SOFT FERRITES

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SOFT FERRITES

T. Postupolski

To cite this version:

T. Postupolski. SOFT FERRITES. Journal de Physique Colloques, 1985, 46 (C6), pp.C6-159-C6-167.

�10.1051/jphyscol:1985628�. �jpa-00224876�

J O U R N A L DE PHYSIQUE

CoHoque C6, supplgment a u n09, Tome 46, s e p t e m b r e 1985 p a g e C6-159

SOFT FERRITES

T. P o s t u p o l s k i

POLFER Research Laboratory, uZ. DzieZna 60, 01-029 Warszawa, Poland

R6sumd - La s i t u a t i o n du march6 d e s f e r r i t e s doux e s t p r c s e n t d e . AprBs un b r e f r a p p e l d e s c l a s s e s p r i n c i p a l e s de f e r r i t e s doux, l e u r s domaines p a r t i c u l i e r s d ' a p p l i c a t i o n s e t l e s a g e n t s d i v e r s q u i d 6 t e r m i n e n t l e s p r o p r i d t d s r d s u l t a n t e s s o n t d d c r i t s . Quelques r 6 s u l t a t s e x p d r i m e n t a u x e t une e x p r e s s i o n g d n d r a l e p o u r l a p e r m g a b i l i t h i n i t i a l e p e r m e t t e n t de s o u l i g n e r l e r z l e d e s a g e n t s non-chimiques d a n s l e f o r m a t i o n d e s p r o p r i 6 t d s r d s u l t a n t e s d e f e r r i t e s doux.

A b s t r a c t - The s i t u a t i o n on t h e m a r k e t r e l a t e d t o s o f t f e r r i t e s i s r e v i e w e d . Next, t h e main c l a s s e s o f s o f t f e r r i t e s , t h e i r s main a p p l i c a t i o n g r o u p s and t h e b a s i c f a c t o r s d e f i n i n g t h e r e s u l t a n t p r o p e r t i e s o f s o f t f e r r i t e m a t e r i a l s a r e summarized. Some e x p e r i m e n t a l r e s u l t s and g e n e r a l i z e d f o r m u l a f o r i n i t i a l p e r m e a b i l i t y a r e u s e d t o emphasize t h e r o l e o f e x t r a - c h e m i c a l f a c t o r s i n f o r m i n g t h e r e s u l t a n t s o f t f e r r i t e p r o p e r t i e s .

I - TO-DAY'S MEANNING OF "SOFT"

S o f t f e r r i t e s (SF J

.

" s o f t l y " r e s p o n d s t o t h e a p p l i e d e x t e r n a l h i g h f r e q u e n c y m a g n e t i c f i e l d . Many s i m i l a r examples c a n b e found among v e r y d i v e r s i f i e d a t p r e s e n t a p p l i c a t i o n s o f m a g n e t i c m a t e r i a l s . Thus, i t seems t h a t r a t h e r a p p l i c a t i o n t h a n a r b i t r a r y t a k e n v a l u e o f c o e r c i v i t y i s a more a d e q u a t e c r i t e r i o n t o c l a s s i f y a g i v e n m a g n e t i c m a t e r i a l a s m a g n e t i c a l l y s o f t o r h a r d .

I 1 - MAIN APPLICATIONS AND MARKET POSITION OF SOFT FERRITES

SF from t h e i r i n d u s t r i a l b i r t h a r e n e a r l y e x c l u s i v e l y r e l a t e d t o e l e c t r o n i c s which s t i l l r e m a i n s t h e dominant SF consumer. E l e c t r o n i c s s t i l l s h a p e s t h e a r e a s o f SF u s e and t h e i r p r o d u c t i o n volume. Novel SF a p p l i c a t i o n s and r e s u l t i n g SF development t r e n d s a r e r e l a t e d c h i e f l y t o e l e c t r o n i c s . The f o u n d s on t h e r e s e a r c h d e v o t e d t o SF, s p e c i a l l y o n t h e a p p l i e d r e s e a r c h , a r e s p e n t m o s t l y by e l e c t r o n i c o r r e l a t e d compa- n i e s . I t i s e v i d e n t t h a t t h e c u r r e n t and p r o s p e c t i v e s i t u a t i o n o f SF i s d e c i s i v e l y i n f l u e n c e d by e l e c t r o n i c s , by i t s p a r t i c u l a r b r a n c h e s .

Hoshino's " a p p l i c a t i o n t r e e " /1/ shows 25 a p p l i c a t i o n s g r o u p s o f SF b e l o n g i n g t o 6 b r a n c h e s o f e l e c t r o n i c s : t e l e c o m m u n i c a t ~ o n s , r a d i o s and t e l e v i s i o n s , microwaves, computers, m a g n e t i c r e c o r d i n g ( h e a d s ) and m i s c e l l a n e o u s u s e s . Each o f t h e s e b r a n c h e s i s c h a r a c t e r i z e d by a c o n t i n u o u s and h e a l t h y d e v e l o p m e n t . E s p e c i a l l y s t r o n g d e v e l o p - ment r a t e i s marked i n t h e c a s e o f computing, r e c o r d i n g and c o n t r o l t e c h n o l o g i e s . I t i s e x p e c t e d / 2 / t h a t i n t h e n e a r f u t u r e t o t h e s e f a s t d e v e l o p i n g b r a n c h e s may j o i n t h e n e x t one: t h e d i r e c t s a t e l l i t e t e l e v i s i o n b r o a d c a s t .

