THE TEMPERATURE DEPENDENCE OF THE ANISOTROPY FIELD IN R2Fe14B COMPOUNDS (R = Y, La, Ce, Pr, Nd, Gd, Ho, Lu)

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THE TEMPERATURE DEPENDENCE OF THE ANISOTROPY FIELD IN R2Fe14B COMPOUNDS (R

= Y, La, Ce, Pr, Nd, Gd, Ho, Lu)

R. Grössinger, X. Sun, R. Eibler, K. Buschow, H. Kirchmayr

To cite this version:

R. Grössinger, X. Sun, R. Eibler, K. Buschow, H. Kirchmayr. THE TEMPERATURE DEPENDENCE OF THE ANISOTROPY FIELD IN R2Fe14B COMPOUNDS (R = Y, La, Ce, Pr, Nd, Gd, Ho, Lu).

Journal de Physique Colloques, 1985, 46 (C6), pp.C6-221-C6-224. �10.1051/jphyscol:1985638�. �jpa-

00224891�

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Colloque C6, supplément au n°9, Tome 46, septembre 1985 page C6-221

THE-TEMPERATURE DEPENDENCE OF THE ANISOTROPY FIELD I N R2F e1 4B COMPOUNDS (R = Y , L a , C e , P r , N d , G d , H o , L u )

R. G r o s s i n g e r , X.K. Sun, R. E i b l e r , K . H . J . Buschow and H.R. Kirchmayr

Institute for Experimental Physios, Technical University of Vienna, Austria

^Philips Research Laboratories, Eindhoven, The Netherlands '

Résumé - La variation thermique du champ d'anisotropie a été déterminée dans les composés du type (^Fej^B (R = Th, Y, La, Ce, Pr, Nd, Gd, Ho, Lu) à des températures, entre 4.2 K et la température de Curie. Dans les composés Nd2Fe^B et P ^ F e ^ B on a observé une réorientation du spin.

A b s t r a c t - The temperature dependence of the anisotropy f i e l d was measured i n v a r i o u s compounds of the type I^Fej^B (R = Th, Y, La, Ce, P r , Nd, Gd, Ho, Lu) at temperatures between 4.2 K and the Curie temperature. I n Nd2Fej4B and

^r2 'r e1 4 ^ a sPi-n r e o r i e n t a t i o n was found t o occur below room temperature.

I - INTRODUCTION

New permanent magnet materials based on Nd-Fe-B were recently developed which have distinct advantages compared to permanent magnet materials based on Sm-Co /!/.

Preliminary measurements of Nd-Fe-B at room temperature / 2 / had characterized this material as having uniaxial magnetic anisotropy, the anisotropy field (H/\ = 75 kG) being of the same order of magnitude as in Sn^Coj^. More detailed measurements made at various temperatures with a Nd2Fe24B single crystal soon revealed that the temperature dependence of the anisotropy in these types of materials is far from simple, as is witnessed by the change in the easy magnetization direction from parallel to the c-axis above 135 K to a direction deviating by 30 degr. from the c axis at 4.2 K /3/. The anisotropy fields in the various R2Fe^4B compounds at 4.2 K were determined from high field measurements on aligned powder samples by Sinnema et al. / 4 / . In this paper we report on the temperature dependence of H/\ in various R2Fe^4B compounds as well as in several materials of a non-stolchiometric composition.

II - EXPERIMENTAL

The anisotropy field was measured on arc melted and vacuum annealed samples using the SPD (Singular Point Detection) technique /5/. The theory of this method predicts a singularity in the second derivative of the magnetization (d^M/dH^) at the anisotropy field Hft if the external field is applied parallel to the hard plane (K^ > 0 ) . From the fact that this method samples the properties of the correctly oriented grains only (Hext parallel to the hard axis), it follows that H/\ can be measured on aligned powder samples but also on polycrystalline samples. This method is well suited for uniaxial materials. If the anisotropy of a material having an easy plane magnetization has to be measured, the theory predicts that the third derivative (d^M/dH^) will show a singularity at H=H/^. Unfortunately, owing to experimental difficulties, measurement of d-'M/dH-' is nearly impossible. Therefore our anisotropy studies will be restricted to R2Fe^4B compounds of uniaxial magnetic anisotropy.

