Liquid phase epitaxial growth of indium substituted magnesium ferrite films

Liquid phase epitaxial growth of indium substituted

magnesium ferrite films

Citation for published version (APA):

Straten, van der, P. J. M., & Metselaar, R. (1977). Liquid phase epitaxial growth of indium substituted

magnesium ferrite films. Materials Research Bulletin, 12(7), 707-715.

https://doi.org/10.1016/0025-5408(77)90130-1

DOI:

10.1016/0025-5408(77)90130-1

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Published: 01/01/1977

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Mat. Res. Bull. Vol. 12, pp. 707-715,1977. Pergamon Press, ~ c . P ~ n t e d i n t h e

U m t ~ ~ a ~ s .

L I Q U I D P H A S E E P I T A X I A L G R O W T H OF I N D I U M S U B S T I T U T E D M A G N E S I U M F E R R I T E F I L M S P . J . M . v a n d e r S t r a t e n and R. M e t s e l a a r L a b o r a t o r y of P h y s i c a l C h e m i s t r y , U n i v e r s i t y of T e c h n o l o g y , E i n d h o v e n , The N e t h e r l a n d s .

(Received May 20, 1977; Communicated by A. Rabenau)

A B S T R A C T S i n g l e c r y s t a l f i l m s of M g ( F e , l n ) 2 0 4 h a v e b e e n g r o w n by m e a n s of l i q u i d - p h a s e - e p i t a x y on ( 1 0 0 ) - M g O s u b s t r a t e s u s i n g a P b O - B 2 0 3 - F e 2 0 3 s o l v e n t . The f i n a l p o l i s h i n g of the s u b s t r a t e s a p p e a r e d to be v e r y i m p o r t a n t in c o n n e c t i o n w i t h the n e c e s s a r y l a r g e s u p e r s a t u r a t i o n of the flux. E l e c t r o n m i c r o p r o b e a n a l y s i s r e v e a l e d Pt and h i g h Pb c o n t e n t s in our films. The e a s y axis of m a g n e t i z a t i o n is i n - p l a n e , p r o b a b l y due to the c o m p r e s s i v e s t r a i n of the f i l m s c a u s e d by the d i f f e r e n c e in t h e r m a l e x p a n s i o n of M g O and M g ( F e , l n ) 2 0 4 . I n t r o d u c t i o n M a g n e t i c f i l m s h a v e a t t r a c t e d m u c h i n t e r e s t b e c a u s e of t h e i r p o t e n t i a l a p p l i c a b i l i t y in m a g n e t i c and m a g n e t o - o p t i c d e v i c e s . M o s t s t u d i e s c o n c e r n the l i q u i d - p h a s e - e p i t a x i a l (LPE) g r o w t h of g a r n e t f i l m s (I) and r e c e n t l y the L P E g r o w t h of h e x a g o n a l f e r r i t e s has b e e n r e p o r t e d (2,3). C o n c e r n i n g the g r o ~ h of s p i n e l f e r r i t e s m o s t w o r k , so far, has b e e n d o n e by m e a n s of c h e m i c a l v a p o r d e p o s i t i o n (CVD). In v i e w of the v e r y l i m i t e d a m o u n t of LPE s p i n e l s t u d i e s (4,5) we h a v e s t a r t e d a p r o g r a m to i n v e s t i g a t e the g r o w t h of s p i n e l f e r r i t e films. In this w o r k we w a n t to r e p o r t on the g r o w t h c o n d i t i o n s of i n d i u m s u b s t i t u t e d m a g n e s i u m f e r r i t e by m e a n s of LPE. A b a s i c r e q u i r e m e n t for h e t e r o - e p i t a x i a l g r o w t h is a c l o s e m a t c h of l a t t i c e s p a c i n g b e t w e e n o v e r g r o w t h and s u b s t r a t e . F o r the g r o w t h of s p i n e l f e r r i t e s n o n - m a g n e t i c c o m p o u n d s w i t h s p i n e l s t r u c t u r e e g M e G a 0 ( M e = N i , M g , Z n , C u ) w o u l d be " " 2 4 s u i t a b l e as s u b s t r a t e s . H o w e v e r , u n t l l now, a p a r t f r o m M g A I 2 0 4 ,

707

708 P . J . M . VAN DER STRATEN, et al. Vol. 12, No. 7 no s i n g l e c r y s t a l s of a d e q u a t e s i z e h a v e b e e n a v a i l a b l e . F o r m o s t f e r r i t e s the use of M g A I g O A (a = 8 . 0 9 ~) as a s u b s t r a t e is n o t a t t r a c t i v e s i n c e the l ~ t ~ i c e ° m i s m a t c h is too l a r g e . I n s t e a d , w e h a v e u s e d M g O w h i c h is c o m m e r c i a l l y a v a i l a b l e . M a g n e s i a has a l a t t i c e c o n s t a n t of 4.21 ~, and e i g h t u n i t c e l l s fit p e r f e c t l y to a s p i n e l c e l l w i t h a = 8 . 4 2 ~. If one is i n t e r e s t e d in f i l m s w h i c h can s u s t a i n ° m a g n e t i c b u b b l e s t h e r e

