Bull. Mate~. Sc~., ',,ol. 5, No. 1, Ma~ch /983, pp. 1-11 c.C) Printed m lndm.
Manganous-zinc ferrites synthesized out of MnO{~
C E D E S H P A N D E , P P B A K A R E a n d M N S A N K A R S H A N A M U R T H Y * [
Physical Chemistry Division, Naliona[ ChermcaI I~ab~namry, Poona 411 008, India IMawrials Research Laboratory, Indian Institu'te ¢~1 Science, Bangalore 560012, India.
MS received 6.July 1982; revised 24 September 1982
Abstract. Manganous-zine t~rrites are best prepared directly from MnO and can be conveniently pressed with non-volatilizable binders such as polyvinyl alcohol which however can be burnt out without oxidizing the M n 2 + in the t~'rri~e by sintering in a static atmosphere generated by the thermal dissociation of active t;erric oxide pellels placed out of contact with the ferrite in the closed sintering unit. Such a process eliminates cracks and rejections in the sintered pieces besides giving very satisfactory and reproducible resuhs.
Keywords. Manganous-zinc ferritcs; sintering; elimination of cracks.
1. I n t r o d u c t i o n
In two p r e v l o u s c o m m u n i c a t i o n s ( M u r t h y ct a / 1978, 1979 w h i c h will be r e f e r r e d to h e r e a f t e r as p a p e r s 1 a n d 2 r e s p e c t i v e l y ) the p r e p a r a t i o n o f m a n g a n o u s - z i n c ferrites d i r e c t l y f r o m M n O itself has b e e n dealt with. P a p e r 1 deals w i t h the use o f M n O p r o t e c t e d f r o m o x i d a t i o n in a closed system, while p a p e r 2 d e s c r i b e s the use of M n O s t a b i l i z e d by solid s o l u t i o n w i t h Z n O . In e i t h e r case the b i v a l e n c y o f m a n g a n e s e is m a i n t a i n e d right f r o m the start. It is worth c o m p a r i n g m a n g a n o u s - z i n c ferrites p r e p a r e d in this w a y w i t h those p r e p a r e d o u t of a stable h i g h e r o x i d e of m a n g a n e s e . In the l a t t e r m e t h o d w h i c h is q u i t e ' p o p u l a r the h i g h e r v a l e n c y of m a n g a n e s e is r e d u c e d to t w o at the t i m e of s i n t e r i n g ( S t r i v e n s a n d C h o l 1971; N a t a n s o h n a n d B a i r d 1969). It w o u l d be i n t e r e s t i n g to see w h e t h e r this is the s a m e as r e t a i n i n g m a n g a n e s e in the c o r r e c t v a l e n c y state r i g h t f r o m the b e g i n n i n g of the p r e p a r a t i v e process.
E v e n if it is a p r e f e r r e d p r a c t i c e to p r e p a r e m a n g a n o u s - z i n c ferrites w i t h o u t a l l o w i n g the m a n g a n e s e to be in a n y v a l e n c y state h i g h e r t h a n the b i v a l e n t at a n y stage, t h e r e still arises the p r o b l e m of b u r n i n g the b i n d e r s e m p l o y e d in p r e s s i n g the ferrite p o w d e r s : the M n 2 ÷ m a y get o x i d i z e d in the process. P a p e r s I a n d 2 dealt with this p r o b l e m in two ways. O n e was to use a v o l a t i l i z a b l e b i n d e r c a p a b l e of b e i n g distilled off in a n i n e r t a t m o s p h e r e ; a l t e r n a t i v e l y t h e ferrite was a l l o w e d to get o x i d i z e d w h i l e b u r n i n g a n o n - volatilizable b i n d e r such as PVA, a n d s u b s e q u e n t l y the M n 3+ was r e d u c e d to M n 2+ . T h i s was d o n e by h e a t i n g the p r e s s e d ferrite at 1 0 5 0 ° C (after b u r n i n g t h e b i n d e r ) in the p r e s e n c e o f a c a l c u l a t e d q u a n t i t y o f r e d u c e d iron p o w d e r in a closed s y s t e m . T h e iron takes up t h e o x y g e n g i v e n o u t by t h e t h e r m a l d i s s o c i a t i o n o f the o x i d i z e d ferrite a n d u l t i m a t e l y r e d u c e s all the M n ~ ÷ to M n 2 +. S u b s e a u e n t l y the ferrite was sintered at 1 1 5 0 ° C
~'l NCL Communicatmn No. 3059
* "Fo whom all communicatmns to he addressed.
