MAGNETOSTRICTION OF AMORPHOUS Co80-xMnxB20 RIBBONS

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MAGNETOSTRICTION OF AMORPHOUS

Co80-xMnxB20 RIBBONS

E. Du Tremolet de Lacheisserie, R. Yavari

To cite this version:

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JOURNAL DE PHYSIQUE

Colloque C8, Supplement au no 12, Tome 49, dbcembre 1988

MAGNETOSTRICTION OF AMORPHOUS C O ~ O - , M ~ Z B ~ ~

RIBBONS

E. du Tremolet de Lacheisserie (I) and R. Yavari (')

(I) Laboratoire Louis Ne'el, C. N.R.S., 166X, 38042 Greno ble Cedez, France (') L.T.P.C.M., ENSEEG, B.P. 75, 38402 ST Martin dYHires Cedex, France

Abstract. - Thermal variation of magnetization and magnetostriction of CoMnB alloys with x = 10 and 20 is studied. ~s/.f5/2 [c-I (m)] is plotted vs. m2/&/2 [L-I (m)] where m is the reduced magnetization: the origin of A, is discussed; for x = 10, annealing 1 hour at 580 K the as-quenched ribbon changes dramatically A, from 3 to 27 x lo-'.

I. Introduction

The origin of the magnetostriction A, in Co-rich metallic glasses has been often investigated, due to its anomalous thermal variation in nearly zero-As alloys, where bumps and change of sign are observed: a competition between a 1-ion contribution, XI, and a 2-ion one, Az, of opposite signs has been suggested for ex- plaining these effects, e.g. in Co.rzMnsB20 [I].

Letting m be the reduced magnetization, A, writes:

where &/2 is a modified hyperbolic Bessel function of the inverse Langevin function [2], which varies as m3 a t

low temperature (m > 0.9) and as 315 m2 near Tc. The

validity of equation (1) can be checked by plotting y =

As/&/2 [L-I (m)] vs.

t

= m2/&/2 [&-I (m)] which should give a linear dependence of y (t)

,

with t ranging from unity a t T = 0 up to 5/3 a t T =

T,.

The data for Co72Mn8B20 [I] do not seem t o agree with equation (I), see figure 2. Moreover, the s dependence of A, through the series Coso-,Mn,Bzo at 300 K seems t o be linear after [3] and quadratic after [I].

The goal of this work is to check the validity of equation (1) and to study the *dependence of A, at 0 OK.

2. Experimental

The melt alloys of the expected compositions have been rapidly quenched into ribbons 35 pm x 10.24 mm in cross section: both surfaces have been checked to be amorphous. DSC data show a crystallisation at 775 K.

Two different kinds of samples have been used: C-samples are cylinders obtained by winding a ribbon onto itself and slipping it into a ring [4], while P-samples are parallelepipeds prepared by piling and gluing together 15 pieces of ribbon, 5 x 10 mm2 [5].

The magnetic moment was measured using standard extraction method from 2 to 600 K up t o 6 x

lo6

~ m - l ;

the thermal expansion and the magnetostriction have been measured by a capacitance method [4] using C-

samples, but only a parallel field was applied, due to the shape of the samples. P-samples were used for verifying that A* = -AI1/2, and for measuring A,. The

All values obtained with C-samples were identical to

A, within experimental uncertainties, while BAll/BH was often larger with P-samples which could slightIy rotate for aligning along the magnetic field. So the data obtained with C-samples were more reliable. The accuracy was 0.2

a

below R.T. and 1 at 580 K. 3. Results a n d discussion

The magnetic moment o is plotted vs. T for x = 20 in figure 1. Due to the antiferromagnetic coupling of Mn atoms [6] a spin-glass like behavior was observed in similar alloys, e.g. Co75-,MnsP16B6A13 for x

>

15 [7]. In our x = 20 alloy, this phenomenon is likely present (see inset: a vs. T ~ / ~ ) and responsible for a large hys- teresis of the magnetostriction below 10 K that was

200 3 00

Fig. 1. -Magnetic moment (open circles) plotted against T (and T ~ / ~ : inset). Thermal variation of the magnetocaloric effect for H = 640 k ~ m - l (full circles).

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C8 - 1328 JOURNAL DE PHYSIQUE

not observed for x = 10. The magnetic moments per [Co

+

Mn] atom are 0.99 and 1.25 p~ and the Curie temperatures, 412 and 760 K, respectively for x = 20 and 10. These two sets are consistent with literature

[6]. Using a (T)

,

and cu (T) taken from thermal expansion data, we can derive m.

Our A, data are analysed in figure 2, together with previous ones after references [I, 81. For x = 20, equation (1) is verified with A1 = 6 x l ~and - A2 ~= -2

x

For x = 10, our results differ markedly from those given in [I]. Moreover, at 580 K, we have observed a continuous negative drift due t o structural relaxation. A further measurement of A, at 300 K gave a A, value of 27 x

loWs

instead of 3

x lo-' before

annealing.

Fig. 2.

-

Plot of ~./fs/z [L-' (m)] versus m 2 / f ~ / 2 (see text). Closed circles: P-sample; open circles: C-samples. Data for x = 0 and 8 are taken from [8] and [I] respectively.

In this alloy, A, = 0 is due to the competition of two single-ion contributions, respectively negative (Co) and positive (Mn). Any slight structural change may perturb markedly this equilibrium: A, appears to be a sensitive probe for the structural relaxation. It varies a t 0 K from -4 x for x = 0 to +4

x

for x = 20, with a nearly zero value for x = 10 : this linear variation confirms the dominant one-ion origin of A,. Large anomalies observed for x = 0 and 8 are probably due to a strong structural relaxation of as- quenched samples; our samples were aged during 5 and 17 months before being studied, for x = 10 and 20 respectively. When dividing the difference between the adiabatic and the isothermal strains by the thermal

Fig. 3. - Thermal variation of the forced magnetostridion.

expansion coefficient a, one gets the magnetocaloric effect ST for any given field. Its thermal dependence is given in figure 1, for x = 20 and H =: 640 k ~ m - ' . The

isothermal forced magnetostriction is given for both alloys in figure 3. The behaviour for x = 20 is nor- mal, and tends towards a A-like peak for T,, while for x = 10, a decrease is observed above 400 K just the temperature where the structural relaxation appears (noisy records of the strain).

[l] O'Handley, R. C., Sullivan, M. O., J. Appl. Phys. 52 (1981) 1841.

[2] Callen, E. R., Callen, H. B., Phys. Rev. A 139 (1965) 455.

[3] Pilipczuk, E., Matyja, H., Acta Phys. Pol. A 68 (1985) 215.

[4] du Tremolet de Lacheisserie, E., Krishnan, R., J.

Appl. Phys. 55 (1984) 2461.

[5] Tsuya, N., Arai, K. I., Shiraga, Y., Masumoto, T., Phys. Lett. A 51 (1975) 12:L.

[6] Obi, Y., Morita, H., Fujimori, H., IEEE trans. Magn. MAG16 (1980) 1536.

[7] Yeshurun, Y., Salamon, M. B.,

h,

K. V., Chen, H. S., Phys. Rev. B 24 (1981) .L536.

[8] O'Handley, R. C., Phys. Rev. Lt 18 (1978) 930. [9] Du Tremolet de Lacheisserie, E., Chamberod, A.,