AMORPHOUS RARE EARTH-TRANSITION METAL FILMS FOR MAGNETO-OPTICAL STORAGE

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FILMS FOR MAGNETO-OPTICAL STORAGE

H. Heitmann, M. Hartmann, M. Rosenkranz, H. Tolle

To cite this version:

H. Heitmann, M. Hartmann, M. Rosenkranz, H. Tolle. AMORPHOUS RARE EARTH-TRANSITION

METAL FILMS FOR MAGNETO-OPTICAL STORAGE. Journal de Physique Colloques, 1985, 46

(C6), pp.C6-9-C6-18. �10.1051/jphyscol:1985602�. �jpa-00224840�

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JOURNAL

DE PHYSIQUE

Colloque C6, supplkment au n09, T o m e 46, septembre 1985 page C6-9

AMORPHOUS RARE EARTH-TRANSITION METAL F I L M S FOR MAGNETO-OPTICAL STORAGE

H. Heitmann, M. Hartmann, M. Rosenkranz and H.J. Tolle Philips Research Laboratory Hamburg, 2000 Hamburg, F.

R.

G.

fGsum6 - Les propristss magndto-optiques des films amorphes terres rares- mdtaux de transition sont trOs prometteuses pour la rdalisation

d'enregistrements optiques effafables. Les meilleures propristds

d'enregistrement sont obtenues dans les alliages ternaires ou quaternaires, dans la gamme de compositions (Gd, Tb) (Fe, Co). Un choix adapts du rapport Gd/Tb permet droptimiser les propriltds magnstiques conduisant 1 une forte densitd d'enregistrement et 1 une stabilitg de l'information. Pour un taux 3 peu prZs 6gal de Fe et Co les propristss magndto-optiques sont maximum.

Cependant, il reste des problOmes concernant la stabilits des films minces en raison de l'oxydation des alliages terres rares-mdtaux de transition. Ceci est la principale cause du vieillissement qui d6pend beaucoup de la microstructure des films.

Des films Gd Tb Co ont St6 obtenus par pulvdrisation en utilisant une diode RF, et en appliquant une tension "bias" au substrat. Pour une tension nsgative de -100V

a

-2OOV, l'anisotropie uniaxiale est maximale. Cependant, ces films sont totalement oxydds en un temps court aprOs exposition 3 l'air, sans couche protectrice. Ceci a st6 attribus 1 la forte teneur en argon des films. En effet, des films presentant une aimantation perpendiculaire peuvent aussi dtre obtenus, sans tension "bias", grbce h un choix approprig de la pression de Ar d'environ 2 Pa. Ces films prssentent une bien meilleure rssistance 2

l'oxydation en raison de leur faible concentration en Argon.

A b s t r a c t

-

Amorphous magneto-optic (m.0.) r a r e e a r t h - t r a n s i t i o n metal (RE-TM) f i l m s are t h e most p r o m i s i n g candidates f o r e r a s a b l e o p t i c a l s t o r a g e t o date.

The best storage p r o p e r t i e s are found i n t e r n a r y o r q u a t e r n a r y a l l o y s o u t o f t h e c o m p o s i t i o n a l range (Gd,Tb)(Fe,Co). A s u i t a b l e c h o i c e o f t h e Gd/Tb r a t i o p e r m i t s t o o p t i m i z e t h e magnetic p r o p e r t i e s y i e l d i n g h i g h storage d e n s i t y and h i g h i n f o r m a t i o n s t a b i l i t y . At about equal amounts o f Fe and Co t h e magneto- o p t i c a l e f f e c t s are maximum. However, t h e r e are s t i 11 some problems concerning t h e s t a b i l i t y o f t h e t h i n f i l m s which are a s s o c i a t e d w i t h t h e o x i d a t i o n o f t h e RE-TM a l l o y s . T h i s i s t h e main reason f o r aging. The aging behavior o f t h e RE-TM f i l m s depends s t r o n g l y on t h e i r m i c r o s t r u c t u r e .

GdTbCo f i l m s were s p u t t e r e d using an RF diode c o n f i g u r a t i o n a t which v a r y i n g DC b i a s v o l t a g e s were a p p l i e d t o t h e s u b s t r a t e . At n e g a t i v e b i a s v o l t a g e s i n t h e range between -100V and -200V t h e u n i a x i a l a n i s o t r o p y was maximum. How- ever, these f i l m s o x i d i z e d t o t a l l y i n a s h o r t t i m e when exposed t o a i r w i t h o u t p r o t e c t i v e l a y e r s . T h i s was a t t r i b u t e d t o t h e l a r g e argon c o n c e n t r a t i o n i n t h o s e f i lms. Indeed, f i l m s w i t h p e r p e n d i c u l a r m a g n e t i z a t i o n can a l s o be sput- t e r e d w i t h o u t b i a s v o l t a g e choosing a proper Ar-pressure o f about 2 Pa. Con- t r a r y t o b i a s s p u t t e r e d f i l m s , these much b e t t e r w i t h s t a n d o x i d a t i o n due t o t h e i r low argon c o n c e n t r a t i o n .

I

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INTRODUCTION

O p t i c a l r e c o r d i n g i s a s t i l l emerging storage t e c h n i q u e /1-4/. Recently e r a s a b l e o p t i c a l r e c o r d i n g a t h i g h d a t a r a t e s on h i g h c a p a c i t y storage media has been demon- s t r a t e d

/3/.