2 C E Deshpande, P P Bakare and M N Sankarshana Murthy
in a static atmosphere of pure nitrogen and in the presence of pellets of active ferric oxide. The Fe203 generates the partial pressure of oxygen necessary to produce the optimum amount of Fe 2÷ required in the ferrite (Winkler 1971; Ohta 1963).
Volatilizable binders as a rule do not possess the binding strength of PVA which - - because of its hydrogen bonding - - can press with advantage complicated shapes such as pot-cores. But if PVA is employed for pressing, and subsequently burnt out with a simultaneous inevitable oxidation of Mn 2 +, all the effort spent so far to keep Mn 2 ÷ as such is in principle wasted. Besides, there arises a serious problem - - that of cracks in the sintered pieces - - whenever oxidation and reduction are carried out on the pressed ferrites. This was indeed noticed on performing a large number of sintering runs of the type described in paper 2 and briefly sketched in the last paragraph. It was therefore necessary to study in greater detail the sintering in a static atmosphere, of manganous- zinc ferrites pressed with PVA.
2. E x p e r i m e n t a l
2.1 Synthesis from different manganese oxides
2.1a Synthesis from MnO : M n O was stabilized (Deshpande et at 1978; Deshpande and Murthy !981) by solid solution with the required amount of Z n O and then reacted at 950°C with the ferric oxide in a closed inert atmosphere in order to yield the ferrite powder (paper 2). The x-ray powder pattern even at this stage revealed a single phase of pure manganous-zinc ferrite. The powder was pressed with a volatilizable binder such as camphor into toroids 2.2 cm OD, 1.2 cm ID and 0.7 cm thickness. The camphor was then distilled off at 100°C in a current of oxygen-free nitrogen. The pressed pieces were then sintered at 1250°C for 8 hr in a closed static system filled initially with pure nitrogen and in the presence of double the weight of active ferric oxide pellets prepared as given in paper 2. The sintered pieces had no cracks and the magnetic measurements on toroids obtained from twenty five such runs are summarized in table 1. The x-ray powder pattern matched fully with that expected for manganous-zinc ferrites. The electron micrograph of this material is given in figure 1.
2.1 b Synthesis from a higher oxide of manganese: Active precipitated MnO2 was thoroughly mixed with the required amounts of active Z n O and Fe203 (prepared as given in paper 2), pressed into pellets and heated at 950°C for 8 hr in a closed system filled with pure nitrogen and in the presence of a calculated quantity of reduced iron (kept out of physical contact with the reaction mixture) in order to reduce all the manganese to Mn 2 +. The x-ray powder pattern of this calcined material showed however notoonly the lines of manganous-zinc ferrite but also additional lines at d = 1.54 and 2.19 A. A volatilizable binder was again employed to press this material and the.sintering of toroids was done exactly as in § 2. la. Again there were no cracks even in the course of a large number of runs and the results from 25 such runs are summarized in table 2. The x-ra~( powder pattern of the sintered material however gave an extra peak at d = 2.028 A besides the lines of manganous-zinc ferrite. The electron micrograph of this material is shown in figure 2. A simple DTA curve in air for the mixture of higher oxide of manganese, Z n O and FezO3 used in the preparation of this material is given in figure 3. An x-ray powder pattern of the material got by a simple air-firing of the above oxide mixture at 950°C for 8 hr showed intense lines of ZnFe204, ZnMn204 and also some free Fe203"
Table 1. Properties of manganous-zinc ferrites p~pared with stabilized MnO and pressed with a volatilizable binder ( § 2. I a) Composition (tool%) Initial Loss factor permeability /a~ tan d/a i × 106 Curie point Density DF TF "013 Fe 2 ÷/g Fe203 ZnO MnO at 4kHz ± 15% at 4 kHz (°C) (gcm "3) x 106 x 106. x 103 51.75 20.25 28.00 3000 1.47 140 4,82 0.85 1.72 0.45 0.016 52.00 19.25 28.75 2800 1.98 150 4.78 0.89 1.45 0,67 0.017 DF Disaccommodation factor defined as (/al-[/a2) logt0~ t2~ where/a t and/a 2 are the initial permeabilities at 10 min (tt) and 100 min (t2) /a~ \t,/ respectively after demagnetization at 23°C. TF Temperature factor defined asg'~, d~T where T is the temperature. rl B Hysteresis material constant. Table 2. Properties of manganous-zinc ferrites prepared with a higher oxide of manganese and pressed with a volatilizable binder (§ 2.1b). Only some significant properties are given to indicate their general deterioration in comparison with table 1. Initial Composition (mol%) permeability Loss factor Density Fe2+/g lai at 4kHz tan d/ai x 106 gcm-3 Fe203 ZnO MnO ± 40%. at 4 kHz 51.75 20.25 28.00 1300 4.9 4.5 0.022 52.00 19.25 28.75 1100 6.2 4.6 0.023
4 C E Deshpande, P P Bakare and M N Sankarshana Murthy
lglgm-e 1. Electron micrograph of manganous-zinc ferrite prepared out of stabilized MnO, pressed with volatilizable binder; binder distilled off and toroids sintered at 1250°C in static N2 atmosphere with Fe203 pellets (§ 2.1 a).
