OPTIMISATION OF THE MAGNETIC PROPERTIES OF CoCr FILMS FOR PERPENDICULAR RECORDING APPLICATIONS

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OPTIMISATION OF THE MAGNETIC PROPERTIES OF CoCr FILMS FOR PERPENDICULAR

RECORDING APPLICATIONS

C. Wright, E. Lacey, P. Grundy

To cite this version:

C. Wright, E. Lacey, P. Grundy. OPTIMISATION OF THE MAGNETIC PROPERTIES OF CoCr FILMS FOR PERPENDICULAR RECORDING APPLICATIONS. Journal de Physique Colloques, 1985, 46 (C6), pp.C6-101-C6-104. �10.1051/jphyscol:1985618�. �jpa-00224865�

JOURNAL d E PHYSIQUE

Colloque C6, suppl6ment au n09, Tome 46, septembre 1985 page C6-101

OPTIMISATION OF THE MAGNETIC PROPERTIES OF CoCr FILMS FOR PERPENDICULAR RECORDING APPLICATIONS

C . D . W r i g h t , E.T.M. L a c e y a n d P . J . G r u n d y

Department o f Pure and Applied P h y s i c s , U n i v e r s i t y o f S a t f o r d , S a l f o r d M5 4W!7', U. K.

Resume - Cet a r t i c l e presente des r e s u l t a t s q u i concernent des experiences pour o p t i m i s e r l e s p r o p r i e t e s magnetiques de couches CoCr d@pos@es p a r " p u l - v e r i s a t i o n " s u r des s u b s t r a t s r i g i d e s e t f l e x i b l e s . Des mesures de couches simples e t couplees, s u r du v e r r e e t p o l y i m i d e s o n t discutees. Les e f f e t s r e - l a t i f s de c i b l e s composPes sont d e c r i t s .

A b s t r a c t - T h i s paper presents t h e r e s u l t s o f some experiments aimed a t o p t i m i s i n g t h e magnetic p r o p e r t i e s o f CoCr f i l m s s p u t t e r deposited onto r i g i d and f l e x i b l e substrates. Measurements o f s i n g l e and double l a y e r s on g l a s s and polyimide are discussed and the r e l a t i v e e f f e c t s o f using composite and a l l o y t a r g e t s a r e described.

1 - INTRODUCTION

There a r e several p o s s i b l e media f o r perpendicular r e c o r d i n g a p p l i c a t i o n s . S u i t a b l e p a r t i c u l a t e media i n c l u d e o r i e n t e d , u n i a x i a l f e r r i t e s such as barium f e r r i t e and deposited f i l m s w i t h e x c e l l e n t p r o p e r t i e s can be prepared from u n i a x i a l c o b a l t -

t r a n s i t i o n metal a l l o y s . Several i n v e s t i g a t i o n s have shown t h a t b o t h s i n g l e l a y e r CoCr /1/ and double l a y e r CoCr/NiFe /2/ c o a t i n g s a r e capable o f v e r y high r e c o r d i n g d e n s i t i e s o f t h e o r d e r 105 f l u x r e v e r s a l s per cm. The best media w i l l e x h i b i t optimum magnetic and t r i b o l o g i c a l p r o p e r t i e s . Both magnetic and mechanical p r o p e r t i e s are, o f course, c l o s e l y r e l a t e d t o t h e composition and m i c r o s t r u c t u r e o f t h e f i l m . This paper describes some of our experiments on t h e o p t i m i s a t i o n o f s i n g l e and double l a y e r s deposited on a v a r i e t y o f s u b s t r a t e s w i t h an emphasis on g l a s s and t h e commercial s u b s t r a t e polyimide. I t complements our previous work /3,4/ and concentrates on magnetic and m i c r o s t r u c t u r a l i n v e s t i g a t i o n s .

11 - EXPERIMENTAL METHODS

Our f i l m s were prepared i n a h i g h r a t e s p u t t e r d e p o s i t i o n system from a m a g n e t i c a l l y a s s i s t e d d.c. t r i o d e source. Two CoCr t a r g e t s were used, an a l l o y t a r g e t of

composition CoglCr19 on a composite t a r g e t c o n s i s t i n g o f a c o b a l t d i s c w i t h a v a r i a b l e number o f chromium ~ n s e r t s . The composite t a r g e t has allowed us t o i n v e s t i g a t e t h e p r o p e r t i e s o f f i l m s o f d i f f e r e n t compositions /3/. I n t h e work r e p o r t e d here we have a d j u s t e d t h i s t a r g e t t o g i v e us f i l m s o f s i m i l a r composition, approkimatel y CoglCrl 9, from e i t h e r t a r g e t . F i l m s f o r magnetic c h a r a c t e r i s a t i o n here deposited on g l a s s , polyimide o r (111) s i l i c o n wafers. Thinner f i l m s f o r transmission e l e c t r o n microscopy and e l e c t r o n d i f f r a c t i o n were prepared on r o c k s a l t and carbon coated glass. NiFe underlayers were deposited from a Nig3Fe17 a l l o y t a r g e t .

