METALLIC INTERFACES AND PROPERTY MODIFICATION

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METALLIC INTERFACES AND PROPERTY MODIFICATION

M. Brodsky

To cite this version:

M. Brodsky. METALLIC INTERFACES AND PROPERTY MODIFICATION. Journal de Physique Colloques, 1984, 45 (C5), pp.C5-349-C5-356. �10.1051/jphyscol:1984552�. �jpa-00224170�

JOURNAL D E PHYSIQUE

Colloque C5, suppl6ment a u n04, Tome 45, avril 1984 page C5-349

M.B. Brodsky

M a t e r i a l s S c i e n c e a n d Technology D i v i s i o n , Argonne N a t i o n a l L a b o r a t o r y , Argonne, I Z Z i n o i s 60439, U . S . A .

RQsumQ - Les p r o p r i 6 t Q . s Q l e c t r o n i q u e s d e Pd, C r e t V s o n t m o d i f i g e s quand c e s m&- t a u x s o n t p r Q p a r 6 s en Q p i t a x i e e n t r e deux c o u c h e s d ' o r ou d ' a r g e n t . Les c o n t r a i n t e s e t l e s d i s t o r s i o n s de c e s f i l m s minces d e Pd, C r e t V provoquent d e n o u v e l l e s p r o - p r i g t 6 s t e l l e s q u ' u n e s u s c e p t i b i l i t k magngtique e x h a l t 6 e du s y s t s m e Au-Pd-Au e t une s u p e r c o n d u c t i v i t 6 a y a n t r e s p e c t i v e m e n t une h a u t e e t une b a s s e t e m p g r a t u r e c r i t i q u e Tc p o u r l e s syst&mes Au-Cr(c.f.c.)-Au e t Au-V(c.f.c.)-Au.

A b s t r a c t - The e l e c t r o n i c p r o p e r t i e s of Pd, C r and V a r e modified by p r e p a r i n g them a s e p i t a x i a l m e t a l f i l m sandwiches i n s i d e Au o r Ag. New p r o p e r t i e s i n c l u d e near- magnetism o r s u p e r c o n d u c t i v i t y on s t r e t c h e d Pd, high Tc s u p e r c o n d u c t i v i t y i n f c c Cr and a lowered Tc i n f c c V.

I - INTRODUCTION

S t r o n g r e c e n t i n t e r e s t i n i n t e r f a c e s , a s w e l l a s f r e e s u r f a c e s , a r i s e s o u t of t h e concurrent development of new experimental and t h e o r e t i c a l techniques t o s t u d y t h e s e i n t e r e s t i n g m a t e r i a l s . The p r e s e n t s t u d y makes use o f u l t r a - h i g h vacuum techniques t o prepare e p i t a x i a l metal f i l m l a y e r s of Pd, C r o r V sandwiched i n s i d e s i n g l e - c r y s t a l ~ ~ ( 1 0 0 ) o r Ag(100). Novel p r o p e r t i e s a r e o b t a i n e d a s a r e s u l t of t h e s t r e t c h i n g and d i s t o r t i o n of t h e Pd l a t t i c e o r by t h e p r e p a r a t i o n of m e t a s t a b l e f c c C r and V. These p r o p e r t i e s i n c l u d e h i g h l y enhanced magnetic s u s c e p t i b i l i t y , X, (and evidence f o r band ferro-magnetism) i n Au-Pd-Au / l / ; s u p e r c o n d u c t i v i t y i n

Ag-Pd-Ag / 2 / ; high Tc s u p e r c o n d u c t i v i t y i n Au-Cr-Au / 3 / ; and low T ' S i n Au-V-Au and Ag-V-Ag (Hamaker, Brodsky, Sowers and Marikar, t o be p u b l i s h e d ) . %and c a l c u l a t i o n s s u p p o r t t h e o b s e r v a t i o n o f enhanced magnetism i n Pd /4,5/ and s u p e r c o n d u c t i v i t y i n f c c C r (Xu, Freeman, J a r l b o r g and Brodsw, t o be p u b l i s h e d ) .

