp, the pattern Rp, Braggs factor RBragg, structurefactor RF, and goodness of fit v2. All these parameters were used asnumerical criteria of the quality of the fit of calculated to experi-mental diffraction data and are represented by the relations [21]:

Table 1Rietveld refined XRD parameters of AxCo1�xFe2O4 (A = Zn, Mg and x = 0

Samples CoFe2O4

Space group Fd3m

Cell parameters (Å)ao 8.3554 ± 0.0004at 8.3532Cell volume (Å3) 583.31

Atomic positionsFe1/Zn, Mgx 0.000y 0.000z 0.000

Fe2/Cox 0.625y 0.625z 0.625Ox 0.247y 0.247z 0.247

R-factorsRp (%) 43.9Rwp (%) 24.3Rexp (%) 23.1RBragg (%) 5.72RF (%) 5.47v2 1.10GOF-index 1.0

Rwp ¼

Pi¼1;n

wi yiðobsÞ � yiðcalÞ

� �2

Pi¼1;n

wi yiðobsÞ

� �2

26664

37775

12

� 100 ð1Þ

RP ¼

Pi¼1;njyiðobsÞ � yiðcalÞjPi¼1;n

yiðobsÞ� 100 ð2Þ

RBragg ¼

PhjIðobs;hÞ � Iðcal;hÞjP

hjIðobs;hÞj

� 100 ð3Þ

RF ¼

PhjFðobs;hÞ � Fðcal;hÞjP

hjFðobs;hÞj

� 100 ð4Þ

v2 ¼Xi¼1;n

wi yiðobsÞ � yiðcalÞ

� �2

n� pþ c� 100 ð5Þ

where yi(obs) is the experimental intensities, yi(cal) is the calculatedintensities, wi = (1/yi(obs)) is the weight experimental observations,n is the number of experimental observations, p is the number of fit-ting parameters, I is the integrated intensity, and F is the structurefactor.

The initial input parameters used for the Rietveld refinementwere taken from the earlier reported data [22,23]. The Refined cat-ionic positions suggest that all the structures have completely dif-ferent compositions corresponding to the chemical formula of thecompound. The value of v2 comes out to be �1, which may be con-sidered to be very good for estimations. The lattice constant iscalculated for all the samples using Rietveld refinement. The latticeconstant of CoFe2O4 and Co0.5Zn0.5Fe2O4 matches well with eachother due to similar ionic radii of Co2+ (0.745 Å) and Zn2+

.0, 0.5) ferrite samples.

Co0.5Zn0.5Fe2O4 Co0.5Mg0.5Fe2O4

Fd3m Fd3m

8.3522 ± 0.0002 8.3221 ± 0.00038.3501 8.3199582.85 576.36

0.000 0.0000.000 0.0000.000 0.000

0.625 0.6250.625 0.6250.625 0.625

0.247 0.2470.247 0.2470.247 0.247

25.4 30.117.3 22.516.0 21.24.25 5.604.10 5.771.16 1.191.1 1.2

CoFe2O4

200 300 400 500 600 700 800Raman Shift (cm-1)

Co0.5Mg0.5 Fe2O4

Co0.5 Zn0.5Fe2O4

Inte

nsity

(arb

. uni

t)

Fig. 3. Room temperature Raman spectra for AxCo1�xFe2O4 (A = Zn, Mg and x = 0.0,0.5) ferrite samples with excitation wavelength of 488 nm.

450 D. Varshney et al. / Journal of Molecular Structure 1006 (2011) 447–452

(0.740 Å) [24]. Whereas the lattice constant of Co0.5Mg0.5Fe2O4

slightly reduce as compared to CoFe2O4, this is due to smaller ionicradii of Mg2+ (0.720 Å). The theoretical value of the lattice constant‘‘at’’ for above three compositions has also been determined byconsidering the cation distribution in these systems estimatedfrom the X-ray diffraction. In order to determine the cation distri-bution we have used the intensities of (220), (422) and (400)planes as these planes are most sensitive to the cations on tetrahe-dral (A) and octahedral (B) sites [25–26]. The relative integrated X-ray intensity ‘‘Ihkl’’ of a given diffraction line was calculated usingthe Buerger formula specified as [27].

