RARE EARTH-TRANSITION METAL PERMANENT MAGNETS

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RARE EARTH-TRANSITION METAL PERMANENT MAGNETS

D. Givord, J. Laforest, H. Li, A. Liénard, R. de la Bâthie, P. Tenaud

To cite this version:

D. Givord, J. Laforest, H. Li, A. Liénard, R. de la Bâthie, et al.. RARE EARTH-TRANSITION METAL PERMANENT MAGNETS. Journal de Physique Colloques, 1985, 46 (C6), pp.C6-213-C6- 220. �10.1051/jphyscol:1985637�. �jpa-00224890�

RARE EARTH-TRANSITION METAL PERMANENT MAGNETS

D. Givord, J. Laforest, H.S. Li, A. Liénard, R. Perrier de la Bâthie and P. Tenaud

Laboratoire Louis Néel, C.H.R.S., 166 X, 38042 Grenoble Cedex, France Résumé - Une température d'ordre élevée et une forte anisotropie sont des pro- priétés essentielles à l'origine de la coercitivité des aimants permanents.

Une combinaison optimum de ces propriétés peut être obtenue dans les composés terres rares-métaux de transition. Parmi les composés binaires, elles caracté- risent la phase SmC05, mais aucune des phase R-Fe. Elles ont été récemment observées dans le composé ternaire NdgFe-ijjB. Dans de tels systèmes, deux pro- cessus métallurgiques essentiels peuvent permettre d'obtenir de la coerciti- vité. Les aimants SmCo5 et Nd2Fe-|i|B sont élaborés par frittage de poudres. La coercitivité est déterminée par la nucléation de domaines à la surface des grains. Des défauts structuraux induisent une réduction très importante de la coercitivité, plus particulièrement dans SmC-05. Dans les aimants Sm(Co-Cu>5 et Sm(Co, Fe, Cu, Zr)f-$, un accrochage des parois entre domaines a lieu sur des défauts structuraux, associés à la coexistence de deux phases présentant des propriétés magnétiques différentes. Dans tous ces systèmes, la décrois- sance de la coercitivité lorsque la température augmente, est plus importante que celle de 1'anisotropie. Ceci montre que les effets d'activation ther- mique, particulièrement importants dans SmCCoCu)^ où le piégeage des parois a lieu sur des défauts de très petites dimensions, doivent être pris en compte.

Abstract - High ordering temperature and large uniaxial anisotropy are basic magnetic properties at the origin of coercivity in permanent magnets. An optimum combination of these may be obtained in rare earth-transition metal compounds. Among binary compounds, they characterize the SmCo^ phase, but none of the R-Fe phase. They were recently observed in the ternary NdgFe-mB compound. In such systems, two main metallurgical processes may allow to develop coercivity. S111C05 and Nd2Feii)B magnets are elaborated by sintering of powders. Coercivity is determined by nucleation of reverse domains at grain boundaries. Structural defects induce a severe reduction of coercivity, especially in SmC.05. In Sm(Co-Cu)5 and Sm(Co, Fe, Cu, Zr)^_g magnets, pinning of domain walls occurs on structural defects, which are associated with the coexistence of 2 phases with different magnetic properties. In all systems, the decrease of coercivity as temperature increases is larger than that of anisotropy. This reveals that thermal activation effects, particularly enhanced in Sm(Co-Cu)5, where pinning of walls occurs on defects at a very small scale, must be taken into account.

INTRODUCTION

Permanent magnets are basically ferromagnets in which ordering temperature above room temperature, e.g. large exchange interactions and high spontaneous magneti- zation, are requested. Magnetostatic energy leads in ferromagnets to the subdivision of matter, at a microscopic scale, into magnetic domains with different magneti- zation directions. The bulk magnetization is zero. The specific property of a permanent magnet is the ability to stay in a metastable state of saturated magnetiza- tion. The phenomenon of coercivity is intrinsically linked, although in an indirect way, to magnetic anisotropy of uniaxial character. 3d and If magnetism, respecti- vely, are characterized by some, but not all, of the basic properties which may allow to obtain permanent magnet. Large exchange interactions but weak anisotropy are associated with the outer character of the 3d shell. On the contrary, to the strong localization of the 4f shell correspond weaker exchange interactions and large magnetocrystalline anisotropy.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985637