I1

-

A l l Pd, C r and V f i l m s were d e p o s i t e d a t room temperature, g e n e r a l l y on 1000 8-thick f i l m s of s i n g l e c r y s t a l Au o r Ag. The i n i t i a l Au o r Ag l a y e r s had been deposited on NaCl(100) a t 300" C t o y i e l d h i g h q u a l i t y c r y s t a l s of (1 00) o r i e n t a t i o n . A covering l a y e r of 1000 8 of Au o r Ag was d e p o s i t e d on t h e Pd o r C r l a y e r s which were 10-20 8 t h i c k . F a r l i e r samples were s t u d i e d by f l o a t i n g t h e f i l m s onto Al s u b s t r a t e s f o r X measurements, s a p p h i r e having pre-deposited Au s t r i p s f o r r e s i s t i v i t y measurements ( P ) o r o n t o TEM sample g r i d s . More r e c e n t l y , samples have been s t u d i e d without removing them from t h e NaCl s u b s t r a t e s . F u r t h e r d e t a i l s may be found elsewhere /l -3/.

I11

-

Au-Pd-Au - TEM s t u d i e s / l / showed t h e s e t o be s i n g l e c r y s t a l s . A s expected, t h e l a r g e r Au l a t t i c e (ao = 4.078 8) c a u s e s a s t r e t c h i n g of t h e Pd l a y e r (ao = 3.890 8) a t t h e i r i n t e r f a c e . Spacings of Moire' f r i n g e s (P. Marikar, p r i v a t e communication) y i e l d s t r a i n s of 1-2%. This a g r e e s q u a l i t a t i v e l y with t h e i o n channeling r e s u l t of Kuk, Feldman, and Silverman /6/ f o r Au on Pd (1 1 1 ) . Fig. 1 shows l / X v s . T f o r a

'work supported by t h e U.S. Department of Energy

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984552

C5-350 J O U R N A L DE PHYSIQUE

number of samples and t h e magnetization curve f o r sample I V ( s e e r e f . 1 f o r sample d e t a i l s ) . The magnitudes of t h e X'S range from 35 t o 2500 times l a r g e r t h a n t h o s e of b u l k Pd. These l e a d , i n t u r n , t o exchange enchancements, S, up t o 350-25000 v s S = 1 0 f o r bulk Pd. C l e a r l y t h i s h a s brought t h e Pd very c l o s e t o t h e S t o n e r c o n d i t i o n f o r band magnetism, T N ( E ~ ) > ~ , where T i s t h e mean i n t e r a c t i o n parameter, N ( E ~ ) t h e d e n s i t y of e l e c t r o n s t a t e s a t t h e Fermi l e v e l and S = l / ( ? - 7 N ( E ~ ) ) . There a r e a d d i t i o n a l i n d i c a t i o n s t h a t t h e sandwiched Pd h a s n e a r l y become magnetic.

Bulk Pd h a s a s p i n f l u c t u a t i o n temperature, Tsf = To/S ' 260 K, where To i s t h e degeneracy temperature. Tsf =

-

CW '

Fig. 1 , T S f ' s c l o s e t o 0 K a r e o b t a l n e d h e r e , compared t o t h e v a l u e of 260 K f o r bulk Pd. T becomes z e r o when t h e system becomes ferromagnetic. Values of S may be d e r i v e d Prom t h e v a l u e s o f A X / x o r from OgTy T h e s e have been found t o be about t h e same f o r t h e samples shown i n Fig. 1 , i n l c a t i n g t h a t t h e paramagnon model used h e r e i s c o r r e c t / l / .

I I I I

- 8 -4 0 4 8

- TEMPERATURE ( K )

Fig. 1

-

Although the'M-H curve i n t h e i n s e t of Fig. 1 i s a p p a r e n t l y l i n e a r t h e r e i s a posi- t i v e i n t e r c e p t on t h e H=O a x i s . This i s n o t due t o i m p u r i t i e s a s a l l of t h e Au-Pd- Au f i l m s l o s e t h e i r s t r o n g X'S when s t o r e d a t room temperature. F i g u r e 2 shows t h i s

MAGNETIC FIELD (kOe)

Fig. 2 - Magnetization c u r v e s f o r a Au-Pd-Au sample a t 4.9 K. Open c i r c l e s a r e f o r t h e f r e s h l y prepared sample and t h e s o l i d c i r c l e s were o b t a i n e d a f t e r two weeks a t room temperature.

