THE RELATIONSHIP BETWEEN INTERFACIAL STRUCTURES AND MAGNETIC PROPERTIES OF METALLIC MULTILAYERED FILMS, Mo/Co AND Ag/Co

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THE RELATIONSHIP BETWEEN INTERFACIAL STRUCTURES AND MAGNETIC PROPERTIES OF

METALLIC MULTILAYERED FILMS, Mo/Co AND Ag/Co

A. Yamaguchi, H. Miyashita, T. Kamemura, Y. Togawa, R. Yamamoto

To cite this version:

A. Yamaguchi, H. Miyashita, T. Kamemura, Y. Togawa, R. Yamamoto. THE RELATIONSHIP BE-

TWEEN INTERFACIAL STRUCTURES AND MAGNETIC PROPERTIES OF METALLIC MUL-

TILAYERED FILMS, Mo/Co AND Ag/Co. Journal de Physique Colloques, 1990, 51 (C1), pp.C1-

927-C1-932. �10.1051/jphyscol:19901146�. �jpa-00230058�

COLLOQUE DE PHYSIQUE

Colloque Cl, suppl6ment au n o l , Tome 51, janvier

1990

THE RELATIONSHIP BETWEEN INTERFACIAL STRUCTURES AND MAGNETIC PROPERTIES OF METALLIC MULTILAYERED FILMS, Mo/Co AND Ag/Co

A. YAMAGUCHI, H. MIYASHITA, T. KAMEMURA, Y. TOGAWA and

R.

YAMAMOTO

Department o f Materials Science, Faculty of Engineering, University

of

Tokyo, Bunkyo-ku, Tokyo 113, Japan

A b s t r a c t - The s t r u c t u r a l and m a g n e t i c p r o p e r t i e s o f two d i f f e r e n t m u l t i l a y e r s y s t e m s , Mo/Co and Ag/Co were i n v e s t i g a t e d . The m u l t i l a y e r e d f i l m s were p r e p a r e d by rf-magnetron s p u t t e r i n g . The s a t u r a t i o n m a g n e t i z a t i o n ( M S ) o f Mo/Co f i l m s d e c r e a s e s r a p i d l y w i t h d e c r e a s i n g m o d u l a t i o n wave-lengths. C o n t r a r y t o t h i s r e s u l t we have o b s e r v e d a s l i g h t i n c r e a s e o f t h e MS o f Ag/Co. One o f t h e r e a s o n s f o r t h e d i f f e r e n c e i n t h e m a g n e t i c p r o p e r t i e s between two s y s t e m s c a n be a s c r i b e d t o t h e h i g h e r s o l i d s o l u b i l i t y o f t h e Mo/Co system compared w i t h Ag/Co. The i n f l u e n c e o f t h e s u b s t r a t e t e m p e r a t u r e and p o s t h e a t t r e a t m e n t on t h e i n t e r f a c i a l s t r u c t u r e and m a g n e t i c p r o p e r t i e s h a s a l s o been s t u d i e d . F u r t h e r m o r e , we have measured t h e m a g n e t i z a t i o n c u r v e s , and K e r r r o t a t i o n a n g l e s a s a f u n c t i o n o f m o d u l a t i o n l e n g t h . The m a g n e t i c p r o p e r t i e s a r e d i s c u s s e d i n more d e t a i l i n r e l a t i o n t o t h e i n t e r f a c i a l s t r u c t u r e s .

1

-

INTRODUCTION

The m a g n e t i c p r o p e r t i e s o f m e t a l l i c m u l t i l a y e r e d f i l m s a r e o f g r e a t i n t e r e s t b o t h from f u n d a m e n t a l and t e c h n o l o g i c a l view p o i n t s . E s p e c i a l l y , m a g n e t i c p e r p e n d i c u l a r a n i s o t r o p y o f m u l t i l a y e r e d f i l m s have been a t t r a c t i n g much a t t e n t i o n i n r e l a t i o n t o m a g n e t i c r e c o r d i n g media /1,2/. S i n c e t h i s u n i q u e c h a r a c t e r i s t i c s which we c a n n o t o b t a i n f o r b u l k m a t e r i a l s s t r o n g l y depends on t h e i n t e r f a c i a l s t r u c t u r e o f t h e f i l m s , it becomes i m p o r t a n t t o s t u d y t h e r e l a t i o n s h i p between m a g n e t i c and i n t e r f a c i a l s t r u c t u r e s .