Magnetic p r o p e r t i e s o f f i l m s between 0.5 and 1 pm i n t h i c k n e s s were measured on a v i b r a t i n g sample magnetometer and t h e c r y s t a l lo g r a p h i c t e x t u r e was determined from X-ray r o c k i n g curves about t h e 10002 1Bragg r e f 1 e c t i o n . The m i c r o s t r u c t u r e and magnetic domain s t r u c t u r e were i n v e s t i g a t e d by transmi ssion and scanning e l e c t r o n micros cop^. Compositional analyses were made by energy d i s p e r s i v e X-ray m i c r o a n a l ~ s i s i n b o t h t r a n s m i s s i o n and scanninq e l e c t r o n microscopes.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985618

C6-102 J O U R N A L DE PHYSIQUE

111 - RESULTS AND DISCUSSION

Our f i r s t i n t e n t i o n has been t o o p t i m i s e t h e p r o p e r t i e s o f s i n g l e l a y e r f i l m s

deposited on g l a s s and polyimide substrates. I n previous work /3/ we have shown t h a t t h e magnetic p r o p e r t i e s o f CoCr f i l m s a r e dependent on t h e process parameters. I n t h i s i n v e s t i g a t i o n we have a d j u s t e d the s p u t t e r i n g power t o g i v e a f a i r l y r a p i d d e p o s i t i o n r a t e (-0.2 pm/flln. ) , optimum t a r g e t - s u b s t r a t e spacing ( 5 cm) and argon pressure (-3mTorr), a nominally room temperature s u b s t r a t e and a v a r i a b l e base pressure P

.

F i g u r e 1 i l l u s t r a t e s t h e f o l l o w i n g r e l a t i o n s h i p s ; i n (a) t h e r a t i o o f t h e uncorrected perpendicular remanence t o t h e in-plane remanence S = Mr /M . and t h e c o e r c i v i t y r a t i o Hcp/Hci as a f u n c t i o n o f Pb - i n (b) t h e c - a x i s d i s ~ e r s i g n as a f u n c t i o n o f Pb.

C l e a r l y a low value of Pb g i v e s magnetisation loops w i t h improved squareness, as r e q u i r e d f o r reasonable perpendicular r e c o r d i n g performance,and a moderate

perpendicular c o e r c i v i t y Hc coup1 ed w i t h a c o e r c i v i t y r a t i o s i g n i f i c a n t l y g r e a t e r than u n i t y i s a measure o f !he m e r i t o f t h e medium. I t i s c l e a r t h a t b e t t e r r e s u l t s a r e obtained f o r g l a s s substrates than f o r polyimide. However, we f i n d t h a t t h e polyimide r e s u l t s a r e much s u p e r i o r t o those f o r s i l i c o n substrates.

- ^2.5

Fiq. 1 (a) The e f f e c t of Base pressure P on remanence squareness S ^(A) and c o e r c i v i t y r a t i o s (+). (b) The e f f e c t o f Pb on t h e

PWHM

Glass

2.0

Polylmlde

1 . 5 -

S 1 . 0 -

0 . 5 -

The dependence o f on Pb, and i t s obvious i n v e r s e r e l a t i o n s h i p t o S and t h e c o e r c i v i t y ratio,shows t h a t t h e c r y s t a l l o g r a p h i c t e x t u r e o f t h e f i l m i s another important f a c t o r i n o b t a i n i n g optimum magnetic p r o p e r t i e s . Again we see t h a t g l a s s g i v e s improved values of A0 ,but a d i s p e r s i o n o f t h e c - a x i s about t h e f i l m normal o f l o o i s q u i t e good f o r a f l e x 7 8 l e s u b s t r a t e a t t h e lowest base pressure o f our system.

The M-H loops of F i g s 2b and c a r e c h a r a c t e r i s e d by reasonable values o f c o e r c i v i t y (H, r n 9 0 0 Oe) and remanence r a t i o and i n t h e case o f t h e g l a s s s u b s t r a t e square perbendicular l o o p shoulders. The moderate c o e r c i v i t y o f these f i l m s i s w i t h i n t h e w r i t i n g c a p a b i l i t y o f e x i s t i n g heads. The perpendicular l o o p o f F i g 2c when c o r r e c t e d f o r demagnetisation reveal s t h e strong perpendicular a n i s o t r o p y i n t h e f i l m and g i v e s a Mr,,/Msat r a t i o o f 0.99.

