MAGNETIC PROPERTIES AND KERR ROTATION IN AMORPHOUS Ce-Fe AND Pr-Fe ALLOYS

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MAGNETIC PROPERTIES AND KERR ROTATION IN AMORPHOUS Ce-Fe AND Pr-Fe ALLOYS

K. Buschow, P. van Engen

To cite this version:

K. Buschow, P. van Engen. MAGNETIC PROPERTIES AND KERR ROTATION IN AMOR- PHOUS Ce-Fe AND Pr-Fe ALLOYS. Journal de Physique Colloques, 1980, 41 (C8), pp.C8-650-C8- 653. �10.1051/jphyscol:19808163�. �jpa-00220264�

JOURNAL DE PHYSIQUE Colloque C8, suppl6ment au n08, Tome 41, ao2t 1980, pagecg-650

MAGNETIC PROPERTIES AND KERR ROTATION IN AMORPHOUS Ce-Fe A N D Pr-Fe A L L O Y S

K.H.J. Buschow and P.G. Van Engen

P h i l i p s Research Laboratories, Eindhoven, The Netherlands.

Abstract.- Several amorphous Ce-Fe and Pr-Fe alloys were prepared by means of vapour-deposition and their magnetic properties were studied. The Cel-,Fe, alloys are ferromagnetic with magnetic

ordering temperatures below room temperature, even for Fe concentrations as high as 90 at% Fe. In the Pr,-xFex alloys magnetic ordering appears to be more complex than ferromagnetic. Ifagnetic ordering temperatures are in excess of room temperature. At low temperatures the intrinsic coercive force is appreciable. At higher temperature it decreases strongly according to the relation

I ~ c - l a ~ . Due to the low ordering temperature the room temperature Kerr rotation is very low in ce~-~Fe,. Much higher values are observed in Prl-xFex. Here the concentration dependence of the Kerr rotation gives rise to a maximum near 80 at% Fe.

1. INTRODUCTION

Considerable attention has been paid in the last decade to thin films of amorphous alloys of rare earths and 3d transition metals. Several of these alloys have a combination of magnetic and magneto-optical properties f I j which make them materials suitable for magneto-optical reading and writing [2j. Most investigations have concentrated on alloys with heavy rare-earth metals. ~n'the present study we have extended the' investigation to some alloys with light rare-earth elements.

2. EXPERIMENTAL PROCEDURES AND RESULTS

The amorphous alloys were prepared by co-eva- porationqfthe pure metals onto quartz or alumi- nium substrates in a vacuum of bar. The absence of crystalline Fe or Fe compounds in the sample we checked by means of X-ray diffraction.

The temperature dependence of the magnetization (0) was determined by means of an adaptation of the Faraday method. Magnetic isotherms at various temperatures were obtained by means of a vibratipg sample magnetometer. The Kerr rotation 2(PK was studied at two different wavelengths (633 nm and 830 nm). In many cases measurements were made by using the reflecting surface of the film adja-

cent to the quartz substrate as well as the free surface of the metal film. The possibility of spurious effects due to surface absorption could be ruled out in this way.

Three amorphous Ce,-xFex alloys (x = 0.7 ,

0.8 and 0.9) were investigated. The results of the magnetic measurements are summarized in Fig.1.

All three alloys are ferromagnetic. As can be seen from the inset in Fig. 1 , the Curie temperature of these alloys is not very sensitive to the Fe con- centration and only slightly higher than 200 K.

The moment per Fe atom (M) was calculated under the assumption that Ce is tetravalent in these alloys

13).

.

The magnetic properties of five amorphous Prl-xFex alloys (x = 0.87, 0.80, 0.75, 0.70 and 0.50) were studied. Results for the x * 0.87 alloy are shown in Fig. 2. The shape of the O(T) curve of the other alloys is essentially the same. There

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19808163

F i g . 1. Temperature dependence of t h e magnetiza- t i o n of two amorphous Ce Fe a l l o y s measured i n a f i e l d of 9 kOe. The i n k g

SHOWS

180 - PI,-, Fe. _{z 2} - ^/

160- ^x = 0.87 140 -

LO.

20

F i g . 2. Temperature dependence of t h e magnetiza- t i o n of amorphous Pr13Fe87 i n t h r e e a p p l i e d f i e l d s . The i n s e t shows t h e c o n c e n t r a t i o n dependence of t h e moment p e r > e atom i n Prl-xFex.

i s almost no c o n c e n t r a t i o n dependence of t h e Curie t e m p e r a t u r e , which o c c u r s c l o s e t o 350 K i n a l l f i v e a l l o y s . The v a l u e s of t h e m a g n e t i z a t i o n a t 4.2 K measured w i t h a f i e l d of 18 kOe a p p l i e d p a r a l l e l t o t h e p l a n e of t h e f i l m a r e 136, 120, 87. 88 and 58 emu/g, r e s p e c t i v e l y . Furthermore t h e Prl-xFex a l l o y s appear t o b e r a t h e r hard mag- n e t i c a l l y . The i n t r i n s i c c o e r c i v e f o r c e (=Hc) i s p a r t i c u l a r l y h i g h i n t h e a l l o y s o f high P r concen- t r a t i o n and a t Sow t e m p e r a t u r e s . For t h e two a l l o y s o f extreme composition we s t u d i e d t h e tem-

p e r a t u r e dependence of IHc i n more d e t a i l . R e s u l t s a r e shown i n F i g . 3. The v a r i a t i o n of IHc w i t h temperature i s seen t o be u n u s u a l l y s t r o n g .