e f f k c t f o r a r e c e n t sample prepared w i t h t e n s e t s of Pd l a y e r s , each with 20

a

s e p a r a t e d by 5 0 a of Au. These measurements, made with a SQUID susceptometer, without removing t h e sample from t h e NaC1, show t h e d e c r e a s e of X with time and t h e r e d h t i o n of t h e p o s i t i v e i n t e r c e p t with time, a s was r e p o r t e d e a r l i e r / l / . It i s l i y l y t h a t t h e magnetic component s e e n a t H = 0 i n Figs. 1 and 2 i s due t o magnetic Pd. Two samples of Au-Pd-Au have y i e l d e d t h e s i g n a t u r e of t r u e band magnetism v i a curved M-H p l o t s and A r r o t t p l o t s w i t h d e r i v e d Tc's o f 3 and 15 K b u t s a t u r a t i o n moments of only 0.01-0.02 %/Pd r rod sky and Hamaker, t o be ~ u b l i s h e d ) . F u r t h e r c o r o b o r a t i o n of t h e s e r e s u l t s

F .

measuring s i g n a l s . This i s n o t helped by making t h e Pd f i l m t h i c k e r a s it l e a d s t o

S t r a i n r e l i e f / 6 , 7 / .

Band c a l c u l a t i o n s of Pd f i l m s /4/ y i e l d S v a l u e s of 400 f o r s t r a i n e d and t e t r a - g o n a l l y d i s t o r t e d Pd and metamagnetism a t r e l a t i v e l y low f i e l d s of = 2 0 T. The combined e f f e c t s of t h e s t r a i n e d Pd i n c o n t a c t with Au have been considered by a c a l c u l a t i o n of t h e d e n s i t y of s t a t e s f o r t h e modulated s t r u c t u r e s , Pd2Au2 and Pd Au /5/. These r e s u l t s s e p a r a t e t h e e f f e c t s due t o i n t e r f a c e and bulk Pd l a y e r s . d e r e s u l t s y i e l d t o t a l d - e l e c t r o n N ( E ~ ) ' s of 2 . 0 l / ( e ~ - a t o m ) f o r Pd2Au2 (lower than t h e v a l u e of 2.40/(e~-atom) f o r bulk Pd) and 2.84/(eV-atom) - f o r Pd Aug. The r e s u l t a n t v a l u e s f o r t h e g e n e r a l i z e d Stoner f a c t o r , S = T N(EF) a r e 0.6 ?or Pd Au and 0.93

? 2

f o r PdjAu3 ( t h a t i s , S = 14 o r double t h e v a l u e f o r bulk P;). Assunlng t h a t i n a Pd Au o r Pd Au s u p e r l a t t i c e one may simply count-up t h e bulk-like" and

"i$te%facen 2d ? a y e r s , Freeman, e t a l . /5/ o b t a i n 3 = 0.98 f o r Pd Au and 3 = 1 .O1 f o r Pd Aug. Thus, they p r e d i c t ferromagnetism f o r a modulated st$uc?ure having

~ d ( l O A ~ ~ ~ ( 1 0 A). These r e s u l t s a r e s t r o n g l y s u p p o r t i v e of t h e r e s u l t s found experimentally which i n d i c a t e p a r t i a l o r t o t a l weak ferromagnetism i n Au-Pd-Au sandwiches.

Ag-Pd-Ag

-

-

6

TEMPERATURE [ K )

Fig. 3 - Low temperature r e s i s t i v i t y - t e m p e r a t u r e curve f o r sample X V I I A ? taken a f t e r 8 and 180 days a t room temperature. The i n s e t shows d a t a f o r sample XVIIA3.

Minima i n t h e P-T curves have been r e p o r t e d f o r Pd-Ag a l l o y s c o n t a i n i n g 20-55 a t . % Ag /8/. The p-minimum and a p T~ f o r lower Ag c o n c e n t r a t i o n s a r e thought t o be a consequence of l o c a l i z e d , exchanged-enhanced Pd d - s t a t e s . This i s suggested by X- r a y p h o t o e l e c t r o n s p e c t r a of Pd-Ag a l l o y s /g/, which show e s s e n t i a l l y no energy s h i f t and a four-fold narrowing o f t h e Pd d-band a s Ag i s added. Thus, t h e s t r o n g e r l o c a l i z a t i o n o f t h e d-band g i v e s r i s e t o t h e +minimum i n a l l o y s . Conversely f o r Au-Pd a l l o y s , t h e Pd d - e l e c t r o n s t a t e s change from v i r t u a l bound s t a t e c h a r a c t e r t o d-band c h a r a c t e r /10/. And no minimum i s found i n a l l o y s . The time-dependent X'S found f o r Au-Pd-Au f i l m s ( s e e Fig. 2) a r e i n d i c a t i v e of conversion of t h e d i s t i n c t

