MAGNETIZATION, ELECTRICAL RESISTIVITY AND THE NON-OCCURENCE OF SUPERCONDUCTIVITY IN NiH

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MAGNETIZATION, ELECTRICAL RESISTIVITY

AND THE NON-OCCURENCE OF

SUPERCONDUCTIVITY IN NiH

D. Mclachlan, I. Papadopoulos, T. Doyle

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C6, supplément au n" 8, Tome 39, août 1978, page C6-430

MAGNETIZATION/ ELECTRICAL RESISTIVITY AND THE NON-OCCURENCE OF SUPERCONDUCTIVITY IN NiH

D.S. McLachlan, I . Papadopoulos and T.B. Doyle.

University of the Wittiatersrand, Johannesburg, South Africa.

Résumé.- Les mesures d'aimantation montrent qu'à 4 K, NiH est un système continu à deux phases (a - B) pour tout x (x = H/Ni). Les résultats de la résistivité de phonon des échantillons NiH M 1

sont bien représentés par une fonction de GrUneisen avec 9p = 365 K. Aucune contribution à la résis-tivité de la part des phonons optiques de basse énergie n a été observée et l'on peut déduire que NiH n'est probablement pas un supraconducteur.

Abstract.- Magnetization measurements show that, at 4 K, NiH is a continuous two phase (a - 3) sys-tem for all x(x = H/Ni). The phonon resistivity results for NiH „j samples fit a GrUneisen function with 9JJ - 365 K. No contribution to the resistivity by low lying optical phonons is observed and it

is concluded that NiH is probably not a superconductor.

Previous magnetization measurements /l/ made at room temperature have shown that the ferromagne-tic saturation moment I„(x) for NiH (x = H/Ni) de-creases with x and vanishes for x in the range 0.6 < x < 0.85. The question then naturally arises whe-ther NiH , like PdH , may not also be a superconductor for sufficiently large x. In PdH it has been esta-blished that the superconducting exchange interac-tion primarily involves optical phonons /2,3/. It is therefore of some interest to investigate the possibility of a low lying optical phonon band in NiH . To this end and to investigate further the ma-gnetization of NiH resistivity measurements in the range 1.5 K < T < 200 K and magnetization measure-ments at 4 K were made on foil specimens of various x.

The specimens used in this work were produced by electrolytically charging 99.99+ pure Ni foils (= 10.0 Jim thick) with hydrogen at 0°C or room tem-perature. After preparation the samples were kept at 140 K or lower and stored, handled and mounted for experimental measurements under liquid nitrogen The hydrogen content x was determined from thermo-electric power measurements at approximately 140 K where it is known that the thermoelectric power va-ries linearly /4/ with x. These results were che-cked against chromotographic measurements in some of the specimens.

Magnetization measurements were made using a Foner (vibrating sample) magnetometer, which has the facility that suitably mounted specimens can be quenched to 4 K in the magnetometer within seconds. For each value of x, I(H,T = 4 K) measurements

(with H in the range H < ± 20 kOe) were made on the hydrogenated specimen and then again after the spe-cimen had been dehydrogenated. (Dehydrogenation was achieved by removing the specimen from the magneto-meter and annealing it in situ in its holder for

60 minutes at 110°C). Using the expansion 1(H) = Is[l " a/H - b/H^|, for H in the range 2.5 kOe < H

< 20 kOe to obtain I„, the reduced saturation mo-ment Ig(NiH )/I„(Ni) was calculated. These results

for I„(NiH )/I„(Ni) are shown as a function of x in figure 1 and it is apparent, within experimental error, that I<,(x) decreases with x as (1-x).

0.8- - \ N 5 06 - NO s \

02 '

v ^

0' I I 1 I ^^^N-0 ^^^N-0 2 ^^^N-0-« ^^^N-0-6 ^^^N-0-8 1-^^^N-0 X

Fig. 1 : The ratio Ig(NiH)/Ig(Ni) as a function of

hydrogen content (x).

The room temperature and 4 K Ig(x) results can be readily understood in terms of the nickel-hydri-de phase-diagram. It is known from X-ray /5/ and electrochemical /6/ measurements that at room tem-perature NiH can exist in three equilibrium phase states, viz ; with increasing x, an a phase, an

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a + ~ phase m i x t u r e and a p u r e

B

phase. The a + a+B

phase boundary o c c u r s a t x < 0 . 0 3 I 6 1 and t h e a+B +

%

B

boundary f o r 0.65 x

2

0.85 / 5 , 6 / . As t h e tempe- r a t u r e d e c r e a s e s t h e a+B +

B

boundary can be expec- t e d t o move towards I , ( x . 9 a t T = 240 K 141). A t T = 4 K t h e a + mixed phase c o u l d t h e n , a s o b s e r - ved, e x t e n d o v e r t h e e n t i r e r a n g e 0 <

_{- -}

x

<

1 . I n prac- t i c e because of t h e reduced m o b i l i t y of the hydrogen a t low t e m p e r a t u r e s e q u i l i b r i u m c o n d i t i o n s w i l l pro- b a b l y n o t be o b t a i n e d . I n t h e p r e s e n t m a g n e t i z a t i o n e x p e r i m e n t s a t 4 K no s i g n of i n c i p i e n t superconduc- t i v i t y was d e t e c t e d even f o r x

=

1 where t h e

6

phase p r e d o m i n a t e s .

