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T2 SATURATION IN RESISTIVITY OF
AMORPHOUS FERROMAGNETS BELOW 0.3K
E. Babić, Ž. Marohnić, J. Cooper, A. Hamzić, C. Rizzuto
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, supplément au n° 8, Tome 39, août 1978, page C6-946
T2 SATURATION IN RESISTIVITY OF AMORPHOUS FERROMAGNETS BELOW 0.3K
E. Babic, Z. Marohmc, J.R. Cooper, A. Hamzic and C. Rizzuto
Institute of Physics of the University, P.O.B. 304, Zagreb, Yugoslavia
*Istituto di Scienze Fisiahe & GNSM de CNR, Genova, Italia
Résumé.- La résistivité de trois alliages amorphes de la composition FexNi8n-xP'11(B6 (avec x égal a 10,20 et 40) a été étudiée entre 50 mK et 50 K. Dans tous ces alliages il y a une saturation de la résistivité au-dessous de 0.3 K. La grandeur du minimum varie fortement avec x.
Abstract.- The resistivities of three amorphous FexNi80-xPi i*B6 alloys (with x=I0,20
and 40 respectively) were measured in the temperature range 0.05-50 K. In all these alloys the resistivities saturate below about 0.3 K. The depths of the resistivity minima depend strongly on x.
Although the occurence of resistance mini-ma in amorphous ferromini-magnets has been known for
several years, little effort was made to
investi-gate the resistivity at the lowest temperatures. At the same time a detailed knowledge of the limi-ting low temperature variation is necessary for a proper theoretical description of the resistivity of these alloys. Here we wish to present a syste-matic study of the resistivities of amorphous fer-romagnets starting from the lowest temperatures and extending over three decades in temperature.
The resistivities of three amorphous Fe Ni 8 0_x : PmBs alloys (with x = 10,20 and 40 res-pectively) were measured from 0.05 to 50 K in a conventional cryostat and ^e-^He dilution refri-gerator. The samples were ribbons 1-2 mm wide and up to 50 urn thick. The resistivities were measured by a potentiometric set up with the resolution of a few parts in 10s. The absolute resistivity values determined from the geometrical shape factor, which in turn was deduced from the measured mass and density of the sample, were accurate to about 5 %.
We note that the resistivities obtained in this manner are ^20 % lower than those obtained from directly measured dimensions which is proba-bly due to roughness (small dentations) of the surfaces of the samples.Data relevant for our samples are given in table I.
The relative changes in the resitivity Ap (Ap=p(T)-p . ) of our alloys are shown in
— ~ v nun J
p m m
figure 1. These data also represent well the abso-lute resistivity variation because the resistivi-ties of all our alloys (table I) were
approximate-ly equal. The resistivity variations shown in figure 1 are similar to those of the Kondo alloys
Alloy Fe10Ni7opl<.B6 F e20N i6oPl4B6 F el,0N ii.0Plll B6 f i . , 2 ( yuAcm) 135 138 138 T . mm ( K ) 32 17 27 S*103 7.55 3-53 2,65 A (ijjlcm/ decade) 0.98 o.tn 0.28 T c ( K ) 60 230 533
Table I : Data for amorphous ferromagnetic alloys: P4_2 i-s t n e resistivity at 4.2 K, Tmin is the temperature of the resistivity minimum, 6 = (Psat _Pmin)/Pmin. A is the coefficient of the logarith-mic slope of the resistivity (dp/d(log T)) and Tc is the Curie temperature.
However in a ferromagnet such a resistivity beha-viour cannot (in principle) be caused by a single impurity Kondo effect because the large internal fields in a ferromagnet quench any spin-flip pro-cesses in the same way that a magnetic field does for to the Kondo alloys. In a view of this fact and recent investigation /2/ it seems more likely that such resistivity behaviour is caused by the structural disorder. Indee'd, recent calculations 73,4/ based on the excitations of a two level sys-tem give results which are qualitatively similar to those of the Kondo model. However we found some difficulties /5,6/ in interpreting our data in terms of these calculations. Also being perturba-tive these calculations cannot be used at the lowest temperatures. A critical survey of these calculations is given in Reference /6/.