C5-352 JOURNAL DE PHYSIQUE

Pd l a y e r i n t o an a l l o y . Color changes due t o a l l o y i n g of t h e f i l m s have a l s o been observed due t o s t o r a g e of Au-Pd-Au f i l m s a t room temperature. We s u g g e s t ,

t h e r e f o r e t h a t t h e occurrence of t h e p-minimum i n Ag-Pd-Ag sandwiches may be due t o a l l o y formation a t t h e expense of t h e Pd l a y e r ( s e e a l s o below).

One sample o f Ag-Pd-Ag (XVIIAI) showed a very d i f f e r e n t p-T curve (Fig. 3 ) . The small drop i n p f i r s t s e e n a t 2.15 K was found subsequently t o be much l a r g e r (22%) when lower temperatures became a v a i l a b l e . Although p d i d n o t drop t o zero, t h e t r a n s i t i o n was found t o have t h e p r o p e r t i e s of a superconductor. Figure 4 shows t h e e f f e c t o f a n a p p l i e d magnetic f i e l d on t h e t r a n s i t i o n temperature. The s t r o n g a n i s o t r o p y and magnitude of dH /dT a g r e e well with t h e e x p e c t a t i o n s f o r a superconductor /2/. Derived p m p e r % i e s a r e c2 = 71 kOe/K;

0 0 -

TEMPERATURE (K1

Fig. 4

-

Hc2(0) = 49 H e , R = 320 8, ⁼ 16 p%cm and 5 = 46 8. These v a l u e s , p l u s t h e N(EF) /l l / allow a c a l c u l a t i o n of Tc a s a f u n c t i o n of Pd t h i c k n e s s f o r v a r i o u s T (bulk) v a l u e s . Figure 5 shows t h e c a l c u l a t i o n o f T f o r sample XVIIAl ( T ~ = 2.1$ K, Ag(1700 8) - Pd(20 8)

-

OF

Pd THICKNESS (L1

Fig. 5 - ^{T 'S} c a l c u l a t e d f o r v a r i o u s Pd t h i c k n e s s f o r Au(1700 8)-~d-Au(1700 A). The upper curv; i s f o r a mean-free-path o f 320 8, and t h e lower one i s f o r II = 32 8 .

Since P does n o t drop t o zero, i t i s l i k e l y t h a t a l l r e g i o n s of t h e Pd f i l m do n o t have t h e same t h i c k n e s s . Due t o t h e normal s t a t e proximity e f f e c t , t h e t h i c k e r r e g i o n s e x h i b i t t h e h i g h e r T ' S and t h e v e r y low Tc t h i n r e g i o n s cause a r e s i d u a l p.

Even with a Pd t h i c k n e s s dougle t h e average v a l u e , Tc (bulk) v a l u e s 8 0 K a r e obtained.

The observed T, was found t o be a f u n c t i o n of time a t room temperature. This i s s e e n i n Fig. 3 where Tc had dropped from i t s i n i t i a l v a l u e of 2.15 K t o 1.6 K a f t e r e i g h t days a t room temperature. A f t e r 180 days, Tc = 1.05. The p-T c u r v e s f o r sample X V I I A 1 began t o show a p-minimum n e a r 12 K a f t e r 58 days a t room temperature, when Tc = 1.4 K. The developnent o f t h e p-minimum w i t h time i s s u p p o r t i v e of t h e proposal t h a t t h e minimum a r i s e s from Pd-Ag i n t e r d i f f u s i o n . It i s p o s s i b l e t h a t t h e o t h e r Ag-Pd-Ag samules, with P-minima i n i t i a l l y , may have become superconducting a t s t i l l lower temperatures. The major d i i f e r e n c e between t h e superconducting Ag-Pd-Ag sample and a l l o t h e r s may l i e i n i t s morphology. When i t was f l o a t e d onto t h e measuring s u b s t r a t e , t h i s sample c u r l e d i n t o a c y l i n d e r whereas a l l o t h e r Ag-Pd-Ag samples were mostly f l a t , b u t wrinkled.

