CRITICAL CURRENTS OF MULTILAYERED FILMS IN SUPERCONDUCTING PROXIMITY

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CRITICAL CURRENTS OF MULTILAYERED FILMS

IN SUPERCONDUCTING PROXIMITY

H. Raffy, E. Guyon

To cite this version:

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JOURNAL DE PHYSlQUE

_Colloque

_{C6, supplkment au}

_no

_{8, Tome 39, aofit 1978, page}

C6-648

CRITICAL CURRENTS

O F

MULTILAYERED FILMS IN SUPERCONDUCTING PROXIMITY

tt

H. ~ a f f ~ ' and E. Guyon.

tt

' ~ a b o r a t o i r e s de Mareoussis,

C . R. C. G. E.

91460 Mareoussis and Laboratoire de Physique des SoZides,

UniversitS Paris-Sud, 91405 Orsay.

R6sum6.- Nous avons 6 t u d i 6 l e s c o u r a n t s c r i t i q u e s dans des systSmes multicouches en proximit6 s u p r a c o n d u c t r i c e .

Abstract.- We p r e s e n t c r i t i c a l c u r r e n t measurements on m u l t i l a y e r e d f i l m s i n superconducting proximity

.

One of t h e causes of f l u x p i n n i n g i n second k i n d superconductors has long been known t o be t h e presence of normal i n c l u s i o n s of t h e o r d e r of magni-

tude of t h e coherence l e n g t h of t h e superconducting m a t r i x . The r e s u l t i n g i n c r e a s e of t h e o r e t i c a l cur- r e n t d e n s i t y h a s been observed i n s e v e r a l c a s e s / I / .

I n o r d e r t o f u l f i l l t h e need f o r c o n t r o l l e d geome- t r i e s we have p r e p a r e d and s t u d i e d f i l m s composed of a l t e r n a t i n g l a y e r s of two m a t e r i a l s w i t h t h e f o l - lowing p e r i o d i c sequence : S Z t

s l ,

s2

.

S1.

s2

(*)

S i s a superconducting a l l o y (XbIn 6 a t . % ) and S

1 2

i s e i t h e r a superconducting a l l o y of lower c r i t i c a l temperature T < T (ZnIn 6 a t . % ) i n t h e supercon-

c2 c1

d u c t i n g o r normal s t a t e , o r a normal m a t e r i a l (Ag). These m u l t i l a y e r s a r e "good" proximity e f f e c t sam- p l e s (no i n t e r m e t a l l i c compound o r s o l u b i l i t y a s i t was checked from t h e absence of ageing e f f e c t )

.

Due t o superconducting proximity, t h e c r i t i - c a l temperature of t h e sandwich was lowered below

T

.

An a n a l y s i s i n terms of t h e de Gennes Wertha- c 1

mer model and using mean f r e e p a t h from d a t a on t e s t f i l m s a g r e e s q u a n t i t a t i v e l y w i t h t h e experimen- t a l Tc 1 3 1 . The d e c r e a s e of t h e r a t i o of t h e c r i t i - c a l f i e l d p a r a l l e l t o t h e f i l m , HI,, t o t h a t perpen- d i c u l a r t o i t , HI

,

i s a l s o c o n s i s t e n t w i t h t h e proximity e f f e c t d e s c r i p t i o n ( t h e Hc from t h e o u t e r 3

l a y e r s S was always lower t h a n t h e upper c r i t i c a l f i e l d of t h e sandwich).

F i g u r e 1 g i v e s c r i t i c a l c u r r e n t v e r s u s f i e l d d a t a i n a t y p i c a l experiment w i t h t h e f o l l o w i n g t h i c k n e s s e s (2nIn : d = 500 ; XbIn : d = 2650

;().

-The. Bpplied magnEtic f i e l d H i s p a r a j l e l o r

he

t o t a l t h i c k n e s s i s 5 p (The r e s u l t s d i s c u s s e d h e r e a r e f a i r l y i n s e n s i t i v e t o t h e number of l a y e r s ) A t h r e e source e v a p o r a t i o n s e t up w i t h t h r e e eva- p o r a t i o n c o n t r o l l e r s and e l e c t r o n i c r e g u l a t i o n s h a s been used 121. p e r p e n d i c u l a r t o t h e l a m e l l a s . I n p e r p e n d i c u l a r geometry t h e monotic decrease of Ic (HI) i s q u a l i t a t i - v e l y comparable t o t h a t of a s i n g l e f i l m .

Fig. 1 : C r i t i c a l c u r r g n t of P&-Ig/S*-In l a y e r e d sample w i t h d l = 2650 A,d2 = 500 A :-

-

i n p a r a l l e l f i e l d ;

---

i n p e r p e n d i c u l a r f i e l d a t T = 5.5 K

(Tc % 6.7K).

