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Chemical synthesis, structural and magnetic properties of nano-structured Co–Zn–Fe–Cr ferrite

Nanoparticles of Co1−xZnxFe2−xCrxO4 (x = 0.0–0.5) ferrites were prepared by chemical co-precipitation technique using metal sulphates. The structural and magnetic properties were investigated by means of X-ray diffraction (XRD), transmission electron
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   Journal of Alloys and Compounds 509 (2011) 5055–5060 Contents lists available at ScienceDirect  JournalofAlloysandCompounds  journal homepage: www.elsevier.com/locate/jallcom Chemical synthesis, structural and magnetic properties of nano-structuredCo–Zn–Fe–Cr ferrite S.T. Alone a , Sagar E. Shirsath a , ∗ , R.H. Kadam b , K.M. Jadhav a a Department of Physics, Dr. Babasaheb Ambedkar Marathwada University, Begampura, Aurangabad 431004 (MS), India b Materials Research Laboratory, ShriKrishana Mahavidyalaya, Gunjoti, Osmanabad (MS), India a r t i c l e i n f o  Article history: Received 2 December 2010Received in revised form 27 January 2011Accepted 2 February 2011 Available online 1 March 2011 PACS: 75.50.Bb61.05.cp73.63.Bd Keywords: Nanostructured materialsChemical synthesisMagnetizationTransmission electron microscopy, TEMMagnetic measurements a b s t r a c t Nanoparticles of Co 1 −  x Zn  x Fe 2 −  x Cr  x O 4  (  x =0.0–0.5) ferrites were prepared by chemical co-precipitationtechnique using metal sulphates. The structural and magnetic properties were investigated by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM),vibrating sample magnetometer (VSM), and AC susceptibility measurements. X-ray diffraction patternsindicate that the samples possess single phase cubic spinel structure. The lattice constant initiallyincreases for  x ≤ 0.3 and thereafter for  x >0.3 it decreases with increasing  x . The saturation magneti-zation(Ms),magnetonnumber( n B )andcoercivity(Hc)decreaseswithincreasingCr–Zncontent  x .Curietemperature deduced from AC susceptibility data decreases with increasing  x . © 2011 Elsevier B.V. All rights reserved. 1. Introduction Ferrites are technologically very important material havingpotential applications and interesting physics. They have beenextensively investigated and being the subject of great interestbecause of their importance in many technological applications[1–3]. The important structural, electrical and magnetic proper-ties of ferrite are responsible for their applications in various fields[4]. Over the past decade, magnetic nanoparticles have attractedmuch attention due to their properties from both application andtheoretical points of view. The nanoparticles have many specialmagnetic and electrical properties that are significantly differentfrom bulk particles [5]. The physical properties of the nanomate- rials are predominantly controlled by the grain boundaries thanby the grains [6]. The nanoparticles of ferrite can be prepared by various techniques such as citrate–nitrate combustion method[7],sol–gel route [8], pulsed laser ablation [9], and solvothermal pro- cess [10]. Ferrites, by virtue of their structure can accommodate a variety of cations at different sites enabling of wide variation intheelectricalandmagneticproperties.Theinterestingphysicaland ∗ Corresponding author. Tel.: +91 2402240950; fax: +91 2402361270. E-mail address:  shirsathsagar@hotmail.com (S.E. Shirsath). chemicalpropertiesarisefromtheirabilitytodistributethecationsamongthetetrahedral(A)andoctahedral[B]sites.Amongthesev-eral spinel ferrites, cobalt ferrite (CoFe 2 O 4 ) containing anisotropyion is the most important ferrite to be used in several applica-tions. It possesses the inverse spinel structure and its degree of inversion depends upon the heat treatment [11]. Many workers have studied the structural, electrical and magnetic properties of cobalt and cobalt-substituted ferrite [12,13]. It has been reported that the substitution of tetravalent ions in cobalt ferrite influencesthe structural, electrical and magnetic properties [14,15]. ZnFe 2 O 4 is normal spinel while CoFe 2 O 4  is an inverse spinel ferrite. CobaltchromateisanormalspinelandCr 3+ hasstrongpreferencetoocta-hedralB-site[16].ThesubstitutionoftrivalentionslikeCr 3+ islikelyto increase the resistivity and decrease the saturation magnetiza-tion[17].Itisalsoreportedthatthesaturationmagnetization(Ms), remanent magnetization (Mr), and coercive force (Hc) decreasesmonotonously whereas markedly improved the complex perme-ability and loss tangent when an appropriate amount of Cr 3+ issubstituted for Fe 3+ ions [18]. The co-substitutional effect of mag- netic Cr 3+ and non-magnetic Al 3+ at Fe site in cobalt ferrite isreportedintheliterature[19].Themethodofpreparationandheat treatment influences the electrical and magnetic properties. In theliterature, studies on magnetic properties of Co 1 −  x Al  x Fe 2 −  x Cr  x O 4 [20,21] and CoCr  x Fe 2 − 2  x O 4  [22] prepared by ceramic method have 0925-8388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2011.02.006  5056  S.T. Alone et al. / Journal of Alloys and Compounds 509 (2011) 5055–5060 60504030 2 θ  (degree)    I  n   t  e  n  s   i   t  y   (  a  r   b .  u  n   i   t   )         (       4       4        0        )        (       5       1       1        )        (       4        2        2        )        (       4        0        0        )        (        2        2        2        )        (        3       1       1        )        (        2        2        0        ) (f)(e)(d)(c)(b)(a) Fig. 1.  XRD patterns of   x =0.0–0.5 of Co 1 −  x Zn  x Fe 2 −  x Cr  x O 4 . been reported. To our knowledge, no reports are available in theliterature for co-substitutional effect of magnetic Cr and non-magneticZnatoctahedralandtetrahedralsiterespectivelyincobaltferrite. The literature survey revealed that the substitution of nonmagneticionsinthespinelferritecausescantingeffect[23–25].The ionic radius of Co 2+ (0.74 ˚A) and Zn 2+ (0.83 ˚A) are comparable andtherefore Co 2+ ions can easily replaced by Zn 2+ ions. Cr 3+ ions aremagnetic having the magnetic moment of 3  B . The ionic radius of Cr 3+ is(0.63 ˚A)comparablewithFe 3+ (0.67 ˚A).Inthepresentpaper,we report our results on structural and magnetic properties of nonmagneticZn 2+ andmagneticCr 3+ substitutedCoFe 2 O 4  preparedbywet chemical co-precipitation method. 2. Experimental details ThespinelferritesystemwithachemicalformulaCo 1 −  x Zn  x Fe 2 −  x Cr  x O 4  withvari-ablecomposition(  x =0.0–0.5)ispreparedbywetchemicalco-precipitationmethod.The starting solutions were prepared by mixing of 50ml of aqueous solution of FeSO 4 · 7H 2 O, CoSO 4 · 7H 2 O, ZnSO 4 · 7H 2 O and Cr 2 (SO 4 ) 3 · 6H 2 O in stoichiometric pro-portion.A2MsolutionofNaOHwaspreparedasaprecipitant.Ithasbeensuggestedthatthesolubilityproductconstant( K  sp )ofalltheconstituentsalwaysexceedwhenthestartingsolutionisaddedintotheprecipitant.Therefore,inordertoachievethesimultaneous precipitation of all the hydroxide, the starting solution (pH ≈ 3) wasadded in to the solution of NaOH. Suspension (pH=11) containing dark interme-diate precipitation was formed. Then the suspension was heated and kept at 60 ◦ Ctemperature,whileoxygengaswasbubbleduniformlyintothesuspensiontostiritandtopromotetheoxidationreactionuntilalltheintermediateprecipitantchangedinto the dark brownish precipitate of the spinel ferrite. The samples were filteredand washed several times by distilled water. The samples were annealed at 600 ◦ Cfor 12h for removing water and hydroxyl ions.The X-ray powder diffraction patterns were recorded by using Cu K  radiationon Philips X-ray difractometer (Model PW 3710) at room temperature. Particle sizeof the sintered powder samples was calculated using transmission electron micro-scope (TEM) (Model CM 200, Philips make). The microstructure and morphologyof the ferrite powder were characterized by scanning electron microscopy (SEM)(Model JEOL-JSM 840). The magnetic data for these samples were obtained withthe help of high field hysteresis loop technique [26]. The low field AC susceptibil- ity measurements on powder samples were carried out in the temperature range300–800K using double coil setup [27] operating at a frequency of 263Hz. 3. Results and discussion  3.1. Structural properties The formation of single-phase cubic spinel structure was con-firmedbyX-raydiffractionpatterns(XRD).AllthepeaksoftheXRDpattern were indexed using Bragg’s law. The XRD patterns of allthe samples are shown in Fig. 1. XRD data was used to determine 0.50.40.30.20.10.0 8.3768.3808.3848.3888.3928.396 Composition x    L  a   t   t   i  c  e  c  o  n  s   t  a  n   t   '  a   '   (    Å   ) 5.2805.2865.2925.2985.3045.3105.316 X-r  a  y d  e n s i   t   y'   d  x '   (   e m u /   gm )   Fig. 2.  Variation of lattice constant and X-ray density with composition. structuralparametersofallthesamples.Thelatticeconstant‘ a ’wasdetermined using the following relation a = d   h 2 + k 2 + l 2 (1)where( hkl )aretheMillerindices, d istheinter-planarspacing.Thevariationoflatticeconstantwithcomposition  x isshowninFig.2.It is clear from Fig. 2 that the lattice constant increases with increas- ing  x upto  x =0.3.For  x >0.3latticeconstantdecreases.Generally,inasolidsolutionserieslinearincreaseordecreaseoflatticeconstantwithinthemiscibilityrangewithcompositionisobserved[28].This may results into initial rise in lattice constant up to  x =0.3 beyondwhichitdecreases.Thisnonlinearbehavioroflatticeconstantsug-geststhattheferritesystemisnotcompletelynormalorinverse.Inthepresentseries,Co 1 −  x Zn  x Fe 2 −  x Cr  x O 4 ,largerFe 3+ (0.67 ˚A)ionsarereplacedbysmallerCr 3+ (0.63 ˚A)andsmallerCo 2+ ionsarereplacedby larger Zn 2+ ions. However the replacement of Co 2+ ions by Zn 2+ ionsisdominantupto  x =0.3.Henceweobservedincreaseinlatticeparameter beyond  x =0.3 replacements of Cr 3+ ions are dominant.Hence we observed decrease in lattice parameter for  x >0.3. Thismaybeduetocrosssubstitutionofions.TheX-raydensity( d  x )wascalculated according to the following relation d  x  = 8 M Na 3  (2)where  M   is molecular weight, N   is Avogadro’s number, and a  is lat-tice constant. The variation of X-ray density with the composition  x  is shown in Fig. 2. The variation of X-ray density with  x  exhibitsexactlyreversebehaviorascomparedtothevariationoflatticecon-stantwith  x .Thisisbecause;X-raydensityisinverselyproportionalto the lattice constant ‘ a 3 ’. The average particle size was calculatedusing the Scherrer equation [29] t   = 0 . 9 ˇ  cos    (3)where  t   particle size,    is wavelength of the X-ray radiation,     isBragg’s angle, and  ˇ  is measure of broadening of diffraction dueto size effect. The average particle sizes calculated using Eq. (3) are listedinTable1,anditisobservedthattheparticlesizeisdecreases from 30nm to 17nm as Cr–Zn content increases. The bulk den-sity( d B )ofthespecimenshasbeendeterminedbythehydro-staticmethod. The values of the bulk density are shown in Fig. 3. This is evidence of decrease in particle size with increase in Cr–Zn con-tent which led to increase in porosity. The percentage porosity ‘ P  ’of the sample was calculated using the values of X-ray density andbulk density, and using the relation, porosity ( P  )=( d  x − d B )/ d  x  [30].Fig. 3 gives the variation of porosity as a function of Cr–Zn con-tent  x . It is clear from Fig. 3 that density of the samples decreases  S.T. Alone et al. / Journal of Alloys and Compounds 509 (2011) 5055–5060 5057  Table 1 Particle size ( t  ), the bond length of tetrahedral (  A ) site ‘ d AX ’ and octahedral [B] site‘ d BX ’,tetrahedraledge‘ d AXE ’,sharedoctahedraledge‘ d BXE ’andunsharedoctahedraledge ‘ d BXEU ’.Comp.  x t   (nm)  d AX  d BX  d AXE  d BXE  d BXEU XRD TEM0.0 30 26 1.901 2.048 3.104 2.819 2.9700.1 27 23 1.903 2.050 3.107 2.822 2.9730.2 24 21 1.904 2.051 3.109 2.825 2.9750.3 22 18 1.905 2.052 3.110 2.825 2.9750.4 20 16 1.904 2.051 3.108 2.824 2.9740.5 17 15 1.902 2.049 3.106 2.822 2.972 and the porosity increases with increase in Cr–Zn composition  x .The bond length of tetrahedral (A) site ‘ d AX ’ and octahedral [B]site‘ d BX ’,tetrahedraledge‘ d AXE ’,sharedoctahedraledge‘ d BXE ’andunshared octahedral edge ‘ d BXEU ’ can be calculated by putting theexperimental values of lattice parameter ‘ a ’ and oxygen positionalparameter ‘ u ’ of each sample in the equations discussed elsewhere[31].Thevaluesof  d AX , d BX , d AXE , d BXE  and d BXEU  arelistedinTable1which indicates that the  d AX  and  d AXE  increases up  x =0.3 and thendecreased after  x >0.3 with Cr–Zn content ‘  x ’, this may be due toZn 2+ replaces Co 2+ of tetrahedral A site. Octahedral bond length‘ d BX ’,unsharedoctahedraledge‘ d BXEU ’andsharedoctahedraledge‘ d BXE ’ increases up  x =0.3 and then decreased after  x >0.3 withCr–Zn content ‘  x ’. This could be related to the smaller radius of Cr 3+ ions as compared to Fe 3+ ions and the fact that Cr 3+ occupiesstrongly tetrahedral B-site.Fig. 4 shows TEM image of typical sample (  x =0.3), this imagewas used to study the particle size. The value of particle size isgiveninTable1.Theparticlesizeisdecreasesfrom26nmto15nm with increasing Cr–Zn content  x . As seen from Table 1, the particle sizemeasuredfromXRDandTEMareingoodagreementwitheachother. It can be considered that Zn 2+ and Cr 3+ ions may diffuse tothegrainboundariesduringthesinteringprocessandinhibitgraingrowthbylimitinggrainmobility.SimilarresultswereobtainedforCrsubstitutedCo 0.5 Zn 0.5 Fe 2 O 4  [32]andZn 2+ -dopedTiO 2  nanopar-ticles [33]. Scanning electron micrographs (SEM) of the surfaces of the three compositions (  x =0.1, 0.3 and 0.5) are shown in Fig. 5.Eachcompositionischaracterizedbyatypicalporousstructureandsmall rounded grains. It is evident that the structure is affected bytheCr–Znsubstitutions.Itcanbeobservedthatthedecreaseinthegrain size and an increase in porosity with increasing Cr and Znsubstitutions. The Cr and Zn substituted ferrite exhibits the finestanduniformgranulation(Fig.5).Theobservedchangesingrainsize suggestthattheincorporationofCr–Zninsolidsolutionoccursdur-ing precipitation preparation which enables a better homogeneity 0.50.40.30.20.10.0 101214161820  P d B Composition 'x'    P  o  r  o  s   i   t  y   '   P   '   (   %   ) 4.34.44.54.64.74.8 B ul  k  d  e n s i   t   y'   d  B '    (   gm /   c m  3   )   Fig. 3.  Variation of porosity and bulk density. Fig. 4.  TEM image of the typical samples  x =0.2. in the powders and, hence, a more controlled microstructure isobtained. It can be observed from the SEM images that the pre-pared samples are amorphous and porous in nature. The particleswere well distributed and slightly agglomerated. The agglomera-tionistheindicationofhighreactivityofthepreparedsamplewiththeheattreatmentanditmayalsobecomefromthemagnetostaticinteractionbetweenparticles[34–37].TheCrandZnincorporation lead to radical changes in microstructure (Fig. 5) which consist in the followings:(i) Asignificantdecreaseintheparticlesize,from26nmtoabout15nm;(ii) The formation of grain bridges around the large pores andinterconnected pores in the form of capillary tubes betweenthe grain chains;(iii) Itispossiblethatthepresenceoftheforeignphaseinhibitsthegrain growth and agglomeration process.  