- ^1.5

The gradual improvement i n p r o p e r t i e s i n going from F i g 2a t o 2c i s , we suggest, associated w i t h t h e form o f t h e i n i t i a l n u c l e a t i o n l a y e r o f t h e f i l m on i t s s u b s t r a t e and t h e subsequent growth o f t h e f i l m . Indeed t h e magnetic p r o p e r t i e s of our f i l m s

6111%

Polylmlde

- ¹ .o

F i g . 2 M-H loops f o r f i l m s deposited on (a) (111) s i l i c o n , ( b ) polyimide and ( c ) glass. ( c ) shows t h e perpendicular l o o p c o r r e c t e d f o r demagnetisation. C0mp. t a r g e t . appear t o c o r r e l a t e more w i t h t h e c - a x i s d i s p e r s i o n and t e x t u r e (as revealed by X-ray and e l e c t r o n d i f f r a c t i o n ) and domain s t r u c t u r e than w i t h g r a i n s i z e o r compositional u n i f o m i t y . The e l e c t r o n micrographs and d i f f r a c t i o n p a t t e r n s o f F i g . 3 i l l u s t r a t e t h e development o f p e r p e n d i c u l a r l y magnetised s t r i p e domains i n a w e l l o r i e n t e d t h i n f i l m . The good t e x t u r e of F i g . 3b i s shown by t h e strong (1010) r e f l e c t i o n and the absence o f (0002) and (1011) d i f f r a c t i o n maxima. The untextured d i f f r a c t i o n p a t t e r n and in-plane magnetised domains o f F i g , 3a a r e t y p i c a l o f f i l m s which develop, w i t h i n c r e a s i n g thickness, i n t o l a y e r s w i t h poor perpendicular magnetic c h a r a c t e r i s t i c s . E l e c t r o n microscope i n v e s t i g a t i o n s o f optimised f i l m s have been described i n a previous paper /4/.

F i g . 3 Lorentz transmission e l e c t r o n micrographs showing t h e magnetic domain s t r u c t u r e i n 35 nm t h i c k Co Crl ,films. Electron d i f f r a c t i o n patterns a r e shown as i n s e r t s . ( a ) a p o o r l y o r i e i l e d ? ~ l m and (b) a w e l l o r i e n t e d f i l m .

r e assume t h a t t h e n u c l e a t i o n o f t h e (0001) basal plane o f t h e CoCr c r y s t a l l i t e s i s i n some way more favoured on amorphous s u b s t r a t e s than on a (111) s i l i c o n surface.

Indeed, o t h e r r e c e n t work /1/ has shown t h a t t h e d i s p e r s i o n o f t h e c - a x i s i s g r e a t l y a f f e c t e d by t h e s u b s t r a t e used and t h a t amorphous substrates can impart b e t t e r growth c h a r a c t e r i s t i c s than some c r y s t a l l i n e ones. F i l m s grown on g l a s s and polyimide, both amorphous substrates, may e x h i b i t d i f f e r e n t magnetic p r o p e r t i e s because o f differences i n s u b s t r a t e surface roughness. P r e l i m i n a r y examination o f our substrates shows t h a t polyimide i s c o n s i d e r a b l y rougher than glass. Experiments on t h e e f f e c t s o f a range o f c l e a n i n g and s p u t t e r e t c h i n g treatments a r e i n progress. Substrate heating e f f e c t s d u r i n g d e p o s i t i o n may a1 so influence t h e n u c l e a t i o n and growth o f tKe i n i t i a l l a y e r s as may a l s o gas e v o l u t i o n from polyimide.

I n double l a y e r NiFe/CoCr media t h e f.c.c. NiFe underlayer shodld grow w i t h i t s (111) plane p a r a l l e l t o t h e medium surface t o promote good (0001),o/rientation o f t h e suuer -imposed CoCr l a y e r . Again, we have found t h a t t h e s u b s t r a t e has a considerable e f f e c t on t h e o r i e n t a t i o n o f t h e growing f i l m . F i g . 4 shows t h a t NiFe f i l m s grown on g l a s s substrates e x h i b i t an improved (111) o r i e n t a t i o n over those grown on polvimide, w h i l s t (1 11 ) s i n q l e c r y s t a l s i 1 ic o n promotes a predominantly (200) NiFe t e x t u r e . The change i n magnetic p r o p e r t i e s r e s u l t i n g from t h e NiFe underlayer manifest themselves

