Cu multilayers

PHYSICAI

Physica B 194-196 (1994) 2383-2384 North-Holland

63Cu AND SSNb NMR STUDY IN SUPERCONDUCTING Nb/Cu MULTILAYERS Yoh Kohori, Takao K o h a r a , Yasukage Oda, Masakazu Nishikawa ^, Eiji K i t a ^, and A k i r a Tasaki ^ Department of Material Science, Faculty of Science, Himeji Institute of Technology, A k o - g u n , H y o g o 6 7 8 - 1 2 , J a p a n ^Institute of Applied Physics, University of Tsukuba, Tsukuba, lbaraki 305, Japan 63 Cu and 9 a Nb NMR have been carried out in Nb/Cu superconducting m u l t i l a y e r e d thin f i l m down to 0.1K, and nuclear spin l a t t i c e r e l a x a t i o n t i m e . T , , in l o w f i e l d (H=100 (De) has been measured by using f i e l d c y c l i n g method. l/T, of 6SCu has a coherence peak just below Tc and decreases at low temperatures. The v a l u e o f l / T , a t low temperatures increases w i t h increasing the mean f r e e path o f e l e c t r o n s in the Cu layers.

1.INTRODUCTION In r e c e n t years, a great deal of interest has been g e n e r a t e d in the field of artificially layered metallic films. The m u l t i l a y e r s o f s u p e r c o n d u c t i n g and normal metals, such as N b / C u or V/As provide a good o p p o r t u n i t y for the study of superconductivity induced by the proximity e f f e c t . F r o m NMR study, the e x c i t a t i o n s p e c t r u m a s s o c i a t e d with the superconducting e n e r g y gap is o b t a i n e d by the t e m p e r a t u r e , T, d e p e n d e n c e o f the nuclear spin lattice relaxation time, T,. NMR is e s p e c i a l l y s u i t a b l e for the study of S/N m u l t i l a y e r s , since it can selectively measure the excitation spectra in the respective layers. In this paper, we report the r e s u l t s of s 3Nb and ~ 3Cu NMR study in the superconducting N b / C u m u l t i l a y e r s . 2. E X P E R I M E N T A L Samples o f Nb/Cu multilayers were prepared with vacuum vapor deposition using a ultrahigh-vacuum evaporation system. The base pressure o f the s y s t e m is on the order o f I0 ,0 T o r r . Nb and Cu metals were e v a p o r a t e d on the Ag f o i l with two independent E-guns. The purities o f the Nb and Cu t a r g e t s w e r e 99.96% and 99.99%, respectively. The superconducting transition temperature, To, of the s a m p l e s w e r e d e t e r m i n e d by measuring the a c s u s c e p t i b i l i t y . The

obtained values w e r e 5K f o r Nb(200~)/ Cu(200/~) and 2.5K f o r Nb(200~)/Cu(400A). A t h i c k Cu m o n o l a y e r ( l / z m) was also prepared in the same c o n d i t i o n . F r o m the resistivity measurement, the ratio of the residual r e s i s t i v i t y ( R R R ) o f the Cu f i l m was obtained to be 18, i n d i c a t i n g that the mean f r e e path for Cu was e s t i m a t e d to be about _5700~ at 0.21<. A conventional pulsed s p e c t r o m e t e r and a superconducting pulse magnet w e r e used for the f i e l d c y c l i n g N M R measurements. The measurements b e t w e e n 0.2K and 1.3K were performed by a 4He c r y o s t a t and below 1.3K were p e r f o r m e d by a c o m m e r c i a l :'He -4 He d i l u t i o n r e f r i g e r a t o r . 3.RESULTS A N D DISCUSSION 9aNb and 63Cu N M R spectra o b t a i n e d a t 9.00MHz and /41.132MHz are shown in Fig. 1. Cu NMR has a n a r r o w l i n e whose w i d t h is w i t h i n 20 Oe. On the c o n t r a r y , 93Nb NMR s p e c t r u m has a broad l i n e width. The 9aNb s p e c t r u m o b t a i n e d a t high frequency consists of a sharp c e n t e r line and a broad line. F r o m the spectrum, we found the electric quadrupole coupling p a r a m e t e r , u 0, of 9SNb d i s t r i b u t e d around 1 ~ 3MHz. The large electric field gradient at 93 Nb was induced by the d i s t o r t i o n o f Nb bcc lattice. By the f i e l d c y c l i n g method, l / T , of ~aCu in the low f i e l d w e r e measured.

