COMPOSITION AND TEMPERATURE DEPENDENCE OF ... - Hal

COMPOSITION AND TEMPERATURE DEPENDENCE OF MAGNETIC TRANSITIONS IN SPUTTERED GdCo FILMS FOR MAGNETO-OPTIC DATA STORAGE R. Carey, D. Newman, B. Thomas

To cite this version: R. Carey, D. Newman, B. Thomas. COMPOSITION AND TEMPERATURE DEPENDENCE OF MAGNETIC TRANSITIONS IN SPUTTERED GdCo FILMS FOR MAGNETOOPTIC DATA STORAGE. Journal de Physique Colloques, 1985, 46 (C6), pp.C6-19-C6-24. .

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J O U R N A L DE PHYSIQUE Colloque C6, suppl6ment au n09, Tome 46, septembre 1985

page C6-19

COMPOSITION AND TEMPERATURE DEPENDENCE OF MAGNETIC TRANSITIONS IN SPUTTERED GdCo FILMS FOR MAGNETO-OPTIC DATA STORAGE R.

Carey, D.M.

Newman and B . W . J .

Thomas

Coventry (Lanchester) Polytechnic, Priory S t r e e t , Coventry CVI SFB, U.K. R6sw6 - Le retournement de l ' a i m a n t a t i o n dans p l u s i e u r s couches minces Gd-Co de composition e n t r e 20 e t 46 % Gd, dPpos6es par pulvCrisation r . f . , a C t C 6tudig par e f f e t Kerr p o l a i r e B A = 633 mm, sous des champs jusqu'b + 10 kOe. Les observations 2 tempgrature ambiante montrent que c e r t a i n e s compositions, qui ne correspondent pas b c e l l e p u r l a q u e l l e une compensation d e l'aimantat i o n e s t a t t e n d u e ("J 20/80), prgsentent une f o r t e c o e r c i v i t g i n h a b i t u e l l e , jusqu'b 6 kOe, e t d e s c y c l e s d t h y s t 6 r 6 s i s t r s s c a r r 6 s qui indiquent une a n i s o t r o p i e p e r p e n d i c u l a i r e i n t r i n s s q u e s i g n i f i c a t i v e . Le tracC de l a c o e r c i v i t 6 en f o n c t i o n de l a composition met donc en Cvidence d e s maxima t r s s marqu6s qui sont a t t r i b u g s d e s t r a n s i t i o n s de s t r u c t u r e du sous-r6seau magnCtique dominant dans l e f i l m , e t semblent ggalement s e manif e s t e r par d e s maxima dans l ' a i m a n t a t i o n b s a t u r a t i o n . Des t r a n s i t i o n s magn6tiques pour d e s compositions c h o i s i e s sont a u s s i prod u i t e s facilement par d e s v a r i a t i o n s thermiques. La dgpendance en tempsrature d e l a c o e r c i v i t g d e s f i l m s correspond b i e n b c e l l e r e q u i s e pour un m i l i e u magngto-optique de stockage p i l o t 6 par un f a i s c e a u .

Abstract - Magnetization r e v e r s a l i n a number o f r . f . s p u t t e r e d GdCo f i l m s with compositions i n t h e range 20-46 a t . % Gd has been s t u d i e d using t h e p o l a r Kerr e f f e c t a t 1 = 633 nm and d r i v e f i e l d s up t o + 1 0 kOe. Room temperature observations show t h a t some compositions o t h e r t h a n t h a t expected t o achieve room temperatue magnetization compensation ( 20/80) have unusually l a r g e c o e r c i v i t i e s , up t o 6 kOe, and very square h y s t e r e s i s loops i n d i c a t i v e o f s i g n i f i c a n t i n h e r e n t perpendicular a n i s o t r o p y . The c o e r c i v i t y v e r s u s composition p l o t t h e r e f o r e shows some extremely sharp c o e r c i v i t y maxima which a r e thought t o b e a s s o c i a t e d with t r a n s i t i o n s i n t h e magnetic dominance o f t h e f i l m subnetwork s t r u c t u r e and a l s o seem t o r e l a t e t o maxima i n t h e s a t u r a t i o n magnetization.

-

Magnetic t r a n s i t i o n s a t s e l e c t e d compositions a r e a l s o e a s i l y produced by a change i n f i l m temperature. The temperature dependent c o e r c i v i t y o f t h e s e f i l m s i s w e l l s u i t e d t o t h a t r e q u i r e d by beam a d d r e s s a b l e magneto-optic s t o r a g e media.

