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Submitted on 1 Jan 1984
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EXPERIMENTAL TEMPERATURE DEPENDENCE OF THE MAGNETIZATION OF Fe AT INTERFACES
WITH NON-MAGNETIC MATERIALS
J. Walker, R. Droste, G. Stern, J. Tyson
To cite this version:
J. Walker, R. Droste, G. Stern, J. Tyson. EXPERIMENTAL TEMPERATURE DEPENDENCE OF
THE MAGNETIZATION OF Fe AT INTERFACES WITH NON-MAGNETIC MATERIALS. Journal
de Physique Colloques, 1984, 45 (C5), pp.C5-357-C5-360. �10.1051/jphyscol:1984553�. �jpa-00224171�
!The Johns Hopkins U n i v e r s i t y , Baltimore, Maryland 21218, U . S.A.
Resume - Nous avons e f f e c t u e des mesures de l a dependance thermique de l a magnetisation du Fe dans une couche de s u r f a c e au c o n t a c t de Ag e t MnF2. Les r e s u l t a t s d i f f e r e n t s dans l e s deux c a s suggPrent que l e s e f f e t s s u r l'echange magnetique en s u r f a c e s o n t egalement d i f f e r e n t s . Abstract - Measurements a r e made of t h e temperature dependence of t h e magnetization of Fe i n a s u r f a c e l a y e r i n c o n t a c t w i t h Ag and MnF2.
The d i f f e r e n t r e s u l t s suggest d i f f e r e n t e f f e c t s of t h e two i n t e r f a c e s on t h e s u r f a c e magnetic exchange.
A number of s t u d i e s 11-31 have been made of magnetic hyperfine f i e l d s a t i n t e r - faces between Fe and non-magnetic metals such a s Ag, A u , and C u . In most cases t h e t h e magnetic hyperfine f i e l d was l a r g e r than bulk values a t 4.2 K and smaller than bulk a t 300 K . I t has become reasonably c l e a r /4/ t h a t two d i f f e r e n t phenomena a r e being seen here. A t low temperatures one sees e f f e c t s of changes i n e l e c t r o n i c s t r u c t u r e a t t h e Fe s u r f a c e l i n t e r f a c e . A t higher temperatures one sees t h e e f f e c t of s u r f a c e c o l l e c t i v e magnetic exci t a t i o n s ( f o r example, spin waves) which decrease t h e magnetization more a t t h e s u r f a c e l i n t e r f a c e than i n t h e bulk. In o r d e r t o s e p a r a t e t h e s t a t i c e l e c t r o n i c e f f e c t s from t h e temperature dependent c o l l e c t i v e e x c i t a t i o n s measurements were made of t h e hyperfine f i e l d s i n t h e l a y e r s of Fe a t o r near t h e s u r f a c e / i n t e r f a c e . This was accomplished by growing r e l a t i v e l y t h i c k (110) Fe f i l m s e p i t a x i a l l y on a f r e s h l y prepared ~ g ( l l 1 ) s u b s t r a t e . The epitaxy of Fe on Ag has been much studied /5/ and t h e f a c t t h a t Fe and Ag do not i n t e r d i f f u s e o r form any a l l o y s makes t h i s an e x c e l l e n t system. In a d d i t i o n , t h e high adhesive energy of Fe on Ag promotes f l a t t h i n f i l m growth.
In o r d e r t o measure t h e hyperfine f i e l d i n s e l e c t e d l a y e r s near t h e Fe s u r f a c e / i n t e r f a c e , t h e bulk of t h e Fe film was made of i s o t o p i c a l l y pure 5 6 ~ e , usually t o a thickness of about 50 l a y e r s . Probe l a y e r s of 57Fe were then grown a t an appropri- a t e p o s i t i o n a t o r near t h e s u r f a c e / i n t e r f a c e . By use of high-count r a t e Mdssbauer
S
ectroscopy, t h e magnetic hyperfine f i e l d was determined f o r just t h e p a r t i c u l a r 57Fe l a y e r s of a given f i l m . The 5 6 ~ e , o f course, produces no Mdssbauer e f f e c t . By l o c a t i n g t h e probe l a y e r a t d i f f e r e n t depths from t h e s u r f a c e i n a number of o t h e r - wise i d e n t i c a l f i l m s , a p r o f i l e of t h e hyperfine f i e l d i n t h e Fe s u r f a c e l i n t e r f a c e region was obtained.
