EFFECT OF A THERMAL ANNEALING ON THE LATTICE SPECIFIC HEAT AND THERMAL CONDUCTIVITY OF SUPERCONDUCTING AMORPHOUS Zr.76Ni.24ALLOYS

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EFFECT OF A THERMAL ANNEALING ON THE

LATTICE SPECIFIC HEAT AND THERMAL

CONDUCTIVITY OF SUPERCONDUCTING

AMORPHOUS Zr.76Ni.24ALLOYS

O. Béthoux, J. Lasjaunias, A. Ravex

To cite this version:

JOURNAL DE PHYSIQUE

CoZZoque C6, supple'ment

au

nO1

2,

Tome

4 2 ,

de'cembre

1982

page

C6-60

EFFECT OF

A

THERMAL ANNEALING ON THE L A T T I C E S P E C I F I C HEAT AND THERMAL C O N D U C T I V I T Y OF SUPERCONDUCTING AMORPHOUS

Z r . ~ ~ N i . 2 4

ALLOYS

0 . BGthoux, J . C . L a s j a u n i a s and A. Ravex

Centre

&

Recherches sur Zes Tr2s Basses Te'empe'ratures,

C.

N.R.S.,

B.P.

166

X , 38042

GrenobZe Cedex, France

A b s t r a c t

-

1Je have measured t h e low t e m p e r a t u r e (from 0.1 R up t o 5 R) s p e c i f i c h e a t and thermal c o n d u c t i v i t y of an amorphous Z r X i s u p e r c o n d u c t i n g a l l o y "as s p u t t e r e d " and a f t e r a n n e a l i n g . We observe a v a r i a t i o n of t h e Debye t e m p e r a t u r e and of t h e thermal c o n d u c t i v i t y r e l a t e d t o a s t r u c t u r a l r e l a x a t i o n caused by t h e a n n e a l i n g .

Two auorphous Zr. 7 6 N i . 24 a l l o y s have b e e n p r e p a r e d by s p u t t e r i n g r e s p e c t i v e l y a t n i t r o g e n and room temperature. They a r e o b t a i n e d i n t h e form of t h i c k f i l m s (100 L~m). The n i t r o g e n t e m p e r a t u r e d e p o s i t e d sample h a s then been a n n e a l e d under vacuum f o r

24 h r s a t 250°C ( t h e c r y s t a l l i s a t i o n t e m p e r a t u r e determined by D.T.A. was 3 5 0 ' ~ ) . The d e n s i t y measurement shows a s l i g h t i n c r e a s e £on t h e a n n e a l e d sample ( s e e t a b l e ) i n d i - c a t i n g a cendency t o a more packed s t r u c t u r e .

f

S p e c i f i c Heat measurements a r e r e p o r t e d i n f i g . ] inaC/T v e r s u s T p l o t . From t h e s u p e r c o n d u c t i n g t r a n s i t i o n t e m p e r a t u r e (T = 3.2 K f o r b o t h "as s p u t t e r e d " samples and T = 3 K f o r t h e a n n e a l e d sample) t o t h e upper l i m i t of o u r measurements (about 6 K)

3

t h e d a t a a r e w e l l f i t t e d by a

yT

+

BT law. The v a l u e s o b t a i n e d f o r y and @ ( s e e t a b l e ) by t h i s a n a l y s i s a r e checked by comparison of t h e e n t r o p y i n b o t h superconduc- t i n g and normal s t a t e below Tc. The p r e s e n c e of t h e B T ~ term a l l % t h e d e t e r m i n a t i o n of a Debye t e m p e r a t u r e

OD

( s e e t a b l e ) which i s n o t p o s s i b l e i n t h e c a s e o f amorphous i n s u l a t o r s . For b o t h "as s p u t t e r e d " samples

B

v a l u e s a r e v e r y c l o s e . For t h e annealed sample t h e l a t t i c e c o n t r i b u t i o n d e c r e a s e s c o r r e s p o n d i n g t o a v a r i a t i o n of about 15 %

of t h e Debye t e m p e r a t u r e . This i n c r e a s e s of 0 c o r r e s p o n d s t o a h a r d e n i n g of t h e

D

s t r u c t u r e . U n f o r t u n a t e l y no sound v e l o c i t y measurements a r e a v a i l a b l e on such amor- phous a l l o y s i n o r d e r t o g e t a n o t h e r d e t e r m i n a t i o n of OD. But s i m i l a r c o n c l u s i o n s have been drawn on PdSi g l a s s y a l l o y ( 1 ) .

