Effect of Ti4+ substitution on the hyperfine interactions in LiZn ferrites

Solid State Communications, Vol. 68, No. 6, pp. 549-554, 1988. Printed in Great Britain.

0038-1098/88 $3.00 + .00 Pergamon Press plc

E F F E C T OF Ti 4~ S U B S T I T U T I O N ON T H E H Y P E R F I N E I N T E R A C T I O N S IN LiZn FERRITES Sumitra Phanjoubam, Deepika Kothari and J.S. Baijal* Department of Physics and Astrophysics, University of Delhi, Delhi-110 007, India and Pran Kishan Solid State Physics Laboratory, Lucknow Road, Delhi-110 007, India (Received 24 February 1988 by P. Wachter)

57Fe M6ssbauer investigations were made on the system Li0.35~0.5,Zn0.3Ti, Fc, 35 r.~,O4(0.0 ~< t ~< 1.2) at 300K and 77K. The effect of variation of Ti concentration on the various hyperfine interactions have been discussed. It is observed that for t ~< 0.6 the M6ssbauer spectra show well defined Zeeman pattern. The spectra for t = 0.8 is relaxed while for t >/ 1.0 there is the existence of a central doublet along with thc relaxed magnetic sextet. This anomalous behaviour of the M6ssbauer spectra at high diamagnetic substitution and the origin of the central doublct havc been discussed.

I. I N T R O D U C T I O N LITHIUM F E R R I T E S are known to possess an attractive set of properties like high Curie temperature, rectangular hysteresis loop, low stress sensitivity and low microwave dielectric losses. These properties have made them find extensive use in industrial, technological and microwave applications. The properties of these ferrites are very sensitive to the method of preparation and the type and amount of impurity substitution. The influence of a number of substitutions have been studied to obtain a high quality ferrite for a specific application. Titanium and zinc are the most commonly used dopants and they modify the saturation magnetization for microwave applications. Such diamagnetically substituted lithium ferrites have been the subject of investigation by. a number of workers to understand the nature of exchange interactions in spinel lattice, spin canting and cation distribution. Lithium ferrites doped with Zn 2~ have been studied [I, 2] and different interpretations were given for the decrease in magnetization at high Zn content. Young and Smit [I] concluded in their zero field, room temperature M6ssbauer study of the system Li~,~ ,5,Zn,Fe25 0~O4 that there is no significant spin canting in this system but merely reversal of the octahedral (B) spins with less than two tetrahedral (A) * Author to whom correspondence should be addres~d.

Fe ~' nearest neighbours. Later, Rosenberg et al. [2] studied the same system at 4.2 K and in the presence of magnetic fields upto 70 kOe. They explained spin canting of the Fee'(B) spins to be the cause of the decrease in magnetization for Zn > 0.4. This is because of the presence of a large number of non-magnetic zinc ions at the A site which is qualitatively explained by the Rosencwaig model [3]. This agrees with the neutron diffraction data of Zhilyakov et al. [4] and high field magnetization data of White et al. [5]. Dormann et al. [6-8] have investigated in detail the system with Ti 4~ and Zn-" substitutions, using different techniques like X-ray and neutron diffraction, a.c. magnetic susceptibility, and M6ssbauer spectroscopy. In a study of the system Li05,0~,Ti, Fez5 ~5,O4he suggested the possibility of spin canting at the tetrahedral sites when t is large. In the present paper we substitute lithium ferrites with Ti 4÷ and Zn -'~ simultaneously, and report the investigations carried out using M6ssbauer spectroscopy. The general formula of the system under study is Li0~5~05,Zn0~Ti, Fe2~5 15,O4 (0.0 <~ t ~< !.2). The quantity of Zn is kept fixed at 0.3 and the influence of varying Ti 4+ substitution on the various hyperfine interactions and cation distribution was investigated. Studies were made at 300 K and at 77 K. The effect of simultaneous substitution of equal amounts of Ti 4+ and Zn 2+ in the lithium ferrite system has been reported elsewhere [9].