3.2. Magnetic properties ThesubstitutionofZnandCrions,whichishavingapreferentialA-andB-siteoccupancyrespectively,resultsinthereductionoftheexchange interaction between A- and B-sites. Hence, by varyingthe degree of Zn and Cr substitution the magnetic properties of thefineparticlescanbevaried.Fig.6showstheroomtemperature (300K) hysteresis loop for the typical samples (  x =0.0, 0.3 and 0.5).The values of saturation magnetization (Ms) coercivity (Hc) andmagnetonnumber( n B )(saturationmagnetizationperformulaunit‘  B ’) obtained from hysteresis loop technique are summarized inTable 2. The observed magneton number ( n B  Obs.) was calculated  Table 2 Saturation magnetization (Ms), coercivity (Hc), observed and calculated mag-neton number ( n B ), Yafet–Kittel angle (   YK ) and Curie temperature ( T  C ) forCo 1 −  x Zn  x Fe 2 −  x Cr  x O 4 .Comp.  x  Ms (emu/g) Hc (Oe)  n B  (  B )    YK  (0 ◦ )  T  C  (K)Obs. Cal.0.0 46.62 1453 1.959 3.00 – 7760.1 59.57 1052 2.568 3.50 – 7200.2 82.88 641 3.489 4.00 – 6520.3 85.47 382 3.603 4.50 27.39 5740.4 49.86 213 2.104 5.00 50.36 5190.5 17.61 135 0.744 5.50 82.81 459  5058  S.T. Alone et al. / Journal of Alloys and Compounds 509 (2011) 5055–5060 Fig. 5.  SEM images of the typical samples (a)  x =0.1, (b)  x =0.3 and (c)  x =0.5. using the formula: n B  = Molecular weight (Mw) × saturation magnetization (Ms)5585(4)It is observed from Table 2 that saturation magnetization andobserved magneton number increases up to  x =0.3 and thendecreases as Zn 2+ and Cr 3+ ions increases. In the present system 6420-2-4-6 -90-60-300306090    M  a  g  n  e   t   i  z  a   t   i  o  n   '   M  s   '   (  e  m  u   /  g   ) Applied field (kOe) (c)(b)(a) Fig. 6.  Magnetization curve for (a)  x =0.0, (b)  x =0.3 and (c)  x =0.5. zinc ions of magnetic moment (0  B ) occupies tetrahedral A-siteand push the Fe 3+ ions of magnetic moment (5  B ) to octahedralB-site. This migration of Fe 3+ ions from A-site to B-site increasesthe net magnetic moment of B-site, resulting overall increase insaturation magnetization up to  x =0.3. The decreasing trend for  x >0.3 is due to the fact that after a certain amount of zinc con-centration, there start fluctuations in the number of ratio of zincand ferric ions on the tetrahedral sites surrounding the variousoctahedralsitesi.e.fluctuationsinthetetrahedral-octahedralinter-actions [10]. Also in the present case Co 2+ (3  B ) are replaced byZn 2+ (0  B )andFe 3+ (5  B )arereplacedbyCr 3+ (3  B )ions,i.e.mag-netic ions are replaced by comparatively non magnetic ions. Thisresults in the weakening of A–B interaction whereas B–B interac-tion changes from ferromagnetic to antiferromagnetic state. Thevariation of   n B  with Zn–Cr content  x  can be understood by consid-ering the cation distribution and the anti-parallel spin alignments,thetwosub-latticesitesfollowingfromtheNeel’smolecularmodelof ferrimagnetisms. According to Neel’s two sub-lattice model of ferrimagnetisms [38], Neel’s calculated magnetic moment in   B, n NB  is expressed as, n NB  = M  B − M  A  (5)where M  B  and M  A  arethemagneticmomentsofBandAsub-latticerespectively.  n NB  values for  x =0.0–0.5 were calculated using aboveequationandalsotakingtheionicmagneticmomentofFe 3+ (5  B ),Zn 2+ (0  B ), Cr 3+ (3  B ) and Co 3+ (3  B ). The values of calculatedmagneticmoment n NB  for  x =0.0–0.3areingoodagreementwiththeobservedmagneticmomentconfirmingthecollinearspinordering,while for  x =0.3–0.5 values of observed and calculated magneticmoment are different from each other. Fig. 7 indicates that signif- icant canting exists on B site suggesting magnetic structure to benon-collinear. Thus, the change of spin ordering from collinear tonon-collinear display a strong influence on the variation of mag-neticmomentperformulaunitasobservedbymagnetizationwithCr–Zn content. The initial increase in observed magnetic momentwith Cr–Zn content  x  is explained on the basis of Neel’s theory,but decrease in observed magnetic moment after  x >0.3 indicatesa possibility of non-collinear spin structure in the system whichcan be explained on the basis of three sub-lattice model suggestedbyYafet–Kittel[39].Yafet–Kittel(Y–K)angleshavebeencalculated using the following formula n B  = M  B  cos   YK − M  A  (6)  S.T. Alone et al. / Journal of Alloys and Compounds 509 (2011) 5055–5060 5059 0.50.40.30.20.10.0 12345 Composition 'x'    M  a  g  n  e   t  o  n  n  u  m   b  e  r   (  n    B    )  Obs. Cal. Fig. 7.  