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I n r a r e e a r t h ( R ) - t r a n s i t i o n m e t a l (M) r i c h compounds, dominant 3d magnetism s t a b i l i z e s o r d e r i n g t e m p e r a t u r e s above 300 K . Large a n i s o t r o p y , a s s o c i a t e d w i t h 4f magnetism, may occur a t low t e m p e r a t u r e s i n compounds o f u n i a x i a l symmetry. I n s u c h i n s t a n c e , 3d exchange i n t e r a c t i o n s a c t i n g on t h e r a r e - e a r t h atoms, l i f t t h e

degeneracy o f t h e 4f l e v e l s . A t room t e m p e r a t u r e , t h e 4f ground s t a t e is s t i l l e s s e n t i a l l y p o p u l a t e d , and t h e v a l u e of t h e a n i s o t r o p y is n o t d r a s t i c a l l y reduced.

T h i s means t h a t a n optimum combination o f l a r g e exchange i n t e r a c t i o n s and a n i s o t r o p y i s o b t a i n e d t h u s g i v i n g account f o r t h e o u t s t a n d i n g permanent p r o p e r t i e s observed i n i t i a l l y i n t h e SmCog compound / 1 , 2 / , and more r e c e n t l y i n t h e R2Fel4B compound / 3 / . We d e s c r i b e i n t h e f i r s t p a r t o f t h i s paper t h e e s s e n t i a l s t r u c t u r a l and magnetic p r o p e r t i e s of r a r e e a r t h - t r a n s i t i o n m e t a l compounds which a r e r e l e v a n t i n t h i s c o n t e x t . I n a second p a r t , t h e d i f f e r e n t m a t e r i a l s and t h e a s s o c i a t e d p r o c e s s e s of c o e r c i v i t y a r e d i s c u s s e d and compared.

I - RARE EARTH-TRANSITION METAL RICH COMPOUNDS S t r u c t u r a l p r o p e r t i e s of b i n a r y compounds

Numerous compounds e x i s t i n t h e phase diagrams between r a r e - e a r t h and t r a n s i t i o n m e t a l s / 4 , 5 / . The RNi5 and RCo5 compounds c r y s t a l l i z e i n t h e hexagonal.CaCu5-type s t r u c t u r e . T h i s s t r u c t u r e may be d e s c r i b e d a s a s t a c k i n g o f a t o m i c p l a n e s o f two k i n d s . The p l a n e s o f t h e f i r s t k i n d a r e o c c u p i e d by R atoms i n a s i t e , l a , of hexagonal symmetry and by M atoms i n a s i t e , 2 c , of symmetry 3/m. Only M atoms occupy t h e o t h e r p l a n e s , on a s i t e . 3g, o f symmetry mmm. I n t e r a t o m i c d i s t a n c e s between M atoms i n t h i s s t r u c t u r e a r e v e r y s h o r t ( - 2.54 A ) .

The c r y s t a l l o g r a p h i c s t r u c t u r e s of most o f M-rich b i n a r y compounds d e r i v e from t h e CaCug-type s t r u c t u r e . I n t h e R2M7, RM3, RM2 p h a s e s , o r d e r e d s u b s t i t u t i o n s o f R atoms f o r M atoms o f t h e 2c s i t e o c c u r . I n t h e R2M17 p h a s e s , p a i r s o f M atoms ( d u m b e l l s ) a r e s u b s t i t u t e d f o r 1 / 3 of R atoms. These s t r u c t u r e s , e x c e p t t h e c u b i c Laves phase s t r u c t u r e o f t h e RM2 compounds, e x h i b i t hexagonal o r rhombohedral symmetries.

However, a s compared t o t h e RC05 s t r u c t u r e , t h e u n i a x i a l c h a r a c t e r o f t h e l o c a l environment a t t h e r a r e e a r t h s i t e s i s g e n e r a l l y reduced.