Attempts t o e x p l a i n t h e s u p e r c o n d u c t i v i t v i n Ag-Pd-Ag f i l m s have followed s e v e r a l approaches. Pd h a s been observed t o become a superconductor when i r r a d i a t e d t o d e s t r o y t h e s p i n f l u c t u a t i o n s /13/. Also, band c a l c u l a t i o n s f o r Pd-Ag a l l o y s pre- d i c t s u p e r c o n d u c t i v i t y a t p a r t i c u l a r compositions /11/ due t o a r a p i d drop i n s p i n f l u c t u a t i o n e f f e c t s b u t a slow drop i n N(EF) and t h e electron-phonon i n t e r a c t i o n with Ag a d d i t i o n s . The s u p e r c o n d u c t i v i t y observed h e r e could be r e l a t e d t o one of t h o s e i d e a s . It could b e due t o a somewhat d i s o r d e r e d Pd f i l m ( n o t observable i n TEM s t u d i e s ) i n which t h e s p i n f l u c t u a t i o n s a r e n o t s u s t a i n e d o r i n t h e l a t t e r c a s e t o t h e p a r t i a l formation of a Pd-Ag a l l o y which may be f a v o r a b l e f o r superconducti- v i t y . Since s u p e r c o n d u c t i v i t y has n o t been observed down t o O.lmK i n Ag-Pd a l l o y s /14/, t h e l a t t e r c a s e seems unreasonable. The l a r g e exchange enhancements found i n Au-Pd-Au s u g g e s t t h e p o s s i b i l i t y of t r i p l e t - p a i r i n g s u p e r c o n d u c t i v i t y i n Pd f i l m sandwiches /15/. The c o n d i t i o n s governing such behavior have been considered by B6al-Monod, e s p e c i a l l y i n confined geometries such a s t h i n f i l m s /16/. These argu- ments would r e q u i r e t h a t f o r T>Tc t h a t t h e X should become n e a r l y c o n s t a n t with T.

That i s , X should drop below t h e v a l u e d e r i v e d from a Curie-Weiss f i t a t T<<T

.

T h i s dependence h a s not y e t been sought i n t h e Ag-Pd-Ag f i l m s . More importan?&-, t h e o b s e r v a t i o n of such behavior would n o t prove t h e presence of t r i p l e t - p a i r i n g s u p e r c o n d u c t i v i t y , b u t would r e q u i r e f u r t h e r proof. Thus f a r , t h e r e i s no c l e a r understanding of t h e s o u r c e of t h e s u p e r c o n d u c t i v i t y i n Ag-Pd-Ag.

Au-Cr-Au

-

m e t a s t a b l e fcc-Cr s t r u c t u r e i s a l s o supported by XANES s t u d i e s (N. Zaluzec, p r i v a t e communication) which show a n e l e c t r o n i c s t r u c t u r e d i f f e r e n t than t h a t of bcc-Cr, and

- . /

TEMPERATURE IKI

Fig. 6

-

C5-354 JOURNAL DE PHYSIQUE

low a n g l e X-ray d i f f r a c t i o n ( B . Batterman, p r i v a t e communication). Also, a LEED s t u d y of C r on ~ u ( l l l ) /l 7/ had concluded t h a t fcc-Cr i s formed. Figure 6 shows p-T d a t a f o r two of t h e Au-Cr-Au samples, which a r e t y p i c a l o f approximately one-half o f t h e Au-Cr-Au samples. These d a t a a r e very s i m i l a r t o t h o s e of t h e superconducting Ag-Pd-Ag, i n t h a t t h e drop i n p i s s i g n i f i c a n t (16%) b u t does not go t o zero, while t h e field-dependence of T i s a s expected f o r s u p e r c o n d u c t i v i t y with r e s u l t s s i m i l a r t o t h o s e i n Fig. 4 /3/. fn b o t h c a s e s ( A g - p d - ~ ~ and Au-Cr-Au) Tc d e c r e a s e s with s t o r a g e a t room temperature and a minimum then occurs. Furthermore, every sample which does n o t become a superconductor h a s a minimum i n t h e p-T curve. Thus, a s t h e d i s t i n c t Pd o r C r l a y e r s a r e converted t o t h e i r r e s p e c t i v e a l l o y s , Ag-Pd o r Au-Cr, minima due t o a l o c a l i z e d d-band o r t o a low TK tend t o d e s t r o y t h e

s u p e r c o n d u c t i v i t y . Presumably, f o r those samples where no s u p e r c o n d u c t i v i t y i s found, a l l o y formation o c c u r s b e f o r e t h e i n i t i a l measurements have been performed, o r t h e y have very low Tc'S.

