354 Journal of Magnetism and Magnetic Materials 83 (1990) 354-356

North-Holland

ELECTRICAL TRANSPORT PROPERTIES OF VANADIUM SUBSTITUTED Fe-B-Si METALLIC GLASSES

Girish CHANDRA “, S. RADHA “. A.K. NIGAM ‘, Shiva PRASAD h, S.N. SHRINGI h and R. KRISHNAN ’

‘I Turo Imrrtute of Fundumental Research, Bomlx~~-5, Indro

’ Phr:rrc.s Depurtment, Indrcrn fnstrtute of Technology:,: Bomhqv-76, Indu

’ Luhorutorre de Mugn~tisme, CNRS, 92195 Meudon Principal Cede\-, Frcmce

The measurements of electrical resistivity on amorphous Fe,,_ ,V, B,*Si, (0 < x < 15) are reported in the temperature range of 4.4 to 300K. With increasing x. double minimum behaviour identical to a-FeeCrrB-Si is observed. In concentrations

above x = 10. only a pronounced maximum is observed unlike other metallic glasses.

The temperature dependence of electrical resistivity in metallic glasses has been a subject of debate since the very beginning. Since the electrical resistivity depends on many factors such as degree of disorder, structural relaxation, etc., it is difficult to isolate the contribution due to the intrinsic mechanism. The results have been particularly complex for the case of metallic glasses containing early 3d transition metals, such as Cr. V or Mn. In spite of extensive experimental and theoretical studies. the complexity of behaviour is still unresolved. We have carried out systematic measurements of resis- tivity in several transition metallmetalloid glasses in the temperature range of 1.5 to 300 K [1,2]. In the present paper, the resistivity measurements on Fe,,_ Y- V,B,,Si, (0 < x’ < 15) in the temperature range of 4.4 to 300 K are reported.

Amorphous ribbons of FeeVB-Si were prepared by melt spinning technique. The ribbons were = 2 mm wide and = 25 urn thick. They were checked for amor- phicity by X-ray with Co-K, radiations [3]. The electri- cal resistivity was measured by the usual four probe dc technique. The electrical leads to the sample were at- tached with non-superconducting Cd-Zn solder. The temperature of the sample was measured with Lake Shore silicon diode thermometer employing a Lake Shore DRC-82C temperature controller. The relative resolution of the resistance measurement was 20-40

ppm. In fig. 1, the relative change in resistance r,,(T)[ =

(R(T) - R(T,,,))/R(T,,,)], where R(T) is the resis- tance at temperature T, is shown as a function of temperature for all the samples studied. It is seen in this figure that r,,(T) curves exhibit a minimum for the .Y = 1 sample at r,,, = 20 K. For x = 3, though r,,(T) shows a single minimum, there is a tendency to develop a second minimum at a higher temperature. For 4.5 < x < 10. r,i( T) shows well defined double minima. In this

0304-X853/90/$03.50 5 Elsevier Science Publishers B.V.

(North-Holland)

case, R( T,,,,) = R( T,,,,,, ), where Thmln is the tempera- ture at which a higher temperature minimum in ro( T) occurs. r,,,, is found to increase with Y. For x > 10. no clear minimum is observed over the temperature range studied. However, there is a tendency which indi-

.

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0 60 I20 180 240 30

TEMPERATURE (K)

Fig. 1. The data of normalised resiativity r,,(T) = [( R(T)-

R(T “,,” ))/R(T “,,” )] for Fe,,,~ ,V,B,,Si,. The curves have been shtfted along the ordinate for clarity.