The above d e s c r i p t i o n o f t h e d i f f e r e n t phases i s , more o r l e s s , i d e a l . I n r e a l compounds, s t r u c t u r a l d e f e c t s , c l o s e l y connected w i t h t h e e x i s t e n c e of s e v e r a l r e l a t e d s t r u c t u r e s , o c c u r . I n p a r t i c u l a r , some s u b s t i t u t i o n s o f CO p a i r s f o r R atoms, which o r d e r e d c o r r e s p o n d t o t h e , b u i l d i n g of t h e R2Col7 compounds, occur i n t h e RC05 phase. By i n c r e a s i n g t h e ~ v a l u e o f t h e c l a t t i c e parameter t h i s a l l o w s t o i n c r e a s e t h e v e r y s h o r t d i s t a n c e s between CO atoms i n two c o n s e c u t i v e p l a n e s . Such a s u b s t i t u t i o n of CO p a i r s is a s s o c i a t e d w i t h a complete change o f t h e l o c a l symmetry f o r CO 2c atoms i n t h e d i r e c t environment / 6 / . F i n a l l y , Den Broader e t a1 /7/ have shown t h a t t h e RC05 phase is a c t u a l l y m e t a s t a b l e a t room t e m p e r a t u r e and t e n d s t o decompose i n t o t h e n e i g h b o u r i n g p h a s e s R2Co17 and R2C07.

Magnetic p r o p e r t i e s o f b i n a r y compounds

I n t r a n s i t i o n m e t a l r i c h compounds, 3d magnetism is dominant. Ferromagnetism o f M moments g e n e r a l l y o c c u r s . Due t o t h e s t r o n g i n t e r a c t i o n between r a t h e r l o c a l i z e d 36 e l e c t r o n s and more d e l o c a l i z e d 5d e l e c t r o n s o f t h e R atoms, t h e 3d moment d e c r e a s e s through 3d - 5d e l e c t r o n t r a n s f e r and h y b r i d i z a t i o n w i t h 5d e l e c t r o n s a s t h e c o n t e n t o f R atoms i n c r e a s e s . T h i s e f f e c t is t h e s t r o n g e s t i n N i compounds, a s shown by t h e P a u l i paramagnetism observed i n Y N i 5 . Among CO compounds, magnetism d i s a p p e a r s a t a c r i t i c a l c o n c e n t r a t i o n c o r r e s p o n d i n g a p p r o x i m a t e l y t o t h e s t o i c h i o m e t r y RCo2. I n RFe2 compounds, t h e Fe moment r e a c h e s s t i l l a b o u t 1.5 VB.

A measure o f t h e v a l u e o f 3d magnetic i n t e r a c t i o n s is g i v e n by t h e v a l u e of t h e Curie t e m p e r a t u r e i n compounds w i t h non-magnetic R atoms, La, Lu o r Y. The C u r i e t e m p e r a t u r e o f Y-CO and Y - N i compounds is approximately p r o p o r t i o n a l t o t h e s q u a r e o f t h e 3d moment ( F i g u r e 1 ) . Thus, exchange i n t e r a c t i o n s a r e n o t 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 change o f band s t r u c t u r e and i n t e r a t o m i c d i s t a n c e s from compound t o

compound. R-Fe compounds e x h i b i t v e r y d i f f e r e n t p r o p e r t i e s . The o r d e r i n g t e m p e r a t u r e of t h e F e - r i c h e r compound, Y2Fel7, is o n l y 300 K ( F i g . 1 ) . With t h i s s t r o n g

r e d u c t i o n of magnetic i n t e r a c t i o n s , is a s s o c i a t e d a l a r g e anomalous thermal expansion i n t h e o r d e r e d s t a t e ( F i g u r e 2 ) which r e v e a l s t h a t magnetic i n t e r a c t i o n s a r e h i g h l y d i s t a n c e dependent /g/. F i n a l l y i n t h e compound Lu2Fe17, where Fe-Fe i n t e r a t o m i c d i s t a n c e s a r e even s h o r t e r , a n h e l i m a g n e t i c s t r u c t u r e is observed which shows t h a t n e g a t i v e i n t e r a c t i o n s a r e competing with t h e dominant p o s i t i v e ones. A l l t h e s e p r o p e r t i e s were shown t o be c o n s i s t e n t /8/ w i t h t h e phenomenological d i s t a n c e dependence of magnetic i n t e r a c t i o n s a s d e s c r i b e d by Ndel /10/ and S l a t e r : 3d p o s i t i v e i n t e r a c t i o n s e x h i b i t a d r a s t i c r e d u c t i o n f o r i n t e r a t o m i c d i s t a n c e s between Fe atoms s h o r t e r t h a n a b o u t 2.5 R , and become n e g a t i v e below 2.4 R.

