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STRUCTURAL AND MÖSSBAUER INVESTIGATION OF EPITAXIAL Cu/γ-Fe/Cu SANDWICH LAYERS
W. Wiartalla, W. Becker, W. Keune, H.-D. Pfannes
To cite this version:
W. Wiartalla, W. Becker, W. Keune, H.-D. Pfannes. STRUCTURAL AND MÖSSBAUER INVES- TIGATION OF EPITAXIAL Cu/γ-Fe/Cu SANDWICH LAYERS. Journal de Physique Colloques, 1984, 45 (C5), pp.C5-461-C5-464. �10.1051/jphyscol:1984569�. �jpa-00224188�
Colloque C5, supp16ment au n04, Tome 45, avril 1984 page '25-461
STRUCTURAL AND MOSSBAUER INVESTIGATION OF EPITAXIAL C U / Y - F ~ / C U SANDWICH LAYERS
W. Wiartalla, W. Becker, W. Keune and H.-D. Pfannes Laboratorim far Angeuandte Physik, U n i v e r s i t d t h i s b u r g , 0-4100 Duisburg 1, F.R.C.
RESUME
Nous avons Q t u d i e p a r LEED, p a r m i c r o s c o p i e dlectronique (TME) e t par l ' e f f e t Mossbauer d e s k l e c t r o n s de c o n v e r s i o n (CEMS) d e s couches minces 6 p i t a x i q u e s de Fe/Cu e t Cu/Fe/Cu d6pos6es s u r s u b s t r a t s d e
(100) NaC1 d a n s 1 ' u l t r a v i d e ( U H V ) . Des f i l m s ( f c c ) y-Fe (100) paramagns- t i q u e s o n t S t 6 o b s e r v g s . Les r e s u l t a t s CEMS m o n t r e n t que l a d e n s i t 6 d e s S l e c t r o n s s au noyau du 5 7 ~ e d 6 c r o f t l o r s q u e 1 1 6 p a i s s e u r du f i l m d e f e r d g c r o l t ; c e c i e s t probablement db 5 une d i l a t a t i o n du r 6 s e a u au v o i s i n a g e d e l ' i n t e r f a c e Fe/Cu.
ABSTRACT
E p i t a x i a l Fe/cu- and ~ u / F e / C u - t h i n f i l m l a y e r s p r e p a r e d on ( 1 o o ) ~ a c l - - s u b s t r a t e s by vapor d e p o s i t i o n i n UHV h a v e - b e e n i n v e s t i g a t e d by LEED, t r a n s m i s s i o n e l e c t r o n microscopy (TEM) and c o n v e r s i o n e l e c t r o n Moss- b a u e r s p e c t r o s c o p y (CEMS) . Paramagnetic ( f c c ) y-Fe ( l o o ) f i l m s have been observed. The CEMS r e s u l t s i n d i c a t e a d e c r e a s e o f s - e l e c t r o n d e n s i t y a t t h e 5 7 ~ e n u c l e u s w i t h d e c r e a s i n g Fe f i l m t h i c k n e s s p o s s i - b l y caused by l a t t i c e expansion n e a r t h e Fe/Cu i n t e r f a c e .
1. I n t r o d u c t i o n
For many y e a r s a c o n t r o v e r s y e x i s t s c o n c e r n i n g t h e magnetic p r o p e r - t i e s of m e t a s t a b l e f c c y-Fe t h i n f i l m s . ESR r e s u l t s on e l e c t r o l y t i - c a l l y d e p o s i t e d ( 1 l o ) y - F e f i l m s on Cu s u b s t r a t e s /1/ and more r e c e n t - l y m a g n e t i z a t i o n measurements on e p i t a x i a l ( I l l ) y-Fe f i l m s e v a p o r a t e d on (111)Cu f i l m s /2-4/ o r (111)-Au f i l m s /5/ r e v e a l e d ferromagnetism a t room t e m p e r a t u r e . On t h e o t h e r hand, 5 7 ~ e M6ssbauer measurements on t h i n e p i t a x i a l y-Fe f i l m s vapor d e p o s i t e d i n UHV on t h i n f i l m s o f
(1oo)Cu / 6 , 7 / , (111)Cu o r ( 1 l o ) C u / 8 / i n d i c a t e d paramagnetism a t 300 K and a n t i f e r r o r n a g n e t i s m a t low t e m p e r a t u r e s / 6 , 8 / . The l a t t e r b e h a v i o r i s w e l l known f o r c o h e r e n t y-Fe p r e c i p i t a t e s i n a Cu m a t r i x /9-11/.