T h e r e i s a v e r y wide s p e c t r u m o f SF-based components and d e v i c e s . Among t h e n t h e r e a r e v e r y s i m p l e p a r t s a s e . g . a n t i p a r a s i t e b e a d s p e r f o r m i n g a s i m p l e f u n c t i o n o f t h e i n c r e a s e o f a c o n d u c t o r impedance a t h i g h f r e q u e n c i e s . The r e s t i s more composed.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985628

JOURNAL DE PHYSIQUE

The wound i n d u c t i v e components comprise t r a n s f o r m e r s , c o i l s , r e a c t o r s and chokes f o r v a r i o u s v o l t a g e and c u r r e n t waveforms, working a s broadband, s e l e c t i v e ( r e s o n a n t ) , p u l s e , c o r r e c t i n g , l i n e a r and n o n l i n e a r , a d j u s t a b l e o r f i x e d components which a r e u s e d a t f r e q u e n c i e s g o i n g from some k i l o h e r t z ( a c o u s t i c ) t o some hundreds o f mega- h e r t z (UHF). The t r a n s m i t t e d power r a n g e s from v a n i s h i n g l y low l e v e l s ( n a n o w a t t s ) t o megawatts o f c o n t i n u o u s waveform. A s e p a r a t e a p p l i c a t i o n group a r e t h e r e c o r d i n g and r e p r o d u c t i o n h e a d s f o r a u d i o - r e c o r d e r s , v i d e o - r e c o r d e r s and f o r t a p e - and d i s k memories f o r d a t a p r o c e s s i n g u n i t s . Another a p p l i c a t i o n gtroup i s formed by n o n - r e c i - p r o c a l f e r r i t e microwave d e v i c e s i n c l u d i n g t h e waveguide d e v i c e s f o r h i g h e r power l e v e l s (up t o s e v e r a l megawatts o f peak power and s e v e r a l k i l o w a t t s o f a v e r a g e power), c o a x i a l d e v i c e s and c h e a p e r low-power l e v e l m i c r o s t r i p d e v i c e s . R e c e n t l y t h e a t t e n t i o n h a s b e e n drawn by t h e m a g n e t o s t a t i c wave d e v i c e s and by n o n - r e c i p r o c a l f e r r i t e d e v i c e s f o r m i l l i m e t e r - w a v e r e g i o n . The most s o p h i s t i c a t e d p r o d u c t s i n t h e SF components f a m i l y a r e t h e b u b b l e memory d e v i c e s .

Not a l l o f SF a p p l i c a t i o n s h a v e b e e n mentioned h e r e b u t t h e s e mentioned i l l u s t r a t e s u f f i c i e n t l y w e l l t h e e x t e n t o f t h e i r v a r i e t y , v a r i o u s f u n c t i o n a l r o l e s , f i e l d s o f a p p l i c a t i o n and m a n u f a c t u r i n g h e t e r o g e n e i t y .

The whole SF-based component market can b e v a l u e d a t SI6% o f t h e whole e l e c t r o n i c component m a r k e t and a b o u t 15% o f t h e p a s s i v e e l e c t r o n i c component m a r k e t . T h i s e s t i m a t e i s a p p r o x i m a t e l y s i m i l a r f o r t h e USA, J a p a n and West Europe e l e c t r o n i c market which f o r p a s s i v e components t o t a l a b o u t 29,OO m i l l i o n USD.

The r e l a t i v e c o n t r i b u t i o n o f p a r t i c u l a r a p p l i c a t i o n g r o u p s u s i n g SF p a r t s o f t h e SF component f a m i l y i s shown i n t h e t a b l e 1 .

T a b l e 1

C o n t r i b u t i o n o f p a r t i c u l a r SF component g r o u p t o t h e SF component m a r k e t ; USA, 1982;

( s o u r c e : / 2 / , E l e c t r o n i c s , Conf. P r o c e e d i n g s )

*/B - b a l a n c e d : f o r 23% o f s h a r e D - d e c l i n i n g : f o r 22% o f s i i a r e R - r i s i n g : f o r 20% o f s h a r e

SR - s t r o n g l y r a i s i n g : f o r 35% o f s h a r e SF a p p l i c a t i o n group

a u d i o t r a n s f o r m e r s low f r e q u e n c y chokes

p l a t e and f i l a m e n t t r a n s f o r m e r s p u l s e and computer t r a n s f o r m e r s RF chokes

RF c o i l s I F t r a n s f o r m e r s

TV t r a n s f o r m e r s and r e a c t o r s t o r o i d a l w i n d i n g s , t r a n s f o r m e r s and r e a c t o r s

b a l u n c o i l s , p e r m e a b i l i t y t u n i n g d e v i c e s m a g n e t i c r e c o r d i n g and r e p r o d u c i n g h e a d s m a g n e t i c c o r e s

microwave d e v i c e s m a g n e t i c b u b b l e d e v i c e s

I n t a b l e 2 some r e l a t i o n s h i p s between tile d i f f e r e n t g r o u p s o f c o r e s f o r t h e most SF volume a p p l i c a t i o n s a r e g i v e n .

I n F i g . 1 t h e r e l a t i v e world-wide growth o f SF s a l e s o v e r t h e y e a r s 1970-1584 i s shown t o g e t h e r w i t h r e l a t i v e SF t o n n a g e growth. For t h e r e f e r e n c e t h e s i m i l a r world- -wide s a l e s growth o f t h e p a s s i v e e l e c t r o n i c components i s a l s o marked.

Over t h e s e 1 4 y e a r s t h e a v e r a g e growth r a t e o f SF s a l e s amounted t o 6.76% p e r y e a r . The a v e r a g e growth r a t e o f SF t o n n a g e consumption was 7.78% p e r y e a r . F o r r e f e r e n c e t h e world s a l e s o f p a s s i v e e l e c t r o n i c components were growing a t a v e r a g e r a t e 9.47%

p e r y e a r (USA: 7 . 4 6 % , J a p a n : 1 1 . 4 5 % , !lest Europe 9 . 0 8 % ) . r e l a t i v e shipment p a r t i c i - p a t i o n , %

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t e n d e n c y */

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F i g . 1 - R e l a t i v e growth o f SF w o r l d s a l e s and p a s s i v e e l e c t r o n i c components w o r l d - -wide s a l e s (compiled on t h e b a s i s o f d a t a t a k e n from E l e c t r o n i c s , Ceramic Age, Ceramic o u l l e t i n , s e r i e s o f Conference P r o c e e d i n g s , p a p e r s e t c . ) and o f world-wide SF tonnage consumption ( a f t e r / 3 / ) .