Figure 2. Same legend as for figure 1 except that the ferrite was made out of a higher oxide of manganese which was subsequently reduced (§ 2. l b).
2.2 Use of PVA as binder
2.2a Burning the binder in air : T h e ferrite powder was prepared from stabilized M n O as described in § 2.1a, a n d pressed into toroids with 2% PVA. It was found, o n slow h e a t i n g of these toroids in air, that the m i n i m u m t e m p e r a t u r e needed for the complete oxidation of the PVA e m b e d d e d in the ferrite was in a n a r r o w t e m p e r a t u r e r a n g e of 3 7 5 ° - 4 0 0 ° C , at which some o x i d a t i o n of the M n 2÷ in the ferrite was inevitable. T h e
Manganous-zinc ferrites synthesized out of MnO 5
rO
<,.3 t n
.o k , .
<
v
1 I I I I , [ I t I
0 200 400 600 800 1000
Temperature °C
Figure 3. DTA in air of a mixture of higher oxide of manganese + ZnO + Fe203.
minimum temperature needed for complete combustion of pure PVA (not embedded in the ferrite) was however found to be ,,o 800°C.
A dozen ferrite toroids pressed with t'vn were then heated in air at 400°C for 2 hr until the binder was totally burnt out. One of the toroids was broken and the extent of oxidation of Mn 2 ÷ was estimated chemically. The partially-oxidized ferrite toroids were reduced in the presence of iron powder in a closed inert atmosphere as mentioned earlier.
The toroids were then sintered in a closed system filled with pure nitrogen and in the presence of active ferric oxide pellets (prepared as given in paper 2).
The results from 25 such runs are summarised in table 3. In each run at least half the number of toroids developed mesh-like cracks.
2.2b Fe203 pellets for burning the binder : Manganous-zinc ferrite prepared from stabilized M n O (§ 2.1 a) was pressed into toroids with 2 % PVA A dozen of these toroids was direct- ly sintered at 1250 ° for 8 hr in the presence of double the weight of active ferric oxide pellets in a closed unit initially filled with pure nitrogen (unit shown in figure 2 of paper 2). It was found that the binder had completely burnt off and none of the toroids had any cracks even in the course of 25 such runs. The results are given in table 4. The reproducibility was very good.
2.2c Sintering in air : The preparation of the ferrite and sintering were done exactly as described in § 2.2b except that the closed sintering unit contained air and not nitrogen to start with. The results were as in § 2.2b and there were no cracks. The electron micrograph of this material is shown in figure 4. The x-ray powder pattern of the sintered material showed only the characteristic lines of manganous-zinc ferrites.
2.2d Sintering in oxygen ." The emire procedure was exactly as in § 2.2c except that oxygen was filled in the closed sintering unit before starting the sintering run. The resulfs are given in table 5. There were no cracks. Figure 5 gives the electron micrograph of this material.