III - RESULTS AND DISCUSSION

The temperature dependences of the anisotropy field H/\ of various R2Fe^4B compounds are compared in Fig. 1. In general one expects the anisotropy field to decrease with increasing temperature. This behaviour is only observed for those R2Fe^4B compounds in which the R component has an orbital moment and where one may assume that the ani- Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985638

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C6-222 JOURNAL DE PHYSIQUE

s o t r o p y o r i g i n a t e s from c r y s t a l f i e l d e f f e c t s . I n t h e R2Fel4B compounds i n which R i s non-magnetic (R = Y,La,Lu,Ce o r Th) t h e a n i s o t r o p y i s due t o t h e 3d s u b l a t t i c e magne- t i z a t i o n . I t i s i n t e r e s t i n g t o n o t e t h a t t h e temperature dependence o f HA i n these m a t e r i a l s shows t h e o p p o s i t e behaviour t o those r e f e r r e d t o above, i.e. t h e aniso- t r o p y f i e l d s l i g h t l y i n c r e a s e s w i t h i n c r e a s i n g temperature ( a t l e a s t a t temperatures w e l l below t h e C u r i e temperature). These r e s u l t s suggest t h a t t h e Fe atoms a t t h e d i f f e r e n t c r y s t a l l o g r a p h i c Fe s i t e s c o n t r i b u t e d i f f e r e n t l y t o HA and a l s o t h a t t h e temperature dependence o f these c o n t r i b u t i o n s i s n o t t h e same. A s i m i l a r s i t u a t i o n was assumed t o e x i s t i n Y2Co17 / 6 / . Attempts were made t o d e s c r i b e t h e 3d s u b l a t t i c e a n i s o t r o p y i n u n i a x i a l i n t e r m e t a l l i c compounds i n terms o f t h e second o r d e r c r y s t a l f i e l d parameter B20 /7/. I n t h i s model a c o r r e l a t i o n between t h e a n i s o t r o p y energy and t h e f a c t o r [l-3/8 ( ~ / a ) ~ l would be expected, where a and c a r e t h e l a t t i c e con- s t a n t s . The value$ o f HA o b t a i n e d i n t h e course o f t h e p r e s e n t i n v e s t i g a t i o n a r e compared w i t h values o f t h e f a c t o r [l-3/8 ( ~ / a ) ~ ] i n Table 1, u s i n g d a t a f o r a and c r e p o r t e d elsewhere /4/. When i n t e r p r e t i n g t h e d a t a shown i n t h e t a b l e one has t o t a k e account o f t h e f a c t t h a t t h e s i t u a t i o n i n compounds where R i s t r i v a l e n t may be s l i g h t l y d i f f e r e n t from t h a t i n compounds where R i s t e t r a v a l e n t . I n g e n e r a l t h e c o r - r e l a t i o n appears t o be q u i t e s a t i s f a c t o r y .

A d e t a i l e d i n s p e c t i o n o f t h e shape o f t h e v a r i o u s d 2 ~ / d ~ 2 versus H curves, measured a t d i f f e r e n t temperatures t o o b t a i n t h e temperature dependence o f HA, makes i t c l e a r t h a t t h e compounds Nd2Fe14B and Pr2Fel4B behave somewhat anomalously a t l o w temperatures. I t i s w e l l known t h a t t h e r e i s a change i n m a g n e t i z a t i o n d i r e c t i o n from an easz a x i s t o an easy cone i n Nd2Fe14B a t 135 K /3/. An a n i s o t r o p y anomaly i n t h e d 2 w / d ~ versus H curves was observed f o r t h e f i r s t t i m e by Grgssinger e t a l . /2/. For temperatures above about 180 K t h e shape o f these curves i s c l o s e t o t h a t expected on t h e b a s i s o f t h e SPD theory, t h e a n i s o t r o p y f i e l d b e i n g determined b y t h e s i n g u l a r i t y appearing as a sharp cusp a t t h e f i e l d HA (see F i g . 2a). However, t h e r e i s a d i s - t i n c t change i n t h e shape o f these curves a t lower temperatures ( F i g . 2b). Closer