The experiments were, a.t f i r s t , performed a t 4.2 K and then a t 295 K. By divid- ing t h e 295 K hyperfine f i e l d a t a given p o s i t i o n R i n t h e f i l m by i t s value a t 4.2 K, t h e constant-temperature e l e c t r o n i c e f f e c t s a t t h e f i l m s u r f a c e a r e normal- ized o u t , leaving only t h e temperature-dependent c o l l e c t i v e e f f e c t s . The r a t i o of t h e hyperfine f i e l d s H%(T)/H%(O) i s expected /6/ t o be approximately t h e same as MRT/MR(0). We t h e r e f o r e a r e a b l e t o f i n d t h e form of t h e normalized magnetization i n t h e s u r f a c e l a y e r s of a 50-layer Fe f i l m as shown i n Figure 1 . In t h i s case t h e completed f i l m was coated with an overlayer of Ag so t h a t t h e magnetization p r o f i l e i s f o r t h e Fe-Ag i n t e r f a c e . I t can be seen t h a t t h e normalized magnetization de- creases from bulk values i n t h e l a s t t h r e e Fe l a y e r s near t h e i n t e r f a c e . The
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984553
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15 20 2 5 30
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( L A Y E R P O S I T I O N )Fig. 1 - Normalized magnetization a t a given f i l m l a y e r ( R ) as a function of t h e l a y e r p o s i t i o n . The data a r e shown from t h e c e n t e r of a t h i r t y - l a y e r f i l m where t h e magnetization i s equal t o i t s bulk value out t o t h e Fe f i l m s u r f a c e where i t forms an i n t e r f a c e with Ag. The r e s u l t s show t h e larger-than-bulk spin deviations a t t h e s u r f a c e / i n t e r f a c e .
horizontal e r r o r bars represent t h e thickness of t h e 5 7 ~ e probe l a y e r s and t h e ver- t i c a l e r r o r bars represent u n c e r t a i n t i e s i n t h e hyperfine f i e l d r e s u l t i n g from e x t r a linewidths of t h e Mbssbauer s p e c t r a . This r e s u l t i s q u a l i t a t i v e l y i n agree- ment with t h e s u r f a c e spin wave p i c t u r e of Mills and Maradudin / 7 / and t h e Monte- Carlo c a l c u l a t i o n s f o r a Heisenberg exchange system by Binder and Hohenberg /8/. A r e c e n t c a l c u l a t i o n by Mathon f 9 / l e a d s t o s i m i l a r r e s u l t s using an i t i n e r a n t Hubbard model. This i s probably a more r e a l i s t i c approach than a Heisenberg exchange model f o r m e t a l l i c Fe. In t h e Mills and Maradudin model and in an e a r l i e r c a l c u l a t i o n by Rado /10/ i t was implied t h a t t h e temperature dependence of t h e s u r f a c e magnetiza- t i o n would go a s
where t h e value of t h e constant BS would be twice as g r e a t a s t h a t f o r bulk s p i n waves. The c a l c u l a t i o n of Binder and Hohenberg, however, suggested t h a t f o r
weakened s u r f a c e exchange a q u a s i - l i n e a r temperature dependence could r e s u l t instead
.-, , A
of t h e T " ' ~ dependence suggested by t h e o t h e r workers.
Although t h e s e c a l c u l a t i o n s a l l assumed t h a t t h e s u r f a c e and bulk magnetiza- t i o n s would be eoual a t T=O ( i n s t e a d of t h e s l i g h t l y increased hyperfine f i e l d which we f i n d / 4 / ) , we have endeavored t o examine t h e temperature dependence of t h e magne- t i z a t i o n by studying t h e hyperfine f i e l d of a 57Fe l a y e r placed just a t t h e surface/
i n t e r f a c e of a 50-layer 5 6 ~ e film. In t h e two cases reported here t h e i n t e r f a c e m a t e r i a l s were Ag, as previously mentioned, and MnF2, an i n s u l a t o r which i s a l s o a weak antiferromagnet.