Ihkl ¼ jFhklj2 � P � Lp ð6Þ

where Ihkl is the relative integral intensity, Fhkl the structure factor, Pthe multiplicity factor, LP refers to Lorentz polarization factor. It isworth mentioning that ions distributed over tetrahedral (A) andoctahedral (B) sites. As reported earlier the intensities correspond-ing to (220) and (422) reflections are most sensitive to cationson tetrahedral sites, while those of (222) reflection to cation onoctahedral sites. As seen from the Fig. 1 that the intensities of(220), (422), (440) and (222) reflections decrease with substitu-tion of Zn2+ and Mg2+ions indicating that both the ions enters inthe tetrahedral (A) sites. Further the intensity ratios of I220/I440,and I422/I220, for all the three samples were calculated to determinethe cation distributions. The values of intensity ratios and cationdistribution are summarized in Table 2. The obtained values arein good agreement with the earlier reported data [28]. On the basisof estimated cation distribution, the value of theoretical lattice con-stant ‘‘at’’ for all the three samples has been determined using therelation [29].

at ¼8

3ffiffiffi3p ra þ rOð Þ þ

ffiffiffi3p

rb þ rOð Þh i

ð7Þ

where ra, rb are the radii of tetrahedral (A) and octahedral (B) sites,respectively, and rO is the radius of the oxygen ion O2� (1.48 Å). Thecalculated theoretical values ‘‘at’’ of all three samples are also re-ported in Table 1. It has been observed that the theoretically calcu-lated values and experimentally observed values are nearly equal.Another important factor that influences physical properties of fer-rite system is the jump length ‘‘L’’. The probability of the electronthat are hopping between the A and B sites are less as comparedto that are hopping between B and B sites. The reason being thatthe distance between the two metal ions placed at B-site is smallerthan the distance if one is placed at A-site and other is at B-site. Thejump length ‘‘L’’ of the tetrahedral (A) and octahedral (B) sites isdetermined from the relation given as [30]:

LA ¼ a0

ffiffiffi3p

4

!and LB ¼ a0

ffiffiffi2p

4

!: ð8Þ

The values of jump length are found to be as for CoFe2O4 (LA =3.6178 Å, LB = 2.9540 Å), Co0.5Zn0.5Fe2O4 (LA = 3.6165 Å, LB =2.9532 Å) and Co0.5Mg0.5Fe2O4 (LA = 3.6034 Å, LB = 2.9421 Å). It hasbeen observed that jump length of CoFe2O4 and Co0.5Zn0.5Fe2O4

Table 2Cation distribution and values of intensity ratio of AxCo1�xFe2O4 samples.

Samples A-site B-site

A = Co (Co0.1Fe0.9) [Co0.9Fe1.1]x = 0.0

A = Zn (Zn0.5Fe0.5) [Co0.5Fe1.5]x = 0.5

A = Mg (Mg0.1Fe0.9) [Mg0.4Co0.5Fe1.1]x = 0.5

are quite similar whereas, the jump length of Co0.5Mg0.5Fe2O4 re-duces. This reduction in jump length is attributed to replacementof larger ionic radii (Fe3+) by smaller ionic radii (Mg2+). The obtainedresult matches well with the earlier reported data [31].