Fig. 1

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Temperature dependence of t h e a n i s o t r o p y f i e l d yoHa i n v a r i o u s R2Fe14B compounds ( R = Y(x), La(.), Ce(A), Nd(O), Pr(+), Gd(o), Lu(v), Ho(@)).

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2 c ) . These r e s u l t s c a n b e i n t e r p r e t e d a s meaning t h a t Nd2Fe14B g i v e s r i s e t o a f i r s t o r d e r m a g n e t i z a t i o n p r o c e s (FOMP) c l o s e t o t h e t e m p e r a t u r e where t h e c h a n g e i n e a s y d i r e c t i o n o c c u r s . A s i m i l a r change i n t h e s h a p e of t h e SPD c u r v e s h a s a l s o been ob- s e r v e d i n Pr2Fel4B below 230 K , a l b e i t a l e s s pronounced c h a n g e t h a n i n Nd2Fe14B. No s u c h c h a n g e s have been found R2Fe14B compounds i n which R i s non-magnetic, s u g g e s t i n g t h a t t h e FOMP t r a n s i t i o n o r i g i n a t e s from t h e r a r e e a r t h s u b l a t t i c e m a g n e t i z a t i o n . The o c c u r r e n c e o f an e a s y c - a x i s m a g n e t i z a t i o n a t room t e m p e r a t u r e a s w e l l a s a t 4.2 K was p r o p o s e d f o r compounds R2Fel4B i n which R i s one of t h e h e a v y r a r e e a r t h m e t a l s Gd,Tb,Ho o r Dy / 1 , 4 / . T h e r e is a t most a s m a l l c o n t r i b u t i o n t o t h e a n i s o t r o p y by Gd, which c a n b e deduced from t h e f a c t t h a t HA i n Gd2Fel4B is a b o u t t h e same a s i n compounds where R is non-magnetic. I n t h e r e m a i n i n g compounds w i t h h e a v y rare e a r t h e l e - m e n t s t h e HA v a l u e s e x p e c t e d f o r t e m p e r a t u r e s below 300 K are t o o h i g h t o b e de- t e c t e d by o u r p u l s e d f i e l d e q u i p m e n t . T h i s is p r o b a b l y t h e r e a s o n t h a t a t t e m p t s t o measure t h e h i g h t e m p e r a t u r e b e h a v i o u r of HA(T) i n R2Fel4B w i t h R = Dy o r Tb were u n s u c c e s s f u l e v e n a t t e m p e r a t u r e s c l o s e t o t h e C u r i e t e m p e r a t u r e . R e s u l t s o b t a i n e d f o r Ho2Fel4B above room t e m p e r a t u r e a r e i n c l u d e d i n F i g . 1.

The c o m p o s i t i o n o f m a t e r i a l s u s e d i n permanent magnet f a b r i c a t i o n i s commonly c h o s e n t o be somewhat d i f f e r e n t (R15Fe77B8) from t h e s t o i c h i o m e t r i c c o m p o s i t i o n (R2Fe14B=