are a d d i t i o n a l r e q u i r e m e n t s b o t h to the s u b s t r a t e and the o v e r - g r o w t h i t s e l f . F o r t h i s p u r p o s e the f i l m m u s t h a v e a u n i a x i a l a n i s o t r o p y K w i t h the p r e f e r r e d a x i s p e r p e n d i c u l a r to the p l a n e of t h e U f i l m . T h e u n i a x i a l a n i s o t r o p y f i e l d H k = K u / 2 M s h a s to be l a r g e r t h a n the d e m a g n e t i z a t i o n f i e l d 4 ~ M s. A low v a l u e of 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 is h e l p f u l in t h i s r e s p e c t . T h e a n i s o t r o p y can be e i t h e r g r o w t h - i n d u c e d or s t r e s s - i n d u c e d . We h a v e f o c u s s e d o u r a t t e n t i o n to M g F e 2 0 . as o v e r g r o w t h a n d M g O as s u b s t r a t e . T h e l a t t i c e c o n s t a n t of 4MgFe~OI, ( h i g h t e m p e r a t u r e f o r m ) is a = 8 . 3 9 ~, c l o s e to the v a l ~ e ~ o f 8 . 4 2 for e i g h t u n i t c e l l s o~ M g O . A b e t t e r fit of l a t t i c e c o n s t a n t s b e t w e e n f i l m and s u b s t r a t e c a n be a c c o m p l i s h e d by s u b s t i t u t i o n of In, Sn or Ti in the film. A d i s a d v a n t a g e of M g O as s u b s t r a t e is its r a t h e r h i g h t h e r m a l e x p a n s i o n c o e f f i c i e n t (6) c o m p a r e d

w i t h s p i n e l f e r r i t e s . A l s o in t h i s r e g a r d M g F e 0 a p p r o a c h e s

2 4

M g O the b e s t (7). C o m p a r e d to o t h e r s p i n e l f e r r l t e s the s a t u r a -

t i o n moment, of MgFe_Ot.. 2 is r e l a t i v e l y l o w ( 4 ~ M s ~ 1500 G), and

g r o w t h l n d u c e d u n l a x l a ~ a n i s o t r o p y of f l u x - g r o w n b u l k c r y s t a l s of A l - s u b s t i t u t e d M g F e 2 0 4 has b e e n r e p o r t e d (8). E x p e r i m e n t a l T h e t h e r m a l e x p a n s i o n of M g O and In, Sn or Ti s u b s t i t u t e d M g F e 2 0 4 has b e e n d e t e r m i n e d u s i n g h i g h t e m p e r a t u r e d i f f r a c t o - m e t r y . S u b s t i t u t i o n is a c c o m p l i s h e d in s u c h a w a y t h a t the r o o m t e m p e r a t u r e l a t t i c e c o n s t a n t s are a b o u t t w i c e the v a l u e of M g O . F r o m the r e s u l t s , s u m m a r i z e d in T a b l e I, it can be c o n c l u d e d t h a t the b e s t t h e r m a l e x p a n s i o n fit w i t h M g O is o b t a i n e d for I n - s u b s t i t u t e d M g F e O . T h e a c c u r a c y of the d e t e r m i n e d l a t t i c e c o n s t a n t s is + 0 . 0 0 5 4~. T A B L E I T e m p e r a t u r e d e p e n d e D c e of l a t t i c e p a r a m e t e r s (~) e x p r e s s e d as a = a + bT + cT z (T in °C) of M g O and In, Sn ~ r Ti - s u b s t i t u ~ e a M g F e 2 0 4 (20 - 1 2 0 0 ° C ) . c o m p o u n d a b x !05 c x 108 O M g O M g F e l . 8 1 1 n 0 . ;904 M g l . 1 8 F e l . 6 4 S n 0 . 1804 M g l . 5 0 F e l . 0 0 T i 0 . 5 0 0 4 4 . 2 1 2 8 . 4 2 5 8 . 4 2 8 8 . 4 2 4 5 . 4 4 7 . 5 3 7 . 8 6 3 . 9 8 0.41 1.77 0 . 8 3 4 . 1 8

Vol. 12, No. ? M A G N E S I U M F E R B I T E F I L M S 709 We h a v e u s e d a P b O - B 2 0 _ - F e 2 0 3 f l u x f o r the L P E g r o w t h of m a g n e s i u m f e r r i t e . F i g . ! g i v e s a s u r v e y of l i t e r a t u r e d a t a on s o l v e n t c o m p o s i t i o n s u s e d for L P E g r o w t h of g a r n e t f i l m s a n d f o r f l u x g r o w t h of b u l k M g F e 2 0 ~ c r y s t a l s (8, 9-15). In the f i g u g e the r e l e v a n t p a r t of the p s e u d o - t e r n a r y p h a s e d i a g r a m is s h o w n . T h e c o m p o s i t i o n of the s o l v e n t u s e d in o u r L P E s t u d y of M g F e 2 0 4 f i l m s , as i n d i c a t e d by p o l n t H in F i g . |, is ! P b O : 0 . 0 8 1 B ^ O 3 : 0 . 0 3 5 F e ^ O ( m o l e s ) . ~o t h i s s o l v e n ~ ~ . 0 6 6 m o l e s of p r e - r e a c t e d M g F e l . 7 8 In_ ~ ^ O _ p o w d e r w a s a d d e d u . z z 4 p e r o n e m o l e of PbO. T h e f l u x c o n s t i t u e n t s w e r e m i x e d a n d p r e m e l t e d in a 75 ml Pt c r u - c i b l e . A c t u a l l y l% P b O w a s r e p l a c e d b y P b O _ f o r p r o t e c - t i o n of the p l a t i n u m (16). I s o t h e r m a l d i p p i n g of the s u b - s t r a t e s is p e r f o r m e d a c c o r d i n g to s t a n d a r d L P E g a r n e t p r o - F I G . 1 P a r t o f t h e t e r n a r y p h a s e d i a - g r a m P b O - F e 2 0 - B 2 0 . S o l v e n t c o m p o s i t i o n s ~n m o ~ % u s e d for f l u x g r o w t h a n d s o l u b i l i t y s t u d i e s of M g F e 2 0 4 a n d f o r L P E g a r n e t s t u d l e s .