3.2. Raman analysis

Room temperature Raman spectra of as synthesized Ax-

Co1�xFe2O4 (A = Zn, Mg and x = 0.0, 0.5) ferrite samples are de-scribed in the in frequency range of 150–800 cm�1. The topmostpanel of Fig. 3 shows the Raman spectra for CoFe2O4. In order todetermine the natural frequency, line width and lattice effect t inall the three samples, a least square fit with Lorentzian line shapewas used to fit the Raman spectra. The spectra of CoFe2O4 consistof broadband nearly at 311, 470, 571, 619 cm�1 and a strong bandat �693 cm�1. Raman modes of AxCo1�xFe2O4 (A = Zn, Mg andx = 0.0, 0.5) samples are illustrated in Table 3. The thick smoothlines are fits to the Lorentzian functions. It has been observed thatRaman band at �690 cm�1 show a shoulder like feature at lowerwave number side against the reported single band to that of

I(220)/I(400) I(422)/I(220)

Obs. Cal. Obs. Cal.

1.0646 1.0454 0.8458 0.7543

1.5786 1.6172 0.7651 0.5422

1.0502 1.0322 0.8273 0.7143

Table 3Raman parameters of AxCo1�xFe2O4 (A = Zn, Mg and x = 0.0, 0.5) samples.

Assignment CoFe2O4 Co0.5Zn0.5Fe2O4 Co0.5Mg0.5Fe2O4

Raman modes (cm�1) Line width Raman modes (cm�1) Line width Raman modes (cm�1) Line width

A1g(2) 693.3 33.97 693.2 43.39 684.5 47.55A1g(1) 619.2 69.88 632.7 121.72 629.4 139.02T2g(3) 571.0 35.19 551.7 111.92 – –T2g(2) 470.5 44.80 468.5 55.59 476.61 83.72Eg 311.5 51.87 314.5 89.95 319.5 99.96T2g(1) – – 207.7 27.26 – –

D. Varshney et al. / Journal of Molecular Structure 1006 (2011) 447–452 451

Fe3O4 [19]. These bands are assigned to A1g1 and A1g2 modesreflecting the stretching vibration of Fe3+ and O2� ions in tetrahe-dral site. The other low frequency modes are assigned to T2g andEg modes reflecting the vibration of that site.

The doublet like feature is due to the local cation distribution. InFe3O4 whole of the tetrahedral and octahedral sites are occupied byFe ions while in CoFe2O4 the octahedral site are occupied by Co andFe ions and tetrahedral site are occupied by only Fe ion. Due to dif-ference in ionic radii of Co and Fe ions in CoFe2O4, the Fe–O, Co–Obond distance redistribute between both the sites resulting in dou-blet like structure. Due to weak Raman signals and broadening theT2g(1) mode in CoFe2O4 plus T2g(1) and T2g(3) mode in Co0.5Mg0.5-

Fe2O4 could not be observed in the spectra. Except Eg mode allother modes of Co0.5Zn0.5Fe2O4 are shifted towards the lower wavenumber side, this red shift is attributed to higher atomic mass of Znas compared to Co ion. Whereas, modes of Co0.5Mg0.5Fe2O4 shows ablue shift owing to lower atomic mass Mg as compared to Co ion.

Another common feature observed in Raman modes of all thethree samples is that the line width changes with the doping ele-ments. The line width increases with Mg and Zn doping. This in-crease in line width is attributed to strong electron–phononinteraction and electronic disorder arising as random arrangementof Cations on the octahedral (B) site. The line width of A1g and T2g

modes are given in Table 3. It is worth mentioning that in case ofthe CoFe2O4 ferrites some controversy regarding the crystal struc-ture, as well as in relation to the features is always observed in theRaman spectra. Wang and researchers have synthesized CoFe2O4

ferrites and found that sample has a tetragonal structure and be-longs to the I41/amd space group [32].

Fourier Transform Infrared Spectroscopy (FT-IR) is one of thepreferred methods of infrared spectroscopy to identify thechemical and structural changes occurring in a particular sample.

