MAGNETORESISTANCE OF Fe-Ni AMORPHOUS ALLOYS

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MAGNETORESISTANCE OF Fe-Ni AMORPHOUS ALLOYS

A. Hamzić, E. Babić, Z. Marohnić

To cite this version:

A. Hamzić, E. Babić, Z. Marohnić. MAGNETORESISTANCE OF Fe-Ni AMORPHOUS ALLOYS.

Journal de Physique Colloques, 1980, 41 (C8), pp.C8-694-C8-697. �10.1051/jphyscol:19808174�. �jpa-

00220276�

JOURNAL DE PHYSIQUE CoZZoque C8, s u p p l h e n t au

Tome 41, aoCt 1980, page C8-694

MAGNETORESISTANCE OF Fe-Ni AMORPHOUS ALLOYS

A . Hamzic', E . ~abic! and Z . Marohnif

I n s t i t u t e o f Physics o f t h e Uniuersity, Zagreb, YugosZavia.

Various p h y s i c a l p r o p e r t i e s o f amorphous d l o y s have been r a t h e r e x t e n s i v e l y i n v e s t i g a t e d

i n t h e l a s t few years. Such i n t e r e s t has been en- couraged by the development of new techniques( ' ⁾

capable o f producing samples o f d e s i r a b l e shape and p r e d i c t a b l e qua1 i t y , t h e p o s s i b i l i t y o f t h e i r i ndu- s t r i a1 a p p l i c a t i o n s and t h e i r unusual p r o p e r t i e s .

There e x i s t a g r e a t v a r i e t y o f d i f f e r e n t amor- phous systems; one o f t h e most s t u d i e d are metal- metal l o i d a l l o y s . Among these, (Fe-Ni )80-(P-B)20 a1 l o y s have some i n t e r e s t i n g c h a r a c t e r i s t i c s , t y p i - c a l o f s o f t ferromagnets.

Generally, amorphous a1 loys have r a t h e r high, b u t very weakly temperature dependent r e s i s t i v i t i e s . For t h e m e t a l - m e t a l l o i d ones, t h e e l e c t r i c a l r e s i - s t i v i t y e x i b i t s some common features: a s a t u r a t i o n a t v e r y 1 ow temperatures, 1 o g a r i thmi c dependence a t h i g h e r temperatures followed by a r e s i s t i v i t y m i n i - mum. The o r i g i n s o f such behaviour a r e s t i l l n o t f u l l y explained; b u t a t present an e x p l a n a t i o n i n terms o f Kondo t y p e s c a t t e r i n g seems more f a v o r a b l e then a non-magneti c s t r u c t u r a l model (').

Extensive s t u d i e s of Fe,Ni80-xP14B6 system have been made and have somewhat c l a r i f i e d t h i s con- t r o v e r s y . The low temperature r e s i ~ t i v i t i e s ( ~ ) and room temperature c o e f f i c i e n t s o f t h e r e s i s t i v i t y ( 3 )

have shown a r a t h e r s t r o n g dependence on t h e Fe/Ni r a t i o i n t h e a l l o y , and such e f f e c t s cannot be..

accounted f o r i n t h e TLS approach('). Likewise, t h e existence o f a magnetic c o n t r i b u t i o n has been obser- ved i n H a l l e f f e c t ( 4 ) and t h e r m o e l e c t r i c power(5)

of t h e same system. The magnetoresistance r e s u l t s presented here support f u r t h e r t h i s conclusion.

The t r a n s v e r s e and l o n g i t u d i n a l magnetoresi- stance were measured w i t h a p o t e n t i o m e t r i c set-up ( w i t h r e l a t i v e accuracy o f few p a r t s i n 10 6 ) on samples i n a form o f ribbon. T h e i r dimensions and r e l e v a n t data have been g i v e n elsewhere(3). The experiments were done m o s t l y up t o 35K, and i n magnetic f i e l d s up t o 8T (superconducting solenoid).

Fig. 1. The r e l a t i v e change o f magnetoresistance f o r t h r e e FexNi80,xP14B6 a l l o y s (ne/rm =(R(H)-Rmin(0)/

/Rmin(0) ). F o r x=O and 10, H=7.5T, f o r x=20, H=3T.

I n t h e i n s e t , t h e f i e l d v a r i a t i o n o f t h e l o g a r i t h - mic slope o f h g / y m (0) and r e s i s t i v i t y minimum temperature (+) i s shown f o r t h e a l l o y w i t h x=10.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19808174

The temperature v a r i a t i o n o f the transverse magnetoresi stance f o r t h r e e Fe,Nigo-,PI $B6 a1 1 oys i s shown i n f i g u r e 1. These data (dashed l i n e s ) were obtained i n f i x e d magnetic f i e l d and are c o w rayed w i t h zero f i e l d r e s i s t i v i ti ~s ( d o t t e d l i n e s )

" i t h r e s p e c t t o the i r o n c o n c e n t r a t i o n ( x ) , t h e da- t a can be summarised as f o l l o w s :

1) x=O. There i s a small p o s i t i v e magnetoresistan- ce which decreases a l i t t l e as t h e temperature i s increased ( f o r T up t o n e a r l y 2Tmin). Both R(HlI) (XI

and R(H1) are p r o p o r t i o n a l t o Hn, w i t h l<n<2, and w i t h i n t h e experimental e r r o r a r e t h e same ( i e .

t h e r e i s no a n i s o t r o p y - F i g 3a). The measured posi- t i v e magnetoresistance seems t o i n d i c a t e t h a t f o r t h i s p a r t i c u l a r a1 l o y the anomalous r e s i s t i v i t y term does n o t come f r o m t h e Kondo effect.

