RESISTIVITY AND MAGNETORESISTANCE OF SILVER-RARE-EARTH AMORPHOUS ALLOYS

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RESISTIVITY AND MAGNETORESISTANCE OF SILVER-RARE-EARTH AMORPHOUS ALLOYS

J. Ousset, J. Ulmet, R. Asomoza, J. Bieri, S. Askenazy, A. Fert

To cite this version:

J. Ousset, J. Ulmet, R. Asomoza, J. Bieri, S. Askenazy, et al.. RESISTIVITY AND MAGNETORE- SISTANCE OF SILVER-RARE-EARTH AMORPHOUS ALLOYS. Journal de Physique Colloques, 1980, 41 (C8), pp.C8-470-C8-472. �10.1051/jphyscol:19808117�. �jpa-00220213�

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JOURNAL DE PHYSIQUE CoZZoque C8, suppZdment au n08, Tome 41, aoct 1980, pagec8-470

RESISTIVITY AND MAGNETORESISTANCE OF SILVER-RARE-EARTH AMORPHOUS ALLOYS

J.C. Ousset, J.P. Ulmet, R. ~somoza', J.B. ~ieri', S. Askenazy and A. ~ert'

Laboratoire de Physique des SoZides, Service des C h q s Intenses, I.N.S.A., 31077 TouZouse-Cddex France.

+Laboratoire de Physique des SoZides, 91405 Orsay, France

Abstract.- We have studied the resistivity and the magnetoresistance of Ag,-, and Ago L U ~ - ~ - ~ R ,

amorphous alloys prepared by sputtering (R = rare-earth). Measurements have been perform&a in pulsed field up to 300 kG. We observe that magnetic ordering can yield positive or negative resistivity changes in these systems. We interpret the experimental results in terms of coherent exchange scattering and two-current conduction.

INTRODUCTION

An upturn of the resistivity at low cemperatu- res has been found in many amorphous alloys and has been ascribed either to magnetic scatterings or to structural mechanisms 11, 21. We present in this communication some examples of this behaviour of the resistivity in alloys of silver, with rare- earths and report on magnetoresistance measurements (in pulsed fields) which have been done to decide between a magnetic and a structural origin.

1

.

THEORETICAL MODELS

The starting point is a standard exchange in- teraction between the conduction electrons and the magnetic ions :

by taking into account the interferences between scatterings on neighbour magnetic ions, Fert and Asomoza [I] find the following expression of the resistivity :

where m(q) is the spin correlation function expres- sed as :

On account of the amorphous structure, this spin correlation function depends only on the local magnetic order.

This model has been mainly applied to account for the experimental results in nickel-rare-earth amorphous alloys. In such alloys with predominant ferromagnetic interactions, the range of the spin correlations can be supposed to be much longer than the range of the atomic order and a local parameter r,of local magnetic order can be defined.

The resistivity is then written as :

J 2

fm = PM (1' ⁺1' ( 2 k ~ ) - ^1]h2)

( 4 ) where aI1(q) is the ~artial structure factor of

- -

the magnetic atoms. The upturn of the resitivity below the temperature of magnetic ordering and the positive magnetoresitance of nickel-rare-earth alloys [l] have been accounted for by a positive value of the factor all ( 2 kF) - 1 (constructive interferences).

In the general case, the expression of the resistivity, Eq (1) - (2), cannot be simplified in

the same way but, the behaviour of the resistivity can be qualitatively predicted. Pairs with constructive interferences will give rise to a positive contribution to the magnetoresistance and to an up-

turn or a downturn of the resistivity at low tempe- ratures according to whether their spontaneous correlation is ferromagnetic or antiferromagnetic. In

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19808117

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c o n t r a s t , p a i r s w i t h d e s t r u c t i v e i n t e r f e r e n c e s , w i l l g i v e r i s e t o a n e g a t i v e c o n t r i b u t i o n t o t h e m a g n e t o r e s i s t a n c e and t o a downturn or an u p t u r n according t o whether t h e i r spontaneous c o r r e l a t i o n i s f e r r o m a g n e t i c o r a n t i f e r r o m a g n e t i c .