2000 1800 1600 1400 1200 1000 800 600

Rel

ativ

e Tr

ansm

ittan

ce

Wave number (cm-1)

CoFe2O4

Co0.5Zn0.5Fe2O4

Co0.5Mg0.5Fe2O4

Fig. 4. FT-IR spectra of AxCo1�xFe2O4 (A = Zn, Mg and x = 0.0, 0.5) ferrite samples.

The transmission IR spectra of as synthesized samples are shownin Fig. 4. It has been mentioned that two main broad metal–oxygenbands are seen in IR spectra of all spinel ferrites [33]. The bandaround 600 cm�1 corresponds to vibration of tetrahedral metal–oxygen [Fe–O] bond and the band at 400 cm�1 to vibration of octa-hedral metal oxygen bond. Furthermore, the absorption band (inthe range of 500–600 cm�1) in the present study corresponds tovibration of Fe3+–O2� bond of tetrahedral (A) site, while a bandaround 1630 cm�1 assigned to the bending vibration of H2O ab-sorbed after calcinations [34,35]. The IR spectra show the bandaround 1384 cm�1 due to the presence of trapped NO�3 (nitrate)in as prepared samples [36].

3.3. Magnetic measurement

To understand the magnetic properties of AxCo1�xFe2O4 (A = Zn,Mg and x = 0.0, 0.5) ferrite samples, the field dependence magneti-zation of all the samples was measured using vibrating samplemagnetometer (VSM) at 300 K with an applied field (�10 kOe 6H 6 10 kOe). The substitution of Co2+ by Zn2+ and Mg2+ ions pro-duces a mixed effect on magnetic properties of these ferrites andis reflected in the magnetic hysteresis loop traced at room temper-ature as shown in Fig. 5. The value of magnetic parameter such assaturation magnetization (Ms), coercivity (Hc), retentivity (MR) hasbeen determined from M–H loop.

It has been observed that hysterisis curve for Zn doped sample istypical S shaped with very low coercivity (Hc � 75.01 Oe) and reten-tivity (MR � 8.12 emu/gm). On the other hand CoFe2O4 representhighest coercivity (Hc � 1202.22 Oe) and retentivity (MR �39.77 emu/gm), whereas, Mg doped CoFe2O4 show less coercivity(Hc � 849.98 Oe) and retentivity (MR � 25.91 emu/gm) as compared

-10000 -5000 0 5000 10000-100

-80

-60

-40

-20

0

20

40

60

80

100

M (e

mu/

gm)

H (Oe)

CoFe2O4

Co0.5Zn0.5Fe2O4

Co0.5Mg0.5Fe2O4

Fig. 5. Magnetic hysteresis loop of AxCo1�xFe2O4 (A = Zn, Mg and x = 0.0, 0.5) ferritesamples at room temperature.

452 D. Varshney et al. / Journal of Molecular Structure 1006 (2011) 447–452

to pristine CoFe2O4. The highest coercivity of CoFe2O4 is attributed tolarger magnetocrystalline anisotropy. The contribution to magneto-crystalline anisotropy from Fe3+ having five unpaired electrons isdifferent in all the three systems, however the Zn2+ and Mg2+ ionshas no unpaired electrons and leads to zero total electron spin, theCo2+ ion have three unpaired electron and have large magnetocrys-talline anisotropy due to strong L–S coupling. Furthermore CoFe2O4

has the highest Saturation value (Ms � 84.95 emu/gm) and this sat-uration value decreases on substitution of Zn2+ (Ms � 80.99 emu/gm) and Mg2+ (Ms � 51.09 emu/gm) ions at Co site.

The present results are compared with the earlier reported data.As per best of our search, the saturation value of CoFe2O4 isamongst the highest values reported so far. The CoFe2O4 preparedby the Layered double hydroxide method reported the maximumMs value of about 86.1 emu/gm as compared to the same sampleprepared by ceramic method and wet chemical method (Ms � 73)[37]. Further more the CoFe2O4 synthesized by wet chemical co-precipitation method has the saturation value of 60 emu/gm for1.3 min digestion time, which was 75% of the corresponding bulkvalue [38]. This value is slightly less than the same sample pre-pared by wet chemical method [39]. Moreover, in a series ofCo1�xZnxFe2O4 (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) prepared bysol–gel method, it has been observed that saturation magnetiza-tion value increases from 84.5 to 91.6 emu/g. as the Zn concentra-tion increases up to 40%. But on further increase in the zincconcentration saturation values decreases. [40].