A ref. | 0 h B ref. If; C ref. 8; D ref. 12; E ref. 13; F ref. 9; G ref. I4; H ref. I5, t h i s w o r k

c e d u r e s in a v e r t i c a l 3 - z o n e r e s i s t a n c e f u r n a c e . M g O s i n g l e c r y s t a l s w e r e o b t a i n e d f r o m S p i c e r L t d . , E n g l a n ~ T h e c r y s t a l s w e r e X - r a y o r i e n t e d w i t h i n 0 . 5 ° a n d cut i n t o ( | 0 0 ) - w a f e r s , w i t h a d i a m e t e r of 12 m m a n d a t h i c k n e s s of 0 . 8 mm. T h e s l i c e s w e r e p o l i s h e d w i t h d i a m o n d p a s t e s w i t h s u c c e s s i v e l y d e c r e a s i n g p a r t i c l e s i z e s r a n g i n g f r o m 30 to 0 . 2 5 ~m. As w i l l be s h o w n l a t e r , a f i n a l t r e a t m e n t w i t h S y t o n - W - 3 0 w a s n e c e s s a r y . P r i o r to the f i l m g r o w t h the s u b s t r a t e s w e r e c l e a n e d w i t h o r g a n i c s o l v e n t s . A f t e r g r o w t h t e r m i n a t i o n , the a d h e r i n g f l u x d r o p l e t s are d i s s o l v e d in a m i x t u r e of h o t d i l u t e a c e t i c a c i d w i t h n i t r i c a c i d . M e a s u r e m e n t s of the l a t t i c e c o n s t a n t m i s m a t c h a r e m a d e by s c a n n i n g the ( 8 0 0 ) - s p i n e l r e f l e c t i o n a n d the ( 4 0 0 ) - M g O r e f l e c t i o n w i t h F e - K ~ r a d i a t i o n . T h e m i s f i t is o b t a i n e d f r o m the r e l a t i o n A a / a = I - v A8 I + 9 t a n @" H e r e Aa = as - af, is the d i f f e r e n c e b e t w e e n t h e u n s t r a i n e d l a t t i c e p a r a m e t e r s of s u b s t r a t e a n d f i l m , @ is the B r a g g r e f l e c t i o n a n g l e a n d 9 is the P o i s s o n r a t i o , w h i c h is a s s u m e d to be 0 . 2 5 (17). F i l m t h i c k n e s s is m e a s u r e d b y g r i n d i n g a s p h e r i c a l h o l e , j u s t t h r o u g h the f i l m , u s i n g a 40 m m s t e e l b a l l c o a t e d w i t h d i a m o n d p o l i s h i n g compofind a n d m e a s u r i n g the d i a m e t e r of the c o n c e n t r i c c i r c l e s f o r m e d by the f i l m s u r f a c e s . R e s u l t s w e r e c h e c k e d by i n t e r f e r o m e t r i c m e a s u r e m e n t s .

710 P . J . M . V A N D E R S T R A T E N , et al. Vol. 12, No.7

R e s u l t s

A f t e r s t i r r i n g the m e l t for a b o u t 100 h at 1 0 0 0 ° C all s o l i d p a r t i c l e s w e r e d i s s o l v e d , b u t a f t e r c o o l i n g to a b o u t 9 0 0 ° C t i n y c r y s t a l s c o u l d be o b s e r v e d f l o a t i n g on top of the m e l t . T h e s e c r y s t a l s h a d an o c t a h e d r a l s h a p e and c o u l d be i d e n t i f i e d as s l i g h t l y In, Pb and Pt s u b s t i t u t e d M g F e 2 0 ~a ; 8 . 4 2 5 + 0 . 0 0 5 A). E l e c t r o n m i c r o p r o b e a n a l y s i s i n d i c a t e ~ g i n c l u s i o n s in s o m e of t h e s e c r y s t a l s . C o n t i n u o u s s t i r r i n g of the m e l t at e l e v a t e d t e m p e r a t u r e s d i m i n i s h e d the a m o u n t of s o l i d p a r t i c l e s at l o w e r t e m p e r a t u r e s b u t the p a r t i c l e s n e v e r d i s a p p e a r e d c o m p l e t e l y . O b v i o u s l y s o m e s p o n t a n e o u s n u c l e a t i o n o c c u r s d u r i n g c o o l i n g b e l o w the l i q u i d u s t e m p e r a t u r e of the m e l t .