Various k i n d s o f magnetic and non-magnetic m a t e r i a l s have been used

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985602

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131, e.g. t h i n f i l m s based on t e l l u r i u m suboxide where t h e phase change between t h e amorphous and t h e c r y s t a l l i n e s t a t e r e s u l t i n g i n a r e f l e c t i v i t y change i s u t i l i z e d 151.

The most advanced e r a s a b l e o p t i c a l s t o r a g e medium t o d a t e i s founded on magneto-op- t i c a l f i l m s 161. M a g n e t o - o p t i c a l s t o r a g e t e c h n i q u e s combine t h e m e r i t s o f both mag- n e t i c and o p t i c a l storage, i . e . p r i n c i p a l l y u n l i m i t e d r e v e r s i b i l i t y w i t h h i g h b i t d e n s i t y , c o n t a c t l e s s w r i t e , read, and erase o p e r a t i o n s . D i g i t a l i n f o r m a t i o n i s s t o r e d as a sequence o f small magnetic domains; w r i t i n g and e r a s u r e i s performed b y h e a t i n g t h e s t o r a g e l a y e r by t h e focussed l a s e r beam (e.g. AlGaAs l a s e r ) i n combina- t i o n w i t h a magnetic f i e l d . N o n - d e s t r u c t i v e read o u t i s accomplished t h r o u g h sensing p o l a r i z a t i o n changes i n t h e r e f l e c t e d l a s e r beam induced by magneto-optical e f f e c t s i n t h e s t o r a g e m a t e r i a l /7,8/.

The h i g h s t o r a g e d e n s i t y i n h e r e n t t o o p t i c a l s t o r a g e can o n l y be e x p l o i t e d i f t h e m a g n e t o - o p t i c a l f i l m s m a i n t a i n c y l i n d r i c a l domains. T h i s r e q u i r e s t h a t t h e f i l m s e x h i b i t an easy a x i s o f m a g n e t i z a t i o n p e r p e n d i c u l a r t o t h e f i l m , i.e. t h e i r 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 p y constant, Keff=KU-po~P/2, must be p o s i t i v e . Ku denotes t h e growth induced u n i a x i a l a n i s o t r o p y c o n s t a n t and t h e second term t h e energy den- s i t y o f t h e demagnetizing f i e l d o f t h e p e r p e n d i c u l a r l y s a t u r a t e d f i l m . I n o r d e r t o o b t a i n a h i g h s t o r a g e d e n s i t y , i.e. t o s t a b i l i z e small domains, t h e c o e r c i v e energy d e n s i t y , Ec=uoMSHc, has t o be l a r g e /9,10/ Ec i s t h e p r o d u c t o f t h e s a t u r a t i o n i n - d u c t i o n , uoMs, and t h e c o e r c i v e f i e l d , Hc.

The read-out s i g n a l depends on t h e magneto-optical e f f i c i e n c y o f t h e s t o r a g e medium,

~ 0 where R i s t h e r e f l e c t i v i t y and 0 t h e m a g n e t o - o p t i c a l r o t a t i o n , as w e l l as on ~ , t h e r a t i o o f t h e domain diameter and t h e l a s e r focus. Thus, t h e read-out c a r r i e r t o n o i s e r a t i o (CNR) depends not o n l y on R O ~ , b u t a l s o on t h e c o e r c i v e energy d e n s i t y t h a t determines t h e s i z e s t a b i l i t y o f t h e domains. The l a t t e r has t o be m a i n t a i n e d a t temperature v a r i a t i o n s o c c u r i n g i n a p r a c t i c a l environment and a l s o a t t h e tempe- r a t u r e s determined by t h e read-out power l e v e l .

The r e q u i r e m e n t s on a m a g n e t o - o p t i c a l s t o r a g e m a t e r i a l 111,121 are met by t h e amorphous heavy r a r e e a r t h - t r a n s i t i o n metal (RE-TM) a l l o y s . These are m o s t l y based on t h e f e r r i m a g n e t i c t e r n a r y o r q u a t e r n a r y a1 l o y s o f c o m p o s i t i o n (Gd,Tb)x(Fe,Co)l-x /6,11-20/. The p r o p e r t i e s d e t e r m i n i n g t h e s t o r a g e c h a r a c t e r i s t i c s o f t h e magneto- o p t i c a l media are d i s c u s s e d i n s e c t i o n 11.