The Kerr r o t a t i o n v a s measured on e l e v e n d i f - f e r e n t amorphous P r -xFex a l l o y s ( 0 . 3 ,( x 6 0.87)

.

Alloys w i t h h i g h e r o r lower Fe c o n t e n t could n o t b e o b t a i n e d i n t h e amorphous s t a t e . R e s u l t s of t h e o p t i c a l measurements a r e shown i n F i g . 4 . I t can b e seen t h a t w i t h b o t h wavelengths a p p l i e d t h e Kerr r o t a t i o n g i v e s r i s e t o a maximum a t about 80 % Fe.

3. DISCUSSION

I t i s w e l l known t h a t t h e combination o f 3d m e t a l s and l e s s e l e c t r o r l e g a t i v e m e t a l s l e a d s t o a d e c r e a s e of t h e moment p e r 3d atom [ 3 ] . P a r t of t h i s decrease may b e due t o changes i n 3d band occupation r e s u l t i n g from charge t r a n s f e r e f f e c t s . There a r e i n d i c a t i o n s t h a t t h e main e f f e c t i s due a r e d u c t i o n i n t h e i n t r a - a t o m i c Coulomb r e p u l s i o n between t h e 3d e l e c t r o n s caused by t h e presence of t h e non-3d atoms. I n t h e c a s e of amorphous Fe a l l o y s experiments have shown t h a t a l l o y i n g w i t h Y o r Th l e a d s t o a g r a d u a l d e c r e a s e of t h e Fe moments w i t h d e c r e a s i n g Fe c o n t e n t and f i n a l l y t o a disappearance of t h e Fe moments a t c o n c e n t r a t i o n s below about 30 a t % Fe [5]. Amorphous Co a l l o y s show e s s e n t i a l l y t h e same behaviour r7J. I n t h i s r e s p e c t t h e s t r o n g r e d u c t i o n i n Fe moment i n t h e r a t h e r Fe-rich Cel-*Fex a l l o y s d e s c r i b e d above i s s u r p r i s i n g . The same h o l d s f o r t h e magnetic o r d e r - i n g t e m p e r a t u r e , which a l r e a d y f a l l s below room t e m p e r a t u r e a t Fe c o n c e n t r a t i o n s a s h i g h a s 90

%.

Presumably t h i s stems from t h e a b i l i t y of t h e Ce atoms t o adopt an abnormal v a l e n c e ( 4 + ) . When com- b i n e d w i t h 3d m e t a l s t h e Ce atoms can l o s e t h e i r l o c a l i z e d 4f e J e c t r o n t o t h e 5d conduction band.

c8-652 JOURNAL DE PHYSIQUE

Fig. 3. Temperature dependence of t h e i n t r i n s i c c o e r c i v e f o r c e i n two amorphous P r Fe a l l o y s . The i n s e t shows t h e h y s t e r e s i s loo$-gf ? r ~ e measu- r e d a t 60 K.

. ,

- - .

.

h = 633 nm; f u l l symboHshX= 830 nm.

H y b r i d i z a t i o n of 5d s t a t e s of t h e r a r e e a r t h atoms with t h e 3d s t a t e s of t h e Fe atom may b e p a r t i - c u l a r l y pronouced i n Ce a l l o y s and can t h u s account f o r s t r o n g l y reduced 3d e l e c t r o n magnetism a s compared w i t h o t h e r amorphous r a r e - e a r t h i r o n a l l o y s [5,6).

-

phous a l l o y s of Fe w i t h t e t r a v a l e n t Th o r t r i v a - l e n t Y shows t h a t it i s not merely t h e d e v i a t i n g v a l e n c e of Ce which causes t h e a d d i t i o n a l e f f e c t .

The Fe moments i n t h e amorphous Prl-xFex a l - l o y s a r e l e s s e a s i l y o b t a i n e d from t h e experimen- t a l d a t a s i n c e t h e P r atoms a l s o c a r r y a moment.

Due t o t h e e l e c t r o s t a t i c : charges of t h e surroun- d i n g i o n s c r y s t a l l i n e e l e c t r i c f i e l d e f f e c t s (CEF) w i l l reduce t h e 4f moment of P r from i t s f r e e i o n v a l u e ( 3 . 2 / u g / P r ) . I n p r a c t i c e a v a l u e between

l / % and 2/% i s observed !8!.