The model c a l c u l a t i o n s mentioned above were a l l made f o r Fe s u r f a c e s w i t h a vacuum i n t e r f a c e . We have assumed, i n comparinn our r e s u l t s with t h e s e c a l c u l a t i o n s t h a t t h e primary e f f e c t of t h e i n t e r f a c e material w i l l be t o change t h e s u r f a c e exchange i n t e r a c t i o n s o r t o introduce magnetic anisotropy e f f e c t s which would probably appear a s e f f e c t i v e changes in t h e s u r f a c e exchange.
The r e s u l t s f o r t h e measurements of t h e temperature dependence of t h e s u r f a c e
magnetism of Fe an Ag i n t e r f a c e a r e shown i n Figure 2. I t can be seen t h a t t h e
r e s u l t s f i t a T 378 dependence much b e t t e r than a T dependence. From t h e s e data we
can e x t r a c t a value of Bs of 12*1
X10-6 which can be compared w i t h t h e value of
F i g . 2 - F o r t h e measurements shown h e r e a p p r o x i m a t e l y two a t o m i c l a y e r s o f 5 7 ~ e were grown on a 5 0 - l a y e r (110) f i l m o f 56Fe. An i n t e r f a c e was t h e n formed w i t h s e v e r a l hundred l a y e r s o f Ag. The d a t a p o i n t s show t h e n o r m a l i z e d m a a n e t i z a t i o n ( o b t a i n e d f r o m t h e h y p e r f i n e f i e l d ) as a f u n c t i o n o f temperature. The d a t a f i t t h e form
Ms(T)
z
( l - B ~ T ~ " + . . . ) w i t h B, about 3.5 t i m e s g r e a t e r t h a n b u l k values.
3.4
X1 0 - ~ f o r b u l k Fe. T h i s i s a b o u t 3.5 t i m e s g r e a t e r t h a n t h e b u l k value, r a t h e r t h a n t h e f a c t o r o f g p r e d i c t e d by Rado / l 0 1 and M i l l s and Maradudin / 7 / .
The r e s u l t f o r t h e t e m p e r a t u r e dependence o f t h e s u r f a c e magnetism o f Fe a t an i n t e r f a c e w i t h MnF2 i s shown i n F i g u r e 3. I t can be seen t h a t t h e d a t a f i t w e l l t o a l i n e a r t e m p e r a t u r e dependence as p r e d i c t e d by B i n d e r and Hohenberg / 8 / f o r a weakened s u r f a c e exchange.
These r e s u l t s r a i s e a number o f q u e s t i o n s . F i r s t , does t h e d i f f e r e n t i n t e r f a c e m a t e r i a l produce a d i f f e r e n t exchanae f o r t h e Fe s u r f a c e by i n t e r a c t i o n between t h e
Fe and t h e Mn i o n s ? Secondly, i s t h e f a c t t h a t t h e MnF2 i s an i n s u l a t o r i m p o r t a n t ? T h i r d l y , does s u r f a c e a n i s o t r o p y induced b y t h e i n t e r f a c e m a t e r i a l p l a y an i m p o r t a n t r o l e ? These and o t h e r q u e s t i o n s suggest a d d i t i o n a l experiments u s i n g o t h e r
m a t e r i a l s . Measurements u s i n g MgO as an i n t e r f a c e m a t e r i a l a r e i n p r o g r e s s . Care must be t a k e n t o c o n s i d e r o n l y i n t e r f a c e s where a l l o y i n g o r i n t e r d i f f u s i o n does n o t t a k e p l a c e .
One may s t i l l be a l l o w e d t h e c o n c l u s i o n t h a t i t may be p o s s i b l e t o i n f e r i n t e r -
e s t i n g t h i n g s a b o u t t h e magnetic b e h a v i o r o f f r e e Fe s u r f a c e s b y s t u d y i n g t h e maane-
t i z a t i o n o f Fe a t s u i t a b l e i n t e r f a c e s and t h a t t h e maanetic b e h a v i o r o f Fe a t i n t e r -
faces i s i n t e r e s t i n g i n i t s own r i g h t .
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0 4 0 8 0 120 160 2 0 0 240 2 8 0 3 2 0 Temp.