2) x=10. This a l l o y represents a b o r d e l i n e between ferromagneti c and paramagnetic r e g i o n , and i t s magnetoresistance depends very s t r o n g l y on tempera- t u r e and magnetic f i e l d . I n f i g u r e 1 we have given o n l y t h e R(H1) curve f o r t h e h i g h e s t f i e l d , and i t shows t h a t t h e l o g T term o f t h e r e s i s t i v i t y i s su- pressed t o n e a r l y h a l f o f i t s value f o r H=O. T h i s i s a l s o e v i d e n t from t h e i n s e t o f Fig. I , where t h e c o e f f i c i e n t A ( + ) of t h e l o g T t e r n i s p l o t t e d as a function o f t h e a p p l i e d f i e l d : we be'lieve t h a t f o r magnetic f i e l d s >10T, c o e f f i c i e n t A would be comple- t e l y saturated. The e f f e c t o f t h e f i e l d i s a l s o t o s h i f t t h e temperature o f t h e minimum t o lower values and t h i s i s n e a r l y p r o p o r t i o n a l t o H ( i n s e t of Fig.!>

S i m i l a r A(H) and Tmin v a r i a t i o n s have been r e p o r t e d f o r o t h e r amorphous system(6), and c l e a r l y are the r e s u l t o f t h e presence o f a magnetic e f f e c t i n t h i s a l l o y . The shape o f t h e R(H) curves ( F i g . 3 ~ ) does

small p o s i t i v e anisotropy which decreases a l i t t l e as T i s increased. The f i e l d dependence o f R(H,T=

c o n s t ) - which i s i n F i g . 3 ~ shown o n l y f o r low f i e l d s - i n d i c a t e s t h a t t h i s system does n o t behave as a r e a l ferromagnet ( i n which s u f f i c i e n t l y s t r o n g f i e 1 d would reduce t h e magnetic s c a t t e r i n g t o zero and therefore l e a d t o s a t u r a t i o n o f t h e magnetore- s i s t a n c e ) . Here t h e r e s u l t s resemble more t o the

behaviour o f a quasi ferromagnetic E M n a l l o y ( 7 ) f o r which t r u e s a t u r a t i o n i s reached o n l y f o r ex- t r e m l y h i g h a p p l i e d f i e l d s .

Fig.2. I s o t r o p i c magnetoresi stance, (R(H 11) +

+ 2R(HI) )/3), vs. temperature f o r x=10 a l l o y . Dashed l i n e represents t h e e f f e c t f o r x=20 a l l o y i n 3T.

n o t change s i g n i f i c a n t l y up t o 65 K; R(H1) i s always

s l i g h t l y more n e g a t i v e then R(HII), g i v i n g a v e r y

C8-696

However, we t h i n k t h a t i n FeloNi70P14B6al~oy t h e p h y s i c a l s i t u a t i o n i s even more complex. If t h e i s o t r o p i c magnetoresistance i s p l o t t e d versus temperature ( f i g u r e 2), i t s s t r o n g f i e l d and tempe- r a t u r e dependence i s again e v i d e n t . There i s a change o f slopes a t 55K which corresponds r a t h e r w e l l t o t h e C u r i e temperature(8)

t h i s a l l o y - Also the t r a n s i t i o n through Tc i s r a t h e r wide i n temperature which one would n o b e x p e c t f o r a proper ferromagnet. (We n o t e t h a t , s i m i l a r l y , t h e c o e f f i - c i e n t o f anomalous Hal 1 e f f e c t ( 4 ) does n o t change i n a t y p i c a l ferromagnetic f a s h i o n around Tc). B u t even f o r quasi ferromagnetic a l l o y t h e i s o t r o p i c ma- g n e t o r e s i s t a t ~ c e should show a c l e a r peak a t Tc as a f u n c t i o n o f temperature f o r f i x e d f i e l d , and t h i s we have n o t observed: f o r T< Tc t h e magnetoresistart ce i s even more negative. We b e l i e v e t h a t t h i s can be understood, a t l e a s t q u a l i t a t i v e l y , i f t h e Kondo type magnetic s c a t t e r r n g i s a l s o taken i n t o account, where t h e dominant magnetoresistance dependence(')