Problems s t i l l a r i s e c o n c e r n i n g t h e long term s t a b i l i t y owing t o t h e o x i d a t i o n o f t h e RE-TM f i l m s g i v i n g r i s e t o a r e d u c t i o n o f 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 c o e r - c i v e energy d e n s i t y 1131. T h i s can be g r e a t l y reduced by e n c l o s i n g t h e f i l m s between d i e l e c t r i c l a y e r s t h a t a l s o serve f o r t h e enhancement o f t h e magneto-optical e f f i c i - ency /13,15,20/. Nevertheless, t h e c o r r o s i o n r e s i s t a n c e of t h e magneto-optic f i l m s themselves has t o be o p t i m i z e d . The s t a b i l i t y o f t h e RE-TM f i l m s depends n o t o n l y on t h e c o m p o s i t i o n 1211 b u t a l s o on t h e i r m i c r o s t r u c t u r e 122,231. The l a t t e r i s r e l a t e d t o t h e p r e p a r a t i o n t e c h n i q u e 122,241 and t h e p r e p a r a t i o n parameters 122,231. The Co a l l o y s are s a i d t o be l e s s s e n s i t i v e t o aging t h a n t h e F e - a l l o y s 1121. I n s e c t i o n I 1 1 t h e dependence o f t h e aging b e h a v i o r on t h e p r e p a r a t i o n parameters o f rf d i o d e s p u t t e r e d GdTbCo f i l m s i s discussed. It w i l l be shown t h a t optimum f i l m p r o p e r t i e s i n c o n s i d e r a t i o n o f t h e aging b e h a v i o r are a t t a i n e d when s p u t t e r i n g parameters a r e a p p l i e d t h a t y i e l d t h e l o w e s t s p u t t e r i n g gas i n c o r p o r a t i o n o f t h e f i l m s .

I 1

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MAGNETIC AND MAGNETO-OPTICAL PROPERTIES OF AMORPHOUS RARE EARTH-TRANSITION METAL FILMS FOR MAGNETO-OPTICAL RECORDING

The most i m p o r t a n t magnetic parameters o f a magneto-optic s t o r a g e m a t e r i a l are t h e m a g n e t o - o p t i c a l e f f i c i e n c y , ~ 8 ~

,

f o r systems u s i n g r e f l e c t e d l i g h t and t h e c o e r c i v e energy d e n s i t y . I n f i l m s w i t h l a r g e u n i a x i a l a n i s o t r o p y domain n u c l e a t i o n i s o n l y p o s s i b l e near t h e C u r i e temperature, Tc, 125). Thus, t h e a n i s o t r o p y as w e l l as t h e C u r i e temperature have t o be considered. E r a s i n g r e q u i r e s a f i e l d s t r e n g t h o f about t h e demagnetizing f i e l d , Hd=-MS. The f i e l d s f o r e r a s u r e and w r i t i n g can be made equal by choosing a c o m p o s i t i o n w i t h a compensation temperature, Tcomp, near room temperature.

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I n Table 1 t h e room t e m p e r a t u r e values o f t h e above mentioned parameters are c o l l e c - t e d f o r a number o f b i n a r y and t e r n a r y a l l o y s o f t h e (Gd,Tb)(Fe,Co) system. The com- p o s i t i o n s and t h e p r e p a r a t i o n method are l i s t e d i n t h e f i r s t and second column, r e - s p e c t i v e l y . Various p r e p a r a t i o n t e c h n i q u e s are a p p l i e d , e v a p o r a t i o n (EV) 1131, d i o d e (DS) 1351 o r magnetron s p u t t e r i n g (MS) 1241. For s p u t t e r i n g o f these RE-TM f i l m s b o t h DC and RF e l e c t r i c f i e l d s are a p p l i e d . E i t h e r m u l t i source t e c h n i q u e s w i t h elemental t a r g e t s and s u b s t r a t e r o t a t i o n 1321 o r composite 1361 o r a l l o y t a r g e t s 1351 are used. With s p u t t e r i n g m o s t l y DC o r RF b i a s i s a p p l i e d t o t h e s u b s t r a t e s . The i n f l u e n c e o f b i a s w i l l be discussed i n s e c t i o n 111. A l l f i l m s l i s t e d i n Table 1 were d e p o s i t e d on g l a s s s u b s t r a t e s .

TABLE 1

Magnetic and m a g n e t o - o p t i c a l p r o p e r t i e s o f some t y p i c a l b i n a r y and t e r n a r y r a r e e a t h - t r a n s i t i o n metal a l l o y s f o r m a g n e t o - o p t i c a l r e c o r d i n g .

c o m p o s i t i o n prep. BK

e~

K u poHcMs fcomp TC Ref

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meth. (deg) (107degm-1) (104Jm-3) (104Jm-3) (K) (K)

[

13

I

[ 2 6 ] a[27]

[

281 1171

[

291 t h i s work

[

271

[

301 t h i s work t h i s work [311 t h i s work t h i s work t h i s work [ 3 2 1 1181 [331 The K e r r r o t a t i o n v a l u e s o b t a i n e d u s i n g s u b s t r a t e i n c i d e n t l i g h t o f A=633nm are c o l -