I n t h e s e a l l o y s o f u n i a x i a l symmetry, s i g n i f i c a n t a n i s o t r o p y o r i g i n a t i n g from 3d e l e c t r o n s may o c c u r . I n R-CO a l l o y s u n i a x i a l CO a n i s o t r o p y is observed. The

a n i s o t r o p y e n e r g y i n YCo5 is 7 . 4 ~ 1 0 7 erg/cm3 a t 4.2 K / l l /

.

When c o n s i d e r i n g a l l o y s w i t h magnetic r a r e - e a r t h , t h e behaviour o f R atoms is e s s e n t i a l l y determined by combined e f f e c t s o f c r y s t a l e l e c t r i c f i e l d (CEF) and c o u p l i n g between 3d and 4f moments. The weak magnetic c o u p l i n g between 4f moments c a n be n e g l e c t e d . Ferromagnetism i n a l l o y s w i t h l i g h t r a r e - e a r t h s and f e r r i m a g n e t i s m w i t h heavy r a r e - e a r t h s o c c u r , due t o t h e a n t i p a r a l l e l c o u p l i n g between 3d and 4f s p i n s . The a d d i t i o n a l magnetic i n t e r a c t i o n s , a s compared t o a l l o y s w i t h La, Y , Lu, i n d u c e a n i n c r e a s e of t h e C u r i e t e m p e r a t u r e which is a p p r o x i m a t e l y p r o p o r t i o n a l t o t h e s q u a r e of t h e 4f s p i n / h / . I n compounds o f u n i a x i a l symmetry, second-order terms i n t h e c r y s t a l p o t e n t i a l , B:o;, a r e dominant. A t low t e m p e r a t u r e s , t h e 4f ground s t a t e is o n l y p o p u l a t e d . The e a s y m a g n e t i z a t i o n d i r e c t i o n i n t h e d i f f e r e n t compounds c a n be e x p l a i n e d by c o n s i d e r i n g t h a t t h e parameter AO of t h e environment is

n e g a t i v e . Thus, among l i g h t r a r e - e a r t h atoms Sm e x h i g i t s u n i a x i a l a n i s o t r o p y . On t h e c o n t r a r y , b a s a l p l a n e a n i s o t r o p y c o r r e s p o n d s t o compounds w i t h Pr and Nd atoms. A f a i r l y l a r g e v a l u e f o r t h e a n i s o t r o p y is maintained up t o room t e m p e r a t u r e and above by 3d - 4f magnetic i n t e r a c t i o n s .

Re2Fe 1 QB compounds

None of t h e R-Fe b i n a r y compounds e x h i b i t l a r g e exchange i n t e r a c t i o n s t o g e t h e r w i t h u n i a x i a l a n i s o t r o p y . Such a combination o f magnetic p r o p e r t i e s , needed i n permanent magnet m a t e r i a l s , may be e n c o u n t e r e d i n R 2 F e 1 4 ~ t e r n a r y compounds. The t e t r a g o n a l

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c r y s t a l s t r u c t u r e /12,13/ of t h e s e compounds can be described a s a sequence of l a y e r s of d i f f e r e n t kinds. I n t h e l a y e r c o n t a i n i n g R and B atoms, atomic arrangement reminiscent of t h e CaCug-type s t r u c t u r e occurs /13/. Only Fe atoms occupy t h e t h r e e atomic l a y e r s which s e p a r a t e two l a y e r s c o n t a i n i n g R atoms, they form a t y p i c a l arrangement observed i n t h e sigma phase /14/. F i n a l l y , i n t h i s s t r u c t u r e , a s i n binary R-M s t r u c t u r e s , very s h o r t i n t e r a t o m i c d i s t a n c e s between M atoms ( < 2.50 A ) occur.