i s through ' M (M i s t h e sample magnetization). A t low temperatures t h i s would ( f o r a f i x e d f i e l d and supposed small c h a r a c t e r i s t i c temperatu- r e ) y i e l d R(H)- T - ~ . Consequently, t h e observed v a r i a t i o n o f t h e magnetoresistance could be regar- ded as a r e s u l t o f t h e presence o f t h e two effects;

one o f which i s weakly temperature dependent f o r T < T c , has a peak a t T, and decreases again f o r T>T,, and a second one (more dominant a t T< Tc) p r o p o r t i o n a l t o T-'. Even i f t h i s q u a l i t a t i v e p i c - t u r e i s n o t f u l l y c o r r e c t , i t i s c l e a r t h a t i n Fe10~i70P14B6 t h e anomalous low temperature beha- v i o u r i s n o t compatible w i t h t h e s t r u c t u r a l modef 2 ,

b u t t h a t i t has a magnetic o r i g i n .

F i g . 3. The l o n g i t u d i n a l and transverse magneto- resistances (normal i s e d t o R(H=O) values) of o u r a l l o y s vs. magnetic f i e l d . T=4.2K.

3) x=20. This a l l o y has a l s o a negative magnetore- s i s t a n c e (Fig.1) b u t t h e e f f e c t o f t h e f i e l d i s n o t so pronounced, n o r we were a b l e t o d e t e c t any s i g n i f i c a n t change o f T , m i w i t h f i e l d . F o r tempe- r a t u r e s up t o 30K (which i s much less then ~ ~ ( 2 3 6 K)) t h e i s o t r o p i c magnetoresistance ( i n a f i x e d f i e l d ) does n o t vary very much w i t h temperature, as

pared t o t h e effect f o r x=10 a l l o y . Data a t 3T a r e shown i n Fig.2 as a dashed l i n e . Although t h e e f f e c t i s much smaller, i t can s t i l l be seen t h a t t h e r e i s a small n e g a t i v e c o n t r i b u t i o n a t lowest temperatures. This again c o u l d be associated w i t h Kondo e f f e c t and n e i t h e r t h i s a l l o y c o u l d be con- s i d e r e d as a completly homogenous ferromagnet.

There i s a l s o a q u a l i t a t i v e l y d i f f e r e n t behaviour

of R(HII ) and R(HL) (Fig.3b) a t low f i e l d s , which

i s due t o t h e demagnetizing f i e l d .

I n order t o determine the t y p e (weak o r strong) o f ferromagnetism present, we have looked a t the r e l a t i v e r e s i s t a n c e anisotropy. This q u a n t i t y i s defined('0) as t h e r a t i o o f a n i s o t r o p i c (?(HI1 ) - - 7 ( H I ) ) and i s o t r o p i c m a g n e t o r e s i s t i v i t i e s and, a t low temperatures, i t i s d i r e c t l y r e l a t e d ( l l ) t o t h e r a t i o oC o f s p i n 4 and s p i n f r e s i s t i v i t i e s :

( I n o u r c a l c u l a t i o n s we have taken value d e t e r - mined f o r Ni-based a l l o y s ( ' ' ) ) . The a n a l y s i s o f da- t a f o r x=20 a l l o y y i e l d s d>> 1, which i n d i c a t e s t h a t t h e s c a t t e r i n g i s dominated by t h e m i n o r i t y - s p i n e l e c t r o n s . T h e o r e t i c a l arguments(12) have sug- gested t h e same conclusion i f e(H 11 ) - ^{?(HI )} > 0.

Therefore, FeZ0Ni 60P14B6 a1 l o y seems t o t h e case of s t r o n g ferromagneti sm.

We have so f a r discussed i n d e t a i l s t h e r e s u l t s f o r a l l o y s a i t h low x, as more i n t e r e s t i n g ones.By i n c r e a s i n g t h e i r o n concentration, ferromagnetic c h a r a c t e r o f Fe,Ni80,xPj4B6 a1 l o y s i s more prono- unced ( 4 ) ( 8 ) . L o n g i t u d i n a l and t r a n s v e r s e magneto- resistances r e t a i n t h e f i e l d dependence as found f o r x=20 a l l o y (Fig,3b), and from t h e p o s i t i v e a n i - s o t r o p y i t f o l l o w s again t h a t d > > 1, i e . they behave as s o f t ferromagnets. ( S i m i l a r behaviour was found(I3) as w e l l f o r Fe40Ni40B20 a l l o y ) . A more d e t a i l e d study o f t h e magnetoresi stance f o r t h e a l l o y s w i t h x 7 2 0 i s i n progress.

We would l i k e t o thank Dr. R: Asonoza f o r h e l p i n some experiments, Dr. F.E. Luborsky f o r g i v i n g us samples and Dr. I . A . Campbell f o r u s e f u l suggestions.

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(+I A = dp/d l o g T ( r e f . 3)

(*) We use t h e n o t a t i o n R(H) z ARfH) = R(H)-R(H=O)