The difference in the value of (Ms) could be explained on thebasis of cation distribution. As discussed earlier CoFe2O4 forms in-verse spinel ferrite with Co2+ ions occupying the octahedral (B) siteand Fe3+ are equally distributed among tetrahedral (A) and octahe-dral (B) site. On the other hand Zn2+ and Mg2+ have a site prefer-ence for A-site. The replacement of Co2+ with non-magnetic Zn2+

and Mg2+ ions causes a reduction in A–O–B super exchange inter-action. This would further disturb some magnetic coupling andlead to an overall reduction in magnetism as to the large magneticmoment of Fe3+ ions. The magnetic moment (lB)/atom in Bohrmagnetron is calculated from expression:

lB ¼ ðM �MsÞ � ðN � bÞ ð9Þ

where M is the molecular weight of the sample, Ms is the value ofsaturation magnetization, N is the Avogadro number, b is the con-version factor (�9.27 � 10�21). The values of magnetic momentare 4.46 lB, 3.45 lB and 1.98 lB for CoFe2O4, Co0.5Zn0.5Fe2O4 andCo0.5Mg0.5Fe2O4 respectively. The substitution of nonmagneticZn2+ and Mg2+ ions replacing the magnetic ions (Fe3+) might haveaffected the exchange interaction between A- and B-site, leadingto the decrease in magnetic moment. The experimental value Co-Fe2O4 is much higher than the theoretical value (�3 lB) due to con-tribution from the orbital magnetic moment remaining unquenchedby the crystalline field [1].

4. Conclusions

All the polycrystalline ferrite AxCo1�xFe2O4 (A = Zn, Mg andx = 0.0, 0.5) samples were successfully prepared by chemical co-precipitation method. The effect of Zn and Mg doping on structuraland magnetic properties of CoFe2O4 has been studied. X-raydiffraction confirms the formation of single-phase crystallinestructure without any trace of impurity. All the three samples fit-ted with Rietveld refinement using FullPROOF program revealedthe existence of cubic structure (space group Fd3m). A slight reduc-tion in the lattice parameter of Co0.5Mg0.5Fe2O4 has been observedas compared to pristine and Co0.5Zn0.5Fe2O4 ferrite.

Evolution of Raman spectra reveals the five active phononmodes for all the three samples. Due to the cation distribution atboth the sites, a doublet like feature has been observed for A1g

mode in all the three samples. Red shift in Co0.5Zn0.5Fe2O4 is attrib-uted to higher atomic mass of Zn as compared to Co; where as blueshift in Co0.5Mg0.5Fe2O4 is due to lower mass of Mg. The linewidthof phonon modes are found to increase with Zn and Mg doping inCoFe2O4. The absorption band at about 500–600 cm�1 in the IRspectra corresponds to the vibration of Fe3+–O2� bond related totetrahedral (A) site with some traces of NO�3 peak and hydroxylgroup. Magnetic study concludes that saturation magnetization va-lue (Ms) is maximum for CoFe2O4. This saturation value decreaseson substitution of Zn2+ and Mg2+ ions at Co site.

Acknowledgements

Authors are thankful to UGC-DAE CSR, Indore for providingcharacterization facilities, and the Department of Physics M.L.Sukhadia University, Udaipur, for magnetic measurement underUGC-DRS and DST-FIST programs. One of the authors (KavitaVerma) is gratefully acknowledged the RGNF UGC, New Delhi, In-dia for financial assistance.

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