F i r s t o f a l l i t c a n b e r e a l i z e d t h a t i n s p i t e o f t h e d e c l i n e i n 1970s o f one o f t h e SF market l e a d e r s - memory c o r e s - t h e SF market h a s remained growing. I t i s w o r t h t o keep i n mind t h a t t h e memory c o r e s i n t h e i r apogee s h a r e d up t o 50% o f t h e whole SF m a r k e t . (The s t o r y o f f e r r i t e c o r e memories, key f a c t s and a c h i e v e m e n t s , accompany- i n g e v e n t s , t h e l a r g e s t s i n g l e p a t e n t payment and o t h e r s - s e e / 4 / ) .

The g e n e r a l m i n a t u r i z a t i o n o f SF components k e p t p a c e w i t h t h e more r a p i d i n c r e a s e o f t h e SF p r o d u c t i o n volume and w i t h t h e d i v e r s i f i c a t i o n o f SF forms and c o r e s h a p e s . A few y e a r s ago a new and huge SF a p p l i c a t i o n h a s been born: t h e s o c a l l e d s w i t c h -

-mode power s u p p l i e s , i n v e r t e r s o r c o n v e r t e r s , o r r e c e n t l y - s h o r t l y - s w i t c h e r s . Otiier a p p l i c a t i o n s - s i n g l e c r y s t a l and d e n s e p o l y c r y s t a l l i n e f e r r i t e h e a d s f o r a u d i o , v i d e o and d i g i t a l r e c o r d i n g - a l s o have c o n t r i b u t e d t o t h e growth o f SF market.

The e x p e c t e d development o f d i r e c t s a t e l l i t e TV b r o a d c a s t may c r e a t e a l a r g e demand f o r low c o s t microwave and m i l l i m e t e r wave f e r r i t e d e v i c e s . S i m u l t a n e o u s l y t h e growing i n t e r e s t a r o s e i n t h e m i l l i m e t e r wave components f o r p r o f e s s i o n a l a p p l i c a - t i o n s a s w e l l a s i n r e c e n t l y e x t e n s i v e l y i n v e s t i g a t e d m a g n e t o s t a t i c wave d e v i c e s and o t h e r p l a n a r and m i n i a t u r i z e d microwave f e r r i t e d e v i c e s .

The b u b b l e memories, b e c a u s e o f t h e i r f u n c t i o n a l c i r c u i t r y d e p o s i t i o n t e c h n i q u e r a n k e d r a t h e r among s e m i c o n d u c t o r p r o d u c t s , have a l s o g a i n e d a n i n c r e a s i n g c o n t r i b u - t i o n t o t h e growth o f SF economics.

O t h e r a p p l i c a t i o n , f r e q u e n t l y u n u s u a l , h a v e a s y e t no s i g n i f i c a n t impact on t h e SF

C6-162 J O U R N A L DE PHYSIQUE

s a l e s o r t o n n a g e c o n s u m ~ t i o n .

On t h e o t h e r hand it i s s e e n t h a t from 1976 on t h e SF component growth r a t e i s l o w e r t h a n t h a t o f p a s s i v e components a s a whole, and i t i s e x p e c t e d t h a t t h i s t e n d e n c y w i l l i n c r e a s e . The p r e s e n t p a r t i c i p a t i o n o f b u b b l e memories i n t h e s t o r a g e s y s t e m s , though s t r o n g l y r a i s i n g , i s s i g n i f i c a n t l y lower t h a n t h a t o f c o r e memories i n t h e p a s t . I t i s n o t o u t o f p l a c e t o add t h a t a b u b b l e g a r n e t f i l m w a f e r c o s t s a b o u t 50 t i m e s a s much a s a s i l i c o n w a f e r .

The TV d e f l e x i o n s y s t e m s , t h e main t o n n a g e SF consumer, must o b s e r v e t h e developments i n t h e f l a t d i s p l a y t e c h n o l o g y . The l a r g e TV p i c t u r e t u b e s become s p e c t a c u l a r l y i n - a d e q u a t e t o t h e r e s t o f t h e t o - d a y ' s T V - s e t s . A e c e n t l y , t h e i n d u s t r i a l l y a c c e p t a b l e n o n - l i q u i d - c r y s t a l - b a s e d b u t e l e c t r o l u m i n e s c e n t , b r i g h t , s h a r p c o n t r a s t and l o n g l i f e t i m e , d i g i t a l l y c o n t r o l l e d f l a t m o n i t o r s have been announced / 6 / . They have o n l y 5% o f t h e volume and 5% o f t h e w e i g h t o f c o n v e n t i o n a l c a t h o d e - r a y t u b e a t t h e com- p a r a b l e d i s p l a y a r e a ( ~ 2 0 0 cm2j.

The s e r i e s o f w e l l e s t a b l i s h e d c o n v e n t i o n a l a p p l i c a t i o n s a s h i g h f r e q u e n c y c o i l s , t r a n s f o r m e r s , c h o k e s , a d j u s t a b l e d e v i c e s e t c . seem t o have f a i r l y a s s u r e d , s l i g h t y i n c r e a s i n g p o s i t i o n , d e s p i t e of more t h a n 15 y e a r s ' c o n t i n u a l p r e v i s i o n s o f a r a p i d and i n e v i t a b l e d e c l i n e . The p r e v i s i o n s f o r t h e SF ~ r o d u c t s a s a whole seem t o b e p r o m i s i n g , e s p e c i a l l y f o r t h o s e which a r e b e i n g g r a d u a l l y improved and become b e t t e r a d a p t e d t o c u r r e n t and f u t u r e n e e d s . The t r a d i t i o n a l and s i n g l e p r o d u c t o r i e n t e d m a n u f a c t u r e r s may s u f f e r from t h e i n c r e a s i n g s c a r c i t y o f o r d e r s . Those advanced and o f f e r i n g new p o s s i b i l i t i e s i n SF a p p l i c a t i o n s a r e e x p e c t e d t o m a i n l y c o n t r i b u t e t o SF development and demand. Four growing g r o u p s a r e now c l e a r l y o u t l i n e d : s w i t c h e r s , microwave and m i l l i m e t e r wave components, m a g n e t i c r e c o r d i n g h e a d s and m a g n e t i c b u b b l e memory d e v i c e s . The a b i l i t y t o r e d e s i g n many o f t r a d i t i o n a l SF i n d u c t i v e com- p o n e n t s t o f l a t s u r f a c e mounting may f a v o u r more f l e x i b l e m a n u f a c t u r e r s .