The magnetic p r o p e r t i e s of t h e s e compounds can be discussed i n t h e l i g h t of those observed i n binary compounds. Owing t o Fe r i c h stoichiometry, t h e 3d moment, 2.10 pb/Fe atom i n Y2Fel4B, is c l o s e t o t h a t of pure Fe metal /15/. The Curie temperature i n YzFelhB, 573 K , although h i g h e r than room temperature, is l a r g e l y reduced compared t o t h a t of pure Fe metal. A s i n the R2Fel7 compounds, a l a r g e anomalous expansion occurs i n t h e ordered s t a t e /16/ (Figure 2 ) . A l l t h e s e p r o p e r t i e s r e v e a l t h a t , due t o very s h o r t i n t e r a t o m i c d i s t a n c e s , magnetic i n t e r - a c t i o n s between Fe atoms a r e reduced and s t r o n g l y d i s t a n c e dependent. Accordingly, p a r t i a l s u b s t i t u t i o n s of CO atoms, but a l s o N i atoms, f o r Fe induce an increase of the Curie temperature / I T / , although i n t h e l a t e r c a s e t h e value of t h e 3d moment i s s t r o n g l y reduced.

Magnetic coupling between R and Fe atoms is of s i m i l a r s t r e n g t h a s observed i n binary compounds. The change i n l o c a l environments from c r y s t a l s t r u c t u r e s of b i n a r y compounds, corresponds t o a change of s i g n of t h e second-order c r y s t a l - f i e l d term along t h e c - d i r e c t i o n /15,18/. A s a r e s u l t , Sm compound e x h i b i t b a s a l plane a n i s o t r o p y , while Pr and Nd compounds have a x i a l symmetry.

I1 - RARE EARTH-TRANSITION METAL PERMANENT MAGNETS.

Anisotropy and c o e r c i v i t y

According t o t h e arguments developed i n t h e i n t r o d u c t i o n , SmCo5 and R2Fe14B com- pounds appear a s b e s t p o t e n t i a l permanent magnet m a t e r i a l s . When t h e magnetization i n a ferromagnet is s a t u r a t e d , an energy b a r r i e r , equal t o t h e a n i s o t r o p y energy, should be overcome i n o r d e r t o r o t a t e t h e magnetization. This g i v e s a t h e o r e t i c a l maximum value f o r t h e coercive f i e l d , which i n SmCo5 is, a t 4.2 K, 650 KG /19/. The a c t u a l value of c o e r c i v e f i e l d i n bulk specimens is reduced by s e v e r a l o r d e r of magnitudes. I n e f f e c t , magnetic domains, with magnetization d i r e c t i o n opposed t o t h a t of t h e bulk specimen, a r e nucleated a t s t r u c t u r a l d e f e c t s , and a subsequent f r e e displacement of domain w a l l s may occur. M e t a l l u r g i c a l processes a r e t h u s needed i n o r d e r t o recover a s i g n i f i c a n t value f o r t h e c o e r c i v i t y . A general purpose is t o o b t a i n an inhomogeneous m a t e r i a l i n which t h e metastable s t a f e of s a t u r a t e d magneti- z a t i o n may be more e a s i l y kept. Powder metallurgy is used f o r e l a b o r a t i o n of SmCo5 /1,2/ and R2Fe14B magnets / 3 / . I n t h e magnets obtained a f t e r s i n t e r i n g , domain w a l l s cannot propagate from g r a i n t o g r a i n . Thus, magnetization r e v e r s a l involves

n u c l e a t i o n of domains a t g r a i n boundaries. I n some g r a i n s , n u c l e a t i o n may occur very e a s i l y a t s t r u c t u r a l d e f e c t s , but t h e a s s o c i a t e d change of magnetization involves magnetization r e v e r s a l i n i n d i v i d u a l g r a i n s : t h e coercive f i e l d may reach a s i g n i - f i c a n t f r a c t i o n of t h e anisotropy f i e l d . I n a second approach, advantage is taken of t h e e x i s t e n c e of a m i s c i b i l i t y gap occurring i n t h e phase diagram of r a r e e a r t h a l l o y s c o n t a i n i n g CO and Fe t o g e t h e r with Cu. Appropriate thermal t r e a t m e n t s i n bulk specimens l e a d t o a s e p a r a t i o n i n t o two phases with d i f f e r e n t magnetic p r o p e r t i e s . Domain w a l l s tend t o be pinned i n t h e less-magnetic phase where t h e i r energy is reduced.