l e c t e d i n t h e t h i r d column. For most compositions t h e K e r r r o t a t i o n a t 780nm being a wavelength o f t e n used f o r magneto-optical r e c o r d i n g i s about 15% l a r g e r compared t o O K a t A=633nm /12,18,34/. The K e r r r o t a t i o n v a l u e a t t h e f i l m g l a s s i n t e r f a c e i s more r e a s o n a b l e t o compare t h a n t h a t o f t h e a i r f i l m i n t e r f a c e s i n c e t h e l a t t e r depends s t r o n g l y on t h e s u r f a c e o x i d a t i o n . Even t h e former values d i f f e r between v a r i - ous a u t h o r s f o r n e a r l y equal composition. One reason may be t h a t some a u t h o r s cor- r e c t f o r t h e component w i t h o u t m.o. i n t e r a c t i o n t h a t has been r e f l e c t e d from t h e a i r g l a s s i n t e r f a c e . T h i s i s superposed on t h e component t h a t i s r e f l e c t e d from t h e sub- s t r a t e f i l m i n t e r f a c e t o be determined. The c o r r e c t e d and t h e u n c o r r e c t e d values d i f f e r by up t o 10%.The h i g h e s t K e r r r o t a t i o n s i n Table 1 a r e found f o r t h e t e r n a r y compound Tbx(FesoCoso )l-x /18,33/. The Faraday r o t a t i o n values o f t h e i r o n based a l l o y s show o n l y s m a l l v a r i a t i o n s between 1.6 and 1 . 8 - 1 0 ~ d e ~ m - ~ f o r TbFe and GdFe, r e s p e c t i v e l y . It i s caused m a i n l y by t h e t r a n s i t i o n metal com onent 1371. The CO- based a l l o y s have l a r g e r Faraday r o t a t i o n s up t o 2.9-107degcm-i 1131. The a b s o l u t e values, however, s h o u l d be compared f o r c o m p o s i t i o n s o f s i m i l a r m a g n e t i z a t i o n (Tcomp) s i n c e t h e m a g n e t o - o p t i c a l e f f e c t s i n c r e a s e w i t h i n c r e a s i n g TM-concentra- t i o n , except f o r Gd,Fe100-~ where t h i s i s v a l i d o n l y f o r x<80. I n t h e range x>80 t h e y decrease a l s o w i t h i n c r e a s i n g Fe c o n t e n t f o r t h i s a l l o y .

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An example o f a c o m p o s i t i o n a l dependence o f t h e m a g n e t o - o p t i c a l e f f e c t s i s shown i n F i g . 1 f o r TbCo f i l m s . These were prepared by dual source e v a p o r a t i o n 1131. Decreas- i n g t h e RE c o n t e n t r e s u l t s i n a s t r o n g i n c r e a s e o f b o t h t h e Faraday and t h e K e r r r o - t a t i o n . For s t o r a g e a p p l i c a t i o n s , however, o n l y t h e c o m p o s i t i o n s TbxColOO-x w i t h x=20

+

2 a r e s u i t a b l e whose compensation temperature i s found r o u g h l y i n t h e i n t e r - v a l 100K<Tcomp<350K. Otherwise, t h e demagnetizing f i e l d w i l l become t o o l a r g e a t ambient temperature. The s t r o n g decrease f o r x>20 i s p r o b a b l y due t o a decrease o f t h e C u r i e t e m p e r a t u r e approaching room temperature a t about x=0.28.

Tb concentration (at%)

F i g .

1 -

Dependence o f Faraday (OF) and K e r r r o t a t i o n (OK) o f TbxColOO-x on t h e Tb c o n t e n t .

The u n i a x i a l a n i s o t r o p y o f t h e amorphous r a r e e a r t h f i l m s depends n o t o n l y on t h e c o m p o s i t i o n b u t a l s o on t h e p r e p a r a t i o n c o n d i t i o n s . I n case o f GdCo t h e extreme values o b t a i n e d by Katayama e t a l . /26/ (see Table 1) by v a r y i n g t h e s u b s t r a t e b i a s v o l t a g e and t h e s p u t t e r - g a s p r e s s u r e are g i v e n i n d i c a t i n g t h a t s t r o n g i n - p l a n e as w e l l as s t r o n g v e r t i c a l a n i s o t r o p y can be o b t a i n e d b y v a r y i n g t h e process parame- t e r s . I n t h e o t h e r cases t h e maximum values o b t a i n e d w i t h t h e r e s p e c t i v e p r e p a r a t i o n method are g i v e n .

C o n s i d e r a b l e e f f o r t has been made t o e l u c i d a t e t h e o r i g i n o f t h e growth induced u n i - a x i a l a n i s o t r o p y i n t h e s e a l l o y s . A u n i a x i a l a n i s o t r o p y i n amorphous magnetic f i l m s has been a t t r i b u t e d t o d i f f e r e n t mechanisms t h a t can be d i v i d e d i n t o two groups i n - v o l v i n g s p i n - o r b i t c o u p l i n g and magnetic d i p o l a r i n t e r a c t i o n s /19,39/. I n t h e f i r s t case t h e a n i s o t r o p y i s caused by s i n g l e - i o n a n i s o t r o p y , a n i s o t r o p i c exchange and m a g n e t o s t r i c t i o n a p p l y i n g p r i m a r i l y t o a l l o y s c o n t a i n i n g non-S-state RE atoms /19, 391. T h i s i s r e f l e c t e d by t h e a p p r e c i a b l y l a r g e r a n i s o t r o p y values o f t h e Tb con- t a i n i n g a l l o y s i n t a b l e 1. The second group i s based on d i p o l a r i n t e r a c t i o n s e i t h e r on atomic o r m i c r o s t r u c t u r a l scales. Several o r i g i n s o f these a n i s o t r o p i c i n t e r a c - t i o n s have been proposed: ( i ) atomic p a i r o r d e r i n g /26,39/, i i ) s t r e s s /40/, i i i ) shape o r v o i d e f f e c t s /41-431, i v ) l o c a l a n i s o t r o p y c o o r d i n a t i o n /44/, and v ) t h e presence o f m i c r o c r y s t a l l i n e r e g i o n s /45-471. M i c r o s t r u c t u r a l inhomogeneities can be caused by p h y s i c a l growth f e a t u r e s /42,43/ and chemical s e g r e g a t i o n /42,43/ which i n t u r n can be due t o s e l e c t i v e o x i d a t i o n /43/. For s p u t t e r e d f i l m s t h e s e l e c t i v e r e - s p u t t e r i n g has been suggested as a cause o f a n i s o t r o p i c p a i r o r d e r i n g /48,49/. It has been proposed t h a t t h e a n i s o t r o p y energy can be expressed i n terms o f 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 , Mi, by