R12Feg2B6). I n o r d e r t o c h e c k w h e t h e r s u c h c o m p o s i t i o n a l c h a n g e s have a n y b e a r i n g o n t h e a n i s o t r o p y and i t s t e m p e r a t u r e dependence we i n c l u d e d i n o u r i n v e s t i g a t i o n mate- r i a l s w i t h n o n - s t o i c h i o m e t r i c c o m p o s i t i o n s . The t e m p e r a t u r e dependence o f HA i n R15Fe77B8 (R = Nd,Ce,Y) is compared w i t h t h a t i n t h e c o r r e s p o n d i n g s t o i c h i o m e t r i c m a t e r i a l s i n F i g . 3. The d a t a show t h a t moderate c h a n g e s i n c o m p o s i t i o n do n o t l e a d t o a d i f f e r e n t a n i s o t r o p y b e h a v i o u r . The SPD t e c h n i q u e p r e f e r e n t i a l l y s a m p l e s t h e m a g n e t i c p r o p e r t i e s o f m a g n e t i c a l l y h a r d g r a i n s p r e s e n t i n t h e m a t e r i a l , t h e s i m u l - t a n e o u s p r e s e n c e of one o r more i m p u r i t y p h a s e s w i t h weak m a g n e t i c p r o p e r t i e s b e i n g u n i m p o r t a n t . A p p a r e n t l y t h e a n i s o t r o p i c p r o p e r t i e s o f t h e s e h a r d m a g n e t i c g r a i n s a r e s i m i l a r f o r b o t h c o m p o s i t i o n s . S i n c e t h e a n i s o t r o p y c a n be e x p e c t e d t o depend s t r o n g - l y on t h e l o c a l e n v i r o n m e n t o f t h e a t o m s ( o f t h e r a r e e a r t h a t o m s i n p a r t i c u l a r ) t h e s e r e s u l t s i n d i c a t e t h a t t h e c o m p o s i t i o n o f t h e m a g n e t i c main p h a s e is n o t changed when c h a n g i n g t h e o v e r a l l c o m p o s i t i o n o f t h e m a t e r i a l .

F i g . 2

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F i e l d dependence o f d 2 ~ / d t 2 i n Nd2Fe14B a t T = 300 K ( a ) and T = 1 3 5 K ( b ) , and f i e l d dependence o f dM/dt a t T = 1 3 5 K ( c ) . For H i n c r e a s i n g l i n e a r l y w i t h t i m e t h e t i m e d e r i v a t i v e s of M c a n b e t a k e n t o r e p r e s e n t f i e l d d e r i v a t i v e s .

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C6-224 JOURNAL DE PHYSIQUE

Fig. 3 - Temperature dependence of the anisotropy field u0Ha in various R-Fe-B alloys Nd15Fe77B8 (+), Nd2Fe14B (o); Y1 5Fe77B8 (x), Y2Fe14B (&); Ce1 5Fe7 7B8 (•), Ce2Fe14B (o)

Table 1 : Comparison between the values of £l-3/8 (c/a)^] derived from the lattice constants a and c and the values of the anisotropy field H/\ in various R2Fei4B compounds.

Compound Lu2Fe14B Y2Fe14B Gd2Fei4B Ce2Fej.4B Th2Fe14B La2Fe14B

[1-3/8 (c/a)2j 0.302 0.293 0.291 0.284 0.278 0.167

M A

< T )

2.64 2.35 2.36 2.64 2.03 1.97

R valency 3+

3+

4+

3+

REFERENCES

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on Magn. MAG-20 (1984) 1584.

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and Sassik, H., Mat. Letters 2 (1984) 539.

/3/ Givord, D., Li, H.S., Moreau, J.M., Perrier de la BSthie, R. and Du Trgmolet de Lacheisserie, E., Proc. of REACIM (1984) St. Polten (Austria).

/ 4 / Sinnema, S., Radwanski, R.H., Franse, J.J.M., De Mooij, D.B. and Buschow, K.H.J., J. Magn. Magn. Mater. 44 (1984) 333.

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/(,/ Inomata, K., Jap. J. Appl. Phys. 15_ (1976T"821.

/!/ Szpunar, B. and Lindgard, P.A., J. Phys. F£ (1979) L 55.