IEEE TRANSACTIONS ON MAGNETICS. VOL. 2 5 . N O . 4. JULY I Y X Y 3109

4+ The Effect of Substitutions of Sn and Zn2+ Ions on the Magnetic Properties of Nickel Ferrites

USHA VARSHNEY A N D RAJINDER K . PUR1

Abstract-The present work is intended to investigate the variations in saturation magnetization, Curie temperature, magnetocrystalline anisotropy, initial permeability, and magnetic losses of nickel ferrites substituted with Sn4+ and Zn2+ ions. An additional aini is to find the zone of utility from the frequency spectra of initial permeability and variation of temperature coefficient and percentage of disaccommo- dation from the point of view of stability of these ferrites. A compar- ison of the above mentioned magnetic parameters has also been made as a function of increasing tin as well as zinc concentrations.

I. INTRODUCTION WIDE VARIETY of ferrite materials has been de- A veloped for application in electronics and communi-

cation industries. Since an ideal ferrite capable of cover- ing a complete spectrum of applications cannot be obtained, a compromise has to be made in improving upon one characteristic property at the expense of another. Ex- tensive work has been done by various workers to upgrade the properties of ferrites by substituting various types of amounts of impurities. It has been reported that by incor- porating small amounts of tetravalent tin additive, the electrical [ 11-[3] and microstructural [2], [3] properties and Mossbauer parameters [3] - [6] of the basic nickel-zinc ferrites are significantly influenced.

The present work is aimed at studying the characteristic magnetic properties of nickel ferrites by successively in- creasing the quantity of Sn4+ and Zn2+ ions. The three series of polycrystalline solid solutions of ferrites having the following composition were investigated:

Ni, +x-,ZnJSnxFe2-2x04 ( 1 ) where i) y = 0.1, x = 0.1 to 0.9, ii) y = 0.3, x = 0.1 to 0.5, and iii) y = 0.5, x = 0.1 to 0.5, and where x varies in steps of 0.1 for all the three series.

11. EXPERIMENTAL DETAILS The samples under study have been prepared by a con-

ventional ceramic techique involving slow cooling in air. The desired quantity of analytical grade NiO, ZnO, Fe203, and Sn02 was wet-blended with deionized water

Manuscript received August 31, 1987; revised November 1, 1988. A part of the work was presented at the 4th International Conference on Fer- rites, San Francisco, CA, October 31-November 2 , 1984.

U. Varshney is with the American Research Corporation of Virginia, P.O. Box 3406, Radford, VA 24143-3406.

R. K . Puri is with the Department of Physics, Indian Institute of Tech- nology, New Delhi, 110016, India.

IEEE Log Number 8927727.

001 8-9464/89/0700-3 1

in an agate mortar and pestle. The mixed powder was cal- cined in an alumina crucible for 4 h at 1000°C in air. The calcined powder was pulverized with deionized water, dried at 3SO"C, and granulated through the sieves of 85- 120 mesh (BSS), using 1-percent weight of polyvinyl al- cohol as a binder. These granules were compressed under a pressure of 46.5 kg/cm2 to form pellets of 15-inm di- ameter and toroids of IO-mm inner diameter and 15-mm outer diameter. The pellets and toroids were finally sin- tered for 2 h at 1300°C in an alumina combustion boat and were slowly cooled in air to room temperature at a rate of 100"C/h. The X-ray diffraction patterns were ob- tained, using Fe5'( K,) radiations with Mn filter for all the samples. A single-phase spinel structure was observed for all the compositions except for samples having y = 0.1, x = 0.8 and 0.9, and y = 0.5, x = 0.4 and 0.5, where a second phase was observed. The dispersed phase was at- tributed to the presence of SnO?.