The e l e c t r i c a l r e s i s t i v i t y measurements P(T) were made, u s i n g a f o u r probe t e c h n i q u e and D.C. cur-

r e n t of 100 mA, i n a t e m p e r a t u r e r a n g e 1 . 5 K

$

T

<

200 K, a f t e r i n i t i a l l y quenching t h e specimens f o r 77 K t o 4 K. Temperature measurements were made u s i n g a g o l d (0.06 % I r o n ) - c o n s t a n t a n thermocouple (which had been c a l i b r a t e d a g a i n s t a p l a t i n u m r e s i s t a n c e

thermometer) i n c l o s e thermal c o n t a c t w i t h t h e specimen. The p(T) r e s u l t s shown i n f i g u r e 2 f o r a specimen w i t h x = 0.985 a r e s i m i l a r t o t h o s e f o r a s p e cimen w i t h x = 0.933. The upper c u r v e shows t h e p(T) b e h a v i o u r a f t e r t h e i n i t i a l quench. At a p p r o x i m a t e l y 110 K, where t h e hydrogen becomes s i g n i f i c a n t l y mo- b i l e , t h e r e i s a p p a r e n t l y some c o n f i g u r a t i o n a l r e d i s - t r i b u t i o n of t h e hydrogen

-

p r o b a b l y towards e q u i l i - brium. A f t e r c o n t i n u i n g t o h e a t t h e specimen t o 180 K and then requenchcng t o 4 K t h e lower c u r v e i s ob- t a i n e d f o r i n c r e a s i n g t e m p e r a t u r e s . The b e s t f i t s t o t h e e x p e r i m e n t a l r e s u l t s u s i n g t h e GrUneisen func- t i o n ( s o l i d l i n e ) were o b t a i n e d w i t h s-d and o p t i c a l ban phonon s c a t t e r i n g terms i n s i g n i f i c a n t l y s m a l l o r n e g l e c t e d . The Debye t e m p e r a t u r e s

BD

f o r t h e two so-

l i d c u r v e s a r e a p p a r e n t l y d i f f e r e n t , w i t h t h a t o f t h e lower c u r v e (OD

-

366 K) lower t h a n OD f o r p u r e N i (OD

=

390 K), t e n d i n g towards OD f o r Cu (310 K ) . The u p p e r , p r o b a b l y n o n - e q u i l i b r i u m c u r v e , h a s a OD of 345 K. The most s i g n i f i c a n t a s p e c t of t h e p r e s e n t r e s u l t s i s however t h e a p p a r e n t absence, f o r T

$

180 K, of a marked c o n t r i b u t i o n t o p(T) i n N i H due t o o p t i c a l phonon s c a t t e r i n g . I n PdH t h e c o n t r i b u - - 9 9 5 t i o n t o t h e r e s i s t i v i t y by t h e o p t i c a l phonons becomes n o t i c e a b l e between 80 K and 100 K 2 . I n N i H t h e p o i n t s a t a b o u t 200 K, may i n d i c a t e t h e s t a r t of a c o n t r i b u t i o n by t h e o p t i c a l phonons, t h i s a l l o w s us

t o s a y BE(NiH)

>

1.8 BE(PdH)2

>

990 K. Thus i f o n l y

t h e dominant o p t i c a l phonon-electron exchange i n t e r - a c t i o n / 2 , 3 / i s c o n s i d e r e d we c a n t a k e Tc = eEexp

( - l / A e f f ) where

Xeff

= h-u*

=

N ( O ) I ~ / ( M B ; ) .

F i g . 2 : shows t h e b e s t f i t of t h e Gruneisen func- t i o n t o t h e e x p e r i m e n t a l r e s i s t i v i t y p o i n t s f o r a sample w i t h x = .985. The dashed l i n e s show t h e t e m p e r a t u r e dependent r e s i s t i v i t y of p u r e n i c k e l .

Assuming N ( O ) I ~ / M i s t h e same i n N i H and PdH t h e n Tc(NiH) = Tc(PdH) (BE(NiH) /BE(PdH)) exp

-

( B ~ ( N ~ H ) 18;

(PdH)), which t a k i n g T = 10 K f o r PdH g i v e s Tc(NiH)

< 0.01 K. A more a c c u r a t e c a l c u l a t i o n b a s e d on t h e e q u a t i o n s of r e f e r e n c e 161, which i n c l u d e t h e acou- s t i c phonon c o n t r i b u t i o n , g i v e s Tc(NiH) < 0.003 K.

R e f e r e n c e s

/ I / See f o r i n s t a n c e : Kozlowski, L., and Kubiak, S.T., Phys. S t a t . S o l .

2

(1963) K177.

121 McLachlan, D.S., M a i l f e r t , R., Burger, J.P. and S o u f f a c h e , B., S o l i d S t a t e Comun

17

(1975) 281 131 Burger, J.P. and McLachlan, D.S., J . P h y s i q u e

( P a r i s )

37

(1976) 1227.

141 Skoskiewicz, T., Phys. S t a t . S o l .

5

(1971) 29 151 See f o r i n s t a n c e : Boniszewski, T. and Smith,

G . C . , J. Phys. Chem. S o l i d s

2

(1961) 115. I 6 1 Baranowski;B., and Szlarska-Smialowska, Z . ,