. -

of 1..5/%/Pr and a f e r r o m a g n e t i c coupling between t h e P r and Fe moments [ 3 3 t h e c o n c e n t r a t i o n de- pendence of t h e Fe moments i n P r Fe i s r e p r e -

1-x x s e n t e d i n t h e i n s e t of F i g . 2. I n view of t h e u n c e r t a i n t y of t h e P r moments t h i s c o n c e n t r a t i o n dependence of t h e Fe moments should b e regarded a s t e n t a t i v e . Although s l i g h t l y s t e e p e r , t h e Fe mo- ment r e d u c t i o n i n t h i s s e r i e s of a l l o y s seems not much d i f f e r e n t from t h a t i n Yl-xFex a l l o y s , f o r i n s t a n c e .

The p r e s e n c e o f CEF e f f e c t s f u r t h e r m o r e im- p l i e s a s t r o n g magnetic a n i s o t r o p y , which i n t u r n i s r e s p o n s i b l e f o r t h e l a r g e i n t r i n s i c c o e r c i v e f o r c e observed. I n p a r t , t h e s t r o n g temperature dependence of can e x p l a i n t h e remarkable tem- p e r a t u r e dependence of t h e m a g n e t i z a t i o n i n Prl-xFex when t h e a p p l i e d f i e l d i s s m a l l . F o r i n s t a n c e , i n t h e lowest curve shown i n F i g . 2 t h e f i e l d a p p l i e d approaches t h e c o e r c i v e f o r c e only a t about 20 K ( s e e Fig. 3 ) . Below t h i s tempera- t u r e t h e f i e l d a p p l i e d w i l l h a r d l y b e a b l e t o move t h e domain w a l l s and r e v e r s e t h e wrongly orien-' t e d domains. I n low a p p l i e d f i e l d s and' a t low t e m p e r a t u r e s t h e magnekization has t h e r e f o r e t h e tendency t o v a n i s h . It i n c r e a s e s i n accordance w i t h t h e d e c r e a s i n g c o e r c i v e f o r c e when t h e tem- p e r a t u r e i s r a i s e d . The maximum a t about 230 K

F i g . 5. I n v e r s e c o e r c i v e f o r c e a s a f u n c t i o n of t e ~ a p e r a t u r e of two amorphous Prl-xFex a l l o y s .

i n t h e lowest o ( T ) curve shown i n Fig. 2 i s o f a d i f f e r e n t o r i g i n . I n t h e same manner a s i n YlwxFex a l l o y s one can argue t h a t t h e random atomic arrangement e n t a i l s a d i s t r i b u t i o n of d i f f e r e n t Fe-Fe n e a r e s t neighbour d i s t a n c e s which l e a d s t o a corresponding d i s t r i b u t i o n i n p o s i t i v e and nega- t i v e magnetic c o u p l i n g s between t h e Fe moments [51.

A t high t e m p e r a t u r e s , where t h e P r moments a r e n o t y e t o r d e r e d m a g n e t i c a l l y , and i n z e r o o r low a p p l i e d f i e l d s t h e Fe s u b l a t t i c e w i l l behave l i k e a s p i n g l a s s r a t h e r t h a n a ferromagnet. The maximum i s t h e n t o b e a s s o c i a t e d w i t h t h e f r e e z i n g t e m p e r a t u r e of t h e s p i n g l a s s .

The o b s e r v a t i o n of a s t r o n g l y t e m p e r a t u r e dependent c o e r c i v e f o r c e i n Prl-xFex i s i n keeping w i t h t h e r e s u l t s of p r e v i o u s i n v e s t i g a t i o n s of amorphous r a r e - e a r t h a l l o y s r 9 ) . F u r t h e r a n a l y s i s o f t h e t e m p e r a t u r e dependence of IHc i n Prl-xFex

shows t h a t t h i s temperature dependence can be

1 1 0

expressed a t /Hc = /Hc + cT ( s e e F i g . 5 ) . This behaviour i s remarkable s i n c e i n p r e v i o u s i n v e s t i -

g a t i o n s [9,10] of amorphous r a r e - e a r t h a l l o y s we observed an e x p o n e n t i a l behaviour of t h e form

H CA exp(- a T ) . The o r i g i n of t h e two t y p e s I c

of T dependences i s s t i l l u n c l e a r and needs f u r t h e r i n v e s t i g a t i o n .

I t h a s been shown i n F i g . 4 t h a t t h e Kerr r o t a t i o n i n t h e amorphous system Prl-xFex g i v e s r i s e t o a maximum n e a r 80 a t % F e . The c u r r e n t l y a v a i l a b l e experimental information i s i n s u f f i c i e n t t o t r a c e t h e o r i g i n of t h i s maximum. Our r e s u l t s show, however, t h a t t h e Kerr r o t a t i o n i s not simply p r o p o r t i o n a l t o t h e Fe moment s i n c e no such

maximum appears i n t h e c o n c e n t r a t i o n dependence o f t h e l a t t e r q u a n t i t y .

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F.E. ~ u b o r s k y , H.H. Liebermann, J. J. Becker and J.L. Walter i n : Rapidly quenched Metals 111 ,Cameleon P r e s s , London ( 1978), B. Cantor,Ed.

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