The saturation magnetization of samples having dimen- sions 5 mm x 5 mm X 2 mm was measured at room temperature by the Ponderometer Method [7]. The Curie temperature was measured with an accuracy of 4°C by a setup given by Sohoo [8]. Using a Siemens Impedance Bridge model R2077 at a frequency of 10 kHz, the initial permeability was calculated [9] at room temperature from the inductance values measured by winding 60 turns of 30 SWG enameled copper wire on the sintered toroids. The percentage of disaccommodation was measured by using the method of ac demagnetization [lo]. The percentage of disaccommodation was calculated at 5 min after demag- netization and 24 h and 15 days after demagnetization, at a frequency of 10 kHz. The temperature coefficient was computed 191, [lo] from the respective values of initial permeability of the samples at 50°C and 20°C at a fre- quency of 10 kHz. In order to study the variation of initial permeability and loss factor as a function of frequency, measurements were made at room temperature by using Siemens Impedance Bridge model R2077 in the frequency range of 200 Hz to 1.5 MHz and RF Impedance Meter, Hewlett-Packard 4815A3, in the frequency range of 0.5 to 108 MHz.

111. RESULTS A N D DISCUSSION Fig. 1 shows the variation of saturation magnetization

per gram with increase of tin and zinc concentration at 27°C. A decrease of saturation magnetization has been

09$01 .OO 0 1989 IEEE

lEEE TRANSACTlONS ON MAGNETICS. VOL. 25 . NO. 4. J U L Y 1989

0 y . 0 1

0 y = 0.5

A y = 0.3

X

Fig. 1 , Saturation magnetization per gram U as function of tin concentra- tion x for zinc concentrations y = 0. I , 0.3 , and 0.5 at 27°C.

observed with an increase of tin concentration whereas saturation magnetization has been observed to increase with an increase of zinc concentration for constant tin value. It has been inferred from the Mossbauer study [3]- [6] that these ferrites follow the general ionic distribution formulas

( Zn:+Snf+Fe:?, -,) (Ni:: -, Sn,"?,Fe:?2x + ,,+,) 0;-

( 2 )

where z is negligible for low tin concentration samples and has a small value for high concentration samples. The replacement of 2Fe'+ ions of magnetic moment, 5pB, each by one Ni" ion of lower magnetic moment, 2pg, and an- other diamagnetic Sn4+ ion primarily at the octahedral B site, reduces the net magnetization of the B sublattice, whereas the negligibly small quantity of Sn4+ replacing Fe" ions at the tetrahedral A site does not considerably reduce the magnetization of the A sublattice. Since the net saturation magnetization is the difference between the magnetization of the B sublattice and the A sublattice, it follows that the magnetization of the system decreases with an increase of tin concentration for all the three se- ries. The marked change in the slope of saturation mag- netization for higher tin concentration samples having y = 0.1 and x > 0.5 indicates that a significant amount of tin replaces iron ions in the A sublattice, with the result that the net difference in the magnetization of the two sub- lattices does not show a rapid variation like that for sam- ples of low tin value.

The observed increase in saturation magnetization with increasing zinc concentration for constant tin value fol-

lows the ionic distribution formula of mixed nickel-zinc ferrites as

( Znt+Fe: ? \ ) ( N: ,Fe:: \ ) 0:- . ( 3 ) As Zn2+ has a marked preference to go to an A site [4], [7], the magnetization of the A sublattice decreases whereas that of the B sublattice increases, similar to that of nickel zinc ferrites, thereby resulting in an increase of net magnetization of the system.

The variation in Curie temperature with increase of tin and zinc concentrations is shown in Fig. 2. Curie tem- perature has been observed to decrease with the increase of tin as well as zinc substitution. The observed decrease can be explained on the basis of exchange interactions between ions. These exchange interactions are dependent on the density of magnetic ions and their magnetic mo- ments. According to Neel's [ l l ] molecular field model, A B exchange interactions are dominant as compared to those of AA or BB interactions.

The inferences drawn from Mossbauer studies showed that Sn4+ mainly replaces Fe3+ ions at the octahedral site following the ionic distribution (2) with z = 0, i .e. , 2xFe3' ions are being replaced by xSn4+ and xNi2+ ions at the B site. Since Ni2+ ions have lesser magnetic mo- ments than Fe3+ ions, and Sn4+ ions are diamagnetic, the increasing value of x for constant y value results in the reduction of density of magnetic ions, thereby reducing the net magnetic moment at the B sublattice. This, there- fore, results in the weakening of A B exchange interac- tions.