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550

Ti 4~ S U B S T I T U T I O N IN LiZn F E R R I T E S

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Vol. 68, No. 6

the samples was confirmed. The variation of the lattice constant with titanium content is shown in Fig. I. The room temperature saturation magnetization (4xMs) and Curie tcmpcrature (T,) measurements were also carried out. A fall in their w~lucs is observed with progressive Ti 4' substitution as is shown in Figs 2(i) and (ii). The M6ssbauer spectra wcre recorded at 300K and 77K using a source of ~:Co in Pd matrix mounted on a constant acceleration drive coupled with a multichannel analyzer. 3. RESULTS A N D DISCUSSION

2. E X P E R I M E N T A L The samples were prepared by the standard ceramic technique using analytical reagent grade Li2CO~, ZnO, TiO2 and Fe.,O~. The suitable proportion of these oxides were taken and mixed thoroughly in an agate jar-mill in the presence of distilled water to improve the homogeneity, and then calcined at 800°C in air for 5 h. A small amount (0.5 wt %) of Bi,O3 was added in order to lower the sintcring temperature and hence avoid volatilisation of lithia and loss of oxygen which occur at higher sintering temperature. The whole mixture was wet milled again, dried and a small quantity of polyvinyl alcohol was added as a bindcr. The granulated mixture was pelleted and finally sintcred in air at 1025°C for 4h. The samples were then furnace cooled in air at room temperature. In order to remove any oxide layer formed on the surface of the pellets during sintering, necessary grinding of the pellets was done. Monochromatic CuK~ radiation was used to characterize the samples by X-ray diffraction. The tbrmation of a well-defined single spinel phase in all

The M6ssbauer spectra taken at 300 K are shown in Fig. 3. At this temperature all the samples upto t = 0.6 exhibited well defined Zeeman pattern which could be resolved into two separate 6-line Zeeman patterns, one due to l:e ~' ions at thc tetrahedral or A site and the other due to the Fe 3' ions at the octahedral or B sites. The sample with t = 0.8 showed relaxation as is evidenced from the broadening of the lines and the enhancement in the intensity of the inner lines. The t - - 1.0 and 1.2 samples are strongly relaxed and showed the co-existence of a central paramagnetic doublet, the intensity of which is greater in the t = 1.2 sample. Almost similar M6ssbauer spectra were observed at 77K as shown in Fig. 4. For higher values of t the spectra showed relaxation. For t = 1.0 and 1.2, along with the broadened magnetic sextet an enhanced central doublet was also observed. The isomer shifts (I.S.) corresponding to Fe ~' coordination at both 300 K and 77 K do not show any significant variation with t. This indicates that the s-electron charge distribution of the Fc ~' ions is

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Vol. 68, No. 6

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Fig. 5(a) and (b) depict the variation of magnetic fields at A and B sites with Ti content at 300 K and 77 K, respectively. In both cases, the internal magnetic fields at B site (HB) was found to be greater than that at A site (HA). Also it was observed that both H~ and HA decreased with increasing t.

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Vol. 68. No. 6

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Fig. 4. M6ssbauer spectra recorded at 77 K o f the system Li,, ~ It has been established by various workers [3 8,] that when non-magnetic Zn and r i ions are substituted in Li fcrrite, Zn:* goes to A site and "Fi 4' goes to B site. and proposed the cationic formulae (Fc~ :Zn )[Li,, s .,.Fcr,.,,s:]O~ and (Fc~ ,,~,Li,,,,)ILi,,sFe~ ~ ,ri,]o4 respect-

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Vol. 68, No. 6

Ti 4~ S U B S T I T U T I O N IN LiZn F E R R I T E S

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553

non-magnetic neighbours for an Fe 3÷ ion increases at both sites. This results in a decrease of the HSTHF at both A and B sites. It is also observed that the field at A site decreases faster than at B site. Considering equation (1), the relative increase in the non-magnetic ion content with increasing t in the formula unit is more in the case of B site. The effect of this is that there is a faster decrease in the number of magnetic bonds as well as HsrHr for an A site Fe 3÷ ion, and this leads to the faster decrease in HA. The occurrence of localized spin canting with increasing diamagnetic ion concentration can also be considered in this context. With the increasing Ti 4÷ content on B-site, the spin canting is supposed to take place at A-site [6, 8, 17], thus supporting the observed variation in the hyperfine fields. The relaxation behaviour observed at high Ti 4+ content could be related to localized canting [6] which destroys the long range correlation between Fe 3÷ spins. With the increase in titanium content the relaxation phenomenon has been observed to increase. At even higher Ti 4÷ substitution for t = 1.0, 1.2 at both 300 K and 77 K, there is an enhancement in the intensity of the central portion as compared to the intensity of the broad outer peaks, and we observe the existence of an additional central paramagnetic doublet along with the relaxed sextet. This observation is in agreement with that of other workers [18-21]. The presence of any impurity phase, which could lead to the central doublet, could not be detected even after careful X-ray studies. The X-ray spectra showed the presence of single spinel phase only. Various explanations have been given for this central paramagnetic doublet and the probable cause has been proposed to be due to superparamagnetic effects [19-22]. lshikawa [23] has proposed that superparamagnetic clusters are present in a magnetically dilute system. In such a system some iron ions are magnetically isolated from other magnetic ions and have a short range of magnetic order. This results in the formation of magnetic clusters of various sizes, which have very little magnetic interaction with the surroundings, but within themselves there can be any type of magnetic ordering. These clusters give rise to a paramagnetic spectrum due to their fast relaxation rate and could be the origin of the central doublet [22]. Coey [24] has also favoured a similar explanation of his study of rare earth iron garnets where part of iron is replaced by diamagnetic Ca 2~ and Sc 4. ions. Some authors [18, 19, 25] have shown that at high diamagnetic substitution, two kinds of spin clusters, (1) entropic clusters and (2) pinned clusters are formed due to the frustration inherent in such a lattice. The formation of these clusters are believed to be responsible for the central