INTRODUCTION We r e p o r t t h e e x i s t e n c e o f previously unknown magnetic t r a n s i t i o n s i n GdCo. The s i g n o f t h e p o l a r Kerr e f f e c t a t 633 nm i n GdCo films has been observed t o be a n o s c i l l a t o r y f u n c t i o n o f f i l m composition. Abrupt r e v e r s a l s have been recorded a t Gd c o n t e n t s o f 34 and 41.5 a t . % . Sharp peaks i n c o e r c i v i t y (H ) measured along t h e f i l m normal a l s o occur a t t h e s e compositions. Moreover, filmsCwith compositions immediately a d j a c e n t t o both 36 and 41.5 a t . % Gd may a l s o b e d r i v e n through t h e t r a n s i t i o n by varying t h e i r temperatures. Lachowicz has noted t h a t t h e Kerr r o t a t i o n i n GdCo o r i g i n a t e s p r i n c i p a l l y from t h e

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985603

C6-20

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Co subnetwork and t h a t consequently i t s s i g n may b e used t o determine which o f t h e two subnetworks i s dominant a t any p a r t i c u l a r composition and temperature. I n GdCo t h e s a t u r a t i o n magnetization (M ) has a s t r o n g compositional and temperature dependence. It i s well k n o h t h a t a t room temperature M passes through zero a t some composition between 1 8 and 22 a t . % Gd a s t h e mainetic c o n t r i b u t i o n s from t h e a n t i - f e r r o m a g n e t i c a l l y coupled Co and Gd subnetworks compensate. Compensation i s accompanied by a peak i n c o e r c i v i t y a s w e l l a s a change o f s i g n i n t h e p o l a r Kerr e f f e c t a s t h e Gd subnetwork assumes dominance from t h e p r e v i o u s l y dominant Co subnetwork. Compensated o r near compensated f i l m s a i s o e x h i b i t a c h a r a c t e r i s t i c perpendicular a n i s o t r o p y with easy a x i s along t h e f i l m normal. Films with compositions such t h a t t h e i r behaviour is Co-dominant a t room temperature r e q u i r e cooling t o a c h i e v e compensation, t h e i r compensation temperature ( Tcomp) i s below room temperature Conversely, Tcomp i s above room temperature f o r

.

Gd-dominant f i l m s with compositions on t h e h i g h Gd s i d e of t h e room temperature compensation p o i n t . Such f i l m s may b e heated through compensation. I m p l i c i t i n most o f t h e published l i t e r a t u r e on GdCo i s t h e understanding t h a t once t h e Gd-network e s t a b l i s h e s dominance a t room temperature ( i . e . when t h e Gd content reaches r 20 a t . $ ) it t h e r e a f t e r remains dominant a t a l l compositions with higher Gd c o n t e n t . A l l such compositions should t h e r e f o r e a t t a i n compensation a t temperatures above room temperature. The f u r t h e r t r a n s i t i o r s i n dominance , implied by t h e p o l a r Kerr e f f e c t r e v e r s i n g s i g n a t c o m p s i t i o r s w e l l removed from t h e room temperature compensation p o i n t , a r e not a n t i c i p a t e d i n any e x i s t i n g l i t e r a t u r e . PREPARATION AND MEASUREMENT TECHNIQUES A Nordiko 2000M r . f . s p u t t e r i n g system running a t a n argon p r e s s u r e o f 1 0 mtorr was used t o prepare a s e l e c t i o n o f f i l m s with compositions i n t h e range 22-46 a t . % Gd. Compositional v a r i a t i o n was achieved by t h e u s e o f mosaic t a r g e t s o f Co on Gd o r Gd on Co augmented a s necessary by negative s u b s t r a t e b i a s f o r f i n e t u n i n g . The deposited composition was determined from t h e a c t u a l sample using a P h i l i p s 9100/60 energy d i s p e r s i v e X-ray a n a l y s i s u n i t . A l l f i l m s were deposited t o a t h i c k n e s s o f 1 urn on t h ~ i g r l a s s s u b s t r a t e s and coated with a p r o t e c t i v e l a y e r o f S i 0 2 b e f o r e removal from t h e system. P o l a r Kerr r o t a t i o n s were measured a t normal incidence u s i n g a He-Ne l a s e r source i n a Newport A-type electromagnet producing f i e l d s up t o k 1 0 kOe. The h y s t e r e s i s loops were obtained by p l o t t i n g t h e r o t a t i o n a s a f u n c t i o n o f a p p l i e d f i e l d . During t h e s e measurements t h e samples were mounted on e i t h e r a P e l t i e r s t a g e p e r m i t t i n g temperature excursions between -20°c t o +50°c o r on a n e l e c t r i c a l l y h e a t e d aluminium block t o achieve temperatures up t o +x)oOC. RESULTS Table I l i s t s t h e s i g n and magnitude o f t h e p o l a r Kerr r o t a t i o n measured a t 633 nm f o r f i l m s with d i f f e r e n t compositions. When t h i s i s examined i n conjunction with Fig. ( 1 ) it can b e seen t h a t t h e s e r e v e r s a l s i n s i g n c o i n c i d e w i t h peaks i n coercivity.

I

Gd c o n t e n t a t .%

>

22.0 23.1 25.5 30.6 33.7 34.6 37 .O 37.7 39 .O 40.9 43.0 45.3 45.9

p o l a r Kerr r o t a t i o n min

- 15.01 - 12.43 - 11.01

-

11.50

- 13.86

+

13.70

+ 11.91 - 10.55 - 9.10 +

9.10

+ 7.11 + 7.82 +

7.98

Table I:

The v a r i a t i o n of p o l a r Kerr r o t a t i o n with f i l m composition.