Therefore we w i l l r a t h e r t r y t o f i n d t h e e m p i r i c a l The x dependence of o u r r e s i s t i v i t y curve r e l a t i o n s which can d e s c r i b e t h e observed r e s i s t i - i s r a t h e r i n t e r e s t i n g . We n o t e ( t a b l e I ) t h a t t h e v i t y v a r i a t i o n s . depths of t h e r e s i s t i v i t y minima, t h e l o g a r i t h m i c s l o p e s of t h e r e s i s t i v i t y and a l s o t h e c o e f f i c i e n t s of a T2 r e s i s t i v i t y v a r i a t i o n depend r a t h e r s t r o n g - l y on x. A d i s c u s s i o n of t h i s c o n c e n t r a t i o n depen- dence i s given i n Reference
/ 5 /
and we w i l l n o t r e p e a t i t h e r e . S t i l l we n o t e t h a t i n t h e s e a l l o y s t h e r e seem t o be some c o r r e l a t i o n between t h e de- c r e a s e i n t h e C u r i e temperature ( t a b l e I) and re- s i s t i v i t y . The c a l c u l a t i o n s / 3 , 4 / based on t h e s t r u c t u r a l model do not seem t o b e a b l e t o account f o r such c o n c e n t r a t i o n dependences.We thank D r F.E. Luborsky f o r g i v i n g us t h e amorphous ribbons and P r . B. L e o n t i c and D r . I .
~ o r i 6 f o r experimental h e l p .
Fig. 1 : Ap/p = (P-Pmi ) / p m i n f o r t h r e e amorphous FexNi
,,-,
P.,,B, a l l o y s $=10,20, 40) v s . l o g T. Numbers denote x. I n t h e i n s e t , Ap/p f o r x=20 (a)and x=40
(m)
i s p l o t t e d V S . T ~ .We n o t e t h a t t h e r e s i s t i v i t i e s o f our samples show a tendency t o s a t u r a t e below about 0 . 3 K. (At t h e s e temperatures s p e c i a l c a r e was t a k e n t o avoid any s e l f h e a t i n g of t h e samples). From t h e i n s e t of f i g u r e 1 i t can be seen t h a t below 0 . 3 K our samples obey r a t h e r w e l l a T2 law. We n o t e t h a t i n Kondo systems a t temperatures below about 0.15 0 / I / t h e r e s i s t i v i t y v a r i e s a s
R
p ( T ) = Ap (I
-
T2 ). I f we approximate Ap a s about-2 BR
twice t h e change i n t h e r e s i s t i v i t y from T=O t o T=T we o b t a i n
eR
v a l u e s around 2K which i n t u r nmin
a g r e e with t h e s a t u r a t i o n below 0.3 K. However t h i s formal analogy w i t h t h e Kondo a l l o y s b r e a k s down a t h i g h e r temperatures. For t h e sake of com- p l e t e n e s s wa a l s o t r i e d t o f i t our r e s i s t i v i t y c u r v e s (above 1.5 K) t o t h e e x p r e s s i o n p = Alln
2
(I - T ) + B T ~ d e r i v e d from t h e s t r u c t u r a l model / 3 / A~
and o b t a i n e d t h e A v a l u e s ranging from 1.5 t o 3.5 K f o r d i f f e r e n t a l l o y s .
References
/ I / Rizzuto, C . , Rep. Prog. Phys.
37
(1974) 47. / 2 / MarohniC',i.,
~ a b i c , E. and Pavuna, D. Phys.L e t t .
fi
(1977) 348/3/ Cohrane, R.W., H a r r i s , R., Strom Olsen, J . O . and Zuckermann, M.J. Phys. Rev. L e t t .
21
(1975) 676./4/ Kondo, J . , Physics
84B
(1976) 207./5/ Babi6, E . , MarohniC, and Ivkov, J . , This Conference.
/ 6 / ~ a b i 6 , E . , ~ a r o h n i c ' ,