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For anorphous GdFe a l l o y s t h e c o u p l i n g c o n s t a n t s : CFe-Fe = 5.84, CFe-Gd = 8.59, and C G ~ - G , ~ = 5.06 (a1 1 i n S1 u n i t s o f

lo4

m/H) have been r e p o r t e d t o s a t i s f a c t o r i l y r e - p r e s e n t t h e c o m p o s i t i o n dependence o f Ku a t room t e m p e r a t u r e 1501. The f a c t t h a t a l l c o n s t a n t s a r e p o s i t i v e means t h a t c o u p l i n g between unequal atoms leads t o an i n - p l a n e (Ku<O s i n c e Mi,M. have o p p o s i t e s i g n s ) and t h e c o u p l i n g between equal atoms leads t o perpendicu? a r p r e f e r r e d o r i e n t a t i o n o f t h e m a g n e t i z a t i o n

.

The c o e r c i v e energy d e n s i t y o f t h e amorphous RE-TM f i l m s o f t h e Tb c o n t a i n i n g a l l o y s i s by an o r d e r o f magnitude l a r g e r t h a n t h a t o f t h e Gd based a l l o y s . T h i s can be a t - t r i b u t e d t o t h e s p i n - o r b i t c o u p l i n g o f t h e non-S-state atom Tb. The l a r g e c o e r c i v e energy d e n s i t i e s o f t h e Tb c o n t a i n i n g a1 l o y s a1 low t h e s t o r a g e o f small domains.For assessment o f t h e m i n i m m s t a b l e domain diameter, 2R, t h e r e l a t i o n 2R = 2 m U / ~ o ~ c ~ S has been d e r i v e d 191. Assuming a v a l u e o f t h e exchange c o n s t a n t on t h e o r d e r o f

~ = l 0 - 1 2 ~ / m 1511, a minimum domain diameter on t h e o r d e r o f O.lum can be estimated.

The C u r i e t e m p e r a t u r e s o f t h e amorphous C o - a l l o y s mentioned i n t a b l e 1 are h i g h e r t h a n 450K. I n t h i n f i l m s it cannot be measured w i t h o u t i n d u c i n g s t r u c t u r a l changes caused by p r e f e r e n t i a l o x i d a t i o n o f t h e r a r e e a r t h component and phase s e p a r a t i o n o f t r a n s i t i o n m e t a l c l u s t e r s 142,431.

Aging o f t h e amorphous f i l m s i s m a i n l y caused by t h e p r e f e r r e d o x i d a t i o n o f t h e r a r e e a r t h atoms. At low oxygen c o n c e n t r a t i o n s i n t h e f i l m s i t causes o n l y a decrease o f t h e compensation temperature. If t h e r a r e e a r t h atoms a r e t o t a l l y o x i d i z e d , however, t h e f i l m s are f e r r o m a g n e t i c 1301. The o x i d a t i o n d e s t r o y s a l s o t h e growth induced a n i s o t r o p y . The a g i n g b e h a v i o r o f t h e amorphous f i l m s depends b o t h on t h e i r composi- t i o n 1211 and on t h e i r m i c r o s t r u c t u r e 1221. The l a t t e r i s i n f l u e n c e d by t h e prepara- t i o n parameters. T h i s i s e x e m p l i f i e d i n t h e n e x t s e c t i o n d i s c u s s i n g t h e dependence o f t h e o x i d a t i o n speed o f RF diode s p u t t e r e d GdTbCo f i l m s on t h e argon p r e s s u r e and t h e s u b s t r a t e b i a s v o l t a g e .

I 1 1

-

INFLUENCE OF THE THE PREPARATION PARAMETERS ON THE AGING BEHAVIOR OF RF-SPUTTERED GdTbCo FILMS

The u n i a x i a l a n i s o t r o p y o f s p u t t e r e d amorphous RE-TM f i l m s depends s t r o n g l y on t h e s u b s t r a t e b i a s v o l t a g e . The l a t t e r , however, has a l s o a l a r g e i n f l u e n c e on t h e m i c r o s t r u c t u r e o f t h e f i l m s . Thus, a r e l a t i o n between t h e s t r u c t u r e dependent aging b e h a v i o r and t h e growth induced u n i a x i a l a n i s o t r o p y i s expected. We i n v e s t i g a t e d t h e oxygen p e n e t r a t i o n i n t o RF s p u t t e r e d GdTbCo f i l m s comparing t h o s e s p u t t e r e d a t a b i a s v o l t a g e r e s u l t i n g i n maximum u n i a x i a l a n i s o t r o p y and those s p u t t e r e d w i t h o u t b i a s .