Nucleation of domains a t g r a i n boundaries : SmCo5 and R2Fel4B magnets

The inhomogeneous c h a r a c t e r of SmCo5 magnets a t a microscopic s c a l e is revealed by o p t i c a l microscopy /20/. Sm-rich phase, such a s Sm2C07, and Sm oxides a r e

p r e c i p i t a t e d a t boundaries of g r a i n s of t h e .@Cog phase. Reduced magnetic i n t e r - a c t i o n s and a n i s o t r o p y correspond t h u s t o t h e phase which s e p a r a t e SmCo5 g r a i n s . Magnetic domains a r e s t r o n g l y pinned a t g r a i n boundaries. Coercivity is t h u s governed by domain n u c l e a t i o n . The coercive f i e l d may reach about 45 KG a t 4.2 K.

disordered p a i r s of CO atoms may be s u b s t i t u t e d f o r Sm atoms i n SmCo5, inducing a l a r g e decrease of a n i s o t r o p y . A p a r a l l e l decrease of c o e r c i v i t y occurs. A s pointed out by D6portes e t a1 /21/ i s o l a t e d s u b s t i t u t i o n s a r e not e s s e n t i a l i n determining t h e c o e r c i v i t y . I n o r d e r t o decrease s i g n i f i c a n t l y the value of t h e coercive f i e l d , s t r u c t u r a l d e f e c t s must extend over a volume approximately equal t o t h a t of a domain w a l l . Considering the metastable n a t u r e of the RC05 phase / 7 / , it can be expected t h a t c l u s t e r s of s u b s t i t u t i o n s , corresponding t o t h e preformation of t h e R2Col7 phase, occur. Such c l u s t e r s may s t r o n g l y reduce t h e coercive f i e l d , and t h u s account q u a l i t a t i v e l y f o r t h e value observed.

I n Nd-Fe-B magnets, t h e phase p r e c i p i t a t e d a t g r a i n boundaries a r e not magnetic a t room temperature / 2 2 / , thus e n s u r i n g s t r o n g pinning of domain w a l l s a t g r a i n

boundaries. A s i n t h e SmCo5 magnet, c o e r c i v i t y i s determined by domain n u c l e a t i o n i n i n d i v i d u a l g r a i n s . A t 150 K, t h e c o e r c i v e f i e l d H, is about 35 kG. This value, weaker than i n SmCog, c o r r e s onds t o t h e weaker u n i a x i a l a n i s o t r o p y i n t h i s system

( a t 150 K , K1 + K2 = 1.75 10 erg/cm3), where K1 and K2 a r e t h e usual second-order

8

and fourth-order anisotropy c o n s t a n t s . However, t h e r a t i o of c o e r c i v i t y t o anisotropy represented by MHc/2(K1 + K2) is equal t o 1/10 a s compared t o 1/15 i n SmCo5. A s a matter of f a c t , t h e R2Fe14B phase is very s t a b l e and c r y s t a l l o g r a p h i c phases c l o s e l y r e l a t e d t o i t do not e x i s t . Therefore, and i n agreement with e l e c t r o n microscopy /23/ t h e d e n s i t y of d e f e c t s , which would favor n u c l e a t i o n of r e v e r s e domains, is r a t h e r low.

Pinning of domain walls a t magnetic inhomogeneities ^: S ~ ( C O - C U ) ~ and Sm(co-Fe-C~-Zr)~-8