I 1 1 - MAIN CLASSES OF SOFT FERRITES AND THEIR BASIC CHEMISTRY

The main c l a s s e s o f f e r r i t e m a t e r i a l s o r i e n t e a t o s p e c i f i e d a p p l i c a t i o n g r o u p s have b e e n r a t h e r e s t a b l i s h e d f o r a l o n g t i m e .

For a p p l i c a t i o n s a s t h e c o r e s f o r i n d u c t o r s , c o i l s , t r a n s f o r m e r s , r e c o r d i n g h e a d s e t c . t h e r e a r e t h e s p i n e l f e r r i t e s o f g e n e r a l f o r m u l a :

M ~ + $ T ~ w ~ + ~ ~

At h i g h e r f r e q u e n c i e s i n t h e r a n g e o f 300 t o 1000 MHz p l a n a r "I- and Z-type h e x a f e r - r i t e s a r e a l s o u s e d i n t h i s a p p l i c a t i o n g r o u p .

For t h e microwave and m i l l i m e t e r wave a p p l i c a t i o n s t h e r e i s t h e h i g h e s t d i v e r s i f i c a - t i o n o f f e r r i t e m a t e r i a l s . T h e r e a r e t h e p o l y c r y s t a l l i n e s p i n e l s Hn Ug Fez04

X' Y

(x+y+z = 31, NiFe2C4, L i 5Fe2 5 0 4 , p ~ l y - and m o n o c r y s t a l l i n e g a r n e t s o f g e n e r a l f o r - mula i l $ * ~ e : + o ~ ~ where R s P ' i s m a i n l y Y and Gd i o n . T i e most expanded g a r n e t s a r e y t r i u m - i r o n g a r n e t s (YIG) w i t h v a r i o u s p a r t i a l s u b s t i t u t i o n s f o r Y by d i f f e r e n t r a r e e a r t h i o n s (R3+) and s u c h a d d i t i v e s a s t h e i o n s o f ~ 1 ~ + , C O ~ + , 1n3+, c a 2 + , z r 4 + , v5+,

~ i ~ + . F o r t h e m i l l i m e t r i c and h i g h e r microwave f r e q u e n c i e s t h e M- and N-type da- and S r - b a s e d h e x a f e r r i t e s a r e u s e f u l . T h e i r s g e n e r a l f o r m u l a e a r e a s f o l l o w s . F o r 2+

ivl-type: baFel2Gl9 and SrFe12019 w i t h a d d i t i v e s o f ~ 1 ~ + , c a 3 + , ~ i ~ + , c o 2 + , ~ i ~ + , Zn . i o n s ; f o r # - t y p e : ~ a ~ e ~and ~+ ~ re~ ~l e~ ~0where l,1e2+ +~~ ~e ~= ~N i , 3 Co, Zn, Mg, F e . ~ ~ A t t h e m i l l i m e t e r wave r e g l o n t h e r a r e e a r t h o r t n o f e r r i t e s R 3 + ~ e 3 + 0 3 c a n a l s o b e u s e f u l / 7 / .

Tne s q u a r e - l o o p f e r r i t e s f o r memory a p p l i c a t i o n a r e t h e s p i n e l s c o n t a i n i n g t h e Mn, Mg, Cu, N i , L i , Co and Zn i o n s .

M a g n e t i c f i l m s f o r b u b b l e memory a p p l i c a t i o n s a r e m a i n l y t h e compounds o f t h e r a r e e a r t i i g a r n e t s w i t h s u c h a d d i t i v e s a s ~ a ~ + , c a 2 + , ~ e l o w e r i n g t h e m a g n e t i z a t i o n , ~ + and o t h e r s i o n s a s ~ 1 3 + and si4+.

The e x c e l l e n t g u i d a n c e i n f o r m a t i o n s on p a r t i c u l a r SF c l a s s e s and t h e i r a p p l i c a b i l i t y f o r a p p l i c a t i o n s can b e found i n t h e S n e l l i n g ' s book / 8 / , i n t h e W o h l f a r t h ' s hand- book / 9 / ( S l i c k - s o f t f e r r i t e s f o r non-microwave a p p l i c a t i o n s ; N i c o l a s - microwave f e r r i t e s ; G i l l e o - g a r n e t s ; E s c h e n f e l d e r : c r y s t a l l i n e f i l m s f o r b u b b l e s ) and i n t h e monographs d e v o t e d t o t h e f e r r i t e s /10 ^t 1 5 / .

IV - EXTRA-CHEMICAL FACTORS AN3 RESULTANT SOFT FERRITE PROPERTIES

The b a s i c c h e m i s t r y a l o n e i s q u i t e i n s u f f i c i e n t t o d e f i n e t h e m a g n e t i c and o t h e r p r o p e r t i e s o f t h e f e r r i t e s . T h i s i s a d i r e c t c o n s e q u e n c e o f two f a c t s : l o - f e r r i t e s a r e t h e compounds of b e r t h o l l i d e t y p e and 2' - p o l y c r y s t a l l i n e f e r r i t e s a r e t h e g r a n u l a r s u b s t a n c e and m o n o c r y s t a l l i n e f e r r i t e s (and c r y s t a l l i t e s ) a r e h i g h l y

" d e f e c t i g e n e o u s " i n t h e m s e l v e s . T h e s e f a c t s d e t e r m i n e t h e whole p h i l o s o p h y o f f e r - r i t e s f o r m a t i o n and m a n u f a c t u r i n g .