T h i s v a r i a t i o n may be used a s a f l u x pinning back ground t o c h a r a c t e r i z e t h e l a r g e r v a l u e of t h e c r i - t i c a l c u r r e n t o b t a i n e d i n t h e p a r a l l e l geometry and' t h e pinning e f f e c t of t h e m u l t i l a y e r e d s t r u c t u r e : i n t h i s case t h e c r i t i c a l c u r r e n t i s k e p t perpen- d i c u l a r t o t h e f i e l d s o t h a t t h e Lorentz f o r c e d r i - v i n g t h e v o r t e x l i n e s i s e x e r t e d p e r p e n d i c u l a r l y t o t h e l a y e r s . The peak s t r u c t u r e observed on t h e Ic

( H I / ) curve i s s t r o n g l y r e m i n i s c e n t of t h a t o b t a i - ned i n previous s t u d i e s on superconducting XbBi a l - l o y s modulated p e r i o d i c a l l y i n c o n c e n t r a t i o n / 4 / and appears t o be due t o a matching of t h e f l u x

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l i n e l a t t i c e and t h e p e r i o d i c s t r u c t u r e of t h e f i l m . The peak f i e l d i s of t h e same o r d e r a s t h a t f o r a modulated f i l m having t h e same p e r i o d : h e r e d =

d + d . 1 2

A c h a r a c t e r i s t i c f e a t u r e of t h e proximity geometry i s t h e non monotonic v a r i a t i o n of t h e peak amplitude : t h e peak d i s a p p e a r s a t h i g h temperature ( t y p i c a l l y when the coherence l e n g t h becomes compa- r a b l e w i t h d)*, I t a l s o d e c r e a s e s a t low temperatu- r e s a s t h e e x i s t e n c e of s u p e r c o n d u c t i v i t y i n ZnIn tends t o a t t e n u a t e t h e modulation i n t h e o r d e r pa- rameter v a r i a t i o n

151.

This behaviour i s more pro- nounced f o r t h i n S f i l m s which a r e more coupled t o

2

t h e S ones. I n some i n s t a n c e s / 6 / . t h e peak disap- 1

p e a r s a t low temperatures. I n t h e same systems, a peak amplitude maximum i s o b t a i n e d around 4 K, i . e . s l i g h t l y above T

.

No such maximum i s o b t a i n e d i n

c2

PbBi/Ag m u l t i l a y e r e d systems.

'L

I f d i s k e p t c o n s t a n t , and d and d v a r y , 1 2

t h e p o s i t i o n of t h e peak does n o t vary much whereas t h e shape of t h e Ic (HI/ ) curve i s d i f f e r e n t . Figu- r e 2 corresponds t o a s l i g h t l y s m a l l e r p e r i o d than f i g u r e 1. Note the d e c r e a s e of t h e peak f i e l d v a l u e c o n s i s t e n t w i t h t h e f l u x l i n e matching mechanism.

Fig. 2 : C r i t i c a l c u r r e n t I (H ) of twooKb-In/xn- I n l a x e r e d samples w i t h (1):: = 2300 A,

$,

= 2500 A and (2) : d l = 1500 A and d 2 = 3500 A.

The peak s t r u c t u r e i s s t r o n g l y reduced when d 2 becomes l a r g e compared w i t h d ; t h e I c ( H

_/I

) v a r i a t i o n s u g g e s t s decoupled S f i l m s . The ~ r o g r e s s i v e in- c r e a s e of t h e excess c r i t i c a l c u r r e n t i n p a r a l l e l geometry o b t a i n e d i n some samples i n low f i e l d may

a l s o be i n d i c a t i v e of t h e f o l l o w i n g mechanism : As H

i s

i n c r e a s e d , t h e s u p e r c o n d u c t i v i t y induced by pro- x i m i t y i s p r o g r e s s i v e l y reduced i n S ( t h i s t a k e s p l a c e f o r low f i e l d s ) with a corresponding i n c r e a s e of jc(Hll ) .

The p o s s i b l e decoupling scheme i s emphasized on t h e c u r v e s of f i g u r e 3 . The same XbIn

6

% l a y e r t h i c k n e s s (d = 2650

i)

was used i n sandwiched of d i f f e r e n t normal l a y e r (Ag)thicknesses: For t h i c k e r Ag f i l m s t h e v a r i a t i o n does n o t show a peak any longer. F i g . 3 : Normalized c r i t i c a l c u r r e n t

---

4.8 K of t h r e e l a y e r e d ~ b - l n / A g o s a m p l e s wizh t h e same d = 2600 A and d 2 = 2900 A ( I ) , 500 A ( 2 ) , 380

1

t 3 ) . 0 On t h e o t h e r s i d e , when d i s reduced below 500 A, t h e peak amplitude d e c r e a s e s a g a i n as t h e Ag l a y e r l o s e s i t s i n d i v i d u a l i t y due t o s t r o n g proximity e f - f e c t .

We wish t o thank W. Weber f o r h e l p i n sample p r e p a r a t i o n and J.P. Burger, L. D o b r o s a v l j e v i c , L. Dumoulin and J . C . Renard f o r many u s e f u l discus- s i o n s . This work was supported by D.R.M.E.

k

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R e f e r e n c e s

/ 1 / L i v i n g s t o n , J . D . , Appl. Phys. L e t t .

8

(1966) 319 S e v e r a l c o n t r i b u t i o n s a l o n g t h i s l i n e a r e found i n Proc. I n t e r n . Disc. Meeting o n F l u x P i n n i n g , ed. Haasen ( G 6 t t i n g e n Akad. Wissen).

/ 2 / R a f f y , H., Weber, W . , Peninou, J . J . e t Renard, J.C.,

t o b e p u b l i s h e d . 131 R a f f y , H. and N e d e l l e c , P., t o be p u b l i s h e d . / 4 / a . R a f f y , H . , Renard, J.C. e t Guyon, E . , S o l . S t . Comun

3

(1974) 427. b. R a f f y , H., Renard, J . C . e t Guyon, E . , S o l . S t . Commun

14

( 1974) 431.

/5/ Note t h e weak t e m p e r a t u r e dependence of Ic(T) around HI % 600 Oe, T % 3 K ; such a b e h a v i o r i s

important! f o r s t a b i l i z a t i o n p u r p o s e s .