A) Experimental

GdTbCo f i l m s were prepared by RF d i o d e s p u t t e r i n g o n t o S i and g l a s s s u b s t r a t e s . A 10 i n c h Co p l a t e w i t h a mosaic p a t t e r n o f Gd and Tb p e l l e t s o f 8mm diameter was used as a t a r g e t . The s u b s t r a t e s were f i x e d t o a water c o o l e d anode l o c a t e d 5cm above t h e t a r g e t . The power was h e l d c o n s t a n t a t 500W. A DC b i a s v o l t a g e , Vb, was a p p l i e d t o t h e s u b s t r a t e s v i a t h e anode v a r y i n g between 0 (grounded s u b s t r a t e s ) and -300V.

The f i l m s were analyzed w i t h r e s p e c t t o Gd,Tb,Co, and Ar b y EPMA 1521. The oxygen c o n t e n t was o b t a i n e d f r o m SIMS depth p r o f i l e s . The u n i a x i a l a n i s o t r o p y c o n s t a n t was determined by means o f a t o r q u e magnetometer.

B) R e s u l t s and D i s c u s s i o n

F i g . 2a shows t h e c o n c e n t r a t i o n o f Gd,Tb,Co, and Ar i n dependence on t h e b i a s v o l - t a g e f o r a c o n s t a n t Ar pressure, PArz2Pa. The a r e a l RE-TM r a t i o on t h e t a r g e t was 0.3 and t h e Tb-Gd r a t i o was 0.33. The c o m p o s i t i o n v a r i a t i o n w i t h PAr a t Vb=O i s shown i n F i g . 2b. The c o n c n t r a t i o n s o f t h e main components Gd,Tb, and Co are c a l - c u l a t e d w i t h o u t c o n s i d e r a t i o n o f t h e A r - i m p u r i t y .

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F i g . 2

-

Dependence o f t h e c o m p o s i t i o n o f GdTbCo f i l m s on ( a ) t h e DC b i a s v o l t a g e , Vb, a t f i x e d argon pressure, PAr=2Pa, and ( b ) on PAr a t Vb=O prepared b y R F - s p u t t e r i n g f r o m a composite t a r g e t o f Gd22TbllCo67 ( a r e a l r a t i o ) .

The v a r i a t i o n o f t h e RE-TM r a t i o w i t h PA, and Vb i s caused by r e s p u t t e r i n g o f t h e growing f i l m 1351. The r e s p u t t e r i n g y i e l d of RE atoms i s l a r g e r t h a n t h a t o f TM atoms r e s u l t i n g i n a decrease o f t h e RE component i n t h e f i l m w i t h i n c r e a s i n g Vb 1351. Reducing t h e Ar p r e s s u r e i n c r e a s e s t h e plasma p o t e n t i a l 1531 and has a i i m l l a r e f f e c t as i n c r e a s i n g ( V b ( . The A r - c o n c e n t r a t i o n i n t h e f i l m i n - creases d r a s t i c a l l y w i t h i n c r e a s i n g

avbl

i n t h e low b i a s r e g i o n and reaches a maximum o f 13 at.% a t Vb=-150V (PAr=2P ) . Then i t decreases upon f u r t h e r i n c r e a s e o f

approaching n e a r l y a s y m p t o t i c a l l y a v a l u e 3.5 at.%. T h i s tendency was a l s o r e - ed by s e v e r a l a u t h o r s i n e a r l i e r papers 154-561.

Oxygen d e p t h p r o f i l e s were measured by SIMS a t 80nm and lOOnm t h i c k f i l m s . They were s p u t t e r e d a t P ~ ~ r 3 P a a p p l y i n g b i a s v o l t a g e s of 0 and -100V, r e s p e c t i v e l y . Both f i l m s were i n v e s t i g a t e d i n t h e as-deposited s t a t e and a f t e r d i f f e r e n t s t o r a g e times, r a , i n a normal ambient. F i g . 3a shows t h e oxygen d e p t h p r o f i l e s o f t h e zero b i a s f i l m a f t e r ra=O and 'a = 28weeks and F i g . 3b those o f t h e -150V b i a s f i l m a f t e r ra=O, Ta = 3weeks, and r a = 38weeks. The oxygen c o n t e n t o f t h e zero b i a s f i l m drops w i t h i n t h e f i r s t lOnm from t h e oxygen s a t u r a t e d s t a t e a t t h e s u r f a c e o f about 60 at.% t o t h e low c o n c e n t r a t i o n o f t h e b u l k . The l a t t e r i s about 2 at.%. The -150V b i a s e d f i l m shows d i r e c t l y a f t e r p r e p a r a t i o n a n e a r l y i d e n t i c a l b e h a v i o r b u t a l i t t l e s m a l l e r oxygen " b u l k " l e v e l . T h i s may be caused by t h e r e j e c t i o n of t h e p r e - f e r a b l y n e g a t i v e l y charged oxygen i o n s i n t h e s p u t t e r i n g plasma by t h e n e g a t i v e sub- s t r a t e b i a s p o t e n t i a l . A f t e r t h r e e weeks t h e t h i c k n e s s o f t h e o x i d e a t t h e s u r f a c e o f t h e -150V b i a s f i l m has d i s t i n c t l y i n c r e a s e d i n d i c a t i n g a l a r g e o x i d a t i o n r a t e . A f t e r even t e n t i m e s l o n g e r s t o r a g e i n a i r t h e o x i d e t h i c k n e s s o f t h e zero b i a s f i l m