A s e r i e s o f f a c t o r s , non p e r t a i n i n g t o b a s i c c h e m i s t r y and b e i n g o f upmost i m p o r t a n c e f o r t h e f e r r i t e f o r m a t i o n , w i l l b e b r i e f l y r e v i e w e d below.

The d i s t r i b u t i o n o f c a t i o n s o f t h e g i v e n v a l e n c i e s o v e r t h e c r y s t a l l a t t i c e s i t e s i s t h e c r u c i a l and v e r y d e l i c a t e p o i n t . The p r o p o r t i o n s between t h e c o n t e n t o f i o n s o f t h e s p e c i f i e d v a l e n c i e s w i t h i n s e p a r a t e s u b - l a t t i c e s and t h r o u g h t h e whole s u b s t a n c e a r e a l s o e s s e n t i a l . The u n i f o r m i t y o f t h e d e s i r e d i o n i c d i s t r i b u t i o n , c r e a t i o n and r e g u l a r c o n c e n t r a t i o n o f i o n i c v a c a n c i e s and o t h e r d e f e c t s a r e t h e f a c t o r s h a r d t o c o n t r o l . A l l t h e s e a t o m i c s t r u c t u r e p e c u l i a r i t i e s , n e a r l y a s a r u l e , change from t h e s u r f a c e t o t h e f e r r i t e i n s i d e . The p r o c e s s e s r e s p o n s i b l e f o r t h e s e f e r r i t e f o r m a t i o n r e l a t e d f a c t o r s t a k e p l a c e d u r i n g t h e s i n t e r i n g p r o c e s s and i n f l u e n c e a l l t h e m i c r o - s t r u c t u r a l , m e c h a n i c a l , e l e c t r i c a l and m a g n e t i c p r o p e r t i e s o f t h e f e r r i t e s .

The s o - c a l l e d m i c r o s t r u c t u r a l p r o p e r t i e s a r e r e l a t e d t o g r a n u l a r s t r u c t u r e which i s c h a r a c t e r i z e d by s u c h f a c t o r s a s : d i m e n s i o n a l and m o r p h o l o g i c a l s t a t i s t i c s o f g r a i n s

(and p o r e s ) , g r a i n b o u n d a r i e s , i n t r a - and i n t e r g r a n u l a r p o r o s i t y , d e n s i t y . T h e s e m i c r o s t r u c t u r a l f a c t o r s d e c i s i v e l y i n f l u e n c e t h e m a g n e t i c , e l e c t r i c a l , m e c h a n i c a l and t h e r m a l p r o p e r t i e s o f t h e f e r r i t e s .

A l l t h o s e f a c t o r s a c t t o g e t h e r and t h e y , o n l y a s a whole, d e f i n e t h e r e s u l t a n t s t a t i c and dynamic p r o p e r t i e s o f t h e f e r r i t e m a t e r i a l a s w e l l a s t h e k i n e t i c s o f i t s i n - t r i n s i c p r o c e s s e s .

Owing t o t h e exchange i n t e r a c t i o n s and m a g n e t o s t a t i c p r i n c i p l e s , t h r o u g h o u t t h e s o c h a r a c t e r i s e d m a t r i x , t h e m a g n e t i c s t r u c t u r e below t h e C u r i e t e m p e r a t u r e i s p r o d u c - e d . T h i s s t r u c t u r e i n c l u d e s t h e o r d e r e d a r r a n g e m e n t o f s p i n s and t h a t o f m a g n e t i c domains s e p a r a t e d by domain w a l l s . The m i c r o s t r u c t u r a l f a c t o r s a c t a l s o d e c i s i v e l y b o t h on t h e s t a t i c p r o p e r t i e s o f m a g n e t i c s t r u c t u r e ( e . g . s p i n a l i g n e m e n t , number, s h a p e s and s i z e s o f d o m a i n s j a s w e l l a s on i t s dynamic p r o p e r t i e s ( s p i n r o t a t i o n , d i s p l a c e m e n t s and jumps o f domain w a l l s , m a g n e t i z a t i o n r e v e r s a l e t c . ) .

T h i s s h o r t o u t l i n e seems t o b e s u f f i c i e n t t o show t h a t t h e s e q u e n c e : i n p u t c h e m i s t r y - p r e p a r a t i o n - m u l t i p l e s t r u c t u r e s ( t r a n s i t i o n a l and f i n a l ) - r e s u l t a n t p r o p e r t i e s - i s complex and h i g h l y c o m p l i c a t e d . T h e s e s e q u e n t i a l s t a g e s i n v o l v e e x t r e m e l y d e l i c a t e a n d / o r c o n t r a r y f a c t o r s h a r d t o b a l a n c e .

Over t h e y e a r s much t h e o r e t i c a l and p r a c t i c a l a c q u i r e m e n t s o n . f e r r i t e s have b e e n a t - t a i n e d b o t h by i n v e s t i g a t o r s a s w e l l a s by p r o d u c e r s . A l l o f t h e s e knowledge, e x - p e r i e n c e and e x p e r i m e n t a l f i n d i n g s i s b e i n g p u t i n t o s e a r c h i n g aimed a t improvements, enhancement and c o n t r o l o f SF p r o p e r t i e s . I n t h e c a s e o f SF f o r mass p r o d u c t i o n t h e economic a s p e c t s a r e n o t n e g l e c t e d . S e l e c t i o n and p r e p a r a t i o n o f raw m a t e r i a l s and s t a r t i n g powder m i x t u r e s a r e t h e b a s i c s t a g e s . The s u b s t i t u t i v e i o n s , a d d i t i v e s and s o - c a l l e d " c o n t r o l l e d i m p u r i t i e s " may b e r o u g l y c l a s s i f i e d i n t o two c a t e g o r i e s : I ) t h o s e i n f l u e n c i n g m a i n l y t h e f e r r i t e i n t r i n s i c p r o p e r t i e s ( s a t u r a t i o n ~ ~ ~ a g n e t i z a t i o n , c r y s t a l l i n e a n i s o t r o p y , C u r i e p o i n t , r e s i s t i v i t y ) and 2 ) t h o s e c o n t r o l l i n g t h e

