Short Notes K185

phys. stat. sol. (a) 100, K185(1987) Subject classification: 75.30; 511.2

Disaccomodation in Ge02/Sn02-Doped - Ni- Zn Ferr i tes

By SATBIR SINGH, R. B. TRIPATHI, and B. K. DAS

Division of Material, National Physical Laboratory, New Delhi 1)

Disaccomodation (DA) in ferrites i s attributed to the diffusion of various cations (Fe2+, Fe3+, metallic dopants, etc.) in the presence of cation vacancies in the ferrite matrix /1,2/. Most ferrite materials generally exhibit two DA peaks in the temperature range of - 20 to + 100 OC. As per standard nomenclature these peaks are identified as peak 111 and peak 11 as the temperature increases. The Fe2+ anisotropy model of Slonczewski /3/ is often used to interpret these mag- netic relaxations. A large DA peak, which occurs around ambient temperature, named peak 111 by Krupicka 141, is caused by interchange of sites between non- localised Fe2+-vacancy pairs and formation of Fe2+-Fe 2+ pairs is considered

to be responsible for peak I1 which is observed a t higher temperatures. The dissolution of a Me4+ ion in the ferrite requires rebalancing of the ionic

charge. It can be shown that by the substitution of each Me4+ ion, two Fe ions are replaced and a Fe2+ ion is generated in the ferrite. In addition a certain number of vacancies will also be created. It has been observed from our studies /5/, that Ge02 is partially soluble in the Ni-Zn ferrite and the addition of up to 0.32 mol% of Ge02 helps to promote densification by facilitating pore elimina- tion. Sn02, however, has been found to have ready solubility in the Ni-Zn fer- rite even up to 1.28 mol% concentration level. In the present studies, the ef- fect of Ge4+/Sn4+ substitution on the disaccomodation spectra of Ni- Zn ferrite sample is reported.

3+

Toroidal Ni- Zn ferrite samples of nominal composition Nio. 58Zn0.40Fe2, 0404 + x(Me02) were prepared by the standard ceramic tech- nique, where Me02 stands fo r either Ge02 o r Sn02 and x varies as 0.00, 0.02,

and 0.04. Thesampleswere sintered at 125OOC for 2 h in each case. Disaccomo- dation was studied using a Siemens bridge (model R 1077) in the temperature range - 20 to + 80 OC. Thetemperature of the sample was stabilized within + 0.05 K

~- ~~

1) New Delhi 110012, India.

K186 physica status solidi (a) 100

Fig. 1. Thermal spectra of disaccomoda- tion of GeO2-doped Ni-Zn ferrite samples. G e 0 2 (in mo1%):0 0, 0 0.02, A 0.04; sin- tering temperature1250OC for 2 h - S 70

0 8

06

L

-x Q

+ 0.5 K for each measurement. In our case DA was defined as follows:

DA(%)=[{u1(603 - r2(600))/ p1(6o)l x 100, (1)

where p1(60) and ~ ~ ( 6 0 0 ) a r e values of ini- tial permeability p at 60 s and 600 s, re- spectively, after demagnetisation.

-

%O 0 LO LO 60 r ( 0 ~ ~ - The observed disaccomodation spectra

of pure and doped Ni- Zn ferr i te samples are plotted in Fig. 1 and 2. The behaviour of process I11 in mixed crystals is charac-

terised by (a) the independence of i ts temperature position of both cation vacan- cy concentration and the kind of attached ions and (b) a linear dependence of i ts magnitude upon the product of cation vacancy concentration and the concentration of Fe2+ ions present (when the concentrations of bothcation vacancies and ferrous ions are comparable). The observed spectra in the case of Sn02 and G e 0 2 addi- tions show a similar feature with regard to effect 111. The observed spectra also display a marked splitting of the relaxation region 111 into two effects 111' and 111 as also observed by Knowles and Rankin /6/ and Ram Naravan /7/ in V205-doped Ni-Zn ferrite samples. The effect 111' which also increases in both cases propor- tional with impurity ion concentration is more intense in the case of Ge02-doped samples as seen from Fig. 1. This DA peak i s attributed to Fe2+-vacancy pairs in the neighbourhood of Me4+ ions (Ge /Sn ) whereas effect 111 is attributed to non-localised Fe2+-vacancy pairs.

As already mentioned, extra Fe2+ ions are generated in the ferr i te as a con- sequence of Ge /a4+ substitution. The concentration of cation vacancies, how- ever, is determined more by the degree of oxidation of the sample. Due to the smaller size of Ge4+ ions, Ge4+-Fe2+ pairs have greater stability (than Sn -Fe pairs) and F?2+-vacancy pairs localised by Ge4+ ions have higher con-

centration. Hohne et al. /8/ have shown the direct proportionality between cation

4+ 4+

4+

4+ 2+

Short Notes Kl87

u r pc) - 020 0 20 LO 60 80

Fig. 2. Thermal spectra of disaccomo- dation of Sn02-doped Ni-Zn ferri te sam- ples. Sn02 (in mol%):O 0 , O 0.02, A 0.04, sintering temperature 1250 OC for 2 h

vacancy concentration and the magnitude of DA peak 111, in Ti-doped magnetite. The effect II according to Braginski /9/ is as- sociated with the formation of Fe2+-Fe2+ (110) pairs which are formed, annihilated

and reoriented by electron diffusion via the vacancies. The tendency of ferrous ion pairing is resisted by the substituted im-

4+ purities (Sn4+ or Ge ). But from the con- 2+ siderations /lo/ of ionic sizes, it is assumed that Sn&-Fe pairs are fairly un-

stable. Based on similar arguments it is believed that the effect I1 magnitude is only weakly influenced by Sn4+ ions present in the host matrix. Therefore, ef- fect I1 in both cases is smallelrthan other DA peaks and in Fig. 1 we observe a decrease in peak 11 magnitude with increase in dopant addition. Postupolski et al. /11/ have observed a splitting of the effect 11 region peak in%- Zn ferrite sam- ples, which is attributed to two different electron diffusion processes at 375 and 478 K. This splitting could not be observed in the present case.

In the DA spectra of undoped samples as shown in both figures, peak 11 is absent as expected. A low intensity peak 111 in the ambient temperature region suggests the presence of an insignificant concentration of Fe2+ ions. DA peak ID' is also not observed since there can be no localised Fe2+ ions present in the undoped sample.

References / l /K. OHTA, J. Phys. SOC. Japan 16, - 250 (1961). /2/ T.G.W. STIJNJES e t al., Proc. Internat. Conf. Ferr i tes (Japan), 1971

/3/ J.C. SLONCZEWSKI, J. appl. Phys. - 32, 2535 (1961). /4/ S. KRUPICKA, Proc. Internat. Conf. Magnetism and Crystallography,

(p. 191).

Kyoto 1961.

K188 physica status solidi (a) 100

/5/ B.K. DAS, R.B. TRIPATHI, and SATBIR SINGH, communicated toIEEE

/6/ J.E. KNOWLES and P. RANKIN, J. Physique,Suppl. - 32, C-1, 845 (1971). /7/ RAM NARAYAN, PH.D. Thesis, University of Delhi, 1980. /8/ R. HtlHNE, K. MELTZER, and G. LIBOR, phys. stat. sol. (a) E, K69

/9/A. BRAGINSKI, phys. stat. sol. - 11, 603 (1965). /10/J.E. KNOWLES, Philips Res. Rep. - 29, 930 (1974). / l l /T . POSTUPOLSKI et al . , C.R. Hebd. S a n c e s Acad. Sci. S, 179 (1974).

Trans. Magnetics.

(1975).

(Received February 2, 1987)