F r o m our f i r s t LPE e x p e r i m e n t s two f e a t u r e s b e c a m e c l e a r . F i r s t l y a r a t h e r l a r g e s u p e r s a t u r a t i o n of the f l u x is n e c e s s a r y to n u c l e a t e f i l m g r o w t h , and s e c o n d l y the s u r f a c e f i n i s h of the M g 0 s u b s t r a t e s is e x t r e m e l y i m p o r t a n t . We c o n s i d e r the l a t t e r p o i n t in m o r e d e t a i l . At t e m p e r a t u r e s b e t w e e n 9 0 0 ° C and 8 5 0 ° C a b a d l y a d h e r e n t o v e r g r o w t h w a s o b t a i n e d on the d i a m o n d p o l i s - h e d s u b s t r a t e s , w h i l e on the s y t o n p o l i s h e d s u b s t r a t e s no g r o w t h c o u l d be o b s e r v e d . At l o w e r t e m p e r a t u r e s we a c h i e v e ~ a f i r m l y a d h e r e n t f i l m on the d i a m o n d p o l i s h e d s u b s t r a t e s . T h i s o v e r g r o w t h , h o w e v e r , l o o k e d v e r y r o u g h and o p a q u e . F i l m g r o w t h on the s y t o n p o l i s h e d s u b s t r a t e s s t a r t e d o n l y at t e m p e r a t u r e s b e l o w a p p r o x i m a t e l y 780°C. O f t e n i s l a n d g r o w t h was o b s e r v e d (Fig. 2) w i t h c r y s t a l l i t e s in the f o r m of i d e n t i c a l l y o r i e n - ted t r u n c a t e d p y r a m i d e s w i t h b a s a l e d g e s a l o n g < I 0 0 > d i r e c t i o n s . At t e m p e r a t u r e s b e t w e e n 6 7 5 ° C and 7 5 0 ° C a c l o s e d , s m o o t h and t r a n s p a - rent, red c o l o r e d o v e r g r o w t h was o b t a i n e d . S o l i d i f i e d a d h e r e n t f l u x d r o p l e t s l o c a l - ly s p o i l e d the o t h e r w i s e c o m p l e t e l y s h i n y s u r f a c e s . As s h o w n in Fig. 3 the g r o w t h r a t e s s h o w e d a q u i t e d i f f e - rent t e m p e r a t u r e d e p e n d e n c e for the s y t o n and d i a m o n d p o l i s h e d s u b s t r a t e s . On the d i a m o n d p o l i s h e d s u b s t r a t e s i • m

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FIG. 2 I s l a n d g r o w t h on a s y t o n p o - l i s h e d M g O s u b s t r a t e , g r o w t h t e m p e r a t u r e 808°C. 0B 0.6 E ':D., ~' 0.~ 0.2 o ' D / D _{j l} i 1 1 1 [] / I I [] z / / / ' I 2 Q " 5 m r n , O r p m I i I I / I 725 750 775 800 8 2 5 ~°C FIG. 3 G r o w t h r a t e s as a f u n c t i o n of g r o w t h t e m p e r a t u r e for a d i a - m o n d p o l i s h e d s u b s t r a t e (das- h e d line) and for a s y t o n p o - l i s h e d s u b s t r a t e ( s o l i d line).

Vol. 12, No.7 MAGNESIUM FERRITE FILMS 711 F I G . 4A F I G , 4B S t e r e o s c a n p i c t u r e of a f i l m g r o w n on a d i a m o n d p o l i s h e d s u b s t r a t e (a) or on a s y t o n p o l i s h e d s u b s t r a t e (b). t h e g r o w t h r a t e d e c r e a s e ~ l i n e a r l y f r o m 0 . 9 ~ m / m i n at 8 5 0 ° C to l e s s t h a n 0.1 ~ m / m i n at 7 0 0 ° C . F o r the s y t o n p o l i s h e d s u b - s t r a t e s t h e g r o w t h r a t e s h o w e d a m a x i m u m of a b o u t 0 . 5 p m / m i n n e a r 7 6 0 ° C ( d i p p i n g t i m e 5 m i n . ) . T h e d i f f e r e n c e in g r o w t h b e h a v i o r m u s t be d u e to t h e s u r f a c e m o r p h o l o g y of the s u b s t r a - tes. S t e r e o s c a n p i c t u r e s r e v e a l a m u c h s m o o t h e r M g O s u r f a c e f o r t h e s y t o n p o l i s h e d s u b s t r a t e s c o m p a r e d w i t h t h e d i a m o n d p o l i s h e ~ s u b s t r a t e s . T h i s d i f f e r e n c e in s u r f a c e m o r p h o l o g y is c l e a r l y r e f l e c t e d in t h e r e s p e c t i v e f i l m s u r f a c e s . F i g . 4a s h o w s the s u r f a c e of a r o u g h , o p a q u e f i l m g r o w n on a d i a m o n d p o l i s h e s s u b - s t r a t e . T h e p i c t u r e s h o w s a r a t h e r o p e n s t r u c t u r e , l o o k i n g l i k e a c r o c k e t - w o r k , w i t h n e e d l e s p o i n t i n g a l o n g < I O 0 > d i r e c - t i o n s . F i g . 4b s h o w s t h e s u r - f a c e of a t r a n s p a r e n t f i l m g r o w n on a s y t o n p o l i s h e d s u b - s t r a t e . T h e s t r u c t u r e is m o r e d e n s e a n d r a t h e r f l a t in t h i s c a s e . T h e s u r f a c e m o r p h o l o g y of a r e a s w i t h a f t e r - g r o w t h on an o t h e r w i s e s m o o t h s u r f a c e is i d e n t i c a l w i t h t h a t s h o w n in F i g . 4a. F i g . 5 s h o w s t h e g r o w t h r a t e of f i l m s g r o w n on s y t o n p o l i s - h e d M g O , b y m e a n s of h o r i z o n - tal d i p p i n g at a r o t a t i o n s p e e d of 125 r p m , or b y v e r t i - cal d i p p i n g . R o t a t i o n of t h e s u b s t r a t e d u r i n g the g r o w t h p r o d u c e s a m u c h s m o o t h e r o v e r - g r o w t h a n d l e s s c o n t a m i n a t i o n of the s u r f a c e b y f l u x d r o p -