The p r e s e n t work i s a c o n t i n u a t i o n o f o u r i n v e s t i g a t i o n s of v e r y t h i n p a r a m a g n e t i c ( l o o ) y-Fe f i l m s on (1oo)Cu by c o n v e r s i o n e l e c t r o n Moss- b a u e r s p e c t r o s c o p y (CEMS) i n b a c k s c a t t e r i n g geometry; i n t h i s c a s e i t i s n o t n e c e s s a r y t o remove t h e f i l m s from t h e i r s u b s t r a t e a s i n Moss- b a u e r t r a n s m i s s i o n /7/. I n view o f t h e above-mentioned c o n t r a s t i n g r e - s u l t s i t may be i m p o r t a n t t o c h a r a c t e r i z e t h e samples by a d d i t i o n a l s t r u c t u r a l i n v e s t i g a t i o n s .
2. Experimental
Layered C U / F ~ / C U samples were p r e p a r e d i n UHV ( b a s e p r e s s u r e < l o - l o T o r r ) by electron-beam e v a p o r a t i o n o f Cu from a w a t e r - c o o l e d e l e c t r o n gun and o f Fe ( ~ 9 5 % e n r i c h e d i n 5 7 ~ e ) from a Ta c y l i n d e r c o n t a i n i n g an A1203 c r u c i b l e f o r 5 7 ~ e /12/. The d e p o s i t i o n r a t e s ( ~ 3 W / s f o r Cu and QIW/S f o r 5 7 ~ e ) and f i l m t h i c k n e s s were measured by a c a l i b r a t e d q u a r t z c r y s t a l monito During e v a p o r a t i o n t h e p r e s s u r e i n c r e a s e d t o 4 x 1 0 - ~ T o r r and 4x10-'.Torr i n t h e c a s e of Cu and Fe, r e s p e c t i v e l y , b u t dropped r a p i d l y t o t h e o r i g i n a l b a s e p r e s s u r e a f t e r t u r n i n g o f f Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984569
C5-462 JOURNAL DE PHYSIQUE
t h e e v a p o r a t i o n s o u r c e s . F i r s t a b a s e l a y e r of 650 - €300 2 Cu was de- p o s i t e d o n t o t h e ( l o o ) s u r f a c e of a c l e a v e d and w e t - p o l i s h e d NaCl c r y s t a l ( 1 5 x 1 5 ~ 2 mm3) which was k e p t a t a h i g h e r t e m p e r a t u r e Ts (250 OC i T s i 335 O C ) d u r i n g t h e e n t i r e sample p r e p a r a t i o n p r o c e s s . A f t e r a LEED s t u d y a t h i n l a y e r o f 5 7 ~ e (12-90 2 t h i c k ) was d e p o s i t e d o n t o t h e Cu b a s e l a y e r and t e s t e d a g a i n by LEED. S u b s e q u e n t l y t h e 5 7 ~ e f i l m was c o a t e d by a t h i n ( s 5 0 - l o o 8) p r o t e c t i v e l a y e r (which was t h i n enough t o a l l o w e s c a p e of Mossbauer c o n v e r s i o n and Auger e l e c t r o n s e m i t t e d from t h e s 7 ~ e f i l m ) and i n v e s t i g a t e d by LEED b e f o r e exposing t h e sample t o a i r . The room-temperature CEMS-spectra were t a k e n by p l a c i n g t h e sample ( s t i l l unremoved from i t s NaCl s u b s t r a t e ) i n t o a He-4% CHq flow-gas p r o p o r t i o n a l c o u n t e r . For t r a n s m i s s i o n e l e c t r o n microscopy t h e f i l m s were s t r i p p e d o f f from t h e NaCl c r y s t a l . 3. R e s u l t s and D i s c u s s i o n
