~ < ~ ~ journal of magnetism and magnetic
, 4 4 4 materials ELSEVIER Journal of Magnetism and Magnetic Materials 131 (1994) L297-L300
Letter to the Editor
High magnetic field studies in amorphous Fevz_xYxHO8B20 alloys
R. Krishnan, H. Lassri, L. Driouch
Laboratoire de Magnetisme et Materiaux Magnetiques C.N.I~S., 92195 Meudon, France
F.E. Kayzel, J.J.M. Franse
Van der Waals-Zeeman Laboratorium der Universiteit van Amsterdam, Valckenierstraat 65, 1018 X E Amsterdam, The Netherlands (Received 25 January 1994)
Abstract
High field magnetic studies on amorphous Fe72_xYxHO8B20 alloys have been performed at 4.2 K for fields up to 35 T. The results are interpreted in terms of a model originally proposed for crystalline materials. The critical field at which the antiferromagnetic coupling becomes unstable is well predicted by the model. The exchange integral JHo-Ve increases with a decrease in Fe moment.
A m o r p h o u s alloys based on rare earth metals with strong spin orbit coupling present random anisotropy. This arises from the topological disor- der which causes the local symmetry axes to be randomly oriented. These materials are of funda- mental interest [1-3]. As in the intermetallics, in amorphous alloys the magnetic m o m e n t of the heavy rare earths (R) couples antiferromagneti- cally to that of the transition metal (T). However u n d e r very high magnetic fields this coupling breaks down and the field d e p e n d e n c e of the magnetization shows very interesting behaviour.
We have already reported such a study in amor- phous F e - ( E r , G d ) - B - S i alloys [4]. We also showed that the results could be explained using a model developed for intermetallic alloys by V e r h o e f et al. [5]. F r o m such a study several magnetic parameters of fundamental importance could be derived.
The important aspects of the model are the following: As the applied field is increased from zero, the magnetization first attains technical sat- uration, then, at a certain critical field Bcr 1 it starts increasing linearly until another critical field Bcr 2 is reached when it shows a plateau. This situation corresponds to the forced ferromagnetic state where the magnetic moments of the rare earths and the transition metal are aligned paral- lel. Fig. 1 schematically illustrates this behaviour.
These two critical fields are related to the sub- network magnetic m o m e n t by the relations [5]
Bet1 -- nRx I MT --MR 1, (1)
O c t 2 = nRT I MT + M R l, (2)
where M R, M T are the magnetic moments of the rare earth and the transition metal sub-networks and the molecular field coefficient nRT is the 0304-8853/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved
SSDI 0304-8853(94)00261-0
LETTER TO THE EDITOR
L298 R. Krishnan et al. /Journal of Magnetism and Magnetic Materials 131 (1994) L297-L300
E _.u
=E
...-- B B ~ B--,--
M R ~ M T M R ~ M T "MR' M T
~ M= MT+M R
i - - 4 . _dM -aE
M=IMT-I~I
Bcrl Bcr2
i i
F i e l d "
Fig. 1. Schematic diagram showing the magnetic coupling as a function of external field.
inverse of the slope of the linear portion ( d M / d B ) - 1.
Also from the experimental value of nRx the molecular field (B R) arising from the T sub-net- work magnetization acting on R can be calculated using the relation
B R = nRTMT . (3)
F u r t h e r the exchange field strength Bex acting on R can also be obtained, from the relation
B e x = B R / T R (4)
where YR = 2(gR -- 1)/gR, gR being the spectro- scopic splitting factor of the rare earth R.
It is also possible to calculate the exchange integral JHo-F~ using the relation [6]
Bex = JHo- Fe Z H o - re SFe///'~ B
where ZHo_F~ denotes the number of nearest neighbours of Fe of H o atoms and the other symbols have their usual meaning. We have as- sumed Z n o _ F e = 12, corrected to the Fe concen- tration, in accordance with other authors [7,8].
It can be seen that for a given nRT, Bcr 1 can be decreased by decreasing the net magnetic mo- ment ( M ) of the alloy. One way of achieving this is to increase the R content close to the compen- sation point. However BorE, being the sum M R A- MT, will increase with R content since the rare
earth moment per a t o m (~.LR) is much higher than that of the transition metal (/ZX). So we have used another method of decreasing I M x - M R l, by diluting the T content by a n o t h e r non-mag- netic metal, which in our present study is yttrium.
