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ON THE ANISOTROPY IN AMORPHOUS Gd-BASED ALLOYS
H. Algra, K. Buschow, R. Henskens
To cite this version:
H. Algra, K. Buschow, R. Henskens. ON THE ANISOTROPY IN AMORPHOUS Gd-BASED AL- LOYS. Journal de Physique Colloques, 1980, 41 (C8), pp.C8-646-C8-649. �10.1051/jphyscol:19808162�.
�jpa-00220263�
JOURNAL DE PHYSIQUE CoZZoque C8, suppZ6ment au n08, Tome 41, aoct 1980, page C8-646
ON THE ANISOTROPY I N AMORPHOUS Gd-BASED ALLOYS H.A. Algra, K.H.J. Buschow and R.A. Henskens
Phizips Research Laboratories, Eindhoven, The Netherlands
Abstract.- The magnetization as a function of temperature and the uniaxial anisotropy of a series of amorphous Gdx C O , - ~ alloys, with x >, 0.45 are reported. Two GdX CU!-~ alloys were studied at the same time to distinguish the influence of the cobalt atoms. The uniaxlal anisotropy increases strongly for x + 0 . 7 4 . This result can not be explained with calculations of dipolar interactions due to a microstructure in amorphous alloys. The gadolinium is thought to contribute mainly to the anisotropy, whereas the Co-Co pairs are of less importance.
I n t r o d u c t i o n
I n t h i s p a p e r we r e p o r t b r i e f l y on a s t u d y o f t h e m a g n e t i c p r o p e r t i e s o f Gd- -based amorphous a l l o y s . Numerous p a p e r s have been p u b l i s h e d by d i f f e r e n t a u t h o r s on t h e amorphous GdxCol-x a l l o y s , which a r e o f t e c h n i c a l i m p o r t a n c e . These i n v e s t i g a - t i o n s r e v e a l e d t h a t t h e c o n t r o l o v e r t h e magnetic p r o p e r t i e s o b t a i n e d i n d i f f e r e n t - l y p r e p a r e d samples i s v e r y c o m p l i c a t e d . The r e l a t i o n between t h e u n i a x i a l a n i s o - t r o p y and 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 was found t o b e o f t h e p s e u d o - d i p o l a r t y p e ( 1 ) . The c o u p l i n g c o n s t a n t s i n v o l v e d , however, t u r n e d o u t t o depend s t r o n g l y on t h e compo- s i t i o n and t h e method of p r e p a r a t i o n . Re- c e n t l y Mizoguchi e t a l . ( 2 ) p u b l i s h e d a p a p e r i n which t h e y d e r i v e d a s i m p l e ex- p r e s s i o n f o r t h e u n i a x i a l a n i s o t r o p y due t o c l a s s i c a l d i p o l a r i n t e r a c t i o n s , t a k i n g i n t o a c c o u n t t h e o c c u r r e n c e o f a p a r t i c u - l a r a t o m i c arrangement o r m i c r o s t r u c t u r e i n t h e s e a l l o y s . A s t h e d a t a p u b l i s h e d s o f a r on t h e amorphous GdxGolmx a l l o y s were r e s t r i c t e d t o t h e s m a l l r e g i o n
0.15
<
x<
0.45, we d e c i d e d t o e x t e n d t h e i n v e s t i g a t i o n s t o gadolinium r i c h a l l o y s i n o r d e r t o g e t e x p e r i m e n t a l i n f o r m a t i o n i n a w i d e r i n t e r v a l . A t t h e same t i m e we a l s o s t u d i e d two GdxCul-x a l l o y s i n o r d e r t o d i s t i n g u i s h t h e p o s s i b l e i n f l u e n c e o f t h e Co atoms.E x p e r i m e n t a l
The amorphous a l l o y s were p r e p a r e d by a r c m e l t i n g f o l l o w e d by m e l t s p i n n i n g i n an atmosphere o f p u r i f i e d argon. X-ray d i f f r a c t i o n , u s i n g CuK, r a d i a t i o n , was performed on s m a l l p a r t s o f t h e r i b b o n s . I n a l l c a s e s t h e X-ray diagrams c o n s i s t e d o f d i f f u s e h a l o s . Sharp r e f l e c t i o n l i n e s , which would i n d i c a t e t h e p r e s e n c e o f c r y s t a l l i n e m a t e r i a l , were n o t found. The s a t u r a t i o n m a g n e t i z a t i o n a s a f u n c t i o n o f t e m p e r a t u r e was d e t e r m i n e d w i t h a v i b r a t i n g sample magnetometer. The magnetic f i e l d s t r e n g t h c o u l d be v a r i e d up t o 19 kOe.