0921-4526/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved S S D I 0921-4526(93)1824-6

2384

63Cu

d

Z tU I--Z

"

C2~

.

f =41.13 2MHz

~I 3~

:E

.

38

Nb , ~ •

~,0

~:2

f=9.0OMHz

Z

e n(kOd 9 Fig. 1. N M R s p e c t r a o f C u and Nb obtained in Nb(200~ )/Cu(#O0~).

The results obtained s h o w n in F i g . 2 . I/T~ follows I/T~ T= 1.0 slightly larger than field, 1/TnT=l.3(secK)10

....

I

.

.

.

.

.

at H,,,=100 Oe are in t h e n o r m a l s t a t e ( s e c K ) - ~, which is the value at high ~. In t h e s u p e r "1

.

'

'

I

Nb(200/~) /Cu(200A)

~

I I I IL

.~

1121

o o*.," _ o ,*"

•. "'1Tc Nb(200~ . . ' o /Cu(4OOA .

1 T o

.

.o.~

",O O t(5" -If.._

0""

0

0.I

-

0.1

,

/"

TIoc T H=100

""

,,,I

Fig.2

1D

.

I

I

,

temperatures were about 1/ 5 ol our results in the samples h a v i n g the same N b - a n d C u - l a y e r thickness. T h e y r e p o r t e d that the Cu film prepared by the sputtering method had RRR of 10 indicating the mean free path to be 3000~ a t 4.2K, a n d t h a t t h e v a l u e o f 1/T ~ in the superconducting state decreased with decreasing the mean Dee p a t h in C u l a y e r s by a d d i n g s m a l l a m o u n t o f G e i m p u r i t i e s . As C u f i l m s p r e p a r e d in high v a c u u m had t h e m e a n f r e e p a t h o f 5700.~, the difference between present s t u d y a n d t h a t by Z h e n g e t a l . w o u l d b e explained qualitatively by the d i l f e r e n c e o I t h e m e a n f r e e p a t h in t h e respective Cu layers. With further decreasing temperatures, 1/T~ b e c o m e s T - i n d e p e n d e n t . The values a r e n e a r l y t h e s a m e f o r t w o s a m p l e s . In our experiment, the external field is s m a l l (100 Oe). H e n c e only the small amount of magnetic impurity would become the fast relaxation center, which would govern 1/T ~ of 6 aCu at low t e m p e r a t u r e s ( 0 . 1 ~ 0.5K). The systematic study including the sample dependence of 1/T I at low t e m p e r a t u r e s is n o w in progress.

I

i I I II

T( K )

Temperature

Oe I

I

[

1

dependence

I i i I

I0 of

l/Tn.

conducting state, 1/T~ increases just below T c and then decreases at low temperatures. In usual superconductors, 1/T ~ decreases exponentially at low temperatures. 1/TI ol ,~aCu in N b / C u multilayers decreases rapidly as decreasing temperature, however, the rapid decrease of 1/T i is gradually r e p l a c e d by a s l o w one o f I/T~ ¢× T at l o w t e m p e r a t u r e s in b o t h o f o u r samples. S i m i l a r T - d e p e n d e n c e was o b s e r v e d by G q. Zheng et al. in N b / C u multilayers prepared by the A r s p u t t e r i n g method~) , w h e r e the values o f I/T~ at l o w

L~.CONCLUSIONS N M R study in N b / C u m u l t i l a y e r s has shown that there is large e l e c t r i c field g r a d i e n t a t 9aNb, w h i c h is i n d u c e d by t h e i n h o m o g e n e o u s d i s t o r t i o n o f Nb b c c lattice. The distribution oI u ~ is around I~3MHz. On the c o n t r a r y , the electric tield gradient associated with the lattice distortion has not been observed m ~:~CuNMR. 1/T~ oI C u measured at 100 Oe has a c o h e r e n t peak just b e l o w Tc, and decreases with decreasing temperatures. The v a l u e ol 1/T~ w h e r e 1/T~ varies in p r o p o r t i o n to T increases w i t h increasing the mean | r e e p a t h ot the c o n d u c t i o n e l e c t r o n s in Cu layers. REFERRENCE 1.G-q. Zheng, Y. K i t a o k a , Y. Oda, Asayama, Y. Obi, H. F u j i m o r l a n d A o k i , 3. Phys. Soc. 3pn. 60 (1991) 599.

K. R.