It is also inferred from Mossbauer studies that for higher x values, Sn4+ ions replace Fe3+ ions at the A site as well as the B site, following the ionic distribution (2) where z has a small but finite value, thereby resulting in the dilu- tion of magnetic ions at both the tetrahedral and octahed- ral sublattices. This further leads to the reduction of A B exchange interactions. It is expected that with the weak- ening of the A B interactions, the thermal energy required to offset the spin alignment also decreases, resulting in the decrease of Curie temperature with increasing tin con- centrations for all the three series. The samples, y = 0.1, x = 0.8 and y = 0.1, x = 0.9 were observed to be par- amagnetic, which agrees with the results obtained from Mossbauer analyses of these samples.

The lattice constant values of these three series [2], [?I have been observed to increase from 8.35 and 8.48 A with the increase of tin concentration. The increase in lat- tice constant results in the decrease of overlap of orbitals thus reducing the exchange interactions between the ions. The variation in the slope of curves of Curie temperature for all the three series follows the above mentioned con- tentions.

For the same tin concentration, the decrease of Curie temperature with increase of zinc concentration follows a trend similar to that of mixed nickel-zinc ferrites. The addition of diamagnetic ions at either site invariably re- sults in the weakening of A B exchange interactions, thereby decreasing the Curie temperature.

VARSHNEY AND PURI: MAGNETIC PROPERTIES OF NICKEL FERRITES

0.1 0.2 0.3 0.6 0.5 0.6 0.7

X

Fig. 2 . Curie temperature T, as a function of tin concentration .r for zinc concentrations v = 0.1, 0.3, and 0.5.

Fig. 3 shows the variation of corrected initial permea- bility pic as a function of tin and zinc concentrations. The initial permeability has been corrected by using the Glo- bus relation [12]

d d

1), = ( p , - I ) ? (4)

where ( p, - 1 ), represents the corrected permeability, ( pi - 1 ) is the observed permeability, d, is the X-ray density, and d is the observed density of the material. The corrected initial permeability has been observed to de- crease with the increase of tin concentration whereas it increases with an increase of zinc concentration for a con- stant value of tin. The variation has been explained on the basis of the Globus model [13]. According to this model, the contribution to initial permeability is predominantly due to reversible-domain wall motion. The permeability corrected for density is given as

where M, is saturation magnetization, D,, is the average grain diameter, and k , is magnetocrystalline anisotropy. The optical micrographs 121, [3] indicated that the aver- age grain diameter [3] does not differ significantly and is not expected to effect a large variation in the values of initial permeability. Since initial permeability is propor- tional to the square of saturation magnetization, the vari- ation of saturation magnetization mainly affects the initial permeability. It is seen from Fig. 3 that the variation of permeability with the increase of tin concentration fol- lows a trend similar to that of decreasing saturation mag- netization with increasing tin concentration in all the three

0 y =0.1

0 y .0.5

A y :0.3

X

3111

9

Fig. 3. Corrected initial permeability p,, as a function of t in concentration .r for three concentrations of zinc Y at 10 kHz and 27°C.

series. The deviation between the experimentally ob- served pic values and the calculated p,, values (according to ( 5 ) ) , indicates that initial permeability depends on the ratio of average grain diameter to magnetocrystalline an- isotropy apart from saturation magnetization in a manner similar to that given by the Globus model [ 131.

It has been observed by the present authors, from the microstructural and electrical properties [ 11-[3] apart from Mossbauer studies 131-[6] of these samples, that tin starts segregating at the grain boundaries for higher concentra- tions. The property of limited solubility of dopant ion in the matrix would reduce permeability due to precipitation of nonmagnetic phase at the grain boundary which pro- duces a magnetic discontinuity between the grains. As seen from Fig. 3 , the change of slope of p,, for higher concentrations for all the three series can be attributed to the possibility that the amount of segregation reaches a limit beyond which it becomes insensitive to the variation of magnetic discontinuity.

Percentage disaccommodation has been observed to de- crease with the increase of tin concentration in all the three series as seen from Fig. 4. It has been found by many workers [15]-[17] that FeZf ions play a major role in the disaccommodation process of these ferrites. A negligibly small value of percentage disaccommodation may perhaps be due to a negligibly small amount of Fe2+ ions present in these samples. The absence of Fe2+ ions in these sam- ples has been further confirmed from the Mossbauer stud- ies [3]-161. The decrease in percentage disaccommoda- tion with increasing tin concentration is due to localization

6 0.07-

' 006-

- U

E d 0.05-

0

a, [5)

F 0.OL- 0 a

a,

003-

0.m-

0.01

31 12

I

-

0 y i 0.1

A y : 0.3

0 v = 0.5

of Sn4+ to the vacancies, thus forming pairs of Sn4+ ions and vacancies. A similar pair formation, responsible for influencing the disaccommodation process, has also been reported by other workers [18]-[20].

