S o l i d S t a t e C o m m u n i c a t i o n s , Vol. 73, No. 7, pp. 4 6 7 - 4 6 9 , 1990. 0 0 3 8 - 1 0 9 8 / 9 0 5 3 . 0 0 + . 0 0
Printed in Great Britain. Pergamon Press plc
H I G H FIELD MAGNETIZATION STUDIES IN AMORPHOUS Cos0_xGdxB20
ALLOYS.
R. Krishnan and H. Lassri
Laboratoire de Magn~tisme, C.N.R.S. 92195 Meudon cedex France
( Received on september 29 1989 by P. Burlet )
ABSTRACT
We have studied amorphous Co80_ x Gd x B20 alloys prepared by melt spinning technique. These alloys are magnetically soft attaining saturation in fields on the order of 1 tesla. The Gd moment is found to be 7 ~ B indicating a collinear spin structure. The Co moment decreases with increasing Gd content.Even under external fields up to 15 tesla the antiferromagnetic coupling Gd-Co is stable. The temperature dependence of the magnetization is described satisfactorily by the mean field theory model.
INTRODUCTION
Melt spun amorphous alloys containing rare earth metals are interesting from fundamental point of view as they offer a rich possibility to study the various aspects of amorphous magnetism, such as, random anisotropy, nature of magnetic interactions the dilution of magnetic moments etc.. Some reports on such work have been published 1,2 We have reported on Co-Er-B system where we have shown that under high magnetic fields in the range 3 to 10 teslas the antiferromagnetic coupling between Co and Er breaks down 3,4.
This phenomenon was attributed by us to the non-collinear spin structure of Er arising from the random local anisotropy, characteristic of rare earths with a large spin orbit coupling moment 5. It was hence interesting to examine the case of Gd which is in S state and therefore does not present random anisotropy in the first approximation.
In this paper we describe our studies on amorphous Co-Gd-B alloys.
EXPERIMENTAL DETAILS
Amorphous Co80_ x C-d x B20 ribbons with 0 < x < 14 were prepared by the usual melt spinning technique under an inert atmosphere.
The details of the ,preparation have been reported by us earlier ~ . Amorphous state was verified by X-ray diffraction studies. The composition of the alloys were determined by electron probe micro analysis. The magnetization (M) was measured in the range 4 to 300 K under applied fields upto 15 tesla.
The Curie temperature T c was measured under a small applied field of 0.01 tesla.
The concentration dependence of the magnetization (M in emu.g -I) and the Curie temperature (T C in Kelvins)are shown in Fig.
1. When x increases, M decreases first faster and then for x > 10 it decreases more slowly.
The T c for x < 5 could not be determined due to the fact that the alloy crystallizes
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Fig. 1 The Gd concentration dependences of M and T C.
467
468 MAGNETIZATION STUDIES earlier. The result for x=0 is an extrapolated result from a study of Co-B amorphous alloys 5 For x > 5 ,T C decreases. The above results are oharcteristic of the antiferromagnetic interaction between Gd and Co atoms which is well known.
Let us discuss first the results at 4 K.
M saturates for fields on the order of 0.2 tesla, We have expressed the alloy magnetic moment (~) in units of Bohr Magneton. Fig.2 shows that it decreases linearly as the C-d concentration increases and the magnetization compensation is reached for x close to I0 at%.
For x > 10 the alloy moment increases again due to the contribution from Gd atoms. We have calculated from the alloy moment, the moments of Co (~Co) and Gd ( ~ d ) following the same procedure as in ref. 3 and briefly as follows. It is known from intermetallic studies that the transition metal ( T M ) moment is decreased due to the hybridisation of the 5d of the rare earth with 3d of the TM. However for v e r y small Gd concentration ~Co is not perturbed. So we first calculated ~Co from Co80 B20 which is 1.25 ~B and using this value calculated ~Gd using the relation ~= ~Co - for the alloys with x= 1.9 and 3.9, where of course, the Co moment dominates. We find that PGd = 7 ~B in agreement with the theoretical value, This indicates that the Gd spin structure is oollinear which is indeed to be expected in the absence of sizeable random local anisotropy. Now assuming this vlaue of 7
~B for Gd and kowing the alloy moment we calculated back the Co moment for other values of x.The Co moment starts decreasing for x > 6 and for x= 13,5 the decrease amounts to 37% as shown in fig. 2.
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IN AMORPHOUS ALLOYS Vol. 73~ NO. 7
The f i e l d d e p e n d e n c e o f t h e m a g n e t i z a t i o n was s t u d i e d u n d e r h i g h m a g n e t i c fields up to 15 teslas for all the samples. It was seen that it does not show practically any variation for H > 0.2 tesla. This is in contrast to what we had observed in the case of Co-Er-B alloys where under strong external fields the antiferromagnetic coupling was no longer stable3.This difference in the behaviour between Gd and Er leads us to the following conclusions, The antiferromagnetic interaction JCo-C-d > JCo-Er and the ferromagnetic Gd-Gd interaction is stronger than that of Er-Er. The above factors and especially the random local anisotropy of Er seem to be favourable to break the anti-ferromagnetic coupling JCo-Er.
The temperature dependence of M has also been studied. As the Gd content increases, M starts decreasing with decreasing temperature, as is to be expected.Fig. 3 shows the results for x= 7.8 and 13,5 respectively. For the latter composition a compensation occurs at T
= 230 K and for T < 230 K the alloy magnetization increases again strongly due to the contribution from Gd atoms.
The mean field theory has been used successfully in the past by many in order to explain the temperature dependence of the magnetization in many amorphous rare earth-transition metal alloys B,7. We have calculated the temperature dependence of the magnetization in Co-Gd-B alloys with Gd= 13.5, working along the lines developed in ref.6.
The following suppositions, as normally employed have been made:
I. ~Co is constant for T < 230 K.
2, The g factors of Co and Gd are taken as 2.00.
3. The coordinance number of the atoms is 12.
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