F e r r o m a g n e t i c resonance (FMR) e x p e r i m e n t s were performed on a c o n v e n t i o n a l s p e c t r o - m e t e r o p e r a t i n g a t
-
10 GHz, i n magnetic f i e l d s up t o 10 kOe.R e s u l t s
The m a g n e t i z a t i o n measurements on b o t h GdxCol-, and GdxCul-, showed t h a t when t h e m a g n e t i c f i e l d was a p p l i e d i n t h e p l a n e o f t h e r i b b o n o n l y s m a l l f i e l d s
(
<
700 Oe) were needed t o s a t u r a t e t h esample. S a t u r a t i o n c o u l d n o t b e r e a c h e d , however, when t h e magnetic f i e l d ( u p t o
19 kOe) was a p p l i e d p e r p e n d i c u l a r t o t h e p l a n e o f t h e r i b b o n . A s an example w e show i n f i g . 1 t h e m a g n e t i z a t i o n o f Gd60C040 as a f u n c t i o n of t e m p e r a t u r e i n t h e p a r a l l e l c o n f i g u r a t i o n . A t T=O K , assuming a z e r o moment f o r Co, t h e Gd
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19808162
-
T [ K IF i g . 1. : M a g n e t i z a t i o n o f Gd Co a s a f u n c t i o n o f t e m p e r a $ 8 r e q 0 f o r H i n t h e p l a n e of t h e r i b b o n . moment i s s l i g h t l y i n e x c e s s o f 7 ) . I ~ / G ~ , which v a l u e i s t o b e e x p e c t e d f o r a p u r e
S - s t a t e Gd i o n . F o r T
>
100 K t h e magne- t i z a t i o n becomes f i e l d d e p e n d e n t , i n d i c a - t i n g t h a t n o t a l l s p i n s a r e f u l l y a l i g n e d , which e f f e c t i n c r e a s e s d r a s t i c a l l y w i t hi n c r e a s i n g t e m p e r a t u r e . The C u r i e tep-pe- r a t u r e i s e s t i m a t e d from a M~ v e r s u s T p l o t a s Tc = 193 K . The Gd- Cul-x magne- t i z a t i o n c u r v e s e x h i b i t e d a f i e l d depen- dence o v e r t h e whole t e m p e r a t u r e r a n g e . T h i s d e m o n s t r a t e s t h a t Co i n f l u e n c e s t h e m a g n e t i c i n t e r a c t i o n c o n s t a n t . A summary o f t h e m a g n e t i c d a t a i s g i v e n i n t a b l e I . T a b l e I : Summary o f t h e m a g n e t i c d a t a .
4 T M I ~ G I ~ I T M I ~ G ] T ~ I X I x,, [GI g K " [ * I o ~
3 1
4 . 2 K 7 7 X
Note t h a t 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 s o f t h e GdxCul,x a l l o y s a r e a l m o s t e q u a l t o t h e m a g n e t i z a t i o n s o f t h e comparable Gd,Col-, a l l o y s .
I n t h e FMR e x p e r i m e n t s , t h e s a m p l e s u s e d h a d t h e t y p i c a l d i m e n s i o n s o f 4x2 mm2, w h i l e t h e t h i c k n e s s was a b o u t
20
7
m. The s a m p l e s were mounted i n a TElO2 c a v i t y a t t h e p o s i t i o n o f maximum f i e l d s t r e n g t h . The m a g n e t i c f i e l d c o u l d b e a p p l i e d i n d i r e c t i o n s v a r y i n g from p a r a l l e l (H ) t o p e r p e n d i c u l a r (HI) t o t h e p l a n e o f t h e r i b b o n . However, f o rN
H, no r e s o n a n c e phenomenon was o b s e r v e d owing t o t h e h i g h m a g n e t i z a t i o n s i n t h e s e a l l o y s . A l t h o u g h , due t o t h e s k i n e f f e c t , t h e e x c i t i n g r . f . f i e l d i n t h e sample i s r a t h e r inhomogeneous, c l e a r r e s o n a n c e s were o b s e r v e d , a d e s c r i p t i o n o f which i s g i v e n e l s e w h e r e ( 3 , 4 ) . R o t a t i o n o f t h e m a g n e t i c f i e l d i n t h e p l a n e o f t h e r i b b o n , y i e l d s no c h a n g e s i n r e s o n a n c e f i e l d po- s i t i o n n o r i n l i n e w i d t h , i n d i c a t i n g t h a t a n a x i a l symmetry e x i s t s .From t h e r e s o n a n c e d a t a t h e u n i a x i a l a n i s o t r o p y a n d e f f e c t i v e g - v a l u e s c a n b e o b t a i n e d by s o l v i n g n u m e r i c a l l y t h e f o l - l o w i n g e q u a t i o n s :
~ ; f ~
( y
) = {H c o s (ff-f30)+2r c o s*ef f
{H C 0 s
(0-eO)
+2-5i-- U C O s 2 go} I (1)HM s i n ($-e0) = K E s i n ~ ~2 Bo ( 2 ) Eq. ( 1 ) and ( 2 ) d e s c r i b e t h e r e s o n a n c e f o r H a n d M making a n a n g l e B a n d go w i t h t h e normal t o t h e r i b b o n . E q . ( 2 ) d e f i n e s t h e e q u i l i b r i u m p o s i t i o n of t h e m a g n e t i z a t i o n M w i t h r e s p e c t t o t h e a p p l i e d f i e l d H.