s y n t h e s i s and i t s k i n e t i c s , t h e g r a n u l a r s t r u c t u r e a n d / o r making t h e d e n s i f i c a t i o n p r o c e s s more e f f i c i e n t a n d / o r e a s y . I n some c a s e s t h e d e n s i f i c a t i o n p r o c e s s i s a i d e d by i s o s t a t i c o r d i r e c t i o n a l h o t - p r e s s i n g . The g r a i n - o r i e n t i n g c o m p a c t i o n o f c u b i c SF h a s b e e n e x p e r i m e n t e d /19 and r e f e r e n c e s c i t e d t h e r e / . S i n t e r i n g i s c a r r i e d o u t i n t h e more and more improved k i l n s and f u r n a c e s e q u i p p e d w i t h p r e c i s i o n m i c r o p r o c e s - s o r - b a s e d programmers o p e r a t i n g a s a m u l t i - z o n e o r many t i m e - s e g m e n t s h a v i n g con- t r o l l e r s o f t e m p e r a t u r e and o f o x y g e n / n e u t r a l g a s c o n t e n t . V o l u m e t r i c microwave s i n t e r i n g methods a r e a l s o t r i e d / 2 0 / ,

V - INITIAL PERMEABILITY - EXAMPLE OF SENSITIVITY TO EXTRA-CHEMICAL FACTORS

I n t h e p r e v i o u s s e c t i o n a n a t t e m p t was made t o b e a r i n mind how c l o s e b u t d i f f i c u l t t o q u a n t i t a t i v e a n d / o r u n i v o c a l s t a t e m e n t a r e t h e r e l a t i o n s between t h r e e g r o u p s o f f a c t o r s : l o - f e r r i t e c h e m i s t r y and p r e p a r a t i o n c o n d i t i o n s , 2" - i n t r i n s i c and m i c r o s t r u c t u r a l f e r r i t e p r o p e r t i e s , 3" - r e s u l t a n t p r o p e r t i e s and c h a r a c t e r i s t i c s o f t h e f e r r i t e m a t e r i a l .

C6-164 JOURNAL DE PHYSIQUE

To e x e m p l i f y t h e s t a t e m e n t s made above l e t u s c o n s i d e r i n d e t a i l t h e problem of t h e m a g n e t i c i n i t i a l p e r m e a b i l i t y i n t h e l i g h t o f remarks o f t h e p r e v i o u s s e c t i o n . I n i t i a l p e r m e a b i l i t y , p i , i s one o f t h e most i m p o r t a n t magnetic p a r a m e t e r s o f s o f t m a g n e t i c m a t e r i a l s . I t i s w e l l known t h a t p i i s a s t r u c t u r e - s e n s i t i v e p a r a m e t e r . The g r e a t e f f o r t s a r e b e i n g s t i l l u n d e r t a k e n t o i n c r e a s e t h e p i - v a l u e no d e g r a d a t i o n o f o t h e r p a r a m e t e r s , t o make t h e p i - c h a r a c t e r i s t i c s l e s s o r more s e n s i t i v e t o v a r i o u s c o n s t r a i n s ( t i m e , t e m p e r a t u r e , m a g n e t i c f i e l d s , s t r a i n s e t c . ) i n o r d e r t o s h a p e t h e s e dependences t o t h e a p p l i c a t i o n n e e d s .

L e t u s b e g i n by p r e s e n t i n g some remarks r e l a t e d t o p i , b e a r i n g i n mind t h a t it is composed, a s commonly a c c e p t e d , o f two components: domain w a l l d i s p l a c e m e n t compo- n e n t ( w a l l p e r m e a b i l i t y ) , pw, and s p i n r o t a t i o n component ( r o t a t i o n a l p e r m e a b i l i t y ) , ur. S t a r t i n g from t h e Globus' i d e a /21/ o f b u l g i n g w a l l f i x e d a t t h e boundary o f i s o l a t - ed s p h e r i c a l g r a i n , t h e f o l l o w i n g e x p r e s s i o n f o r t h e p w c a n b e d e r i v e d ( P o s t u p o l s k i , WiSniewska, Okoniewska, t o b e p u b l i s h e d ) :

MS d e n o t e s t h e s a t u r a t i o n m a g n e t i z a t i o n and V t h e volume o f m a g n e t i c m a t e r i a l ( e x - c l u d i n g p o r e s ) . I t h a s been assumed t h a t t h e j t y p e s o f domain w a l l s e x i s t (180°, l o g 0 , 90' e t c . /22/ i n t h e m a t e r i a l , depending on t h e s i g n o f m a g n e t o c r y s t a l l i n e a n i s o t r o p y c o n s t a n t ( j = 5 f o r s u b s t a n c e s o f K1 > 0 and j = 7 f o r K1 < 0 ) . Each domain w a l l t y p e , i , i s c h a r a c t e r i z e d by i t s own v a l u e o f s u r f a c e e n e r g y d e n s i t y , y i , r a n g i n g from 0 . 4 6 ~ A f o r /110/ K ~ - 71" w a l l t o 2.76 A A K ~ f o r /110/ - 180' w a l l (A - exchange c o n s t a n t , K1 - f i r s t m a g n e t o c r y s t a l l i n e a n i s o t r o p y c o n s t a n t ) / 2 2 / . Nw - i s t h e number o f w a l l s o f t h e g i v e n t y p e , L - i s t h e s p a n o f g i v e n w a l l which i n ' t h e c a s e o f a w a l l c i r c u l a r l y f i x e d a t t h e s p h e r i c a l g r a i n p e r i p h e r y i s e q u a l t o i t s d i a m e t e r . The r e a l ( s p a t i a l j v a l u e s o f L a r e d i s t r i b u t e d w i t h i n L1 and Lmax l i m i t s