The variation of temperature coefficient 191, defined as

where pil and p i 2 are values of initial permeability at tem- peratures TI and T2, respectively, which is an intrinsic property of the material, is observed to increase with the increase of tin concentration whereas it is found to de- crease with the increase of zinc concentration for a con- stant tin value, as shown in Fig. 5 . The intrinsic param- eters of the material that affect initial permeability are saturation magnetization and magnetocrystalline anisot- ropy. It is also dependent on the incidence of different degrees of wall continuity across the grain boundaries as shown by Globus et al. [21]. Since both M, and k l vary with temperature, p i is a complicated function of temper- ature and it is difficult to derive any quantitative infer- ences. The large increase in the temperature coefficient for higher tin values can be attributed to the increased magnetic discontinuity between the grains due to segre- gation of tin at the grain boundaries which makes initial permeability more sensitive to temperature.

Fig. 6(a)-(c) shows the variation of initial permeability as a function of frequency. The dispersion frequency is observed to be lower for ferrite samples of higher initial permeability than those of lower permeability. The dis- persion frequency shifts toward higher frequency with in- creasing tin concentration whereas it shifts toward lower

IEEE TRANSACTIONS ON MAGNETICS. VOL. 25 . NO. 4. JULY 1989

L O -

- Y I 30- - c a, U

a,

U

- - : m 20- a

+

E

n-

o y z o . 1

A y =0.3

0 y zo.5

X

Fig. 5 . Temperature coefficient a as a function of tin concentration x for zinc concentrations y = 0.1, 0.3, and 0.5 at 10 kHz.

frequency with the increase of zinc concentration for a constant tin value. It is further observed from these curves that the resonance character, shown by the maxima of pi, increases with the increase of tin concentration whereas it decreases with the increase of zinc concentration for a constant tin content. The variation follows a trend as ex- plained by Globus [22], [23] from the relation for the re- laxation character as

( 6 ) 2

( pi - 1) * fr = constant

where pi is the static initial permeability andf, is the re- laxation frequency. The transformation of magnetic spec- tra from the relaxation character to the resonance char- acter changes (6) to the form

( p i - 11' '~ . h = constant. (7 )

It follows from the above mentioned equation that the dispersion frequency is expected to be lower for speci- mens of higher permeability. This is due to the fact that for materials of lower permeability, the demagnetizing fields, which appear during the wall movement, result in an increase of the restoring force thereby increasing the relaxation frequency. Also, the high anisotropy of low- permeability ferrites is known to increase the intrinsic re- storing force of the domain walls. Fig. 6 shows that the increase in relaxation frequency with increasing tin or de- creasing zinc concentration in these samples follows a trend similar to the above mentioned relation.

The samples with increasing tin concentration show a deviation from the relation (6) which shows an increase

VARSHNEY AND PURI: MAGNETIC PROPERTIES OF NICKEL FERRITES

I

Y -0.1

1. I I I I I I I I I I I 1 I I , I , I I I I I I , , ,

10 lo6 l o 7

f (Hd

I

I

3113

Fig. 6. Corrected initial permeability p,< as a function of f requencyffor zinc concentration. (a) v = 0. I . (b) Y = 0.3. (c) Y =

0.5. x = 0. I to 0.5 varying in increments of 0.1.

31 14 IEEk TRANSACTIONS O N MAGNETICS. VOL 25. NO 4. J U L Y 1989

in the resonance character. According to Globus [ 131, 1241, the resonance character is present only in those ma- terials which are not homogeneous from the point of view of their granular structure, i.e., which possess different grain size. It arises from modifications of the wall param- eter due to the interaction between grains of different sizes having different relaxation frequencies. The resonance character, however, vanishes when the grain size is uni- form. I t has been shown that the resonance character is further expected to shift the dispersion threshold to higher frequencies. It has been observed from optical micro- graphs [2], [ 31, that nonuniformity in the granular struc- ture increases with the increase of tin concentration and hence an increase in the magnitude of resonance character is thus expected; whereas with the increase of zinc con- centration, the inhomogeneity is observed to decrease thus reducing the magnitude of the resonance character.