Nesbitt e t a 1 /24/ observed l a r g e c o e r c i v i t y t o develop i n s o l i d s o l u t i o n s of RCo5 and RCu5 compounds. This phenomenon o r i g i n a t e s from t h e formation of two d i f f e r e n t phases i n t h e s e pseudo-binary a l l o y s /25/. A copper-rich and a copper-poor phases a r e formed. I n t h e S m ( C ~ - c u ) ~ system where maximum c o e r c i v i t y is obtained, both phases a r e ferromagnetic and Sm e x h i b i t s l a r g e u n i a x i a l a n i s o t r o p y . I n Cu-rich r e g i o n s , exchange i n t e r a c t i o n s a r e reduced. The r a t i o of exchange energy t o anisotropy energy which determines the wall t h i c k n e s s may be of t h e o r d e r of u n i t y . Domain w a l l s a r e expected t o be very narrow and they w i l l consequently tend t o be pinned on d e f e c t s of l o c a l c h a r a c t e r r e s u l t i n g from t h e s e p a r a t i o n i n t o two d i f f e r e n t phases. I n t h e s e S ~ ( C O - C U ) ~ a l l o y s , t h e s i g n i f i c a n t amount of Cu p r e s e n t induces a l a r g e decrease of t h e magnetization, and t h e r e f o r e of t h e induction i n magnets. Higher i n d u c t i o n s of about 10 kG, and c o e r c i v i t i e s of 30 kG may be obtained i n a l l o y s of t h e type .%(CO, Fe, C U , Zr)7-8 /26,27/. E l e c t r o n microscopy i n t h i s system r e v e a l s a c e l l u l a r s t r u c t u r e of rhombic c e l l s /28/. The c e l l i n t e r i o r s correspond e s s e n t i a l l y t o t h e rhombohedral Th2Zn17 s t r u c t u r e , surrounded by hexagonal RM5 phase a t t h e boundaries. The RM5 phase is r i c h e r i n Cu and a c t s f o r domain w a l l s a s t r a p p i n g c e n t r e s and pinning b a r r i e r s . F i r s t magnetization curves f o r these magnets g e n e r a l l y r e v e a l two d i f f e r e n t processes. I n low f i e l d s , pinning of domain w a l l s is r a t h e r weak. Magnetization i n c r e a s e s l i n e a r l y with t h e f i e l d , and then tends t o s a t u r a t e . I n a second f i e l d range, above about 30 kG, a s t e e p increase of the magnetization, and of the c o e r c i v i t y , occurs. A s discussed by Kronmiiller /29/

t h e low f i e l d behaviour must be a t t r i b u t e d t o movement of domain walls within t h e c e l l i n t e r i o r . The domain w a l l s a r e stopped a t the c e l l boundaries. I n higher f i e l d s , t h e w a l l s a r e pushed over t h e c e l l boundaries.

Temperature dependence of c o e r c i v i t y

Although c o e r c i v i t y i n r a r e - e a r t h permanent magnets is a consequence of t h e high magnetocrystalline a n i s o t r o p y a s s o c i a t e d with t h e r a r e - e a r t h i o n s , its decrease a s temperature i n c r e a s e s is always l a r g e r than t h a t of anisotropy /30-32/. This e f f e c t . r a t h e r small i n f i n e p a r t i c l e s magnets i s d r a s t i c i n systems where c o e r c i v i t y is

JOURNAL DE PHYSIQUE

Fig. 3

-

govered by pinning of domain walls (Fig. 3 ) . I n f a c t , t h e occurrence of thermal a f t e r - e f f e c t i n t h e s e systems r e v e a l s d i r e c t l y t h a t thermal a c t i v a t i o n induces a n a d d i t i o n a l decrease of c o e r c i v i t y . The r e l a t i o n

d e s c r i b e s t h e time dependence of the magnetization M. TO i s a c h a r a c t e r i s t i c time 1- S) and E t h e a c t i v a t i o n energy. Barbara and Uehara /33/ have shown t h a t i n t h e s e h i g h l y a n i s o t r o p i c systems, t h e a c t i v a t i o n energy is p r o p o r t i o n a l t o l/H.

I t can be i n t e r p r e t e d i n terms of n u c l e a t i o n processes which a r e n a t u r a l l y expected t o occur i n SmCo5 and Nd~FeluB magnets. I n magnets where pinning of w a l l s occurs, it can be shown t h a t , due t o t h e high anisotropy i n these systems, displacement of w a l l s occurs s t e p by s t e p , through process which involves simultaneously pinning and n u c l e a t i o n /33/. Thus, a g e n e r a l formula f o r t h e temperature dependence of

c o e r c i v i t y was derived :

The f i r s t term r e p r e s e n t s the dependence of c o e r c i v i t y on a n i s o t r o p y . HA is t h e anisotropy f i e l d . I n an i d e a l system, the phenomenological

c o e f f i c i e n t a would be one, it is reduced t o about 0.1 i n r e a l systems. The second term r e s u l t s from thermal a c t i v a t i o n e f f e c t s . EA i s t h e energy a n i s o t r o p y , H, t h e molecular f i e l d , V is a c h a r a c t e r i s t i c volume, cc' is a phenomenological c o e f f i c i e n t which g i v e s account f o r the decrease of a n i s o t o r p y and ( o r ) exchange i n t e r a c t i o n s a t d e f e c t p o s i t i o n s .