A s i n t h e c a s e o f Ag-Pd-Ag, a proximity e f f e c t c a l c u l a t i o n may be used t o d e r i v e a bulk Tc value. Again, a n unexpectedly high Tc (bulk) lOK i s found if a n e f f e c t i v e

t h i c k n e s s twice t h e mean t h i c k n e s s i s used i n t h e c a l c u l a t i o n / 3 / . Band

c a l c u l a t i o n s (XU, Freeman, J a r l b o r g and Brodsky, t o be published; have been c a r r i e d o u t f o r f c c C r w i t h t h e l a t t i c e parameter (3.52 8) d e r i v e d i n t h e LEED s t u d y / I T / . The r e s u l t s i n d i c a t e a very h i g h d e n s i t y of s t a t e s comparable t o t h a t o f P t ( s e e a l s o r e f . 3 ) , no magnetism b u t a superconducting TC = 4.5 K i f t h e

%

Au-V-Au and Ag-V-Ag

-

0

o

!

,b,

,Ao loko 12i0

V THICKNESS

Fig. 7 - T C 1 s f o r ~ ~ ( 5 0 0 8)-V-Au(500 8) sandwiches. Open c i r c l e s a r e t h e measured T ' S while t h e s o l i d curve i s c a l c u l a t e d f o r t h e parameters of bcc-V. The s o l i d c l r c l e s show t h e Tc f o r bulk bcc-V n e c e s s a r y t o y i e l d t h e measured v a l u e s . C Below t h i s t h i c k n e s s , t h e measured v a l u e s drop d r a m a t i c a l l y below t h e p r e d i c t e d curve. TEM s t u d i e s (Marikar, p r i v a t e communication) and low a n g l e X-ray s c a t t e r i n g (Batterman, p r i v a t e communication) i n d i c a t e t h a t f o r t h i n V l a y e r s ( = 25 A), a n f c c s t r u c t u r e i s obtained, while f o r t h i c k e r l a y e r s bcc-V i s observed. Thus, t h e low T c ' s observed with t h i n V samples a r e probably due t o fcc-V which h a s a much lower T t h a n bcc-V. Preliminary band c a l c u l t i o n s ( ~ r e e m a n , p r i v a t e communication) do i E d i c a t e lower N ( E ~ ) ' s and T c ' s f o r f c c V. S i m i l a r e f f e c t s a r e found f o r Ag-V-Ag, w i t h t h e drop i n Tc o c c u r r i n g f o r s l i g h t l y h i g h e r V t h i c k n e s s . Thus, t h i s e f f e c t i s

n o t due t o t h e formation of magnetic Au4V (no such compounds form i n t h e Ag-V system). I n t h e s e sandwiches, p + 0 when t h e drop i n p i s observed. Measurements o f dHc2/dTc follow t h e p r e d i c t i o n s f o r N/S sandwiches.

I V - CONCLUSION

These measurements show t h a t i n t e f a c e s may be used t o s t r a i n m a t e r i a l s and/or s t a b i l i z e m e t a s t a b l e s t r u c t u r e s . I n t h e c a s e of Pd, C r and V major changes can be a f f e c t e d i n magnetic and/or superconducting p r o p e r t i e s . Since t h e tendency i n Au- Pd-Au sandwiches i s t o a c h i e v e , o r n e a r l y achieve ferromagnetism, no

s u p e r c o n d u c t i v i t y i s l i k e l y t o be found i n t h i s system ( s e e a l s o r e f . 1 6 ) . In Ag- Pd-Ag, a p p a r e n t l y t h e s u p e r c o n d u c t i v i t y i s seen only when no a l l o y formation occurs. Although t h e fcc-Cr and V s t r u c t u r e s a r e n o t s t a b l e , t h e y seem t o form r e a d i l y i n t h e sandwiches. Thus, t h e r e s p e c t i v e novel p r o p e r t i e s a r e r e t a i n e d more r e a d i l y t h a n f o r t h e s u p e r c o n d u c t i v i t y of Ag-Pd-Ag. An unexpected r e s u l t i s t h e v e r y high Tc (bulk) r e q u i r e d t o e x p l a i n t h e observed Ag-Pd-Ag and Au-Cr-Au f i l m

!Cc's. Although t h e p o s s i b i l i t y of non-uniform t h i c k n e s s e s w i l l reduce t h e p r e d i c t e d v a l u e s f o r Tc ( b u l k ) , very high T c ' s may be achievable with f c c C r o r sandwiched Pd i f t h i c k e r superconducting f i l m s can be prepared. This p o s s i b i l i t y r e q u i r e s f u r t h e r s t u d y

.

