High field magnetisation studies in amorphous CoDyB alloys

~zSD

Journal of Magnetism and Magnetic Materials 157/158 (1996) 149-150

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ELSEVIER

H i g h field m a g n e t i s a t i o n studies in a m o r p h o u s C o - D y - B

journal of magnetism and magnetic materials

alloys

L . D r i o u c h a R. K r i s h n a n a,* j . V o i r o n b a Laboratoire de Magn&isme et d'Optique de Versailles, CNRS, 92195 Meudon, France b Laboratoire de Magn£tisme Louis N£eL 166 X, 38042 Grenoble cedex, France Abstract

We have prepared amorphous Cos0_xDyxB20 ribbons with 0 < x < 15 by melt spinning and studied their magnetic properties in the temperature range 1.5-300 K under applied fields of H _< 15 T. The Dy moment was found to be 8.6/z B, which is lower than the theoretical value of 10/x B, indicating the non-collinearity of Dy spins due to strong random anisotropy. For x < 10, the local anisotropy K 1 is 1.5 >( 10 7 e r g c m -3, and 3.8 × 10 7 ergcm -3 for x > 10. Keywords: Amorphous alloys; Random anisotropy; Magnetisation

Amorphous alloys based on rare earth metals are interesting candidates for studying various fundamental parameters, such as ferromagnetic and anti-ferromagnetic interactions, random anisotropy, etc. [1,2]. In this paper we describe magnetic studies on amorphous C o - D y - B alloys. We have interpreted the results on the basis of Chudnovsky's model and extracted several magnetic parameters. Amorphous Coso_xDYxB20 alloys with 0 _< x < 15 were prepared by the usual malt spinning technique under inert atmosphere. The amorphous state was verified by X-ray diffraction. The exact composition was determined by electron probe micro-analysis. The magnetisation was measured in the temperature range 1.5-300 K and under applied fields of up to 15 T. The Curie temperature (Tc) was also determined using a vibrating sample magnetometer under applied fields of less than 0.05 T. The ribbons were about 1 mm wide and 30 /zm thick and were all amorphous, as shown by the characteristic broad X-ray diffraction peak. The magnetisation decreases as x increases as is usual in such alloys due to the antiparallel coupling between Dy (/ZDy) and Co (/Xco) moments. At 1.5 K the magnetic compensation occurs for x ~ 8. The alloy moment (/x A) can be written as:

= [(80-- X)Co --

(1)

To calculate /XDy we proceed as follows. For the alloy with x = 0, /Xco= 1.27/z B. Knowing /LA, we calculated /ZDy for the sample with x = 5.3 where the transition

* Corresponding author. Fax: -t-33-1-4507-5822; email: krishnan @bellevue'cnrs-bellevue'fr"

metal moment is not perturbed by hybridisation effects and found it to be 8.6/* B. Since /.LDy is independent of x, we can calculate /Zco for other values of x. Two points can be noted from this analysis. First, the experimental value of ]ZDy is smaller than 10/z B, the ground state value (gJtz B) of Dy, implying non-collinear spin structure of Dy. Using the relation [3]

~t/~Dy= /ZDy(free ion)(1 + cos 0 ) / 2

(2)

we calculate the average apex angle 20 to be 86 °. This also indicates the effect of the local random anisotropy. The second point is that /,co decreases from 1.27 to 0.81/* B when x increases from 0 to 12.7. This decrease is mainly due to the hybridisation of the 5d orbitals of Dy with the 3d ones of Co. As the 4f electrons are deep, they are not perturbed by variation of the Co content nor can there be any charge transfer from the s - p orbitals of the metalloids, hence the Dy moment is constant for all values of x. The temperature dependence of the magnetisation of these samples is typical for a ferrimagnetic system. As the temperature is reduced, the magnetisation begins decreasing and for sufficiently high Dy concentration one observes a compensation point. For x = 9.6 the compensation occurs at a temperature close to 55 K. We have analysed the field dependence of the magnetisation at 1.5 K using the theoretical model developed by Chudnovsky et al. [4]. The model distinguishes two regimes: (1) when the external applied field is smaller than the exchange field Happ << Hex ; and (2) when Happ >> Hex. In the first case the approach to saturation is described by the relation M o -- M = ( M o / 1 5 ) ( H s / H . p p ) I/2,