The d i f f e r e n c e s between temperature dependence of anisotropy and t h a t of c o e r c i v i t y a r e i l l u s t r a t e d i n f i g u r e 4 where t h e q u a n t i t y HO(H,(T) - ( a ~ ~ ( ~ 1 1 - l is p l o t t e d f o r the d i f f e r e n t types of magnets considered. The vglue of t h e c o e f f i c i e n t a is taken s o t h a t a t low temperatures, when a c t i v a t i o n processes can be n e g e l c t e d , H, = a HA.

The f a c t o r H:, which is t h e c o e r c i v e f i e l d a t 0 K , allows a d i r e c t ~0iIIparison Of a l l d a t a on a unique s c a l e .

processes become more and more e f f e c t i v e a t high temperature. Such a behaviour is expected from r e l a t i o n ( 2 ) a s a r e s u l t of t h e decrease of exchange i n t e r a c t i o n s and a n i s o t r o p y a s

temperature i n c r e a s e s . The e f f e c t s of a c t i v a t i o n energy a r e t h e l a r g e s t i n S ~ ( C O C U ) ~ , a s expected from t h e l o c a l c h a r a c t e r of t h e d e f e c t s a c t i n g a s pinning b a r r i e r s . Accordingly, r e p l a c i n g the a n i s o t r o p y energy (150 K / c e l l ) and t h e molecular f i e l d ( 2 107 Oe) by t h e i r values i n r e l a t i o n ( 2 ) , and t a k i n g a r b i t r a r y a' = 0.1 l e a d s t o a c h a r a c t e r i s t i c volume of about one c e l l a t low temperature. I n SmCog or uZ$8 Nd2Fel4B magnets, thermal a c t i v a t i o n

e f f e c t s a r e l e s s important. The volume of n u c l e a t i o n deduced from t h e same a n a l y s i s a s above is of about 10 c e l l s . F i n a l l y , two d i f f e r e n t regimes

c h a r a c t e r i z e thermal a c t i v a t i o n e f f e c t s i n Sm(C~FeCuzr)~-g magnets (Figure 4 ) .

0 200 300 A t low temperature, t h e observed

T [ K 1 behaviour suggests t h a t pinning of

w a l l s on l o c a l s t r u c t u r a l d e f e c t s occurs a s i n S ~ ( C O C U ) ~ . A t 100 K, t h e decrease of c o e r c i v i t y is considerably Fig. 4 - Differences between temperature reduced, and resembles t h e behaviour dependence of anisotropy and c o e r c i v i t y observed i n SmCo5 magnets. According t o a s given by t h e q u a n t i t y r e f . 33, i n systems where c o e r c i v i t y H ~ ( H ~ ( T ) - ( u ( H ~ ( T ) ) - ~ ( s e e t e x t ) depends on propagation of domain w a l l s ,

combination of pinning on d e f e c t s and n u c l e a t i o n determines t h e magnetization processes. The behaviour observed i n S m ( c o F e C ~ Z r ) ~ - 8 suggests t h u s t h a t t h e balance between these two terms is changed a s temperature i n c r e a s e s .

CONCLUSION

Analysis of t h e b a s i c magnetic p r o p e r t i e s of r a r e e a r t h - t r a n s i t i o n metal compounds allows one a b e t t e r understanding of t h e i r permanent magnet p r o p e r t i e s . The l a r g e c o e r c i v i t y i n these systems o r i g i n a t e s from t h e high anisotropy of 4f i o n s . However, s t r u c t u r a l d e f e c t s lower t h e value of t h e c o e r c i v i t y . This is i n p a r t i c u l a r t h e c a s e i n SmCo5 magnets where t h e presence of neighbouring phases, such a s Sm2Co17 is never t o t a l l y avoided. The decrease of c o e r c i v i t y with temperature r e s u l t s from both the decrease of anisotropy and an a d d i t i o n a l term due t o thermal a c t i v a t i o n . This term is predominant i n S ~ ( C O - C U ) ~ magnets where c o e r c i v i t y is determined by pinning b a r r i e r s of l o c a l c h a r a c t e r .

Acknowledgements - We thank f o r t h e i r many suggestions during t h i s work D r . B.

Barbara, J. DBportes, R . Lemaire and M. Uehara.

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