Other s t u d i e s have a l s o used sandwiches (uncovered) t o modify m e t a l l i c p r o p e r t i e s . Thus, Gradmann and I s b e r t /18/ have o b t a i n e d magnetic moments ranging from very low v a l u e s t o 2.6 uB a s t h e l a t t i c e parameter of t h e Cu-Au s u b s t r a t e i s v a r i e d .

P r o p e r t i e s of f c c Fe sandwiched by Cu a r e r e p o r t e d a t t h i s conference ( ~ i a r t a l l a , Becker, Keune and ~f annes)

.

V - ACKNOWLEDGEMENT

The a u t h o r wishes t o thank A. J. Freeman f o r h i s continued i n t e r e s t i n t h i s work and C . H. Hamaker f o r a very f r u i t f u l c o l l a b o r a t i o n . The use of unpublished r e s u l t s by P. Marikar, N. Zalueec and B. Batterman h a s been most h e l p f u l .

REFERENCES

1 . Brodsky, M. B. and Freeman, A. J., Phys. Rev. L e t t . 45, (1980) 133; Brodsky, M.

B., J. Magn. Magn. Matl. 35 (1983) 99.

2. Brodsky, M. B., Phys. R e v x z , (1982) 6060.

3. Brodsky, M. B., Marikar, P., F r i d d l e , R. J., S i n g e r , L. and Sowers, C. H . , Solid S t a t e Commun. 42, (1982b) 675; Brodsky, M. B. and Hamaker, H. C., i n Superconductivity i n d- and f-Band Metals, eds. Buckel, W. and Weber, W. (KFK, 1982) D. L 291. -

4 ~ a r l b o r g , T . and Freeman, A. J . , Physica 107B (1981) 69; J a r l b o r g , T. and Freeman, A. J., Phys. Rev. B=, (1981) 3 m

5. Freeman, A.J., Xu, J.-H. and J a r l b o r g , T., J. Magn. Magn. Matl. 31-34 (1983) 909

6. Kuk, Y . , Feldman, L. C . and Silverman, P. J . , Phys. Rev. L e t t .

2

7. Frank, F. C. and van d e r Merve, J. H., Proc. Roy. Soc. London, Ser. (1949) 205.

8. mwards, L. R . , Chen, C . W. and Legvold, S., S o l i d S t a t e Commun.

8

Murani, A. P., Phys. Rev. L e t t . (1974) 91.

9. Hnfner, S.

,

J., E l e c t r o n Spectrosc. Relat. Phenom. 15 (1 979) 95.

11. Gyorffy, B. L., Pindor, A. and ~ e m m e r m a z W. M., Phys. Rev. L e t t . 47, ⁽¹⁹⁷⁹⁾

1343.

C5-356 JOURNAL DE PHYSIQUE

de Gennes, P. G. and Guyon, E., Phys. L e t t . 3 (1963) 168; Werthamer, N. R . , Phys. Rev. 132 (1963) 2440.

S t r i t z k e r , B., Phys. Rev. L e t t .

42,

Buchal, C. H., S t r i t z k e r , B., Kubota, M , , M u e l l e r , R. M. and P o b e l l , F., S o l i d S t a t e Comun.

2,

Levin, K. and V a l l s , 0. T., Phys. Rev. B

3

%al-Monod, M ~ T . , S o l i d S t a t e Commun. 32 (1 979) 357; Be'al-Monod, M. T., Phys.

Rev. 28 (1983) 368; and Bgal-Monod, M. 'P., t h i s conference.

Thornas, R. E. and Haas, G. A . , J. Appl. Phys.

2,

Gradmann, U. and I s b e r t , H. O., J. Magn. Magn. Matl. 15-18 (1980) 1109.

EL-Bhatanouny, M . , S t r o n g i n , M., Williams, G. P. and C o l b e r t , J. Phys. Rev.

L e t t .

fi