(L1 i s a c r i t i c a l s i z e of s i n g l e domain g r a i n ) and gwi (L) i s t h e d e n s i t y f u n c t i o n of t h a t s p a t i a l s p a n d i s t r i b u t i o n . The ( c o s $ ) ~ r e p r e s e n t s t h e mean v a l u e of c o s $ (where $ i s t h e a n g l e between w a l l s u r f a c e and t h e a p p l i e d e x t e r n a l f i e l d H) c a l c u - l a t e d /23/ u s i n g a formula:

where g a . ( @ ) i s t h e d e n s i t y f u n c t i o n o f $ i - d i s t r i b u t i o n . Because i t h a s b e e n assum- ed t h a t ' i n a d e f e c t e d g r a i n a number o f w a l l s o f v a r i o u s t y p e s s i m u l t a n e o u s l y c a n e x i s t and t h a t t h e y c a n c o n t a c t a t d i f f e r e n t a n g l e s , i t is r a t h e r h a r d t o keep up t h e a s s u m p t i o n t h a t t h e w a l l s form e x c l u s i v e l y t h e c i r c l e s . T h e r e f o r e , t h e p r o d u c t

~ n f e - ~ h a s been i n t r o d u c e d , 1 c n < 4, t o a c c o u n t f o r t h a t s i t u a t i o n ( f L i s a c o e f - f i c l e n t making a l s o t h i s p r o d u c t d i m e n s i o n l e s s ) and n o t t o i n f l u e n c e t h e i n c r e a s e of m a g n e t i c moment r e s u l t i n g from w a l l b u l g i n g when i t i s n o t s p h e r i c a l ( n o n - s p h e r i c a l membrane p r o b l e m ) . n i i s a n u m e r i c a l c o e f f i c i e n t dependent on t h e w a l l t y p e .

As i t c a n b e s e e n t h e q u a n t i t a t i v e d i s c u s s i o n o f t h e above f o r m u l a r e q u i r e s a p r e c i s e knowledge o f s u c h domain s t r u c t u r e p a r a m e t e r s a s : w a l l t o p o g r a p h y , number, t y p e s , s h a p e s and p o s i t i o n i n g o f g i v e n w a l l t y p e s , d e n s i t y f u n c t i o n s o f w a l l s p a t i a l s p a n s e t c . These d a t a a r e p r a c t i c a l l y u n a v a i l a b l e , e x c l u d i n g t h e l i m i t c a s e o f t h e s i n g l e 180" c i r c u l a r w a l l s h a v i n g no s i z e s p r e a d / 2 4 / , i . e . t h e Globus' c a s e when

p i - 1 = 3 ~ : ~ / 1 6 ~ / 2 1 / .

N e a r l y a l l o f t h e components o f (1, and ( 2 ) a r e d e p e n d e n t o n t e m p e r a t u r e , m a g n e t i c f i e l d e t c . ( e x c l u d i n g V , n i l and some o f t h e s e dependences a r e d i f f i c u l t t o b e p r e d i c t e d .

A s i t i s s e e n , more r e a l approach t o t h e problem o f w a l l p e r m e a b i l i t y makes i t g r e a t l y complex and c o m p l i c a t e d . N e v e r t h e l e s y , t h e f o r m u l a ( 1 ) e n a b l e s more e x t e n s i v e and more d e t a i l e d d i s c u s s i o n of t h e r e s u l t s o n v a r i o u s p, d e p e n d e n c e s . These r e s u l t s a r e sometimes s u r p r i s i n g and d i f f i c u l t t o b e i n t e r p r e t e d u s i n g s i m p l i f i e d o n e - o r two-parameter models ( e . g . Ms and K1).

F i g s . 2 and 3 c a n b e a n example o f such r e s u l t s . F i g . 2 p r e s e n t s t h e dependences o f

F i g . 2 - I n i t i a l p e r m e a b i l i t y u i a t room F i g . 3 - I n i t i a l p e r m e a b i l i t y u i vs t e m p e r a t u r e v s mean g r a i n d i a m e t e r Da. t e m p e r a t u r e f o r p a r a l l e l , 11 , and Samplesof a Ni-Zn f e r r i t e a f t e r t h e r m a l p e r p e n d i c u l a r , I , a . c . d e m a g n e t i z a t i o n s .

( T ) , and a . c . d e m a g n e t i z a t i o n s , 1) ^- pa- T - p o i n t c o r r e s p o n d s t o u i a f t e r r a l l e l and

I-

i n g f i e l d .

p i on mean g r a i n d i a m e t e r , Da ( n o t on t h e w a l l s p a n , L ) f o r t h e samples of a Ni0.726Zn0 2 5 0 C p 0 0 3 2 ~ e ~ + 1 7 ~ e 3 + 9 7 5 ~ 4 - 0 012 f e r r i t e . These samples have t h e same s p e c i f i c m i g n e t ~ z i t i o n arid d l $ f e r I n mean g r a i n s i z e and i n d e n s i t y d . I n F i g . 3 t h e t e m p e r a t u r e dependences o f p i a r e shown f o r one o f t h e s e s a m p l e s , namely f o r t h a t of Da = 0.709 urn. These c u r v e s were measured a f t e r : a ) - t h e r m a l d e m a g n e t i z a t i o n ( T ) , b ) - a . c . d e m a g n e t i z a t i o n s , p a r a l l e l ( 1 1 ) and p e r p e n d i c u l a r (1) t o measuring magnetic

f i e l d o f low i n t e n s i t y . The a . c . d e m a g n e t i z a t i o n s and u i - measurements were c a r r i e d o u t a f t e r t h e r m a l e q u i l i b r i u m was r e a c h e d i n t h e s a m p l e s . The r e s u l t s were o n l y i n - s i g n i f i c a n t l y a f f e c t e d by t h e a f t e r - e f f e c t s . The same b e h a v i o u r a s t h a t p i c t u r e d i n F i g s 2 and 3 f o r t o r o i d a l samples h a s been o b s e r v e d f o r t h e samples b e i n g i n a form of c u b e s .