The magnetic rare earth content was therefore kept constant. The aim of this work is to study the effect of such a substitution on the magnetic coupling and amorphous Fe72_xYxHoaB20 alloys with 0 _< x _< 14 were chosen for the present study.
The H o concentration has been chosen in order that Bcr 1 would be within the range of the avail- able field.
The samples were p r e p a r e d by melt spinning which was carried out in an inert atmosphere.
T h e samples, about 2 mm in width and about 30 /xm thick, were amorphous when X-rayed. T h e exact composition was determined by electron probe micro analysis. High field magnetization measurements up to 35 T were performed at 4.2 K. T h e high field installation has been described previously [9].
Fig. 2 shows the field d e p e n d e n c e of the mag- netic moment (expressed in e m u / g ) for four alloy compositions. First of all it is seen from Fig. 2
120
E m c
¢ E o u E
80-
40-
oet o e e o
"Jr
o 1'o 2'0 do
B(T)
Fig. 2. The field dependence of magnetic m o m e n t M of 4.2 K for different Y(x) concentrations, x =0.0 (*); x =4.6 (*);
x = 10.0 (*) and x = 13.7 ( ).
LETTER TO THE EDITOR
R. Krishnan et al. /Journal of Magnetism and.Magnetic Materials 131 (1994) L297-L300 Table 1
Some magnetic parameters at 4.2 K
L299
Y(x) M(gB) = I MFe - Mrto I nHoF e Bcr 1 (T) Bcrt (T) Bex (T) JHo-Fe (T//z a) c a l c u l a t e d experiment (10- 23 j)
0.0 0.690 38.0 26 25 141.6 15
4.6 0.423 45.9 19.4 18.2 140.3 16
10.0 0.260 46.5 12.1 11 123.2 18
13.7 0.110 51.0 5.6 4.7 116.0 20
that the net magnetic m o m e n t of the alloy de- creases as Y increases. This is due to a decrease in the sub-network m o m e n t MFe due to a de- crease in Fe concentration. Besides the m o m e n t per atom of Fe (~Fe), it also decreases due to the hybridization of the 5d orbitals of Y with 3d of Fe. The results are shown in Table 1. As pre- dicted by the model, one can see that for fields higher than a critical value, M rises steeply and linearly. Also this critical field Bcr 1 decreases as the Y content increases, again as expected. For Y ( x ) = 13.7, Ber 1 is indeed very low. F r o m the slope of the linear portion nHoFe was calculated and the results are shown in Table 1. It is seen that nHoFe increases as the Fe content decreases.
25~
2C
¢.-
O ) L L
6 i
" l - -..n
1.0 1.5 2.0
PFe in PB
Fig. 3. The variation of JHo-Fe with the iron moment /~Fe"
From the experimental value of nHoFe we have calculated Bex using the Eqs. (3) and (4). The value of MFe was obtained from the alloy mo- ment M (Table 1) taking /XHo = 10 tz B as deter- mined from our previous studies [10]. As regards gvlo, this was calculated for the ground state 518 to be 1.25 which yields 3' = 0.4. Thus knowing all the parameters, Bex and JHo-Fe could be calcu- lated and the values are shown in Table 1. It is seen that Jvlo-Fe increases, when the Fe concen- tration and hence /~Fe decreases, where /zFe has been calculated knowing the moment of H o and that of the alloy. Fig. 3 shows the variation of JI-Io-V~ with iron m o m e n t tiFf- A similar increase in JR-T has been reported in intermetallic alloys and amorphous alloys also [11-13]. The 3 d - 5 d interactions depend critically on 3 d - 5 d hybridiza- tion according to Brooks et al. [14]. Therefore the increase in JHo-Fe would indicate an increase in the 3 d - 5 d hybridization when the Fe concentra- tion relative to H o is decreased.
As pointed out earlier, Bcr 1 can also be calcu- lated from the experimental value of nRT using Eq. (1). It is seen that the calculated value agrees very well with the experimental one as shown in Table 1.
We thank Mrs D u m o n d for the determination of the composition of the samples.
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LETTER TO THE EDITOR
L300 R. Krishnan et al. /Journal of Magnetism and Magnetic Materials 131 (1994) L 29 7-L300
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