A t y p i c a l LEED p a t t e r n from a 750 2 t h i c k Cu b a s e l a y e r on NaCl (de- p o s i t e d a t Tss270 OC) i s shown i n f i g . l a . The f o u r f a i r l y s h a r p d i f f r a c t i o n s p o t s i n d i c a t e t h e 4 - f o l d symmetry o f a ( l o o ) l a t t i c e p l a n e a t normal e l e c t r o n i n c i d e n c e and a r e t y p i c a l f o r (1oo)Cu. The d i f f r a c t i o n p a t t e r n d i d n o t change when moving t h e primary e l e c t r o n beam a c r o s s t h e f i l m p l a n e i n d i c a t i n g e p i t a x i a l growth of t h e e n t i r e
(1oo)Cu f i l m a r e a . A s i m i l a r LEED p a t t e r n was observed f o r a l l t h i n Fe f i l m s (between 12 and 33 W t h i c k ) d e p o s i t e d o n t o t h e (1oo)Cu base l a y e r . E . g. t h e t y p i c a l LEED p a t t e r n of 33 8 Fe ( F i g . l b ) e v a p o r a t e d
Fig.1: Low Energy E l e c t r o n D i f f r a c t i o n (LEED) p a t t e r n o f e p i t a x i a l f i l m s : a ) 750 8 Cu(1oo) and b) 33 8 y-Fe(1oo) on 800 2 CU ( l o o ) , b o t h a t e l e c t r o n e n e r of 74 eV: c) 150 .% ~ u ( l o o ) d e p o s i t e d o n t o 16 2
y-Fe ( l o o ) on 6 5 0 3 C u ( l o o ) , a t 71 eV. ( ~ e a m c u r r e n t x l u l ; s u b s t r a t e NaC1 ( l o o ) )
a t T %270 O C i n d i c a t e s e p i t a x i a l growth of (1oo)y-Fe w i t h n e a r l y t h e same l a t t i c e c o n s t a n t s C u . For a l l Fe f i l m s t h e s p o t i n t e n s i t y was found t o be much weaker t h a n f o r Cu f i l m s . No e x t r a d i f f r a c t i o n s p o t s due t o a-Fe c o u l d be o b s e r v e d i n LEED p a t t e r n s of Fe f i l m s , a l t h o u g h e a r l i e r CEMS r e s u l t s unambiguously i n d i c a t e d t h a t 30 - 33 8 Fe f i l m s d e p o s i t e d a t T -270 OC were a phase m i x t u r e of a-Fe and y-Fe /7,13/.
Fig. l c shows ?he LEED p a t t e r n o b t a i n e d a f t e r c o a t i n g a l a y e r of 16 2
(1oo)y-Fe on 650 2 ( 1 o o ) ~ u by 150 2 Cu a t T,=330 OC (150 8 Cu/16 8 Fe/
650 8 Cu-sample). S i m i l a r p a t t e r n s were observed a l s o f o r t h e under- l y i n g 650 2 and 16 8 Fe l a y e r s . Thus a l l t h r e e l a y e r s grew e p i t a x i a l l y
a b 1 1 P
Fig. 2: Transmission e l e c t r o n microscopy (TEM) a t l o o keV beam e n e r g y on e p i t a x i a l 150 2 Cu/16 8 Fe/ 650 ?. Cu t h i n - f i l m l a y e r s : a ) d i f f r a c - t i o n p a t t e r n b ) micrograph showing i s l a n d s t r u c t u r e o f 150 R.Cu ( l o o ) t o p l a y e r
w i t h ( l o o ) o r i e n t a t i o n and n e a r l y t h e same l a t t i c e c o n s t a n t . The TEM r e s u l t s a r e i n agreement w i t h LEED. The TEM d i f f r a c t i o n p a t t e r n ( F i g . 2a) o b t a i n e d from t h e 150 8 Cu /16 2 Fe/650 Cu-sample i n d i c a t e s t h a t t h e s e l a y e r e d f i l m s were ( l o o ) s i n g l e c r y s t a l s . The d i f f r a c t i o n p a t t e r n was s i m i l a r t o t h a t o f a s i n g l e (1oo)Cu l a y e r . A s can be s e e n i n t h e TEM micrograph of t h i s composite sample t h e 150
(1oo)Cu c o a t i n g l a y e r was n o t a c l o s e d f i l m b u t had an open i s l a n d s t r u c t u r e w i t h b r i d g i n g ( F i g . 2 b ) .