Fig. 7 shows the variation of magnetocrystalline an- isotropy k , as a function of tin and zinc concentration, as calculated from ( 5 ) . The variation of k , can be explained on the basis of the single-ion anisotropy model [25]-[27] according to which Fe3+ ions, present at the tetrahedral as well as octahedral sites, exclusively contribute to mag- netocrystalline anisotropy energy whereas it is known from the crystal field considerations that Ni2+ ions at the B site do not contribute to k , .

It has been concluded from Mossbauer study [3]-161 of these samples that the following ionic distribution holds good for specimens with y = 0.1, x I 0.5 wherein 2xFe3+ ions are being replaced by xNi2' and xSn4+ ions at the octahedral site.

( Znt'Fe:? ,.) (Ni:?! + ,Sn:+Fe:;? - \-) Oi- . ( 8 ) Here ( 1 - y)Fe3+ ions at the A site and ( 1 + y - 2 x ) Fe3+ ions at the B site contribute to k , . The variation can be understood in a qualitative way from the following expression of k , :

kl = ( 1 - y ) k , + ( 1 - y - 2x)kR (9) where k, is the positive anisotropy contribution due to A-site Fe3+ ions and is compensated by the negative an- isotropy k R of B-site Fe3+ ions. It is also reported [25] that the contribution from B-site Fe3+ ions is twice as large in magnitude as compared to A-site Fe3+ ions. It can be seen from (9) that increasing tin concentration reduces the number of Fe3+ ions at the B site without changing the Fe3+ ions present at A site. A net reduction in k , is thus expected and agrees with the experimental observations.

The insignificant change in the slope of the curve for samples with y = 0. I , x > 0.5 and y = 0.3 and 0.5 series suggests that Sn4+ ions replace Fe3' ions at the A site as well. The amount of SnJC ions replacing Fe3+ ions at the A and B sites may be such that the difference between the contributions to k l from Fe" ions at both the sites re- mains insignificant. It indicates the following ionic dis- tribution :

( Znt'Fe;':, - ,Sn;+) (Nif 1, -,.Sn, 4 f -:Fe, i f - ?, + > + :) O:-.

( 10)

0 y =0.1

A y = 0.3

o y ~0.5

9

X

Fig. 7. Magnetocrystalline anisotropy k , as a function of tin concentration x for zinc concentration Y = 0.1, 0.3, and 0.5.

L

3.

ro c i-"

2.

1. I

0 y.o.1

A y = 0.3

0 y = o s

0.2 0 3 0.4 0.5 3.6 0.7 0.6 C

X

3

Fig. 8. Magnetic loss factor tan 6 as a function of t in concentration .'i for = 0.1. 0.3, and 0.5 series.

Similar results corresponding to the above mentioned ionic distribution have also been obtained from Mossbauer studies of these samples. The variation of k l with the in- crease of zinc concentration similarly follows from the ionic distribution of mixed nickel zinc ferrite given by (3).

VARSHNEY AND PURI. MAGNETIC PROPERTIES O F NICKEL FERRI'IES 31 IS

1.0: I Q . c - 2 :

0.1 -

y I O . 1

0 x z 0.1

a x = 0.2 . x . 0 3

0 x I O L

A x I O . 5

y I 0.3 10

0 x I O . 1

A x = 0.2 x I 0.3

0 x = 0.6 . x = 0.5

10 i Y = 0.5

0 x = 0.1

a x.0.z

x i 0.3 x r 0.L

L x = 0.5

Fig. 9. Magnetic loss factor tan 6 as a function of frequency ,f for zinc concentration. ( a ) y = 0. I . (b) y = 0.3. ( c ) y = 0.5. Tin concentration x = 0. I to 0.5 varying in steps of 0.1.

