Ghatage, A K

Paranjpe, S K

Patil, S A

IACS Institutional Repository

Indian J. Phys. 72A(3), 209-216 (1998)I J P  A—  an in te rn a tional jo u rn alN eutron d iffraction study of tin-substitu ted M g-Zn ferritesA K Ghatage, S A Patil and S K Paranjpe*Department of Physics, Shivuji University, Kolhapur-416 004, Maharashtra, IndiaSolid Slate Physics Division, Bhubhu Atomic Research Centre, Trombay, Mumbai-400 085, IndiaReceived 5 September 1997. accepted 17 February 1998Abstract : The magnetic behaviour ol tin-substituted Mg-Zn ferrites has been studied by powder neutron diffraction and magnetic measurements The net magnetic moment (riff) and Curie temperature Tc decreases with increase in tin content The experimental values of ng obtained from neutron diffraction and magnetic measurements are found to be less than those calculated with Neel's model GilJco's model has also been used to calculate n^ and T{ values These values are higher than the experimental ones. It is concluded that canted spin arrangement is favoured in these ferrites.Keywords : Ferntes, magnetic structure, neutron diffraction PACS Nos. : 61 l2.Ld, 75 50.Gg, 75.30.Cr1. IntroductionA wide variety of ferrite materials has been developed for application in electronic and microwave industries. The cubic spinel structure permits to substitute cations selectively on the octahedral and/or tetrahedral sites. This helps in modifying electrical and magnetic ordering in these systems. Extensive work has been done by various workers to upgrade the properties of ferrites by substituting different types and amounts of impurities. It has been reported that by addition of small amount of tetravalenl ions like titanium or tin, electrical and magnetic properties of basic Ni-Zn and Mg-Zn ferrites are significantly influenced [1-3]. An anomaly in magnetisation was observed in tin substituted Mg ferrite [4], It was also stated that for lower contents of tin. it occupies only B-site and as tin- content increases, it occupies both A and B sites [3]. In order to understand the cation distribution and its effect on the magnetisation behaviour, we have carried out neutron© 1998 I ACS210 A K Ghat age, S A Patil and S K Paranjpediffraction, low field ac susceptibility and magnetisation measurements on tin substituted Mg-Zn ferrites.2. ExperimentalThe samples ol the series Mgo 7+iZn0 3SnjrFe2_2*0 4 (with * = 0.0, 0.1,0.3) were prepared by ceramic method. AR grade MgO, ZnO, SnQ, F^O* were weighed in required proportions and mixed thoroughly. These powders were pre-sintered in air at 800°C for 10 hours, then milled and pellets were made and finally sintered at 1100°C for 24 hours in air and then slowly cooled. The formation of single phase was confirmed by X-ray diffraction using CuK(1 radiation. Magnetisation measurements were carried out on high field loop tracer. AC-susccptibility (xac) was measured using low field ac-susceptihilily technique, and Curie temperature was obtained from the normalised susceptibility versus temperature plots.Neutron diffraction measurements were carried out on polycrystalline samples using the position sensitive detector based powder diffractometer at Dhruva reactor at BARC. Monochromatic neutrons having wavelength of 1.094 A were used for the experiment. The samples were packed m cylindrical vanadium containers. Diffraction profiles were recorded at 300 K in the angular range (20) 10° to 100°. The patterns showed single phase compounds except for a small unidentified impurity phase for x = 0.3 sample. The X-ray patterns, however, did not give any indication of such an impurity phase. The data were analysed using Rietveld profile refinement technique for both chemical and magnetic structures [5,6).3. Results and discussionAs the compounds are magnetically ordered at 300 K, only higher angle data, where the magnetic contribution to the Bragg peaks is negligible, were first refined to get the chemical structure [7]. For the system under study, the occupancies of some of cations were fixed by considering their site preference. It is well known that Zn has strong preference for A-silc. In the refinement, therefore, all the Zn ions were pul on the A-site. The oxygen position parameter («), the isotropic temperature factor (B) and lattice constant (a) were varied in addition to the profile half width parameters. The site occupancies of Fe, Sn and Mg were varied independently within the constraint of satisfying the stoichiometry of the system. The fitted and observed profiles for the composition ;c = 0.0 are shown in Figure 1, along with the difference plot.The results of the refinement are summarised in Table 1. It is observed that the lattice parameter a increases with increase in Mg and Sn content, which may be a direct consequence of the larger ionic radii of the Mg and Sn (i.e. 0.78 A and 0.69 A, respectively) as compared to Fe (0.67 A). The oxygen position