I n t h e s e e q u a t i o n s cd d e n o t e s t h e r e s o - n a n c e f r e q u e n c y , t h e g y r o m a g n e t i c r a t i o a n d K E f t h e e f f e c t i v e u n i a x i a l a n i s o t r o - py, which i s d e f i n e d a s
K e f f 2
u s i n 2 9; = (K;ZT~M ) s i n 2 go. F o r o u r s a m p l e s t h e d e m a g n e t i z i n g f i e l d h a s b e e n t a k e n a s 4 q M , i n a g r e e m e n t w i t h t h e
s h a p e o f t h e r i b b o n s . T h i s i s a r e a s o n e b l e a s s u m p t i o n s i n c e t h e r i b b o n s do n o t ex- h i b i t a p a r t i c u l a r m i c r o s t r u c t u r e l i k e e . g . t h e p i l l a r s , a s f o u n t i n e v a p o r a t e d f i l m s (5)
.
The e x i s t e n c e o f such s t r u c - t u r e would i n f l u e n c e t h e d e m a g n e t i z i n g f i e l d c o n s i d e r a b l y . The K U a n d , g - v a l u e s deduced fr0.m o u r e x p e r i m e n t s a t T=77 K a r e p r e s e n t e d i n t a b l e I.C8-648 JOURNAL DE PHYSIQUE
D i s c u s s i o n
R e c e n t l y Mizoguchi e t a l . ( 2 ) have p u b l i s h e d a p a p e r i n which t h e y c a l c u l a t e d t h e c o n t r i b u t i o n t o KU of c l a s s i c a l d i - p o l a r i n t e r a c t i o n s . T h i s r e l a t i o n r e a d s :
where t h e c o n s t a n t A i n c l u d e s t h e measure p o f a l i g n m e n t o f u n l i k e n e a r e s t n e i g h b o u r p a i r s , a " p a c k i n g f r a c t i o n " and o t h e r s t r u c t u r a 1 c o n s t a n t s . x a n d M d e n o t e
A I B A I B t h e a t o m i c f r a c t i o n s and t h e s u b l a t t i c e m a g n e t i z a t i o n s , r e s p e c t i v e l y . The l a t t e r q u a n t i t i e s a r e r e l a t e d t o t h e a t o m i c mo- ments m v i a t h e r e l a t i o n s M =
A l B A 1 B
X A , B NmABB (N i s Avogrado's number). I n t h e s e c a i c u l a t i o n s it was assumed t h a t t h e m a g n e t i z a t i o n i n t h e s e a l l o y s was made up by two o p p o s i t e l y d i r e c t e d s u b l a t t i c e m a g n e t i z a t i o n s . With e q . ( 3 )
,
u s i n g s m a l l p o s i t i v e v a l u e s f o r p , t h e a u t h o r s were a b l e t o e x p l a i n t h e a n i s o t r o p i e s o f K5 U
10 erg/cm3 f o r x
-
0.3-0.4. Followingeq. ( 3 ) , K U would h a v e an extreme v a l u e f o r x r 0 . 5 . I n f i g . 2 we have p l o t t e d t h e K U v a l u e s deduced from o u r FMR e x p e r i m e n t s a t T=77K, a s a f u n c t i o n of x . I n c l u d e d
F i g . 2. : K as a f u n c t i o n o f composi- tYon of amorphous Gd Co a l l o y s .