R e s u l t s of Thermal C o n d u c t i v i t y measurements f o r b o t h " s p u t t e r e d " and a n n e a l e d n i t r o g e n d e p o s i t e d samples a r e shown i n f i g . 2. The s u p e r c o n d u c t i n g t r a n s i t i o n tempe- r a t u r e s T determined i n t h e s p e c i f i c h e a t measurements a r e r e p o r t e d by arrows : they correspond v e r y w e l l t o t h e change i n t h e regime i n t h e phonon-electron s c a t t e r i n g due t o t h e r a p i d d e c r e a s e of t h e number of normal e l e c t r o n s below Tc.

Below Tc, b o t h e l e c t r o n h e a t t r a n s p o r t and phonon-electron s c a t t e r i n g become r a p i d l y n e g l i g i b l e . T h e n t h e phonon c o n d u c t i v i t y K i s n a i n l y l i m i t e d by t h e i n t e r a c -

ph

t i o n w i t h t h e low e n e r g y e x c i t a t i o n s and t h e sample boundary s c a t t e r i n g . I n f a c t ,

J 4

F i g . 2 : Thermal Conductivity b e f o r e

W

and a f t e r annealing. c II

C6-62 JOURNAL DE PHYSIQUE

above 0.3 K we observe a T I " l i k e v a r i a t i o n , c h a r a c t e r i s t i c i n t h e amorphous mate- r i a l s of t h e r e s o n a n t s c a t t e r i n g of phonons by t h e two l e v e l systems (T.L.S.) : t h e Casimir l i m i t a t i o n j u s t o c c u r s below 0.3 K. The s c a t t e r i n g of phonons by T.L.S. i s d e s c r i b e d a s :

2 V~

, * k p T

-

K p h - ~ ~ ~ 1;1~i2 $2

where n i s the d e n s i t y of s t a t e s of TLS s t r o n g l y coupled t o t h e phonons and

??

i s a n average coupling c o n s t a n t of TLS w i t h phonons. Using an average Debye sound v e l o c i t y vD o b t a i n e d from t h e 0 v a l u e of s p e c i f i c h e a t measurements we determine t h e unique

D

-2

a d j u s t a b l e parameter nM ( s e e t a b l e ) . To t h e observed i n c r e a s e of 50 % of thermal -2

c o n d u c t i v i t y by a n n e a l i n g corresponds a s i g n i f i c a n t d e c r e a s e of nM by a f a c t o r 1.35. This v a r i a t i o n i s w e l l c o r r e l a t e d t o a diminution of t h e TLS s p e c i f i c h e a t observed i n o t h e r experiments ( 2 ) . Q u a l i t a t i v e l y t h e s e r e s u l t s a r e coherent and a g r e e w i t h a t h e o r e t i c a l model of s t r u c t u r a l r e l a x a t i o n developed by B a n v i l l e and H a r r i s (3) :

they assume t h a t a TLS i s a s i n g l e atommoving between two o r more a l t e r n a t e e q u i l i - brium p o s i t i o n s w i t h i n a void of the s t r u c t u r e . This model p r e d i c t s a decrease of TLS d e n s i t y w i t h t h e disappearance of t h e voids during annealing.

References

( 1 ) Uichiro-Mizutani and T.B. Massalski, J. Phys. F : Metal Phys. 10 (1980), 1093- 1100.

(2) To be p u b l i s h e d i n LT 16 Proceedings by t h e a u t h o r s .

(3) M. B a n v i l l e and R. H a r r i s , Phys. Rev. L e t t . (1930), 1135.

Table : Experimental parameters.