The amorphous m a t e r i a l s show s e v e r a l charac- t e r i s t i c anomalies i n t h e i r thermal p r o p e r t i e s a t low t e m p e r a t u r e s . The anomalies have b e e n a s c r i b e d t o s t r u c t u r a l i n s t a b i l i t i e s and e x p l a i n e d i n a mo- d e l c o n s i d e r i n g " t w o - l e v e l systems" [ 3 ] . By c a l c u - l a t i n g t h e r e s i s t i v i t y due t o t h e s c a t t e r i n g b y

" t w o - l e v e l s y s t e m s " , Cochrane e t a l . [2J ~ r e d i c t e d a v a r i a t i o n o f t h e form :

p

^-

en

^{( k ~ 2}⁺

a2,

They proposed t o a s c r i b e t o t h i s mechanism t h e r e s i s t i v i t y u p t u r n s observed i n many amorphous a l l o y s . However o t h e r a u t h o r s [ 4 , 51 have t r e a t e d t h e same problem and predicted a d i f f e r e n t v a r i a - t i o n i n - ⁽ en^)^. Some o f them a l s o claimed t h a t t h e c o n t r i b u t i o n s t o t h e r e s i s t i v i t y from t h e s c a t - t e r i n g b y " t w o - l e v e l systems" are much t o o small t o e x p l a i n t h e e x p e r i m e n t a l r e s u l t s [ 5 ] .

2. RESISTIVITY A N D MAGNETORESISTANCE O F SILVER-RARE-EARTH AMORPHOUS ALLOYS.

EXPERIMENTAL RESULTS A N D DISCUSSION

We have measured t h e r e s i s t i v i t y and t h e ma- g n e t o r e s i s t a n c e ( u p t o 250 kG i n pulsed f i e l d s ) o f AgS0 Gd50, AgS0 DyS0, Ag50 Lu15 Dy35 and AgS0 ErS0 amorphous a l l o y s prepared by s p u t t e r i n g b y B . Boucher (CEA-Saclay, F r a n c e ) . Using a crowbar method we o b t a i n a s l o w l y v a r y i n g magnetic f i e l d which a l l o w s u s t o measure t h e m a g n e t o r e s i s t a n c e a t h i g h f i e l d w i t h a good p r e c i s i o n (about 5% o f t h e mea- sured v a r i a t i o n ) . We make sure o f t h a t b y r e g i s t e r - i n g t h e r e s i s t i v i t y s i g n a l w i t h b o t h i n c r e a s i n g ( t 50 ms) and decreasing ( t ^%400 ms) t h e f i e l d . Examples o f experimental r e s u l t s are g i v e n i n f i g u r e s ( 1 ) and ( 2 ) . I n a l l o f t h e a l l o y s we f i n d a r e s i s t i v i t y u p t u r n i n a temperature range which i s approximately t h e range o f magnetic ordering and a n e g a t i v e m a g n e t o r e s i s t a n c e . Both r e s u l t s can be accounted f o r b y coherent exchange s c a t t e r i n g w i t h a predominant c o n t r i b u t i o n o f a n t i f e r r o m a g n e t i c p a i r s having a n e g a t i v e i n t e r f e r e n c e f a c t o r ( s e e t h e d i s c u s s i o n o f t h e s e v e r a l t y p e s o f e x p e c t e d beha- v i o u r s i n paragraphe 1 a ) . A s t r u c t u r a l mechanism o f t h e r e s i s t i v i t y u p t u r n can be p r a c t i c a l l y r u l e d

Figure 1 : Normalized r e s i s t i v i t y v e r s u s - temperature i n s e v e r a l magnetic f i e l d s f o r

t h e AgS0 G d s 0 amorphous a l l o y . The tempera- t u r e f magnetic o r d e r i n g e s t i m a t e d b y Boucher [6 ] i s i n d i c a t e d on t h e f i g u r e . out as such a mechanism would be f i e l d i n d e p e n d e n t . I n c o n t r a s t an a p p l i e d f i e l d o f 250 kG c o m p l e t e l y suppress t h e r e s i s t i v i t y u p t u r n i n Ag

50 Gd50 and lowers i t c o n s i d e r a b l y i n t h e o t h e r a l l o y s (more d i f f i c u l t t o magnetize a t s a t u r a t i o n (61). We i n f e r t h a t c o n t r i b u t i o n s from s t r u c t u r a l e f f e c t s t o t h e r e s i s t i v i t y u p t u r n , i f t h e y e x i s t , are much weaker t h a n t h e magnetic c o n t r i b u t i o n s i n t h e a l l o y s we i n v e s t i g a t e d .