3. Discussion
The calcined ferrite prepared out of a higher oxide of manganese ( § 2. lb) as the source of manganese gives clear extra peaks in its x-ray diffraction pattern as compared with
Table 3. Results on manganous-zinc ferrite toroids prepared from stabilized MnO, pressed with PVA; oxidized slightly while burning the binder but subsequently reduced (§ 2.2a) Fe203
Composition (mol%) Initial Loss factor permeability /~i tan6/~ x 106 Curie point Density DF TF lqB Fe 2 +/g ZnO MnO at 4kHz + 15% at 4kHz (°C) (gcm -3) x 106 x 106 x 103 51.75 20.25 28.00 2600 1.10 140 4.76 0.82 1.02 0.42 0.016 52.00 19.25 28.75 2650 1.42 150 4.80 1.10 1.38 0.60 0.017 ~ 51.71- 23.13 25.26 2750 1.65 125 4.75 0.42 1.70 0.72 0.018 ~'~ 53.06 17.8 29.14 2700 2.60 160 4.80 1.07 0.15 0.35 0. 02 DF, TFj rl13 as given in the foot-note of table 1, Table 4. Measurements on manganous-zinc ferrite toroids prepared from stabilized MnO, pressed with PVA; binder was burnt without oxidizing the Mn 2 + by enclosing Fe20 3 pellets in an initial atmosphere of N 2 (§ 2.2b).
g~ Composition (mol%) Fe203 ZnO MnO
Initial Loss factor permeability ~, tand//~i x 106 Curie point Density DF TF r[B at 4kHz + 15% at 4kHz (°C) (gcm "3) x 106 x 106 x 103 51.75 20.25 28.00 2840 1.87 140 4.72 0.91 1.10 0.40 52.00 19.25 28.75 2534 2.25 150 4.77 1.10 1.40 0.65 51.71 23.13 25.76 2693 1.65 125 4.76 0.50 1.70 0.70 53.06 17.80 29.14 2744 1.82 160 4.81 1.08 0.15 0.36 DF, TF, rl~ as given in the foot note of table 1.
"Iable 5. Measurements on mnaaganou~-zmc tcrrlte torolds preparea tzom staDihzed MnO, pressed with PVA, Bmdei was omnt without oxidizing the Mn 2 + by enclosing Fe203 pellets in an initial atmosphere of O2 (§ 2.2d). Initial Loss factor Composition (mol%) permeability/a tand/lai × 106 Curie point Density DF TF rlB at 4kHz ± 15% at 4kHz (aC) (gcm -3) × 106 × 106 × 103 ZnO MnO Fe203
r~ 51.75 20.75 28.00 5356 3.11 140 4.83 0.90 1.05 0.38 52.00 19.25 28.75 4509 3.86 150 4.80 1.10 1.41 0.64 51.71 23.13 25.26 4483 4.64 125 4.86 0.44 1.70 0.74 53.06 17.80 29.14 5183 3.21 160 4.82 1.00 0.17 0.40 DF, TF, r/B as given in the foot note of table I.
C E Deshpande, P P Bakare and M N Sankarshana :Viurthy
Figure 4. Electron micrograph of manganous-zinc ferrite prepared out of stabilized MnO, pressed with PVA sintered directly in a closed static atmosphere of air in the presence of Fe203 pellets. (§ 2.2 c)
Figure 5. Same legend as for figure 4 except that air was replaced with oxygen (§ 2.2d).
O
that made out of stabilized M n O _(§2.1@ The peak at d = 2.19 Ais the most intense line and another line at d = 1.54 ~ is also a fairly strong line of M n O . M n O also gives a fairly strong line at d = 2.57~" but this would have merged with the strongest line of the ferrite at d = 2 . 5 4 ~ The x-ray powder patterns of the calcined ferrites clearly show therefore that M n O can straightaway enter into the spinel formation on reacting with ZnO and Fe203; while there are clear difficulties in starting with a higher oxide of manganese even though the latter may be reduced to M n O afterwards. The DTA curve (figure 3) shows that there is appreciable reaction of the higher oxide of manganese
.~anganouz-zznc Jkrrttes ~yntheszzed out of MnO 9 with ZnO and Fe20~ even at 300°C. Any reduction of the higher oxide of manganese to M n O can take place at a much higher temperature than 300°C - the temperature at which a good deal of unwanted ZnMn204 has been formed as shown by the x-ray pat- terns (§ 2. lb). The valencies may get corrected chemically; but once an ion with a wrong initial valency gets into a certain lattice site preferred by the starting valency, the rigidity of the crystal lattice does not permit easy migration of the same ion, after reduction, to its new preferred lattice site. It is significant that even the sintered ferrite (§ 2.1b) still gives an extra peak at d = 2.028 ~kwhich is actually the strongest line of Mn~O4.