A t y p i c a l CEMS-spectrum t a k e n from t h e 150 8 Cu/16 2 Fe/ 650 8 Cu
sample i s shown f i g . 3. The s p e c t r u m shows a predominant p a r a m a g n e t i c s i n g l e l i n e which ( a c c o r d i n g t o i t s isomer s h i f t ) i s a s s o c i a t e d w i t h p a r a m a g n e t i c y-Fe. The s a t e l l i t e l i n e a t a b o u t + 0.8 mms-I o b s e r v e d
i n f i g . 3 i s t y p i c a l f o r ~ e i o n s and i n d i c a t e s p a r t i a l o x i d a t i o n o f ~ + t h e Fe f i l m because ( a s a r e s u l t s o f i t s i s l a n d s t r u c t u r e ) t h e p r o t e c - t i n g 150 8 Cu l a y e r d i d n o t c o v e r t h e Fe f i l m completely.
A summary of y-Fe isomer s h i f t ( I . S . ) v a l u e s o b t a i n e d from CEMS mea- surements of a s e r i e s of e p i t a x i a l Cu/Fe/Cu(loo) f i l m s w i t h d i f f e r e n t v a l u e s o f t h e a v e r a g e y-Fe f i l m t h i c k n e s s d i s g i v e n i n f i g . 4. Here d i s p l o t t e d v e r s u s t h e isomer s h i f t a s o b t a i n e d by a l e a s t - s q u a r e s computer f i t of t h e measured s p e c t r a by two L o r e n t z i a n l i n e s . A l l I . S . v a l u e s a r e v e r y c l o s e t o t h a t o f y-Fe p r e c i p a t e s i n a Cu m a t r i x / 6 , 1 1 / (-0.088 mms-l, a s i n d i c a t e d by " b u l k " v-Fe). However, a t e n - dency towards more p o s i t i v e I . S . v a l u e s w i t h d e c r e a s i n g y-Fe f i l m t h i c k n e s s d i s c l e a r l y o b s e r v e d i n f i q . 4. Consequently t h e s - e l e c t r o n d e n s i t y a t t h e 5 7 ~ e n u c l e u s i s d e c r e a s i n g w i t h d e c r e a s i n g Fe f i l m t h i c k n e s s . High-pressure e x p e r i m e n t s have shown t h a t t h e I . S . o f y-Fe p r e c i p i t a t e s becomes more p o s i t i v e w i t h d e c r e a s i n g p r e s u r e /14/. S i n c e t h e l a t t i c e c o n s t a n t of Cu i s s l i g h t l y l a r g e r t h a n t h a t of r e l a t i v e l y l a r g e y-Fe p r e c i p i t a t e s (by a0.027 2 /15/) t h e t h i n n e s t e p i t a x i a l y-Fe f i l m s (pseudomorphically grown on Cu) w i l l b e i n t h e h i g h e s t s t a t e o f t e n s i o n and w i l l have an a v e r a g e l a t t i c e p a r a m e t e r c l o s e s t t o t h a t o f Cu. T h i s change i n y-Fe l a t t i c e p a r a m e t e r i s t o o s m a l l t o be o b s e r - v a b l e by LEED o r TEM. L i n e a r e x t r a p o l a t i o n o f t h e I . S . v e r s u s p r e s s u r e d a t a i n r e f . /14/ l e a d s t o a t e n s i l e s t r e s s o f a-28 k b a r f o r o u r t h i n n e s t ( l o o ) y-Fe f i l m (d=12 2).