parameter was found to be almost constant (u = 0.258) for all the samples. From u-parameter it is concluded that theNeutron diffraction study of tin-substituted Mg-Zn ferrites 211Table I. Parameters obtained from Rielveld profile refinement, for Mgo 7 +*Zno .iSn^Fc^-^CX* systemAParameter0 0 1 0 3Lattice 8.385 (8) 8 423 (8) 8 4 6 0  (6)c o n s ta n t  a (A )Oxygen 0 .2 5 7 9 0 .258 0 .2 5 8 2p a r a m e te r  u (A )Temperature 0 .5 0 0 .43 0  33f a c to r  B  ( c m 2 )Cation (Z n 0 .3 M g o o 5 6 F e0.644) (Z n 0 3M 80.138F e0  562) ( Z n o 3M g o 2 6 6 Fc0.424)distribution fM 80 644Fc 1.3561 [M 80.662Sn0 .lF e 1.238l [Mgo 734 Sn0 3F e0 966)h 0 8 1 4 8 0  7807 0 7 1 7 8hB 0 .8 1 9 8 0 .7 9 9 5 0 7 5 1 53 75 3 85 4 76R w p 4.7 4 .8 4 6 0 4K exp 2.74 3.6 2 8 1*B 3.42 4.62From magnetic structure analysis5.932.1(1) 1.85 (6) 1 22 (4)Free ion 2.35 1.99 1.44-  VB 1.99(6) 1 .6 5 (6 ) 0.83 (6)Free ion 2.49 2.19 1.65212 A K Ghatage, S A Patii and S K Paranjpecoordination of Fe ion is very little affected by changes in the compositional parameter. From the cation distribution it is observed that Sn occupies B-site. Mg ion, however, occupies both the sites.The A-site magnetic moment was deduced from the intensities of (220) and (422) reflections. The structure factors of both these reflections have contribution from A-sile magnetic moment only. The B-site moment was obtained using (222) reflection, the intensity of which depends only on B-site magnetic moment. These values were used as initial parameters for magnetic profile refinement. The values obtained after refinement are almost same as those derived from the intensities. The fitted profile with magnetic reflections for one of the compositions x = 0,0 is shown in Figure 2. The values of the magnetic moments obtained are given in Table 2. The magnetic moments determined fromhysteresis and from neutron study for different values of x are also given in Table 2. These values are in good agreement with each other and decrease with increase in x.The magnetisation behaviour in ferrites is explained on the basis of Neel's molecular field model. Considering the magnetic ions on both tetrahedral (A) and octahedral (B) sites, the possible exchange interactions are the two intrasite (AA and BB) and one intersite (AB). Amongst the three magnetic interactions, the intersite AB interaction is the strongest. In thisNeutron diffraction study qf tin-substituted Mg-Zn ferrites 213model collinear arrangement of magnetic moments of individual site is presumed i.e. the magnetic ions on each sublattice are ferromagnetically aligned with an opposite alignment of intersite moments. The net magnetisation is the vector sum of octahedral (B) and tetrahedral (A) site magnetisations. The nB values are calculated using Neel's two suhlattice model with suitable correction for the Brillouin function and using the cation distributionTable 2. Magnetic moment and Curie temperature from various methods for Mgo7+vZno3 system.contentjrMagnetic moment (nB) Curie temperature (K)FromneutronFrommagneticmeasurementFromNeel'smodelFromGilleo'smodelFromsusceptibilityFromGilleo'smodel0 1.88 1.71 3.58 34 590 6000 1 1.45 1.54 3.38 3 520 5770.3 0 45 0 54 266 1 78 450 522obtained from neutron data. These values are given in Table 2. The nB values show a decreasing trend with x. The experimental nB values arc lower than the theoretical ones indicating a deviation from the collinear arrangement of moments.4. Gilleo’s modelThe magnetic moment and Curie temperature were calculated using the Gilleo's model[8,9]. With the assumption that magnetic moment actively participates in ferrimagnetism only when it interacts with two or more magnetic ions in different coordination. Gilleo has proposed a statistical model which neglects intrasublattice interaction and thus no canted spin is considered.If K is the fraction of Fe3"*- ions which are replaced by nonmagnetic ions at one site; for each ( 1-tf), the probability of one ion being linked with other m ions isPn(m) =The probability £that an ion is linked with no or almost one magnetic ion is E = Pn(m) = nKn~l - ( n - \ ) K n.In the present systemMmFej_m0 4,where m is the amount of the nonmagnetic cation M. The cation distribution is given asx m ^  l-xm  ) [ ^ ( l - j r )m  2—(I — jr)m ]■The magnetic moment is given by72A<3)-7where M<°> = 2 x 5 ( I - K 0) [ l - E 0( * ( )]and M<'> = 1 x 5 ( 1 - A T ,)[l-£ ,(A f0 )].Here, K,*xm(at Asite)andK0 = U -Jc)m /2(atB  site), nAB = 12 and nBB = 6,and E0(Kt ) = 6 K - 5 K ,E t (K0 ) = l 2 K " - \ \ K n .Thus, calculated magnetic moment values are given in Table 2 and are higher than the observed values.The theoretical calculations to estimate luc Curie temperature for spinel ferrites are given by Gilleo [8]. According to the model the Curie temperature is proportional to the number of active linkages per magnetic ion per formula unit and is cast in the form :Tc = 3*(1-*)[l - £ 0 (* ,)][!  - E , ( K 0)] (3 -m )T 0 /2[x(\ - E 0(K,)+ ( 1 - jc)(1 - £ ( (AT0 )],where Tq is a constant. Milligan et al [10] have evaluated Tq for MgFejC^ with cation distribution Mgo.1Feo.9IMgo.9Fe11], and Curie temperature 440°C. The 70 was calculated to be 96i°C. The calculated Curie temperatures (Tf) are given in Table 2. The theoretically calculated Tc values are higher than those obtained from experiment, the difference being large for ferrites with higher Sn-concentration.5. Non-collinear structureFrom Table" 1, it is observed that the A-site moment is close to the free ion value. The B-site moments on the other hand, are smaller than their estimated free ion values, suggesting that the B-site moments are noncollinear. The occurrence of localised canting has been reported in tetravalent and zinc mixed ferrites [11]. A canting of Yaffet-Kittel [12] type on the B-site and spatial ordering of the transverse spin components of the magnetic moment should give rise to the (200) reflection [6], which is purely magnetic in nature. None of the systems studied here show this reflection. However, the absence of (200) does not rule out the possibility of a canted structure as has been shown in many cubic spinels like.ZnCr^ Ga2_2x0 4 [13] and Z^Coi.^FeCK^ [13]. Such a behaviour can be explained on the basis of a long range ferrimagnetic ordering of the longitudinal component with the A-site moments, and a disordered normal component. The system under study could have similar behaviour.6. ac-susceptibilityThe temperature dependence of the normalised ac susceptibility for the series of samples is shown in Figure 3. From the nature of these plots, it is observed that Xk  remains almost constant in all the samples until the temperature reaches nearer to Curie temperature.214 A K Ghatage, S A Patil and S K ParanjpeNeutron diffraction study of tin-substituted Mg-Zn ferrites 215The normalised ac susceptibility drops rapidly to zero at Curie temperature. However, tailing effect is observed for higher content of Sn.Figure 3. Variation of susceptibility with temperature for MgQ7+rZnQ 3 SnxFe2_2.r0 4  system.A polycrystallinc magnetic material consists of three types of domain slates, viz. multidomain (MD), singledomain (SD) and superparamagnet (SP). It has been observed that for MD gamples Xac does not change appreciably with temperature and drops off sharply at Curie temperature. For SP samples Xac decreases with temperature and become zero at Curie temperature. For SD samples Xac increases and shows a maxima at blocking temperature Th and then decreases to zero at Curie temperature. Murthy and Nandikar [ 13] have explained the magnetic behaviour of ferrites on the basis of shapes of Xac curves. From these observations and based on the concept given above it can be concluded that the samples under study contain MD stales. It is also noted that tailing in Xac curve near Curie temperature is due to canting in ferrites. Our samples show tailing effect for higher content of Sn. This observation supports our conclusion of the presence of a canted magnetic ordering in these materials.7. ConclusionThe neutron diffraction and magnetic measurements on the series of Sn substituted Mg-Zn ferrites show that the magnetic moment on the octahedral site is reduced and can be explained on the basis of a canted spin arrangement.AcknowledgmentAuthors (AKG, SAP) are thankful to IUC-DAE Facilities (Indore) for providing financial assistance to carry out this work. We are thankful to Dr. K R Rao, Ex Head, SolidStale Physics Division, Bhabha Atomic Research Centre, Mumbai, for the encouragement and Dr. V Ganeshan (1UC-DAEF, Indore) for X-ray diffraction work.Referencesf I ] Usha Varashney and R K Puri IEEE. Trans Magn, 25 3109 (1989)[2] A R Das, V S Ananthan and D C Khan J Appl. Phys. 57 4189 (1985)[3 1 S S Suryavanshi, S R Savant and S A Paul Indian J ‘Pure Appl Phys. 31 500 (1993)|4J D R  Sagar, Prakash Chandra, S N Chattarjee and P Kishan Proc ICF-5 India, Advances m Ferrites cd C M Srivastava and M J Patm (New D e lh i, 1BH and Oxford) (1989)[S| M M  Kietveld Acta Cryst 22 151 (1967)|6] H M R ie tve ld / Appl Cryst 2 65(1969)|71 R A Young and D B Wiles J. Appl Cryst. 10 262 (1982)|M] M A Gilleo J Phys Chem. Solids 13 33 (I960)|9| Chen Yang and H eR ui-Y un7 Mag. Materials 116231 (1992)110J W M illigan, Y Tumai and J T Richardson J Appl Phys. 34 2093 (1963)1111 K C Srivastava, D C Khan and A R Das Phys. Rev B41 12514 (1990)112| NS Satya Murthy, M G Natera, S I Yousseff, R J Begum and C M Snvastuva Phys Rev 181 969 (1969)I H | C R K Murthy and N G Nandikar Pramana 13 473 (1979)216 A K (Jliata#e, S A Paul and S K Paranjpe

Neutron Diffraction Study of Tin—Substituted Mg-Zn Ferrites

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Neutron Diffraction Study of Tin—Substituted Mg-Zn Ferrites

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