I t c a n b e s u g g e s t e d i n t h e l i g h t o f f o r m u l a ( 1 ) t h a t t h e number o f domain w a l l s , Nw, t h e d e n s i t y f u n c t i o n s o f L and Q d i f f e r f o r t h e s t a t e s a f t e r T and a . c . demagnetiza- t i o n s . Also g a ( $ ) f o r t h e 11 and L c a s e s c a n d i f f e r s i g n i f i c a n t l y . I t c a n n o t b e excluded t h a t t h e p r o p o r t i o n s between t h e numbers of v a r i o u s w a l l t y p e s a r e changed w i t h t h e change o f domain and domain w a l l topography. Thus, t h e t o t a l w a l l e n e r g y c a n b e changed a s a d i r e c t consequence o f t h e change i n : w a l l s u r f a c e a r e a s f o r e a c h t y p e o f w a l l , s h a r e o f y i - v a l u e s , s p e c i f i c f o r each w a l l t y p e , numbers of p a r t i c u l a r w a l l t y p e s .

The change i n domain topography p r o d u c e s a l s o t h e changes between numbers and volumes o f domains w i t h m a g n e t i z a t i o n v e c t o r d i r e c t i o n a p p r o x i m a t i n g l e s s o r more t h e d i r e c - t i o n of t h e measuring f i e l d . I f one assumes t h a t t h e r o t a t i o n a l p e r m e a b i l i t y , u r , i s commensurate t o uw, t h e e x p l a n a t i o n o f t h e c a s e o f pi(^) > u i ( 1 1 ) becomes r a t i o - n a l , b u t makes t h e s i t u a t i o n more c o m p l i c a t e d .

The t e m p e r a t u r e dependences p i ( T ) i n F i g . 3 a r i s e need f o r a s e p a r a t e comment. The t e m p e r a t u r e dependent i n t r i n s i c p a r a m e t e r s o f a m a t e r i a l (Ms, ~ J A K , e t c . ) s h o u l d n o t b e a f f e c t e d by t h e r m a l o r a . c . d e m a g n e t i z a t i o n s . Thus, t h e more d e t a i l e d d i s - c u s i o n of t h e s e u i ( T j dependences seems t o be i n t e r e s t i n g .

The r e s u l t s o f s i m i l a r meaning c a n b e r e p o r t e d f o r f e r r i t e s o f o t h e r k i n d s and f o r o t h e r p a r a m e t e r s a s e . g . h y s t e r e s i s l o o p , m a g n e t i c l o s s e s , a m p l i t u d e p e r m e a b i l i t y , disaccommodation and accommodation.

JOURNAL DE PHYSIQUE

A l l t h e s e r e s u l t s i n d i c a t e markedly t h e e x t r a - c h e m i c a l o r i g i n o f p r e s e n t e d dependen- c e s . Here t h e y h a v e a v e r y marked c h a r a c t e r and were r e v e a l e d by s p e c i a l l y programm- ed measurements. I n t h e normal measuring p r a c t i c e s u c h dependences may n o t b e d e t e c t - ed and t h e e f f e c t u n d e r s t u d y c a n b e a c c o u n t e d f o r t h e i n a p p r o p r i a t e o r i g i n .

VI - CONCLUDING REMARKS

Many e x c e l l e n t review p a p e r s d e v o t e d t o s o f t f e r r i t e s h a v e been p u b l i s h e d i n a few r e c e n t y e a r s / 2 5 + 3 3 / . The c u r r e n t p r o g r e s s i n t h e f i e l d o f SF m a t e r i a l s r e s u l t s more from l a b o r i o u s , v e r y s p e c i a l i z e d and d e l i c a t e r e s e a r c h work t h a n from f l a s h l y s p e c t a c u l a r r e v e l a t i o n s . Thus t h e s t a t u s i n t h e s o f t f e r r i t e m a t e r i a l <cir.ncc, p r o - p e r t i e s e t c . i s n o t changing t o o r a p i d l y and t h e m a t t e r o f r e c e n t SF-review l i t e - r a t u r e i s a l i v e i s s u e .

I n t h i s p a p e r i t was i n t e n d e d t o emphasize t h a t t h e m a t t e r o f c h e m i s t r y and i n t r i n s i c f e r r i t e m a t e r i a l p r o p e r t i e s , m i c r o s t r u c t u r e and domain s t r u c t u r e , r e s u l t a n t p r o - p e r t i e s o f t h e f e r r i t e m a t e r i a l s h o u l d b e c o n s i d e r e d j o i n t l y , a s a whole. I t would b e h i g h l y d e s i r a b l e t o r e l a t e t h e s e problems, a s c l o s e a s p o s s i b l e , t o i n t r i n s i c mechanisms and t o p h y s i c a l and chemical p r o c e s s e s t a k i n g p l a c e i n t h e m a t e r i a l , and n o t , a s i n many c a s e s , t o s u c h f a c t o r s a s s t a r t i n g c h e m i c a l c o m p o s i t i o n and p r e p a r a - t i o n c o n d i t i o n s .

And t h e f i n a l remark. The s o f t f e r r i t e m a t e r i a l s a r e produced i n v a r i o u s forms and i n v a r i o u s ways: from t h e " b l a c k ceramics" t h r o u g h t h e s i n g l e - c r y s t a l s t o t h e t h i n f i l m w a f e r s . T h i s c a u s e s t h a t t h e s o f t f e r r i t e community i s d i s p e r s e d and s p r e a d . T h e r e i s o n l y one l i n k which j o i n s a l l t h e d i s p e r s e d magnetic p e o p l e : t h e p h y s i c s o f magnetism. I t would b e h i g h l y d e s i r a b l e t o h a v e a s i m i l a r l y c l e a r l y o u t l i n e d "homo- logue" l i n k i n g t h e magnetism w i t h i t s a p p l i c a t i o n s . The Magnetic M a t e r i a l s f o r A p p l i c a t i o n Conference f o r m u l a j u s t b u i l d s t h a t b r i d g e .

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