C6-104 JOURNAL DE PHYSIQUE

i n double-layer M-H loops as an increase i n s a t u r a t i o n magnetisation coupled w i t h a decrease i n c o e r c i v i t v over those values achieved i n t h e s i n g l e l a y e r case. T h i s i s i l l u s t r a t e d i n Fiq. 4b which g i v e s t h e l o o p f o r a double l a y e r o f 0.5 pm t h i c k NiFe beneath 1 pm t h i c k CoCr. The form o f these loops i s much t h e same as found by o t h e r

workers / 6 / . *

M (kG)

200

I

+

, F i g . 4 (a) X-ray d i f f r a c t o m e t e r t r a c e s o f NiFe f i l m s qrown on (a) (111) s i l i c o n , ( b ) g l a s s and ( c ) polyimide. (b) M-H

# loops f o r a NiFe/CoCr double l a y e r deposited on polyimide.

Films deposited from t h e composite t a r g e t .

Our experiments have shown t h a t c o n s i s t e n t l y b e t t e r f i l m s a r e obtained from a composite t a r q e t . T y p i c a l optimum values f o r S, Hcp and Hc /Hci a r e given i n F i g s 1 and 2.

These ma-y be compared w i t h best values from t h e a y l o y t a r g e t on p o l y i m i d e o f 0.1,250 Oe and 0.9 r e s p e c t i v e l y . M i c r o a n a l y s i s measurements have shown t h a t f i l m s from b o t h t a r g e t s a r e c o m p o s i t i o n a l l y uniform i n t h e plane o f t h e f i l m over submicron areas t o b e t t e r than 0.2 %. On a s p a t i a l s c a l e o f 20 nm v a r i a t i o n s o f 1-2 % were observed.

!4e c a l c u l a t e t h a t t h e dimensions of t h e composite t a r g e t a r e such t h a t unlform films should be obtained a t t h e t a r g e t - s u b s t r a t e s e p a r a t i o n used. A n a l y s i s as a f u n c t i o n o f f i l m t h i c k n e s s d i d suqgest t h a t i n i t i a l l a y e r s (-15 nm t h i c k ) from t h e composite t a r g e t can be r i c h i n c o b a l t and depleted i n chromium (+3,-3% r e s p e c t i v e l y ) whereas f i l m s from t h e a l l o y t a r g e t a r e nearer t h e " t h i c k " f i l m composition. Experiments on these p o s s i b l e compositional e f f e c t s are c o n t i n u i n g . X-ray r o c k i n g curve measurements on f i l m s sputtered from t h e a l l o y t a r g e t have so f a r shown poor t e x t u r e . D i f f r a c t - ometer t r a c e s a l s o i n d i c a t e a v e r y l i m i t e d p r e f e r r e d o r i e n t a t i o n f o r t h e c - a x i s i n these f i l m s . T h e i r i n f e r i o r magnetic p r o p e r t i e s noted above r e f l e c t these p u z z l i n g m i c r o s t r u c t u r a l f i n d i n g s .

1V - SUMMARY

We have shown t h a t s i n g l e and double l a y e r CoCr f i l m s w i t h adequate magnetic p r o p e r t i e s f o r perpendicular r e c o r d i n g a p p l i c a t i o n s can be prepared by h i g h r a t e s p u t t e r i n g onto r i g i d and f l e x i b l e amorphous substrates. The important process parameters have been confirmed and h i g h l i g h t e d . The d i f f e r e n c e s i n t h e p r o p e r t i e s o f f i l m s from t h e a l l o y and composite type o f t a r g e t s w i l l be i n v e s t i g a t e d f u r t h e r . I n p a r t i c u l a r , i n i t i a l n u c l e a t i o n and subsequent growth c o n d i t i o n s w i l l be studied.

V - REFERENCES

/1/ Futamoto, M., Honda, Y., Kakibashi, H., and Yoshida, K., paper DB-OlINTERMAG 1985 /2/ Iwasaki, S., IEEE Trans. Maqn. MAG-20 (1984)

/3/ Grundy, P.J. and A l i , M., J. Magn. and Magn. Mat. 40 (1983) 154

/4/ Grundy. P.J., A l i , M. and Faunce, C .A., IEEE Trans. Maqn. MAG-20 (1984) 794 /5/ Weilinga, T.and Lodder, J.C., IEEE Trans. Magn. MAG-20 (1984) 57

/6/ Sugita, R., IEEE Trans. Magn. MAG-20 (1984) 687.