M u l t i l a y e r e d f i l m s s u c h a s MO-CO which a r e composed o f two d i f f e r e n t k i n d s o f e l e m e n t s f o r m i n g a l l o y p h a s e s a t e q u i l i b r i u m can be e x p e c t e d t o have d i f f u s e d r e g i o n s a t t h e i n t e r f a c e s . I t i s , t h e r e f o r e , i n t e r e s t i n g t o i n v e s t i g a t e t h e change o f s t r u c t u r e and magnetism w i t h v a r y i n g b i l a y e r t h i c k n e s s ( A ) . S i n c e Ag-CO b i n a r y system i s a l m o s t i m m i s c i b l e , i t can be c o n s i d e r e d t h a t much s h a r p e r c o m p o s i t i o n a l m o d u l a t i o n s w i l l be formed f o r t h i s s y s t e m .

I n t h i s p a p e r , we r e p o r t t h e r e s u l t s o f x - r a y d i f f r a c t i o n , m a g n e t i z a t i o n and p o l a r m a g n e t o - o p t i c a l K e r r r o t a t i o n a n g l e ( 8 )measurements on Mo/Co a n d Ag/Co m u l t i l a y e r e d f i l m s . F u r t h e r m o r b , t h e i n f l u e n c e o f s u b s t r a k e t e m p e r a t u r e d u r i n g t h e d e p o s i t i o n and p o s t a n n e a l t r e a t m e n t s upon s t r u c t u r a l and m a g n e t i c p r o p e r t i e s o f m u l t i l a y e r e d f i l m s a r e s t u d i e d .

2

-

SAMPLE PREPARATION

Mo/Co and Ag/Co m u l t i l a y e r s were p r e p a r e d by a s i n g l e c a t h o d e r f - m a g n e t r o n s p u t t e r i n g s y s t e m . As shown i n F i g . 1 , i n o r d e r t o s y n t h e s i z e c o m p o s i t i o n a l l y modulated s t r u c t u r e s , s e m i c i r c u l a r t a r g e t s o f molybdenum(99.99%) o r s i . l v e r ( 9 9 . 9 9 % ) a n d c o b a l t ( 9 9 . 9 9 % ) a r e p l a c e d on t h e same c a t h o d e and a s h i e l d i s a r r a n g e d a t t h e boundary o f t h e s e two t a r g e t s s o a s t o p r e v e n t mixing of s p u t t e r e d f l u x e s . The s u b s t r a t e s a r e f i x e d on a r o t a t i n g t a b l e , which a l t e r n a t e l y moves them from one f l u x t o t h e o t h e r . The b i l a y e r t h i c k n e s s was c o n t r o l l e d by t h e r o t a t i n g s p e e d o f t h e t a b l e . The c o n c e n t r a t i o n o f Mo/Co a n d Ag/Co was a d j u s t e d by u s i n g a n a p p r o p r i a t e mask and checked by e n e r m d i s ~ e r s i v e x - r a y r n i c r o ~ r o b e a n a l v s i s . The

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19901146

Cl-928 COLLOQUE DE PHYSIQUE

g p u t t e r i n g was done w i t h h i g h - p u r i t y Ar a t 5 x l 0 - ' ~ o r r , a f t e r pumping t h e chamber t o T o r r . G l a s s s u b s t r a t e s were u s e d and d u r i n g t h e d e p o s i t i o n o f Mo/Co m u l t i l a y e r e d t h e s u b s t r a t e t e m p e r a t u r e ( T s ) was c o n t r o l l e d by a n e l e c t r i c a l h e a t e r t o i n v e s t i g a t ~ i n f l u e n c e o f Ts upon t h e f i l m s t r u c t u r e s . Ts was a b o u t 100°C w i t h o u t h e a t i n g and a b o u t when t h e h e a t e r was u s e d . P o s t a n n e a l t r e a t m e n t was e r f o r m e d f o r t h e Ag/Co m u l t i w i t h i n f r a r e d - g o l d e n - i m a g e - f u r n a c e i n a vacuum ( i l x 1 0 - ' T o r r ) . The a n n e a l i n g t e n p e r a t u r , 300°C and t h e a n n e a l i n g t i m e was 3 0 min.