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712 P . J . M . VAN DER STRATEN, et al. Vol. 12, No. 7

lets. T h e f l a t n e s s of the h o r i z o n t a l l y d i p p e d f i l m w a s I-2 f r i n g e s in N a - D l i g h t , c o m p a r e d w i t h 5 - I 0 f r i n g e s f o r the f i l m s g r o w n b y v e r t i c a l d i p p i n g . T h e r e a l s o is a d i f f e r e n t t e m p e r a t u r e d e p e n d e n c e on t h e g r o w t h r a t e s for the two c a s e s s h o w n in the f i g u r e . X - r a y a n a l y s i s X - r a y d i f f r a c t i o n p a t t e r n s of the f i l m s w e r e i n v e s t i g a t e d b y m e a n s of a S i e m e n s t e x t u r e g o n i o m e t e r . P o l e f i g u r e s of o n e or m o r e f o r m s of l a t t i c e p l a n e s c a n be o b t a i n e d of b o t h s u b s t r a t e a ~ d f i l m b y a p r o p e r s e l e c t i o n of the B r a g g a n g l e 0. T h e { I l l } - p o l e s of M g O c a n n o t be d i s t i n g u i s h e d f r o m the { 2 2 2 } - s p i n e l p o l e s , b u t the { 2 2 0 } - p o l e s of the s p i n e l a p p e a r a l o n e . F r o m the r e s u l - t i n g p o l e f i g u r e w e c o n c l u d e t h a t the f i l m s a r e s i n g l e c r y s t a l - l i n e . F o r the f i l m s g r o w n on s y t o n p o l i s h e d s u b s t r a t e s the < 1 0 0 > ~ i r e c t i o n s of f i l m a n d s u b s t r a t e c o i n c i d e . H o w e v e r , the o p a q u e f i l m s g r o w n on d i a m o n d p o l i s - h e d M g O s h o w a d i f f e r e n c e I-2 ° b e t w e e n the < 1 0 0 > d i r e c t i o n s of f i l m a n d s u b s t r a t e . D u e to t h i s d i f f e r e n c e the m i s f i t c a n n o t be m e a s u r e d b e c a u s e M g O a n d s p i n e l r e f l e c t i o n c a n n o t be a c h i e v e d f o r the s a m e p l a - t e l e t s e t t i n g in the d i f f r a c - t i o n e q u i p m e n t . T h e u n s t r a i n e d m i s f i t Aa = a s - af, b e t w e e n s u b s t r a t e a n d f i l m , is p l o t t e d in Fig. 6 as a f u n c t i o n of g r o w t h t e m p e r a - t u r e Tg for f i l m s g r o w n on s y - t o n p o l i s h e d M g O . A l l f i l m s a r e in c o m p r e s s i o n i.e. Aa < 0. F o r f i l m s g r o w n w i t h o u t r o t a - t i o n the m i s f i t i n c r e a s e s f r o m Aa = - 0 . 0 1 9 f o r Tg = 8 0 0 ° to Aa = - 0 . 0 3 6 A for T~ = 7 3 0 ° C . F o r f i l m s g r o w n u n a e r r o t a t x o n the