G. Chandra et al. / Transport properrres of Fe- V-B-S merullic glasses 355

Table 1 Results of fitting of r,(T) data to r,,(T) = a + b log T for T < Thmln and r,,(T) = A + ET” for T > T,,,,

103b(rmsd) B (coefficient of T”)

1 .o 3.0 4.5 6.0

10.0 12.5 15.0

_

- 1.74(0.74) - 2.88(2.07) - 4.20(3.75) - 5.20(3.68) -5.01(2.81)

n = 1 (rmsd) (10-s)

12.40(8.14) 7.20(7.97) 3.53(2.41) 2.27( 1.48) _ _ _

n = 1.5 (rmsd) (10-6)

5.88(3.90) 3.42(3.06) 1.45(2.11) 0.92( 1.47)

_ _

n = 2 (rmsd) (lo-‘)

3.07( 11.79) 1.79(4.06) 0.67( 1.83) 0.42(1.24)

_

cates that T,,,,, may occur above 300 K while the low temperature minimum could be below 4.4 K. This be- haviour is somewhat different from Fe,, _ $rs B,, Si x metallic glasses where for all concentrations up to x = 12, both the minima were observed in the temperature range of 4.2 to 300 K [2]. The maximum value of T,,,,, (- 240 K) was observed for x = 8 in Cr containing metallic glasses. Also, the maximum observed in the present series of vanadium samples is much more pro- nounced in comparison to Cr containing alloys.

The various theoretical models proposed to explain the negative temperature coefficient of resistivity (TCR) below T,,i, do not give any relationship which could be used to explain the double minimum behaviour. It is also difficult to say whether the origin of both the minima is from the same or different scattering processes. In a-Fe,,-,Cr,B,, systems. Olivier et al. have argued that the origin of the low temperature minimum is structural while that of the higher tempera- ture minimum is magnetic [4]. In order to see the effect of vanadium on the negative TCR, ro( T) was fitted to the expression: r,(T) = a + b log T below the tempera- ture of T,,,,. The value of b for various V concentra- tions is given in table 1. This indicates that the slope increases with x and tends to saturate above x = 10.

In order to determine what kind of scattering mecha- nism is responsible for the resistivity behaviour in the positive TCR region, an analysis of the data was made above Thmln. This had been possible only for samples up to .X = 6 where Thmin is below 300 K. Using the least squares method, the data were fitted to the equation r,(T) = A + BT” with n = 1, 1.5 or 2. Table 1 shows the results of the fitting with values of B and root mean square deviation (rmsd). It can be seen that for .X = 1 and 3 the best fit is in favour of T”‘-dependence of resistivity where rmsd is minimum indicating the pres- ence of incoherent electron-magnon scattering. For x =

4.5 and 6. T’-dependence of resistivity is seen as in the case of a-Fe-Cr-B-Si alloys for all Cr concentrations

PI. The coefficient (B) of the T” term contains the

factor (l/D)“, where D is the exchange stiffness con- stant. The value of D was determined by Velu et al. [3] on the same samples through magnetisation measure- ments. They found a decrease in D with increasing x, which implies that the coefficient of the T” term should increase with x. In table 1, the coefficients of T’/’ and T2 terms are found to decrease with x which is incon- sistent with the dependence of D on x. We therefore tried to fit r,(T) to a combination of two terms out of three terms in the expression

r,(T) =a’+b’T+c’T3”+f’TZ.

This brought an overall improvement in rmsd. The results of the fitting show that the combination of T

and T3j2 terms gives better results. However, it is found here that coefficient (h’) of T changes sign as .x increases and its physical meaning is not clear. The coefficient c’ of the T”’ term increases by more than 100% when x changes from 3 to 4.5. whereas it should change only by = 50% as expected from values of D

obtained by Velu et al. [3]. In conclusion, the maximum in r,(T) observed in

the present samples with .Y > 6 is much more pro- nounced and has not been observed in other metallic glasses. Further measurements and analysis are in pro- gress to establish the origin of this maximum. The behaviour of resistivity in the present samples in the negative TCR region is qualitatively similar lo that found in Cr-substituted Fe-B-Si metallic glasses. Above T hmln. the temperature dependence of r,)(T) is different from that observed for Cr containing metallic glasses.

356 G. Chandra et al. / Transport propertres of Fe- V-B-S{ metallic glasses

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