The b i l a y e r t h i c k n e s s o f m u l t i l a y e r e d f i l m s c a n b e d e t e r m i n e d from x-ray d i f f r ~ a n a l y s i s and showed good a g r e e m e n t w i t h t h e d e s i g n e d v a l u e s . The b i l a y e r t h i c k n e s s o f m u l t i l a y e r s r a n g e d between 18A-180A and t h a t o f Ag/Co m u l t i l a y e r s between 30, r e s p e c t i v e l y , where t h e Mo/Co and Ag/Co t h i c k n e s s r a t i o was f i x e d a t 1 : l an1 r e s p e c t i v e l y , w i t h a c c u r a c y b e i n g w i t h i n '5%.

3

-

X-RAY DIFFRACTION ANALYSIS A

.

Mo/Co M u l t i l a y e r s

I t i s c o n f i r m e d by s m a l l a n g l e x-ray d i f f r a c t i o n p a t t e r n s t h a t a l l t h e s a m p l e s i i

t h e d e s i g n e d p e r i o d i c c o m p o s i t i o n a l m o d u l a t i o n s . F i g . 2 shows t h e t y p i c a l h i g h a n g l e d i f f r a c t i o n p a t t e r n s o f t h e f i l m s w i t h Ts%10O0C. The peak p o s i t i o n s o f t h e main r e f l c s u g g e s t t h a t t h e ( 1 1 0 ) l a t t i c e p l a n e s o f b c c MO and ( 0 0 0 1 ) l a t t i c e p l a n e s o f hcp CO ^I t h e ( 1 1 1 ) l a t t i c e p l a n e s o f f c c CO a r e s t a c k e d p a r a l l e l t o t h e f i l m p l a n e . I t i:

p o s s i b l e t o d e t e r m i n e t h e l a t t i c e s t r u c t u r e o f CO l a y e r o n l y from t h e s e d i f f r ; p a t t e r n s . M u l t i l a y e r w i t h A=18A d o e s n o t show any s t r o n g peak which i n d i c a t e s t h a s t a c k i n g o f a t o m i c p l a n e s i s d i s o r d e r e d . On o t h e r f i l m s , r e l a t i v e l y s t r o n g e r p e a k s d e t e c t e d , b u t t h e peak w i d t h s a r e b r o a d compared w i t h t h e d i f f r a c t i o n p a t t e r n s of m u l t i l a y e r e d f i l m s / 3 , 4 / . The l o n g r a n g e s p a t i a l c o h e r e n c y o f Mo/Co s y s t e m can be c o n s . t o be r a t h e r low. S i n c e t h e a t o m i c r a d i i o f c o n s t i t u e n t e l e m e n t s i n t h e Mo/Co s y s t e l q u i t e d i f f e r e n t , it i s d i f f i c u l t t o o b t a i n c r y s t a l l o g r a p h i c a l l y e x c e l l e n t m u l t i l a y e r s .

F i g . 3 shows two t y p i c a l h i g h a n g l e d i f f r a c t i o n p a t t e r n s o f t h e f i l m s where Tss.

Compared w i t h F i g . 2 , one can s e e t h a t t h e peak w i d t h s become n a r r o w e r when h e a t i n l s u b s t r a t e s . On t h e o t h e r hand, t h e i n t e n s i t i e s o f s a t e l l i t e p e a k s a g a i n s t t h a t o f t h e peak become weaker when t h e Ts i s h i g h e r . One o f t h e r e a s o n s f o r t h i s weakening cc a t t r i b u t e d t o t h e b r o a d e n i n g o f t h e d i f f u s e d r e g i o n s a t t h e i n t e r f a c e s . I n a d d i t i o n , shows t h e comparison o f s m a l l a n g l e d i f f r a c t i o n p a t t e r n s o f t h e f i l m s o f T s ~ 1 0 O 0 ( Ts230O0C. I n t h e c a s e o f Ts%30O0C, t h e 3 r d o r d e r and 4 t h o r d e r p e a k s o f s u p e r l t r e f l e c t i o n s d i s a p p e a r and o t h e r p e a k s a r e weaker t h a n t h o s e o f f i l m s w i t h Ts\10OoC.