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FIG. 6 M i s f i t , a ( s u b s t r a t e ) - a (film), as a f u n c t i o n of g r o w t h t e m p e - r a t u r e . C i r c l e s , v e r t i c a l l y ~ i p p e d ; t r i a n g l e s , h o r i z o n t a l - ly d i p p e d w i t h s u b s t r a t e r o - t a t i o n 125 rpm; d i p p i n g t i m e 15 m i n . m i s f i t is a l w a y s 0.01 l a r g e r . D u e to t h e d i f f e r e n c e in t h e r m a l e x p a n s i o n c o e f f i c i e n t of s u b s t r a t e a n d f i l m , a z e r o m i s f i t at the g r o w t h t e m p e r a t u r e l e a d s to a f i l m u n d e r c o m p r e s s i o n at r o o m t e m p e r a t u r e . F o r T. = 8 0 0 ° C the r e s u l t i n g r o o m t e m p e r a t u r e m i s f i t is Aa = - 0 . 0 1 7 0 A ~ w h i l e f o r Tg = 7 0 0 ° C Aa = - 0 . 0 1 6 5 ~. F r o m F i g . 6 it f o l l o w s t h a t the f i l m s a r e c l o s e to z e r o m i s f i t or s l i g h t l y in c o m p r e s s i o n at Tg. T h e h i g h e r c o m p r e s s i v e s t r a i n of the f i l m s g r o w n u n d e r r o t a t i o n is p r o b a b l y r e l a t e d to the h i g h e r g r o w t h r a t e . M i c r o p r o b e a n a l y s i s G e n e r a l l y d i f f e r e n c e s in l a t t i c e c o n s t a n t s are c a u s e d b y d i f - f e r e n c e s in c o m p o s i t i o n . F r o m s t u d i e s on L P E g r o w n g a r n e t f i l m s it is k n o w n t h a t a p p r e c i a b l e a m o u n t s of Pb c a n be i n c o r p o r a t e d (18). I n d e e d , x - r a y f l u o r e s c e n c e m e a s u r e m e n t s r e v e a l e d the p r e s e n c e of a l a r g e a m o u n t of Pb in o u r s p i n e l f i l m s . In o r d e r to o b t a i n m o r e q u a n t i t a t i v e i n f o r m a t i o n a b o u t the c h e m i c a l

Vol. 12, No. 7 MAGNESIUM FERRITE FILMS 718 c o m p o s i t l o n we h a v e p e r f o r m e a an e l e c t r o n m i c r o p r o b e a n a l y s i s . The m e a s u r e m e n t s w e r e p e r f o r m e a u s i n g a S E M II A e l e c t r o n p r o b e m i c r o - a n a l y s e r . The c o n c e n t r a - t i o n s w e r e c a l c u l a t e d w i t h the aid of a c o m p u t e r p r o g r a m u s i n g m e a s u r e d i n t e n s i t i e s of p u r e Fe, In, Pb, Pt and M g O as s t a n - d a r d s . To o p t i m i z e the e x p e r i - m e n t a l c o n d i t i o n s and to c h e c k the a c c u r a c y of the r e s u l t s s t a n d a r d s of s i n t e r e d M g F e 2 _ x I n x O 4 (x=0, 0.1, 0.2, 0.3) w e r e m e a s u r e d . F o r the d e t e r m i n a t i o n - of M g a low a c c e l e r a t i n g vol- tage is p r e f e r r e d , h o w e v e r , e s p e c i a l l y for Pb a h i g h v o l - tage has to be a p p l i e d . The b e s t c o m p r o m i s e w a s 20kV. At this v o l t a g e the d e p t h of the a n a l y s e d r e g i o n in the f i l m is 1.25 pm. F r o m m e a s u r e m e n t s on the c e r a m i c s t a n d a r d s the f o l - l o w i n g a c c u r a c y w a s o b t a i n e a ( a t o m s p e r f o r m u l a u n i t ) : M g ! 0.2 (due to h i g h a b s o r p t i o n c o r r e c t i o n ) , Fe + 0.05, In + 0.02. The e s t i m a t e d a c c u r a c y of Pb and Pt is + 0.01. A l l f i l m s w e r e f o u n d to be Pb and Pt s u b - s t i t u a t e d M g F e ~ In O., w i t h x = 0 . 0 9 - 0 . l ~ ? X A s X s ~ o w n in Fig. 7 the Pt c o n t e n t is a b o u t 0.05, w i t h s l i g h t l y i n c r e a s i n g c o n c e n t r a t i o n at l o w e r g r o w t h t e m p e r a t u r e s . The Pb c o n t e n t is s t r o n g l y d e p e n d e n t on the g r o w t h p a r a m e t e r s . At the lo- w e s t g r o w t h t e m p e r a t u r e s the c o n c e n t r a t i o n i n c r e a s e s to a b o u t 0.2 a t o m s p e r f o r m u l a u n i t . So far w ~ h a v e a n a l y s e d the c o m p o s i t i o n of our f i l m s n o r m a l to the p l a n e of the p l a - t e l e t w i t h an a n a l y s e d d e p t h of 1.25 Hm. In o r d e r t o s t u d y the c o m p o s i t i o n as f u n c t i o n of f i l m t h i c k n e s s we h a v e g r o w n a 27 H m t h i c k f i l m (60 m i n , ;25 rpm, 7 6 0 ° C ) . A n a l y s i s are p e r f o r m e d on the b r o k e n and p o l i s h e d p l a - t e l e t n o r m a l to the g r o w t h di- r e c t i o n w i t h s t e p s of 2.8 ~ m f r o m s u b s t r a t e to f i l m s u r f a c e . i , , l i 0.20 : P b 0.15 E " " 0.10 o ~ 0.05