It is not unlikely that this material also contains some other unwanted compounds too small in quantity to be detected by x-ray diffraction. Starting with a higher oxide of manganese for preparing manganese zinc ferrites therefore has a distinct disadvantage;
the solid state reaction precedes the correction of the valency of manganese; and since the site preference of a metal ion in the solid state compound is determined by the valency of the ion at the time of reaction, some unwanted compounds as well as some unwanted spinels such as Mn304 or even the ferrite with undesirable cation distributions in its lat- tice sites cannot be avoided in such a preparative process. This explains the less satisfac- tory and less reproducible magnetic properties of table 2 as compared to table 1 cor- responding to the ferrite prepared out of stabilized M n O . It is also significant that the electron micrograph of the ferrite prepared as per § 2.1b (out of a higher oxide of manganese) contains smaller grains (figure 2) than those (figure 1) in the ferrite prepared as per § 2.1a (out of stabilized MnO). This is obviously because the stabilized M n O has already given the ferrite grains even at the stage of calcination (as shown by the x-ray powder pattern of this material); and these grains can grow without any hindrance while sintering. The powder pattern of the calcined ferrite made out of a higher oxide of manganese (§ 2. lb) shows on the other hand some lines of M n O also. That means the spinel formation in this case is incomplete at the stage of calcination: some further structural changes have taken place subsequently while sinterhag. A proper grain growth under such conditions perhaps needs a higher temperature than has been used here - 1250°C. In fact the ferrite prepared out of a higher oxide of Mn - - usual commercial method - - is normally sintered above 1300°C. Such high temperatures are however not desirable for this ferrite which contains the thermolabile ZnO. There would be still another factor contributing to the poorer grain growth as shown in figure 2. The reduction of the valency of manganese during the sintering run means the simultaneous evolution of oxygen from the body of the ferrite; and other things being equal this causes a breaking up of the ferrite grains.
The main object of the experimental work reported in § 2.1 (and being discussed in the present section) was to compare the performance of M n O as raw material for the preparation of manganous-zinc ferrite with that of a higher oxide of manganese.
The use of a non-vol~ttilizable binder at this stage would not have helped in isolating the problem of the starting valency of manganese; volatilizable binders have therefore been employed at this stage. Another point to be mentioned here is that the method of reduction of the valency of manganese at the calcination stage used in this work is different from the conventional method of passing a current of inert gas over the heated reaction mixture (Natansohn and Baird (1969); Strivens and Chol (1971)). This latter method has been deliberately avoided in this work since it entails a greater loss of the volatile ZnO. A last point to be noticed in this section is that the problem of cracks has not appeared at this stage though the initial valency of manganese has been reduced subsequently during the process. The reduction has taken place at the stage of calcination
10 C E Deshpande, P P Bakare and M N Sankarshana Murthy
and not on the pressed pieces. So long as there is no oxidation or reduction of the material after pressing, there are no volume changes (Economos 1955) and no stresses in the pressed material being sintered.
3.1 Use of PVA to press Mn-Zn ferrite
T h e PVA embedded in the ferrite gets completely oxidized below 400°C - - while in the absence of the ferrite pure PVA takes oo800°C to burn off. This is obviously due to the catalytic action of the ferrite. It has been shown (§ 2.2a) that a good deal of organic matter belonging to the binder disappears in a narrow temperature range. Apparently the catalytic oxidation of organic matter in this narrow range of temperature must be very fast; perhaps so fast that the escaping gases cleave through the ferrite matrix thereby initiating cracks which m a y develop further while sintering. Even a very slow programming of temperature while burning the binder helped little to avoid these cracks which however decreased in n u m b e r and in extent by controlling the oxygen available for the combustion (Deshpande 1982). Apparently this is not the only reason for the cracks in the sintered pieces; cracks persisted as long as the binder could not be oxidized without a simultaneous oxidation of the M n 2+. T h e latter would require a partial pressure of oxygen sufficient to oxidize the organic matter but not the M n 2÷ in the ferrite; and the thermal dissociation of active ferric oxide pellets (§ 2.2b) provides such an o p t i m u m oxygen partial pressure. T h e dissociation pressure of ferric oxide also controls the ferrous content in the ferrite (as described in paper 2). This ferrous content needs to be controlled optimally in order to minimize the magnetocrystalline anisotropy and magnetostriction so that the permeability increases (Winkler 1971; O h t a 1963).