Fig.(l)

The compositional v a r i a t i o n

o f c o e r c i v i t y i n GdCo f i l m s .

Hysteresis loops showing ( A ) Co and ( B ) Gd dominated behaviour observed immediately e i t h e r s i d e t h e peak c e n t r e d a t 34 a t . $ a r e shown i n F i g . ( 2 ) . The sequence of loops depicted i n F i g . ( 3 ) shows t h e change i n behaviour t h a t occurs f o r only a 2% i n c r e a s e i n Gd content across t h e t r a n s i t i o n a t approximately 4 1 a t . % Gd. The thermomagnetic p r o p e r t i e s o f t h e f i l m s a r e i l l u s t r a t e d i n F i g s . ( & ) through ( 6 ) . Sample (B) of F i g . ( 2 ) has a composition o f 36.4 a t Gd. I t s h y s t e r e t i c behaviour a s a f u n c t i o n of temperature i s shown i n F i g . ( & ) . The c o e r c i v i t y o f t h i s sample shows a c l e a r peak a s it executes a t r a n s i t i o n i n subnetwork dominance a t approximately 4 7 O ~ ,s e e ~ i g( .5 ) . Sample ( A ) of F i g . ( 2 ) shows s i m i l a r behaviour0 ' b u t being Co dominant a t room temperature, has a t r a n s i t i o n temperature of 2 - 5.5 C , see also Fig. ( 5 ) .

.%

Films with compositions c l o s e t o t h e c o e r c i v i t ~peak a t 4 1 at .%Gd show guch t h e same behaviour. F i g . ( 6 ) shows a t r a n s i t i o n occurrmg a t approximately 36 C i n a f i l m Gd dominant a t room temperature.

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Fig.(2)

Square h y s t e r e s i s loops o b t a i n e d e i t h e r s i d e t h e peak i n C o e r c i v i t y a t ' 34 a t .%Gd. ( A ) Co dominated (B) Gd dominated. Drive f i e l d s i 1 0 kOe.

Fig.(3)

The compositional dependence o f t h e h y s t e r e s i s loops a c r o s s t h e c o e r c i v i t y peak a t 40 a t . % Gd. The Gd content i n c r e a s e s by 2 a t . % between 1 and 6. Drive f i e l d s i 10 kOe.

Fig.(4)

The h y s t e r e t i c behaviour o f a f i l m with a Gd content o f 36.4 a t . % a s a f u n c t i o n of temperature.

Temp. 'C

Fig. ( 5 )

The temperature dependence o f t h e c o e r c i v i t y f o r f i l m s with compositions e i t h e r s i d e t h e peak i n c o e r c i v i t y a t a G d content of 34 a t . % .

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Fig.(6)

The v a r i a t i o n o f c o e r c i v i t y with temperature f o r a f i l m with 4 1 a t . % Gd.

DISCUSSION Apparent t r a n s i t i o s i n magnetic subnetwork dominance have been observed i n GdCo f i l m s a t compositions c l o s e t o 34 and 4 1 a t . % Gd. A change i n subnetwork dominance i s u s u a l l y t a k e n t o i n d i c a t e compensation. However, although t h e magnetic and thennomagnetic behaviour i l l u s t r a t e d i n ~ i ~ s . ( through 2 ) ( 6 ) i s analogous t o t h a t observed a t t h e room temperature compensation p o i n t , it i s not l i k e l y t h a t compensation i s o c c u r r i n g a t t h e s e compositions. The t e m p e r a t u r g a t which t h e s e t r a n s i d i t i o n s occur a r e incornpatable wi h t h e compensation temperatures p r e d i c t e d i n Published work. According t o ChaudhariB c o m p o s i t i o ~ w i t hGd contents around 30 t o 40 a t . % should have compensation temperatures between X)O and 2 5 0 ~ ~ Moreover, . compensation temperatures below room temperature should not b e observed a t t h e s e o r any o t h e r composition above approximately 20 a t . % Gd. Secondly, a t t h e room i s a minimum ( z e r o ) with r e s p e c t t o composition temperature compensation p o i n t 1M and temperature. A t t h e c ~ m ~ o s i t f o n s w h e rwe e o s e r v e t h e s e new t r a n s i t i o n s , IMS i s c l o s e t o a maximum a s a f u n c t i o n of c o m p o s i t i o 8 with a value = 25% t h a t of pure cobalt

1

I

The p r e c i s e o r i g i n o f t h e s e new t r a n s i t i o n s i n GdCo f i l m s remains t h e s u b j e c t o f further investigation. REFERENCES 1 Lachowicz, H.K.

IEEE Trans .Mag. Vol. MAG-20 N0.5,

2

Chaudhari, P . , Cuomo, J.J. and Gambino, R . J . ,

3

Bendson, S.A., Judy, J . H . ,

(1984) 1417-1422

Appl.Phys.Lett.

Vo1.22 No.7 (1973) 337-339.

IEEE Trans .Mag. MAG-9, No.5 (1973) 627-631.