554

Ti 4~ SUBSTITUTION IN LiZn FERRITES

line existing along with the magnetic sextet [18]. It follows therefore that the formation of superparamagnetic clusters either due to the Ishikawa model or the existence of pinned and entropic spins in the frustrated disordered spinel could possibly be the reason for the simultaneous appearance of the central paramagnetic doublet along with the magnetic sextet as has been observed in our present study.

Acknowledgements - The authors (S.P., D.K. and J.S.B.) are grateful to the Ferrite Division, Solid State Physics Laboratory, Delhi for their help in the preparation of samples. The authors are particularly thankful to Dr Chandra Prakash for useful discussions. Two of the authors (S.P. and D.K) wish to thank the University Grants Commision, New Delhi and the Defence Research and Development Organisation, New Delhi, respectively for tinancial assistance.

8, 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

REFERENCES I. 2. 3. 4. 5. 6. 7.

J.W. Young & J. Smit, J. Appl. Phys. 42, 2344 (1971) M. Rosenberg, P. Deppe, S. Dey, V. Janssen, C.E. Patton & C.A. Edmonson, IEEE, Trans, Magnetics, MAG-18, No. 6, 1616 (1982). A. Rosencwaig, (;an. J. Phys. 48, 2857 (1970). S.M. Zhilyakov, V.V. Ivolga, V.I. Mal'tsev & E.P. Naiden, Soy. Phys. Solid State, 19, 1817 (1977). G.O. White, C.A. Edmonson, R.B. Goldfarb & C.E. Patton, J. Appl. Phys..50, 2381 (1979). J.L. Dormann, J. de Phys. CI, 41, 175 (1980). J.L. Dormann, T. Merceron & M. Nogues, Proc. Int. Conf. on Appl. of Mfssbauer Effect, Jaipur, India, 193 (1981).

19. 20. 21. 22. 23. 24. 25.

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M. Nogues, J.L. Dormann, M. Perin, W. Simonet & P. Gibart, IEEE Trans. Magnetics, MAG-15, No. 6, 1729 (1979). C. Prakash, S. Phanjoubam & J.S. Baijal, Hyp. lnts. 2g, 511 (1986). J.B. Goodenough & A.C. Loeb, Phys. Rev. 98, 391 (1955). A. Hudson & H.J. Whitfield, Molec. Phys. 12, 165 (1967), L. Neel, Ann. Phys. 3, 137 (1948). G.F. Dionne, J. Appl. Phys. 45,(8), 3624 (1974). F. Van der Woude & G.A. Sawatzky, Phys. Rev. 134, 3159 (1971). L.K. Leung, B. J. Evans & A. H. Morrish, Phys. Rev. !]8, 29 (1972). J.L. Dormann & R. Renaudin, Proc. Int. Con./'. on Ferrites, Japan, 156 (1980), S.K. Kulshreshtha, J. Magn Magn Materials 53, 345 (1986). J.L. Dormann, D. Fiorani & S. Vitticoli, Proc. Int. ('on/~ on Appl. of M6ssbauer Effect. Jaipur, India 206 (1981). J.K. Srivastava, K. Muraleedharan & R. Vijayaraghavan, Phys. Lett. 104A, No. 9, 482 (1984). W. Kundig, H. Bommel, G. Constabaris & R.H. Lindquist, Phys. Rev. 142, 327 (1966). T.K. McNab, R.A. Fox & A.J.F. Boyle, J. Appl. Phys. 39, 5703 (1968). J.M. Daniels & A. Rosencwaig, Can. J. Phys. 48, No. 4, 381 (1970). Y. lshikawa, J. Appl. Phys. 35, 1054 (1964); Y. Ishikawa, J. Phys. Soc. Japan. 17, 1877 (1962). J.M.D. Coey, Phys. Rev B6, 3240 (1972). R. Rammal, R. Suchall & R. Maynard, Solid State Cornmun, 32, 487 (1979).