r e s u l t s t r o n g l y s u p p o r t s t h e f o l l o w i n g p h y s i c a l l y p l a u s i b l e e x p l a n a t i o n t h a t t h e r e ^I t h i c k e r d i f f u s e d r e g i o n s a t t h e i n t e r f a c e s i n t h e f i l m s p r e p a r e d w i t h highei F u r t h e r m o r e , we have c a l c u l a t e d t h e d i f f r a c t i o n p a t t e r n s o f Mo/Co u t i l i z i n g t h e s o (

" s t e p modelu which assumes t h a t t h e m u l t i l a y e r s c o n s i s t o f p u r e C O and MO l a y e r s and a r e no mixed r e g i o n s a t t h e MO-CO i n t e r f a c e s . Compared w i t h t h i s t h e o r e t i c a l c a l c u l a ' we have f o u n d t h a t t h e i n t e n s i t i e s o f s a t e l l i t e p e a k s o b t a i n e d from t h e e x p e r i m e n t a l a r e r a t h e r weak even when Ts i s 100°C. T h i s shows t h a t t h e s p u t t e r i n g p r o c e s s i n e v i t a b l y form d i f f u s e d r e g i o n s a t t h e i n t e r f a c e s b e c a u s e o f h i g h e n e r g y o f s p u ' p a r t i c l e s .

No p e a k s were o b s e r v e d c o r r e s p o n d i n g t o t h e r e f l e c t i o n s from o x i d e s o r i n t e r m e . compounds.

B. Ag/Co M u l t i l a y e r s

F i g . 5 shows t h e t y p i c a l h i g h a n g l e d i f f r a c t i o n p a t t e r n s o f a s - d e p o s i t e d m u l t i l a y e r e d f i l m s . These p a t t e r n s show t h a t (111) l a t t i c e p l a n e s o f f c c Ag and ' l a t t i c e p l a n e s of hcp CO a n d / o r ( 1 1 1 ) l a t t i c e p l a n e s of f c c C O a r e s t a c k e d p a r a l l e l t(

f i l m p l a n e . S i n c e t h e r a t i o o f Ag/Co was 7:2 (31 a t . % C O ) , t h e p e a k s a t t h e l o w e r anglc s i d e ) a r e much s t r o n g e r t h a n t h o s e a t t h e h i g h e r a n g l e (CO s i d e ) .

F i g . 6 shows t h e d i f f r a c t i o n p a t t e r n s of s a m p l e s a n n e a l e d a t 300°C f o r 30 min. I n o f A=62A, t h e w i d t h s o f p e a k s were n a r r o w e r and t h e i n t e n s i t y r a t i o s o f s a t e l l i t e peal main peak were h i g h e r t h a n t h o s e o f a s - d e p o s i t e d sample. T h e s e r e s u l t s i n d i c a t e t h a i s t r u c t u r a l and c h e m i c a l o r d e r o f m u l t i l a y e r e d f i l m s became h i g h e r by t h e a n n e a l i n g , 30(

3 0 min. As mentioned a b o v e , Ag and CO a r e m u t u a l l y i n s o l u b l e e a c h o t h e r and t h e i n t e r i e n e r g y o f Ag and CO l a y e r s i s l a r g e p o s i t i v e v a l u e . I t c a n b e c o n s i d e r e d , t h e r e f o r e , u n d e r a n a p p r o p r i a t e c o n d i t i o n , a "back" d i f f u s i o n o f Ag and CO w i l l o c c u r a t t h e d i f f u s e d i n t e r f a c e s and t h i s c a n l e a d t o s h a r p e r i n t e r f a c e s . The c h e m i c a l o r d e r i n g dt t h e h e a t t r e a t m e n t c a n be e x p l a i n e d by t h i s b e h a v i o r .

I n c a s e o f A=30A, however, s a t e l l i t e p e a k s a n d main peak d i s a p p e a r e d by t h e t r e a t m e n t and new s e g r e g a t e d two p e a k s l o c a t e d a r o u n d t h e peak p o s i t i o n s of b u l k An ar

+

2 8 ( d e g ) 28 (deg)

Fig.1 Schematic diagram of Fig.2 Typical high angle diffraction patterns of Mo/Co sputtering system used multilayered films of Ts%10O0C.

for preparation of magnetic multilayers.