C5-464 JOURNAL DE PHYSIQUE
F i g . 3: C o n v e r s i o n E l e c t r o n Mossbauer S p e c t r u m (CEMS) a t room tempe- r a t u r e o f e p i t a x i a l 150 2 Cu/16 8 5 7 ~ e / 6 5 0 8 Cu t h i n - f i l m l a y e r s . S o u r c e : - 5 m ~ i 5 7 ~ 0 .
F i g . 4: Average f i l m t h i c k n e s s d ( 8 ) o f e p i t a x i a l y - 5 7 ~ e ( l o o ) v e r s u s m e a s u r e d i s o m e r s h i f t ( m m s - l ) ( r e l a t i v e t o a-Fe a t room t e m p e r a t u r e )
Acknowledqement. F i n a n c i a l s u p p o r t o f t h e D e u t s c h e Forschungsgemein- s c h a f t i s g r a t e f u l l y a p p r e c i a t e d .
8.
9.
l o .
W r i g h t J . G . , P h i l - M a g . 24 ( 1 9 7 1 ) 217
Gradmann U . , Kumrnerle W., a n d T i l l m a n n s P . , T h i n S o l i d F i l m s 3,
( 1 9 7 6 ) 249
K i i m e r l e W., a n d Gradmann U . , S o l i d S t a t e Commun. 2 4 , ( 1 9 7 7 ) 33:
p h y s . s t a t . s o 1 . ( a ) 45, ( 1 9 7 8 ) 1 7 1
Gradmann U . , a n d T i l l m a n n s P . , p h y s . s t a t . s o 1 . ( a ) 44, ( 1 9 7 7 ) 539 Gradmann U., a n d I s b e r t H . D . , J.Mag.Magn.Mater. 15-18, (1980) 1109 Keune W . , H a l b a u e r R . , G o n s e r U . , L a u e r J . , a n d W i l l i a m s o n D.L., J . A p p l . P h y s . 48, ( 1 9 7 7 ) 2976: J.Mag.Magn.Mater. 6, ( 1 9 7 7 ) 192 B e c k e r W., P f a n n e s H.,D., a n d Keune W., J.Mag.Magn.Mater. 3 5 ,
( 1 9 8 3 ) 53
H a l b a u e r R . , a n d G o n s e r U . , J.Mag.Magn.Mater. 35, ( 1 9 8 3 ) 55 K o n d o r s k i E. J . , and Sedov V. L., S o v i e t . JETP 35, ( 1 9 5 8 ) 1104 Abraham S.C., Guttman L., a n d K a s p e r J . S . , P h y s - R e v . 127, ( 1 9 6 2 ) 2052
G o n s e r U . , Meechan C. J., Muir A. H., a n d W i e d e r s i c h H., J - A p p l - P h y s . 34, ( 1 9 6 3 ) 2373
Keune W., Nucl. I n s t r u m . Methods 116, ( 1 9 7 4 ) 5 9 1
E r r o n e o u s l y t h e s u b s t r a t e t e m p e r a t u r e s Ts g i v e n i n r e f . 7 w e r e s y s t e m a t i c a l l y %60 OC t o o h i g h .
L i u C.M., a n d I n g a l l s R., J . A p p l . P h y s . 50, ( 1 9 7 9 ) 1751 E h r h a r t P . , S c h o n f e l d B . , E t t w i g H . H . , a n d P e p p e r h o f f W., J.Mag.Magn. Mater. 2 2 , (1980) 79