It has been observed (Economos 1955) previously that any process of oxidation or reduction on the pressed ferrite pieces is liable to produce cracks due to the consequent volume changes and stresses. Even in the course of a hundred sintering runs of the type described in § 2.2b, no cracks were observed and the results were highly reproducible. The presev.ce of air to start with, instead of nitrogen, in the closed sintering unit made no difference to the outcome of the sintering runs. The limited air enclosed in the sintering unit was far from sufficient to burn the binder and the oxygen needed for burning off the binder had to come, by and large, from FezO 3 even in this case. The air initially present did no harm since a competition between the organic binder and M n 2 + in the ferrite for the limited oxygen filling the unit went as expected in favour of the organic binder. It was even found possible to carry out the above sintering run (§ 2.2d) with oxygen filling the closed sintering unit provided this oxygen was too insufficient to burn the PVA present. The corresponding improved results given in table 5 and better grain growth shown in figure 5 can perhaps be explained on the basis of increased cation vacancies which in turn enhance the mobility of cations. At least part of the Fe203 present over and above 50 mol% in the ferrite changes to the Y variety which m a y be in equilibrium with Fe30~ (Slick 1980).
4 Fe~ + 03 (rh) -~ 3 Fe38/; :,/3 04 (sp)
3Fe38/; 0,/3 04 ~ - 8/3 Fe 2÷ Fe32 +O4 + 2/3 0 2
The presence of oxygen in the atmosphere shifts the second equilibrium backwards and causes cation vacancies which are favourable to grain growth.
Manganous-zinc ferrites synthesized out of M n O 11 4. C o n c l u s i o n
M a n g a n o u s - z i n c ferrites are best p r e p a r e d directly from stabilized M n O . T h e ferrites pressed with PVA are best sintered directly in the static a t m o s p h e r e of a closed system a n d in the presence of a sufficient n u m b e r of active ferric oxide pellets. T h e s e ferric oxide pellets n e e d not be v e r y p u r e since they do not c o m e in contact with the ferrite.
T h e y m a y be p r e p a r e d b y dissolving scrap iron in 30% nitric acid a n d processing the ferric nitrate as given in p a p e r 2. T h e used ferric oxide can be reprocessed b y dissolving in h y d r o c h l o r i c acid a n d p r e c i p i t a t i n g the ferric h y d r o x i d e with an alkali. T h e static a t m o s p h e r e in the closed sintering unit can be air itself or even o x y g e n so long as the o x y g e n is too insufficient to b u r n the b i n d e r . T h i s m e t h o d o f sintering c o m p l e t e l y e l i m i n a t e s cracks besides giving highly satisfactory a n d r e p r o d u c i b l e properties.
A c k n o w l e d g e m e n t
T h e a u t h o r s are grateful to the P r o t o t y p e P r o d u c t i o n U n i t o f the A r m a m e n t R e s e a r c h and D e v e l o p m e n t L a b o r a t o r y , for the excellent dies which they p r e p a r e d a n d for their help in pressing the ferrites. T h e y are grateful to M r s J J Shrotri for general help in the work, to S h r i J S G u j r a l for s c a n n i n g the x - r a y d i f f r a c t o g r a m s and to M r s A M i t r a for t a k i n g the electron m i c r o g r a p h s . T h e a u t h o r s are also grateful to D r A P B Sinha for e n c o u r a g i n g this work.
References
Deshpande C E 1982 Unstable oxides MnOand FeO;their stabilization and use in some ferrites, Ph.D. Thesis, University of Poona.
Deshpande C E and Murthy M N S 1981 Bull. Mater. Sci. 3 261
Deshpande C E, Pant L M and Murthy M N S 1978 Indian,]. Chem. A16 251 Economos G 1955J. Am. Ceram. Soc. 38 241
Murthy M N S, Deshpande C E and Shiotri J J 1978 Proc. Indian Acad. Sci. A87 49
Murthy M N S, Deshpande C E, Bakare P P and Shrotri J j 1979 Bull. Chem: Soc. Jpn. 52 571 Natansohn S and Baird D H 1969J.Am. Ceram. Soc. 52 129
Ohta K 1963 J. Phys. Sac. Jpn. 18 685
Slick P I 1980Ferromagnetic materials (ed) E P Wohlfarth (New York: North-Holland Publishing Co) Vol. II p.202
Strivens M A and Chol G 1971 Ferrites Proc. Int. Conf. Jpn 1970 (eds) Y Hoshino, S Iida and M Sugimoto (Tokyo: University Press) p.239
Winkler G 1971 Magnetic properties of materials (ed)J Smit (New York: McGraw Hill) p.52