i s o n l y about one h a l f o f t h a t o f t h e -150V b i a s f i l m a f t e r r a = 3 weeks. A f t e r an a i r s t o r a g e o f comparable t i m e t h e l a t t e r i s o x i d i z e d c o m p l e t e l y . A s i m i l a r b e h a v i o r has been r e p o r t e d b y Esho e t a l . /41/ and Kusuda e t a l . /57/. The oxygen concentra- t i o n s i n t h e b u l k o f t h e s p u t t e r e d f i l m s , i.e. below t h e h i g h l y o x i d i z e d s u r f a c e oxide, are shown i n F i g . 4 as a f u n c t i o n o f t h e s u b s t r a t e b i a s v o l t a g e a f t e r expo- s i n g t h e f i l m s 1 week and 26 weeks t o t h e a i r . For r a = l week t h e b u l k oxygen con- c e n t r a t i o n i s s t i l l v e r y low and i s even lower i n f i l m s prepared w i t h i n c r e a s i n g n e g a t i v e b i a s . A f t e r 26 weeks, however, t h e f i l m s prepared a t b i a s v o l t a g e s -100V and -150V a r e s a t u r a t e d w i t h oxygen, i.e. t h e c o n c e n t r a t i o n i n t h e whole f i l m volume i s as h i g h as i n t h e s u r f a c e l a y e r .

The b u l k oxygen c o n c e n t r a t i o n a f t e r extended exposure t o a i r depends s i m i l a r l y on t h e b i a s v o l t a g e as t h e A r - c o n c e n t r a t i o n . The lowest o x i d a t i o n r a t e i s observed f o r t h e zero b i a s f i l m s e x h i b i t i n g t h e lowest A r - c o n t e n t s o f about 0.3 at%.

The i n c r e a s e o f t h e A r - c o n t e n t was r e p o r t e d by Esho e t a l . 1411 t o c o r r e l a t e w i t h a decrease o f t h e f i l m d e n s i t y w i t h i n c r e a s i n g ( f o r IVb1<170V). The A r - i n c o r p o r a - t i o n was argued t o be t h e reason f o r a p o r o s i a m i c r o s c o p i c s c a l e 1411,

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as deposited film

I

8 : 28 weeks later

l o L

10

-

_m

-...

-

m A

5

^{l o 2} ^5at%

-

>- k V)

Z W l-

z

10'

A: as deposited film B : 3 weeks later C . 38 weeks later

a) DEPTH (nm) b) DEPTH lnm)

F i g . 3

-

SIMS oxygen depth p r o f i l e s o f GdTbCo f i l m s s p u t t e r e d a t a) Vb=O and b) Vb=-150V a t P A R = ~ P ~ measured d i r e c t l y a f t e r d e p o s i t i o n and a f t e r s t o r a g e o f t h e f i l m s i n a normal ambient.

L 0 .

-

+

p,,= 2 Pa

ta=

26weeks c 0.1

w

a ~ t ~ = l

week

cn - -

^--,-

g- 0.01

o 0 -100 -200 -300

F i g . 4

-

Oxygen c o n c e n t r a t i o n i n t h e b u l k o f t h e f i l m , i.e. below t h e s u r f a c e o x i d e l a y e r , i n dependence on t h e b i a s v o l t a g e .

B. Magnetic P r o p e r t i e s

The growth induced u n i a x i a l magnetic a n i s o t r o p y depends on t h e b i a s v o l t a g e i n a s i m i l a r f a s h i o n as t h e A r - c o n c e n t r a t i o n . S p u t t e r i n g from a t a r g e t w i t h a r e a l f r a c - t i o n Gd22Tb8Fe70 a t P A ~ = Z P ~ t h e r e s u l t s shown i n F i g . 5a were obtained. With i n c r e a s i n g Vb t h e a n i s o t r o p y passes t h r o u g h a maximum a t Vb=-100V and then it de- decreases c

b

^{n t}

1

nuously. T h i s behavior was a l s o r e p o r t e d by numerous authors /41,48, 54,55,57/. A t low b i a s v o l t a g e s a s i g n i f i c a n t dependence of Ku on PA, was observed b e i n g s t r o n g e s t a t Vb=O. T h i s was a l s o r e p o r t e d by Katayama e t a l . /26/ f o r RF s p u t t e r e d GdCo f i l m s . The argon p r e s s u r e dependence o f t h e a n i s o t r o p y a t Vb=O of our f i l m s i s shown i n F i g . 5b. A p o s i t i v e a n i s o t r o p y i s o b t a i n e d i n t h e range 0.7<PAr (Pa) <2 and a maximum o f Ku=3*

lo4

jm-3 a t PAr=l .5 Pa.

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Fig. 5

-

Dependence o f t h e growth induced u n i a x i a l anisotropy o f the sputtered GdTbCo f i l m s ( a ) on the s u b s t r a t e bias voltage a t P~,=2pa and b) on the argon pressure a t zero bias.