28 (deg)

Fig.3 High angle diffraction patterns of Mo/Co

(a) multilayered films of Tsz30OoC.

5 10

28 (deg)

( b )

Fig.4 Small angle diffraction patt.erns of Mo/Co films.

(a):Ts%10O0C (b):Ts%30OoC

2 8 ( d e g ) 2 8 (dog)

Fig.5 Typical high angle diffraction patterns of as-deposited Ag/Co multilayered films.

2 8 (de9) I 8 (deg)

Fig.6 High angle diffraction patterns of post- annealed Ag/Co multilayers.

Fig.7 Small angle diffraction patterns of Ag/Co multilayered films.

(a) :as-deposited

(b)

:annealed

Cl-930 COLLOQlJE DE PHYSIQUE

were observed. This means that the layered structure was partially destroyed by treatment. The small angle diffraction patterns shown in Fig.7 also support this conclus By annealing, the diffraction pattern of the film with thicker

A(66A)

hardly changed, the intensities

of

superlattice diffractjon decreased when A was thinner than A=LOA,

T'

results suggest the existence of two competitive heat-induced reactioris, the inter sharpening and the partial crystal-grain coarsening. The latter reaction will be predomi in the cases of thln

A

or excess annealing. Therefore, optimum annealing conditions in o to improve the quality of multilayers differ with varying

A

and the determination of t conditions will be important hereafter.

f+ - MAGNETIC PROPERTIES

A.

Mo/Co multilayers

(i) Magnetization measurements

The magnetization of Mo/Co and Ag/co multilayered films was measured at temperature with ;he V ~ ~ ( v i b r a t i n ~ sample magnetometer), applying the magnetic field up lSkOe, both parallel and perpendicular to the film plane. A sequence of hysteresis loops the Mo/Co multilayered films wi.th bilayer thicknesses 32,

60

and 120A are shown in Fi The easy magnetization directions of all the samples were parallel to the film plane.

samples of A=60 and 120A show typical ferromagnetic features and the shape magn anisotropy is predominant. However, the magnetization curve of the film of A=32A show weak magnetism which suggests that these involved two or more kinds of magnetism, such ferromagnetism and paramagnetism or superparamagnetism.

The bilayer thickness dependence of saturation magnetization (MS) is shown in

F

where all the cases the magnetic field was applied in the plane of the samples. The decreases consistently with A and this dependence sLrongly indicates that there e magnetically-dead regions at MO-CO interfaces. The physical origin of the "dead" layers uncertain until now. Since the CO-MO alloy shows a weak magnetism and there exist diff layers at interfaces from x-ray analysis the width of which increases with the subst temperature Ts, the most possible reason of the reduction of MS is that the magnetic mom of CO atoms are much reduced in the mixed regions at MO-CO interfaces.

(ii) Magneto-optical Kerr rotation angie measurements

The wavelength dependence of polar magneto-optical Kerr rotation angle (0 k ) of M multilayers was measured by the Kerr spectrometer in the wavelength region from 250nm 800nm. The measurement was performed under the condition of saturation magnetization.

incident angle of light was 10° from the normal to the film plane.

Fig.10 shows the wavelength dependence of Bk of Mo/Co multilayered films with sev kinds of A . The €lk spectrum of pure CO film are also shown for comparison. 8 multilayered films 1s smaller than that of pure CO film. Xatayama et al. has reportei enhancement of 0 of Cu/Fe multilayered films and concluded that the enhancement originated from tke absorption edge transition /5/. In the case of MO, the position of absorption edge is around 1000nm. In addition, the saturation magnetization of M multilayers were smaller than that of pure CO because of mixed regions at interfaces mentioned above. We have considered that these were the reasons for the small

Ok

of

M

multilayers.

B.

Magnetization measurements of Ag/Co multilayers

The sequential changes of hysteresis loops for as-deposited Ag/Co multilayers varying the thickness of CO layers (t ) are demonstrated in Fig.11. The resi magnetization with the applied field garF?lel to the film plane ( ~ r ~ ) decreased and with the field normal to the plane (Mr ) increased with a decrease in t up to ~ r " / ~ r

0.7.