+
d a t a from %ef!-g.4
-
d a t a o f t h e amorphous GdxCul-x a l l o y s .a r e a l s o t h e d a t a o f Ref. 6. Although t h e l a t t e r a r e o b t a i n e d a t room tempera- t u r e , j u s t i f i c a t i o n f o r a comparison i s
g i v e n by t h e f a c t t h a t b o t h s e t s o f d a t a p e r t a i n t o a b o u t t h e same reduced tempe- r a t u r e ( T
<
0.5 T c ).
KU i s o b s e r v e d t o i n c r e a s e s t e e p l y , e s p e c i a l l y f o r x > 0.65 and r e a c h e s K u = 4x10 6 erg/cm3 f o rGd74C026 ( t h e u l t i m a t e Gd c o n c e n t r a t i o n r e a c h e d w i t h o u r t e c h n i q u e w i t h o u t t h e sample becoming c r y s t a l l i n e ) . C o n f i r m a t i o n o f such h i g h a n i s o t r o p i e s was found i n a t o r q u e e x p e r i m e n t w i t h an a p p l i e d f i e l d o f 2 4 kOe. I n t h e a n a l y s i s o f t h e t o r q u e c u r v e a l s o t h e t e r m IZL2) s i n 4
e
was t a k e n i n t o a c c o u n t . I t t u r n e d o u t , however, t h a t t h e l a t t e r t e r m i s an o r d e r o f magnitude s m a l l e r t h a n t h e~z~~
s i n 2 0 term. The Ku v a l u e f o r e . g . G d 7 4 C ~ 2 6 i s w i t h i n 10%e q u a l t o t h e r e s u l t o f t h e FMR e x p e r i m e n t . The q u e s t i o n a r i s e s i f t h i s s t r o n g i n - c r e a s e o f K U can b e e x p l a i n e d w i t h d i p o l a r i n t e r a c t i o n s , u n l e s s p depends s t r o n g l y on t h e c o m p o s i t i o n and moreover, p must b e v e r y much l a r g e r i n t h e g a d o l i n i u m r i c h a l l o y s t h a n i n t h e r e g i o n f o r x 0 . 4 . E m p i r i c a l l y K U i s p r o p o r t i o n a l t o x 2
,
a t l e a s t i n t h e r a n g e where KU i s s t r o n g l y c o n c e n t r a t i o n dependent. The magnetiza- t i o n depends l i n e a r l y on t h e c o n c e n t r a - t i o n f o r c o n s t a n t t e m p e r a t u r e ( s e e t a b l e I ) . Thus, phenomenologically w e may w r i t e f o r x>
0 . 5 , K U ~ M 2.
T h i s r e s u l t a g r e e s w i t h e a r l i e r o b s e r v a t i o n s ( 6 ) .I n c l u d e d i n f i g . 2 a r e a l s o t h e r e s u l t s on t h e G ~ , C U ~ - ~ a l l o y s . I t i s s u r p r i s i n g t h a t KU i s o f t h e same o r d e r o f magnitude a s i n t h e Co-alloys. T h i s i n d i c a t e s t h a t t h e magnetic moment o f t h e t r a n s i t i o n m e t a l atoms does n o t p l a y a p a r t . T h i s i s i n agreement w i t h e a r l i e r i n v e s t i g a t i o n s
( 6 , 7 ) , i n which it was r e p o r t e d t h a t w i t h i n c r e a s i n g g a d o l i n i u m c o n t e n t t h e c o b a l t moment d e c r e a s e s .
I n c o n c l u s i o n , w e have found t h a t t h e a n i s o t r o p y of GdxCol-x amorphous a l l o y s i n c r e a s e s s t r o n g l y f o r x -0.74.
A t c o n s t a n t t e m p e r a t u r e , i n t h i s compo- s i t i o n r a n g e , t h e u n i a x i a l a n i s o t r o p y Yaries w i t h t h e m a g n e t i z a t i o n s q u a r e d . Comparison w i t h t h e r e s u l t s on GdxCul-x a l l o y s i n d i c a t e s t h a t it i s m a i n l y t h e g a d o l i n i u m t h a t c o n t r i b u t e s t o t h e a n i s o -
t r o p y , making t h e o c c u r r e n c e o f CO-Co p a i r s r a t h e r u n i m p o r t a n t .
Acknowledgement
H e l p f u l l d i s c u s s i o n s w i t h D r s . J.W.
S m i t s a n d R.P. v a n S t a p e l e are g r a t e f u l l y acknowledged.
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