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714 P . J . M . V A N D E R S T R A T E N , et al. Vol. 12, No. 7 As c a n be s e e n f r o m Fig. 8 the Pb a n d In c o n t e n t s d e c r e a s e w h e n g o i n g to the f i l m s u r f a c e w h i l e the Pt c o n t e n t s l i g h t l y i n c r e a - ses. T h e s e c o m p o s i t i o n a l v a r i a t i o n s a r e p r o b a b l y r e l a t e d to c h a n g e s in g r o w t h w i t h d i p p i n g time. M a g n e t i c p a r a m e t e r s T h e s a t u r a t i o n m a g n e t i z a t i o n of the f i l m s w a s m e a s u r e d w i t h a v i b r a t i n g s a m p l e m a g n e t o m e t e r . T h e 4 ~ M s v a l u e s d e c r e a s e c o n t i - n u o u s l y f r o m 2 0 0 0 ~ 100 G a u s s for a f i l m g r o w n at 7 9 0 ° C to 1300 G a u s s for a f i l m g r o w n at 7 2 4 ° C . R o t a t i o n of t h e s u b s t r a t e l e a d s to a b o u t I0% l o w e r v a l u e s of 4 ~ M s. M a g n e t i c t o r q u e m e a s u r e m e n t s r e v e a l e d t h a t the e a s y a x e s of m a g n e t i z a t i o n a r e i n - p l a n e . In a c c o r d a n c e w i t h t h i s r e s u l t no s t r i p e d o m a i n s w e r e o b s e r v e d . D i s c u s s i o n We h a v e s e e n t h a t a r a t h e r h i g h s u p e r s a t u r a t i o n of the f l u x (AT ~ 2 0 0 o c ) is n e c e s s a r y to o b t a i n f i l m g r o w t h on ( 1 0 0 ) - s y t o n p o l i s h e d M g O s u b s t r a t e s . A n e x p l a n a t i o n c a n be f o u n d in the h i g h e r s u r f a c e f r e e e n e r g y of a ( 1 0 0 ) - p l a n e of M g O c o m p a r e d to a ( I I I ) - p l a n e . F i l m g r o w t h on ( I l l ) - M g O s t a r t s a b o u t 1 0 0 ° C h i g h e r t h a n on ( 1 0 0 ) - M g O s u b s t r a t e s (19), so w h e n an i n s u f f i c i e n t l y p o l i s h e d (IO0) s u b s t r a t e is d i p p e d at a t e m p e r a t u r e b e t w e e n t h e s e s a t u r a t i o n t e m p e r a t u r e s , i n i t i a l g r o w t h c a n o c c u r on t h o s e p a r t s of t h e s u r f a c e w h i c h h a v e a l o w e r s u r f a c e f r e e e n e r g y (20). T h i s e x p l a i n s w h y g r o w t h on d i a m o n d p o l i s h e d s u b s t r a t e s s t a r t s at a h i g h e r t e m p e r a t u r e t h a n on s y t o n p o l i s h e d s u b s t r a t e s . As a r e s u l t f i l m s on d i a m o n d p o l i s h e d s u b s t r a t e s h a v e an i n f e r i o r q u a l i t y a n d < I 0 0 > d i r e c t i o n s of f i l m a n d s u b s t r a t e c a n e v e n m a k e a s m a l l a n g l e w i t h e a c h o t h e r . We f i n d an a p p r e c i a b l e d r o p of the s a t u r a t i o n m a g n e t i z a t i o n w i t h d e c r e a s i n g g r o w t h t e m p e r a t u r e . T h i s e f f e c t c a n be a t t r i b u - ted to the i n c r e a s i n g Pb c o n c e n t r a t i o n . T h e l e a d i o n s a r e p r o - b a b l y s u b s t i t u t e d at o c t a h e d r a l s i t e s , w h i c h g i v e s a d e c r e a s e of 4 ~ M s. At t h e s a m e t i m e the C u r i e t e m p e r a t u r e d e c r e a s e s d u e to the d e c r e a s i n g m a g n e t i c i n t e r a c t i o n and t h i s l e a d s to a f u r t h e r l o w e r i n g of t h e m a g n e t i z a t i o n . T h e i n f l u e n c e of Pb is c l e a r l y p r o v e d by the e f f e c t of s u b s t r a t e r o t a t i o n . F i n a l l y the e q u i l i - b r i u m d i s t r i b u t i o n of M g i o n s a l s o d e p e n d s on the t e m p e r a t u r e . At l o w e r t e m p e r a t u r e s m o r e M g is p r e s e n t at o c t a h e d r a l s i t e s , a n d t h i s a l s o c o n t r i b u t e s to the l o w e r i n g of 4 ~ M s. So f a r w e d i d n o t s u c c e e d in g r o w i n g f i l m s w i t h the p r e f e r r e d m a g n e t i z a t i o n d i r e c t i o n p e r p e n d i c u l a r to the f i l m p l a n e . T h i s is p r o b a b l y d u e to the c o m p r e s s i v e s t r e s s in c o m b i n a t i o n w i t h a n e g a t i v e m a g n e - t o s t r i c t i o n c o n s t a n t ~ , n n of the f e r r i t e . A s s u m i n g Y o u n g ' s m o - d u l u s E ~ 2 x I0 L a y n / ¢ m 2 , a m i s f i t Aa = - 0 . 0 2 ~ c o r r e s p o n d s to a s t r e s s ~ ~ -6 x 109 d y n / c m 2. If we f u r t h e r t a k e 4 ~ M s 1500 G and ~ I O 0 ~ - I 0 - 5 , the s t r e s s i n d u c e d a n i s o t r o p y f i e l d is H s ~ r e s s ~ - 1 5 0 0 Oe. A c c o r d i n g to B o r r e l l i (8) the g r o w t h - i n d u - c e a a n l s o t r o p y in b u l k c r y s t a l s is of the o r d e r H ro t + I 0 0 0 Oe. If H r o w t h h a s a s i m i l a r m a g n i t u d e in t~e ~ 1 ~ m s a s l i g h t l y p o s i t i v e m l s ~ i t is n e c e s s a r y . We h a v e t r i e d to a c h i e v e this by a d d i n g s m a l l a m o u n t s of e i t h e r A I 2 0 3 or G a 2 0 3 to the f l u x . H o w e v e r , the r e s u l t i n g f i l m s a l w a y s s h o w e d l i g h t c o l o r e d , b r o a d , s t r a i g h t l i n e s a l o n g < I 0 0 > d i r e c t i o n s as s o o n as Aa > - 0 . 0 1 4 . T h i s m e a n s t h a t d u e to the o c c u r e n c e of s o m e s t r e s s