This indicates that

a

large perpendicular anisotropy was induced a% this anisot increased with a decrease in t Nevertheless, the hysteresis loop of the films with below

17A

lost this tendency rapldly and looked like a normal - CO' demagnetization predomi - ferromagnetic film. This suggests that some structural disordering occurred at this po Fig.12 shows the tCo dependence on Ms. All the samples exhibited a larger MS than of bulk Co. Moreover, the MS increased slightly with a decrease of t which is q different from the behavior of Mo/Co Multilayers. Krishnan et al. h&' reported on enhancement of magnetization of Ag/Ni multilayer films and concluded that this enhance might be originated from a modification of band structure of Ni because of the presence neighboring Ag atoms /6/. It can be considered that the mutual solubilities of ferromagn and non-magnetic substances play an important role in determining the magnetic propertie Multilayers.

Fig.9 Bilayer thickness dependence of MS of Mo/Co multilayered films.

I ¹

WAVELENGTH ( nm 1

I

L

Fig.10 Wavelength dependence of Kerr rotation angle of Mo/Co multilayers. The upper- most layer is Co.

Fig.8 Magnetization hysteresis loops of Mo/Co multi- layered films.

MODULATION LENGTH.

A ( A )

Fig.11 Sequential change of hysteresis loops for as-deposited Ag/Co multilayered films.

T l I I I I I I ,

20 10

60 80

CO

LAYER THICKNESS

.

^tc,

^{( A )}

Fig.12 The t dependence on MS of Ag/Co multi?gyered f ilms.

tco (A) - 1

- 3L

Fig.13 The t dependence on magnetic anisofropy of Ag/Co multi.l.ayers.

Cl-932 COLLOQUE DE PHYSIQUE

Total magnetic anisotropy, Ku, is defined as the encircled area between perpendicular and the parallel magnetization curves per unit volume of Co. Ku take:

positive value when the magnetization is preferably oriented perpendicular to the J

plane. If we denote Ks as the anisotropy originating from the interface per unit area an(

as the contribution to the anisotropy per unit volume of CO, the total anisotropy phenomenologically be written as

Here the factor 2 arises from the two interfaces of each CO layer. The Kv consists magnetostatic or demagnetization energy, magnetocrystalline anisotropy and magnetoela:

energy. In Fig.13 the anisotropy is shown as function of the thickness of the CO layers.

total anisotropy energy is well described by Eq. (l). From the measurements, we have obta.

and

Carcia et al. performed a similar anejlysi2 for Pd/Co multilayegs wh'ch were made by sputtering /l/. They found Ks =

0.16

X 10 J/m and Kv = -0.37 X 10 J/m

3 .

H.J.G.Draaism, al. rep9rted on Au/Co m~ltil~yers~synthesized by ion beam sputtering and found Ks = 0.

I O - ~

J/m and Kv = -0.75 X 10 J/m /2/. It is still not certain whether the the differe in Ks and Kv are resulted from the system difference or from other reasons.

Using the values (2) and

(31,

we have estimated the critical thickness of CO at W

Ku turns positive, tCo =

7A.

We have not realized, however, positive Ku sample by magnetron sputtering apparatus. It will be relevant to some structural disordering at t 17A as mentioned before and we believe that it may be possible to synthesize

g

perpendicular magnetization films if prepared by other processes such as evapora technique.

5

-

CONCLUSIONS

The interface structures and magnetic properties of multilayered films are stro dependent on the solid solubility of constituent metals. Mo/Co multilayers contained m regions at MO-CO interfaces and because of that,.saturation magnetization and Kerr rota angle decreased continuously with bilayer thickness. On the other hand, Ag/Co multila consisted of rather sharper interfaces and the saturate magnetization was larger than of bulk Co. It is a challenging subject to investigate the sharpening of interface by treatment in more detail and realize perpendicular magnetization films.

ACKNOWLEDGMENTS

The authors wish to thank T.Izumiya, T.Kaizuka and W.Soe for the preparations of samples. This work was partially assisted by Nippon Steel Corporation.

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