Fig. 8 shows the variation of magnetic loss factor, tan 6, with an increase of tin, x, as well as zinc, y, con- centration, at a frequency of 10 kHz. The magnetic losses are found to increase with the increase of tin concentra- tion in all the three series, whereas it is found to decrease with the increase of zinc concentration for a constant tin value. The optical micrographs reveal an increase in in- homogeneity with increasing tin concentration for all three series whereas inhomogeneity decreases with the increase

of zinc concentration. The increasing inhomogeneity gives rise to the resonance character, which results in formation of poles thus giving rise to demagnetizing fields and an increase in magnetic losses. It explains the observed vari- ation of loss factor with the increase of tin and zinc con- centration.

Fig. 9(a)-(c) shows the variation of magnetic loss fac- tor as a function of frequency for the three series. The critical frequency at which tan 6 shows a maximum is ob-

3116 IEEE TRANSACTIONS ON MAGNETICS. VOL. 75. NO 1. J U L Y I Y X Y

served near the frequency for which p, is maximum. Fur- thermore, this critical frequency is observed to shift to- ward higher frequencies with increasing tin concentration whereas it is found to shift toward lower frequencies with increasing zinc concentration. Also, the intensity of tan 6 was found to increase with the increase of t in concentra- tion whereas no systematic significant change has been observed in the intensity of tan 6 with the increase of zinc concentration for constant tin value. It has also been shown by Globus [24] that the frequency dependence of magnetic losses in ferrimagnetic material is controlled by the same parameters as that of the real part in initial permeability. The shift in the critical frequency with con- centration of tin and zinc can be explained from the fol- lowing relation given by Globus:

where

and K = k l + A,$ * a,,, and&. represents the critical fre- quency, D,,, is the average grain diameter, M,\ is saturation magnetization, K is the total global anisotropy, k l is mag- netocrystalline anisotropy, A, is the saturation magneto- striction, and a,, is the internal stress. It is seen from (1 1) that the critical frequency J;. increases with a decrease of M, and an increase of K . The shifts in the critical fre- quency with the increase of tin or zinc concentration, therefore, follows the trend as observed from the above mentioned relation.

111

121

I 31

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151

161

171

181

REFERENCES U . Varshney, R . K . Puri. K . H. Rao. and R . G . Mendiratta, “Anom- alous electrical behavior of nickel-zinc ferrites doped with tetrava- lent-tin impurity.” in Ferrire.\. Proc. I C F 3 (Sept.-Oct. 1980. Japan). p . 207, 1981. U . Varshney and R . K . Puri. “Resistivity and dielectric properties of Z n 2 ’ and Sn“ substituted niche1 ferrite\,” J . A m e r . Certrrti. Soc. ( to be published). U . Varshney. “Microstructural, electrical. magnetic and Mossbauer investigations of tetravalent-tin Substituted nickel-zinc ferrites.“ the- sis. IIT, New Delhi, India, 1982. R. K . Puri and U . Varshney, “ M h s b a u e r study of Zn” and Sn” additive in nickel ferrites.” J . Ph!.s. C/ic>tn. So/ic/.s. vol. 44. n o . 7 . p . 655, 1983. U . Varshney. R . K . Puri. and R . G . Mendiratta. “Miissbaucr study of the tin substituted nickel-zinc ferrites,“ in Pro<.. Ir~ditrri Nuriot~uI Sci. A u d . . Phys. Sci. . I r i r . CoriJ: O J I rhc App/icurion.\ oj the Md.5.s- h i r e r Efecr (Dec. 1981. India). p . 190. 1982. R. K . Puri. U . Varshney. and R . G . Mendiratta, “MBssbauer study of Zn,, , Ni, , , , , .Sn,FeZ >,O,.” / m / i u ~ J . Plrrc, AppI . P h ~ s . . vol. 2 1 . no. 12. p. 686. 1983. J . Smit and H . P. J . Wijn. Fcrri/c>.t. Philips Technical Librnry. 1959. p . 157. R . F. Sohoo, Theory u r d A p p / i w r i o m ofF<,rri tes . Englewood Cliffs. NJ: Prentice-Hall. 1960. p . 109.

191 1. C . Heck. Mugneric Muferiuls urd Their Applicurioti.7. London. England: Butterworths. 1974, p . 30.

[ 101 E. C . Snelling, Soft Ferrite.). London. England: llifee Books, 1969. 1111 L. Neel, Ann. Phys. , vol. 3. p . 137, 1948. 1121 A. Globus. P. Duplex. and M . Guyot , “Determination of initial mag-

netization curve from crystallites size and effective anisotropy field.” IEEE Trcrm. M u g t i . . vol . MAG-7, p . 617. 1971.