Vol. 12, No. 7 MAGNESIUM FERRITE FILMS 715 r e l i e v i n g m e c h a n i s m no p e r f e c t f i l m s c o u l d be g r o w n w i t h Aa > + 0 . 0 0 3 at the g r o w t h t e m p e r a t u r e . C o n c l u s i o n s We h a v e s h o w n t h a t it is p o s s i b l e to g r o w e p i t a x i a l f i l m s of m a g n e s i u m - i n d i u m f e r r i t e on (I00) M g O s u b s t r a t e s by m e a n s of the L P E m e t h o d . To g r o w p e r f e c t f i l m s a g o o d s u r f a c e f i n i s h of the s u b s t r a t e s is n e c e s s a r y . A s e r i o u s d i s a d v a n t a g e of M g O is the l a r g e d i f f e r e n c e in t h e r m a l e x p a n s i o n c o m p a r e d w i t h the f e r r i t e . As a c o n s e q u e n c e g o o d f i l m s are a l w a y s in c o m p r e s s i o n at r o o m t e m p e r a t u r e . A c k n o w l e d g e m e n t s

The a u t h o r s w o u l d like to t h a n k J.M. R o b e r t s o n and P.F. B o n g e r s of P h i l i p s R e s e a r c h L a b o r a t o r i e s , E i n a h o v e n for t h e i r a a v i c e s and s u p p o r t ; H.A. L o g m a n s of the s a m e l a b o r a t o r y p e r f o r m e d the m a g n e t i c m e a s u r e m e n t s . J . W . G . A . V r o l i j k of our l a b o r a t o r y is t h a n k e a for a s s i s t a n c e w i t h the e l e c t r o n m i c r o p r o b e c a l c u l a t i o n s and H. de J o n g e Baas for t e c h n i c a l a s s i s t a n c e .

R e f e r e n c e s

I. H.J. L e v i n s t e i n , S. L i c h t , R.W. L a n s d o r f and S.L. B l a n k , A p p l . P h y s . L e t t e r s 19, 486 (1971).

2. F.S. S t e a r n s and H.L. G l a s s , M a t . Res. Bull. IO, 1255 (1975) 3. F.S. S t e a r n s ana H.L. G l a s s , Mat. Res. Bull. I__~I, 1319 (1976). 4. R.J. G a m b i n o , J. A p p l . Phys. 38, 1129 (1967).

5. D.A. H e r m a n , R.L. W h i t e , R.S. F e i g e l s o n , B.L. M a t t e s and H.W. S w a r t s , A . I . P . Conf. Proc. 24, 580 (1975).

6 B.M. D u t t a , J. Sci. Res. B a n a r a s H i n d u U n i v . 15, 80

( 1 9 6 4 - 1 9 6 5 ) .

7 R.E. C a r t e r , J. Am. Cer. Soc. 42, 324 (1959).

8. N.F. B o r r e l l i , J. A p p l . P h y s . 46, 430 (1975).

9 J . E . D a v i e s , E . A . D . W h i t e and J . D . C . W o o d , J. C r y s t a l G r o w t h 27, 227 ( 1 9 7 4 ) .

l0 T. M a t s u y a m a , Y. N a g a t a and K. O h t a , J a p a n J. A p p l . Phys.

14, 727 ( 1 9 7 5 ) .

[I E.V. R y b a l s k a y a , T.G. P e t r o v and A.G. ~ i t o v a , S o v i e t

P h y s i c s - C r y s t a l l o g r a p h y 15, 958 (1971). 12 H.L. G l a s s and M . T . E l l i o t t , J. C r y s t a l G r o w t h 27, 253 (1974). 13 A.E. M o r g a n , J. C r y s t a l G r o w t h 27, 266 (1974). 14. R. G h e z and E.A. G i e s s , J. C r y s t a l G r o w t h 27, 221 (1974). 15. J.M. R o b e r t s o n , We t h a n k J.M. R o b e r t s o n for his a d v i c e c o n c e r n i n g the f l u x c o m p o s i t i o n . 16. B.M. W a n k l y n , J. C r y s t a l G r o w t h ~, 323 (1969). 17. D.J. D u m i n , J. E l e c t r o c h e m . Soc. 114, 749 (1967).

18. J.E. D a v i e s , E.A. G i e s s , J.D. K u p t s i s and W. R e u t e r , J. C r y s t a l G r o w t h 36, 1971 (1976).

19. J.M. R o b e r t s o n , p r i v a t e c o m m u n i c a t i o n .