0304-8853/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. SSDI 0 3 0 4 - 8 8 5 3 ( 9 5 ) 0 1 2 5 1 - 6

(3)

L. Driouch et al. / Journal of Magnetism and Magnetic Materials 157/158 (1996) 149-150

150 50-

Table 1 Some fundamental parameters of Coso_xDyxB~0 alloys at 1.5 K

40-

x=5.3

"•30-

Dy (x)

A (10 -8 erg/cm 3)

Hex (T)

Hr (T)

K 1 (107 erg/cm 3)

Ra (A)

0 5.3 9.6 11.7 12.7

33.3 28.3 16.9 12.6 11.7

7.6 17.6 21.4 7.2 8.3

3 9.4 18.1 18.4 17.0

1.3 1.5 1.4 3.6 3.9

9.4 7.8

~20x=9.6 10-

0

o.oo

oAo

oAo

o.bo

oAo

1.oo

H"o.s(10-eOe-O.'a)

Fig. 1. The H -°'5 dependence of the magnetisation at 1.5 K.

where the slope is

Hs = H4/H3x .

(4)

The random anisotropy field H r = 2 K1/Mo, where K 1 is the local anisotropy, and Hex = 2 A / M o R2, where A is the exchange constant and R a the correlation length of the symmetry axis. The exchange constant A can be obtained from Tc by combining the models proposed by Hasegawa [5] and Heiman et al. [6]. For want of space we are unable to develop this aspect any further. In the second case, the approach to saturation is described by the relation

M o - M = r M = ( M o / 1 5 ) [ H r / ( H a p p + H e x ) ] 2,

(5)

which can be rewritten as ( r M / M ) -°5 = B ( H + H e x ) , where B = v/15(Hr) -I Therefore, by plotting ( r M / M ) -°5 as a function of H one can obtain both B and Hex [7]. The approach to saturation for x = 5.3 and 9.6 has been analysed under the regime H < He× and Fig. 1 shows the variation of M as a function of H - ° 5 at 1.5 K; the data points are found to align well along a straight line. However, for x = 11.7 and 12.7, the second regime is more appropriate and the results were analysed accordingly.

Table 1 shows the various parameters of fundamental importance that have been extracted from the analyses of the approach to saturation at 1.5 K. The sudden increase in K 1 for x = 11.7 and 12.7 is noteworthy. It is possible that some change in the local order occurs close to this composition. More detailed structural studies would be necessary to discuss this any further. The exchange constant A decreases as x increases, which is a consequence of the decrease in Tc. However, Hex shows complex behaviour, first increasing with x and then decreasing. It is known that Hex is sensitive to the overlap integrals, but this result is not well understood at present. Finally, it can be seen that the correlation length R a is of the order of 9 A, which is quite reasonable for amorphous alloys and it also agrees with the result published by O ' S h e a et al. [8]. Acknowledgement: The authors are grateful to Dr. Y. Dumond for the chemical analyses.

References [1] R. GrSssinger, H. Sassik, R. Wezulek and T. Tamoczi, J. Phys. (Paris) 49 Suppl. (1988). [2] R. Krishnan, H. Lassri and R.J. Radwanski, Appl. Phys. Lett. 61 (1992) 354. [3] J.M.D. Coey, J. Chappert, J.P. Rebouillat and T.S. Wang, Phys. Rev. Lett. 61 (1976) 1061. [4] E.M. Chudnovsky, W.M. Saslow and R.A. Serota, Phys. Rev. B 33 (1986) 251. [5] R. Hasegawa, J. Appl. Phys. 45 (1974) 3109. [6] N. Heiman, K. Lee and R. Potter, J. Appl. Phys. 47 (1976) 2634. [7] H. Lassri, L. Driouch and R. Krishnan, J. Appl. Phys. 75 (1994) 6309. [8] M.J. O'Shea, K.M. Lee and A. Fert, J. Appl. Phys. 67 (1990) 5769.