C. Discussion

The increase o f the Ar-content i n the low b i a s r e g i o n i s caused by an increase o f the Ar-penetration depth w i t h increasing i o n energy reducing i t s desorption probabi- l i t y . The Ar i s thereby trapped by the growing f i l m . I n c r e a s i n g the substrate bias, however, increases also the backsputtering r a t e . Above a c e r t a i n bias voltage t h e backsputtering and t h e re-emission by i o n bombardment overtake the argon incorpora- t i o n leading t o a decrease o f the Ar-content. The growth induced anisotropy i n t h e sputtered f i l m s i s a t t r i b u t e d t o an ordering o f atom p a i r s r e l a t i v e t o the growth d i r e c t i o n caused by p r e f e r e n t i a l r e s p u t t e r i n g 148,491. With increasing bias voltage, i.e. w i t h i n c r e a s i n g energy o f the bombarding ions, the r e s p u t t e r i n g w i l l become less s e l e c t i v e 1481. As a f u r t h e r e f f e c t o f increased b i a s voltage s t r u c t u r a l changes have been observed, i.e. a tendency t o approach a c r y s t a l l i n e s t a t e 1571 and a decrease o f the columnar diameter o f the n e t w o r k - l i k e f i n e s t r u c t u r e whose period o f about lOnm a t zero b i a s diminishes a t l a r g e b i a s by an order o f magnitude 1571. This might i n d i c a t e t h a t t h e r e i s no s i n g l e mechanism determining the appearance o f t h e anisotropy. I n t h i s sense the Ar-content o f t h e f i l m s might only be an i n d i c a t i o n o f the balance o f d i f f e r e n t processes being responsible f o r the observed e f f e c t . The Ar-content i s n o t a cause o f the u n i a x i a l anisotropy since, f i l m s w i t h a very low Ar-content have been prepared t h a t e x h i b i t a p o s i t i v e u n i a x i a l anisotropy e i t h e r by d i o d e - s p u t t e r i n g a t zero bias (see Fig. 5a or r e f . 1261) o r by magnetron s p u t t e r i n g even a t negative b i a s values up t o (Vbl=lOOV 1241.

The o x i d i z e d surface l a z e r i s ferromagnetic and e x h i b i t s a l a r g e negative anisotropy constant Keff = Ku-~oMs/2 owing t o the demagnetizing f i e l d being on the order o f 5.10~ Alm 1301. The growth-induced anisotropy, i f t h e r e i s any, i s too small t o be determined 1301. Thus, even a p a r t i a l o x i d a t i o n changes the magnetic c h a r a c t e r i s t i c s o f t h e f i l m s completely, since t h e u n i a x i a l anisotropy i s p r o p o r t i o n a l t o the u n i - o x i d i z e d f i l m volume /22,30/.

I V

-

CONCLUSIONS

Amorphous r a r e - e a r t h t r a n s i t i o n metal f i l m s e x h i b i t s u f f i c i e n t l y high magneto-optical e f f i c i e n c y f o r a p p l i c a t i o n as an erasable o p t i c a l storage medium. The l a r g e s t e f f e c t s r e p o r t e d so f a r are found i n t e r n a r y o r quaternary a l l o y s w i t h composition (Gd,Tb) (Fe,Co). T h e i r c o e r c i v i t y can be made very 1 arge by a d j u s t i n g the Tb content and thus a l l o w i n g f o r high s t a b i l i t y o f small domains a t high density. They can be prepared by various vacuum d e p o s i t i o n methods.

A c r u c i a l problem w i t h the RE-TM a l l o y s i s o x i d a t i o n . T h e i r aging behavior has t o be improved both by covering them w i t h p r o t e c t i o n l a y e r s and by improving t h e i r s t a b i - l i t y against c o r r o s i o n . Therefore, f i l m s o f optimum d e n s i t y have t o be prepared being c l e a r o f voids and d e n s i t y f l u c t u a t i o n s 1221. I f t h e l a t t e r are present, a r a p i d d i f f u s i o n o f oxygen i n t o the bulk occurs 1221. This was confirmed by i n v e s t i -

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g a t i o n o f oxygen depth p r o f i l e s o f RF s p u t t e r e d GdTbCo f i l m s . When t h e growth i n - duced u n i a x i a l a n i s o t r o p y i s maximized by a p p l i c a t i o n o f a DC b i a s o f about -100V, t h e o x i d a t i o n r a t e i s a l s o maximum. T h i s was a t t r i b u t e d t o t h e l a r g e Ar-concentra- t i o n caused by t h e b i a s . However, f i l m s c o u l d a l s o be prepared e x h i b i t i n g a p o s i t i v e u n i a x i a l a n i s o t r o p y when s p u t t e r e d a t zero b i a s . They had t h e l o w e s t A r - c o n t e n t and t h e i r o x y d a t i o n speed was l a r g e l y reduced. A n i s o t r o p y and c o e r c i v i t y values s u i t a b l e f o r s t o r a g e a p p l i c a t i o n can be o b t a i n e d by choosing a h i g h Tb c o n c e n t r a t i o n .

ACKNOWLEDGEMENTS

We wish t o thank K. W i t t e r f o r measurement and a n a l y s i s o f t o r q u e curves and f r u i t - f u l d i s c u s s i o n s . We thank P. W i l l i c h f o r t h e e l e c t r o n microprobe a n a l y s i s . T h i s work was sponsored by t h e German Federal M i n i s t r y o f Research and Technology (BMFT) under g r a n t number 13 N 5337.

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