Figure 2 : Normalized r e s i s t i v i t y v e r s u s tem- p e r a t u r e i n s e v e r a l magnetic f i e l d s f o r t h e AgS0 L u ~ ~ . D ~ ~ ~ a l l o y . To remove t h e ( s m a l l ) anisotropic c o n t r i b u t i o n t o t h e m a g n e t o r e s i s - tance form quadrupole s c a t t e r i n g (91 we have p l o t t e d T = 113 ft + 213

e,

^where ^pfl^and^fL

are t h e r e s i s t i v i t i e s merCsured i n l o n g i t u d i - n a l and t r a n s v e r s e f i e l d s r e s p e c t i v e l y . The temperature o f magnetic o r d e r i n g i s e s t i m a - t e d t o l i e i n t h e range i n d i c a t e d on t h e f i g u r e .

I n order t o t e s t t h e p r e d i c t i o n s o f t h e cohe- r e n t exchange s c a t t e r i n g model we have s t u d i e d t h e dependence on t h e c o n c e n t r a t i o n o f D i n t h e

L u ~ ~ - ~ Dyx a l l o y s ( x = 50 and x = 35) :

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JOURNAL DE PHYSIQUE

- The i n c r e a s e o f r e s i s t i v i t y a t low tempera- t u r e (down t o 4 . 2 K) i s s t r o n g e r by a r a t i o o f 1 . 8 3 i n t h e more c o n c e n t r a t e d a l l o y ( x = 5 0 ) . T h i s v a l u e i s n o t v e r y d i f f e r e n t from t h e r a t i o o f t h e s q u a r e o f t h e c o n c e n t r a t i o n s ( 2 . 0 4 ) , i n a g r e e m e n t w i t h t h e v a r i a t i o n i n C1 o f t h e two-ion term i n 2 En. ( 2 ) .

- The m a g n e t o r e s i s t a n c e a t 200 kG and 4.2 K i s s t r o n g e r by a r a t i o o f 1.48 i n t h e more concen- t r a t e d a l l o y . T h i s l i k e l y means t h a t n o t o n l y two- i o n t e r m s ( r v C ) 2 b u t a l s o one-ion term c o n t r i b u t e

1

t o t h e magne t o r e s i s t a n c e . The main one-ion c o n t r i - b u t i o n t o t h e m a g n e t o r e s i s t a n c e i n c r y s t a l l i n e ma- g n e t i c a l l o y s a r i s e s from " t w o - c u r r e n t e f f e c t s " [7]

which have n o t been t a k e n i n t o a c c o u n t i n E q . ( 2 ) . F i n a l l y we have compared o u r e x p e r i m e n t a l r e s u l t s w i t h c a l c u l a t i o n s b a s e d on t h e mechanism o f c o h e r e n t exchange s c a t t e r i n g and a n a p p r o x i m a t i o n o f two-spin c l u s t e r s ' w i t h a n t i f e r r o m a g n e t i c i n t e r - a c t i o n s [8]. We f i n d a r e a s o n a b l e q u a l i t a t i v e a g r e e m e n t .

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1886 (1979)

2 R.W. COCHRANE, R. HARRIS, J.O. STROM-OLSEN, M . J . ZUCKERMANN, Phys. Rev. L e t t . , 3, 676 (1975)

3 P.W. ANDERSON, B . HALPERIN, C. VARMA, P h i l . Mag., 5, 1 (1972)

4 J . KONDO, P h y s i c a , 848, 207 (1976)

5 J.L. BLACK and B.L. GYORFFY, Phys. Rev. L e t t . , 41, 1487 (1978)

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6 B. BOUCHER, IEEE, T r a n s . o n Mag., MAG 1 3 , 1601 (1977)

7 T. VAN PESKI-TINBERGEN and A.J. DEKKER, P h y s i c a , 29, 917 (1969)

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8 A.K. BHATTACHARJEE and B . COQBLIN, J . de P h y s . , C5, 225 (1979)

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9 R. ASOMOZA, J . B . BIERI, A. FERT and J.C. OUSSET, t h i s c o n f e r e n c e .