1131 A. Globus, “Influence de la stmcture granulaire sur la dispersion de la pernieabilite des ferrite.” thesis. Univ. of Paris. Paris. France. 1963.

1141 -, presented at 1CF2, Bellevue. France. 1976. 1151 A . Yanese. “Mechanism of disaccommodation in ferrite.” J . Ph!.,.

Soc. Japan, vol. 17. no. 6 , p. 1005, 1962. 1161 S . lida. “Mechanism of disaccommodation in ferrites.” J . Phxs. Soc.

Jupuri, vol . 17, no. I , p . 123, 1962. [ I71 Y. Taward. “Relaxation loss in ferrite as a function of ferrous ion

content,” J . Phys. Soc. Jupnn.. vol. 17. no. 11, p. 171, 1962. [I81 R . Hohne, K. Melzer, H . Hochschild, G . Libor. and R . Krause.

“Magnetic aftereffects in titanium-doped magnetite.” Phys. Sruru.5 Solidi (a) , vol. 27. p. k l 1 7 , 1975.

1191 A . Broese Van Groenou, J . Appl. Phys., vol. 2. p . 47 , 1973. 1201 J . E. Knowles, “Magnetic after-effects in ferrites substituted with

titanium o r tin.” Phi/ ips Res. Reprs . , vol. 29. no. 2. p . 93. 1974. 1211 A . Globus and P . Duplex, “Initial susceptibility of ferrimagnetic ma-

terials and topography of domain walls.” Phys. Sfcitits Solidi ( a ) , vol. 31, no. 2 , p. 765, 1969.

122) J . Gieraltowski and A. Globus, “Domain wall size and magnetic losses in frequency spectra of ferrites and garnets,” IEEE Truti!. Mugn., vol. MAG-13. no. 5 , p. 1357. 1977.

1231 A. Globus, Proc. J . Phys. Colloq.. vol. 38. p . C l . 1977. [24] A. Globus and M. Guyot , “Control of the susceptibility spectrum in

polycrystalline ferrite materials and frequency threshold of the losses,” IEEE Truns. M a p . , vol . MAG-6, p . 614, 1970.

1251 J . Kanamori. G . Rado. and H . Suhl, Mognrtistn. London. England: Academic Press Ina.. 1964. ch . 4.

1261 K . Yoshida and M . Tackiki, Progr. Theorer. Phys. . vol. 17, p . 331. 1957.

1271 A. Broese Van Groeneu and J . A . Schulkes. “Magnetic anisotropy of some nickel zinc crystals.” J . Appl. Phys. , vol . 38. no. 3. p . 1133. 1967.

Usha Varshney was born in Aligarh. India. on July 2 , 1956. She graduated from the Physics Honours School, Panjab University. Chandrigarh. India. in 1977. and received the Ph .D . degree from the Indian Institute of Tech- nology. New Delhi, India, in 1983.

Before joining the American Research Corporation of Virginia, Rad- ford, VA. she was a Research Associate at Virginia Polytechnic Institute and State University, Blacksburg, where she conducted research on phos- phors for an advanced cathode ray display system. From 1985 through 1986 she was a magnetic design engineer at Merrimack Magnetics Corporation. Lowell, MA. She also served as a Research Scientist for Chronar Corpo- ration. Princeton. NJ. She is currently employed as a Research Scientist at the American Research Corporation of Virginia. Her current research in- terests include ferrites technology. high-T, superconducting thick films and devices. and laser processing of materials and coatings.

Rajinder K . Puri was born in Lahore. India. o n June 6, 1938. He received the M.Sc. degree from the Physics Honours School, Panjab Univerhity. Chandrigarh. India. and the Ph .D . degree from the Indian Institute of Technology. New Delhi. India, in 1969.

He has been working in the field of high-energy physics and solid-state physics using Mossbaucr spectroscopy. During 1973 he was a po\t doctoral fellow at Gamma Ray Group, Birmingham University. England. Since 1970 he has been a professor in the Physics Department, Indian Institute of Tcch- nology. New Delhi. His current interest is in magnetic materials.