Variation of observed and calculated magnetic moment of Co 1 −  x Zn  x Fe 2 −  x Cr  x O 4 . ThevalueofY–Kanglesforsampleswith  x =0.0–0.2isnotobserved(Table 2) which indicates that magnetization of this composition canbeexplainedwithNeel’stwolatticetheory.TheincreaseinY–Kangles with further increase in Cr–Zn content suggest that magne-tization in these ferrites can be explained on the basis of cantedspin model. The increase of Y–K angles with Zn content indicatesthe fact that triangular spin arrangement is suitable on the B-siteleading to the reduction in A–B interaction.Coercivity in Co–Zn–Fe–Cr ferrites is given in Table 2. Itdecreases with the increase in Cr–Zn concentration. This is due tothe fact that Hc decreases with the decrease in magnetocrystallineanisotropy.Themagnetocrystallineanisotropyconstant( K  1 )isneg-ative for both Cr and Zn ferrites. The absolute value of   K  1  is largerforCoferritesthanthatofCrandZnferrites.Thetotalanisotropyisequal to the sum of their individual anisotropies. So  K  1  and hencecoercivity decreases with the increase in Cr–Zn concentration.The plots of AC susceptibility   ac ( T  )/  ac ( RT  ) against tempera-ture  T   for the samples  x =0.0–0.5 are shown in Fig. 8. For  x ≤ 0.3the plots of    ac ( T  ) data display two peaks, one sharp peak nearthe Curie temperature ( T  C ) and another broad peak at much lowertemperature. For  x ≥ 0.4–0.5 only a broad maximum is observed.Zubov et al. [40] have studied the temperature dependence of AC 800700600500400300 0123456  0.0 0.1 0.2 0.3 0.4 0.5    T   R   T   χ    /  χ Temperature 'Tc' (K) Fig. 8.  Plots of    T  /  RT   versus temperature. susceptibility for pure cobalt ferrite and they have observed twopeaks one near Curie temperature ( T  C =776K) and second peak at533K. This second peak is referred to as the isotropic peak [41],which could be seen clearly for a magnetic material in a multi-domainstateonlyifthematerialhasthetemperatureatwhichthemagneto-crystalline anisotropy is zero [42]. Beyond the tempera- ture at which the isotropic peak occurs, the shape anisotropy willbedominantandasaresultthecoerciveforcearises.Theadditionof ZnandCrtopureCoFe 2 O 4  reducesthecoerciveforce,whichresultsin decrease in the peak value of susceptibility. Further addition of Zn and Cr shows broad maxima near  T  C  and the suppression of theisotropic peak. Therefore, it can be concluded that the samples of thesesystemcontainmulti-domainspincluster.ThevalueofCurietemperature is given in Table 2. The Curie temperature obtained from  T  /  RT   plotsdecreaseswithincreasingZn–Crconcentration  x which suggest decrease in A–B interaction. 4. Conclusions Analysis of XRD patterns confirms that all the samplespossess single phase cubic spinel structure. The wet-chemical co-precipitation method yields fine particles of the order of fewnanometerswhichareconfirmedbyTEManalysis.Latticeconstantincreasesupto  x =0.3andthereafteritdecreases,thisbehaviorsug-gests that the ferrite system is not completely normal or inverse.Magnetizations and observed magneton number increases up to  x ≤ 0.3 and then decreases with increasing Cr–Zn content. Neel’smodel is applicable up to  x =0.3, above which Yafet–Kittel modelcan be applied and it decreases with increasing Cr–Zn content.Thermal variation of AC susceptibility shows a normal ferrimag-neticbehaviorwhichreduceswithincreasingCr–Zncontent.Curietemperature determined from AC susceptibility plots decreaseswith increasing Cr–Zn content. The decrease in Curie temperaturereflects the weakening in A–B interaction of the respective ions. References [1] A. 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