Magnetic studies in some melt spun amorphous FeHoB alloys
R. Krishnan, O. El Marrakchi, H. Lassri, and P. Rougier
Citation: Journal of Applied Physics 73, 7599 (1993); doi: 10.1063/1.353980 View online: http://dx.doi.org/10.1063/1.353980
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/73/11?ver=pdfcov Published by the AIP Publishing
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[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:
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Magnetic studies in some melt spun amorphous Fe-Ho-B alloys
R. Krishnan, 0. El Marrakchi, H. Lassri, and P. Rougier
Laboratoire de Magnttisme et Matkriaux Magnktigues, C.N. R.S 92195 Meudon, France (Received 30 November 1992; accepted for publication 8 February 1993)
Amorphous Fe,,-$o,Bts alloys have been prepared by melt spinning and their magnetic properties have been studied. The Ho moment at 6 K is found to be 10~~ which agrees with the theoretical value indicating a collinear spin structure. The magnetic compensation at 6 K occurs for x close to 13. The mean-field theory has been used to explain the temperature dependence of the magnetization.
I. INTRODUCTION
We had recently reported our magnetic studies in amorphous Fe-Er-B alloys where we have shown that the Er spin structure is noncollinear.’ We have also recently studied their behavior under magnetic fields as high as 35 T.’ Above a critical field the value of which depends upon the Er concentration, the antiferromagnetic coupling be- comes unstable. We also calculated some magnetic param- eters such as the inter-sub-network molecular-field coeffi- cient, exchange fields, etc.2 We therefore wanted to study the magnetic behavior of Ho which has a ground state of
‘Is (assuming a trivalent state) with a strong spin-orbit coupling and compare it with that of Er. Griissinger et al3 have reported on the magnetization of amorphous Fesc-ao,B,e alloys with x < 8%, at room temperature.
However, our study shows that their magnetization values are much lower than ours probably because the data have been taken with a field of only about 5 kOe. As shown by our study this field is too far below the saturation value. In this work we describe the results of our magnetic studies of amorphous Fesz-,Ho,Bts alloys as a function of tempera- ture in the range 6-300 K, under fields only up to 1.8 T. It is indeed surprising that our results show that the spin structure of Ho is collinear.
II. EXPERIMENTAL DETAILS
The amorphous Fesz-,Ho,Bls alloys with O<x<16 were prepared by the usual melt spinning technique under a protective atmosphere of argon. The amorphous state was checked by x-ray diffraction. The exact composition was determined by electron probe microanalysis.
The magnetization (M) was measured using a vibrat- ing sample magnetometer in the temperature range from 6 to 300 K under applied fields up to 1.8 T.
III. RESULTS AND DISCUSSIONS
For all the samples studied magnetic saturation could be obtained with H< 1.8 T at all temperatures. Figure 1 shows some typical results at 6 K. Figure 2 shows the Ho concentration dependence of the magnetization at 6 K. It is seen that M decreases with increasing Ho concentration indicating the antiparallel coupling between the Fe and Ho
moments, and the compensation of M occurs close to x= 13. The magnetic moment of Ho (1~~~) was calculated from the alloy moment as follows.’ The alloy moment yA can be written as
PA= [ (82--xh,- b)p~,1/100. (1)
For small concentrations (x(4) of Ho, the moment of Fe is not perturbed. So taking the value of ,L+~=~.O~,LL~
obtained from the alloy with x=0 and substituting in Eq.
( 1 ), it is possible to determine pnO for x=2 and 4. The calculated moment at 6 K is found to be 10~~ which agrees well with the theoretical value of gfig. It is noteworthy that the spin structure of Ho is therefore collinear unlike that of Er where we had found ,&&= 8~~ which is much smaller than the theoretical value of 9ps, suggesting a conical spin structure.’ Though the spin-orbit coupling constants for Er and Ho are comparable,4 it is seen that local anisotropy of Ho is not high enough and could be overcome with an external field of 1.8 T. This result is in agreement with that found by us for amorphous Co-Ho-B alloys.5 Using this value of ,+rO we then calculated the value of pFe for other compositions richer in Ho. It was found that pu, decreases from 2.06~~ to 1.83~~ when Ho concentration increases from 0 to 14. This decrease in ,$+
150- 7 -
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FIG. 1. The field dependence of the magnetization for the various Fe-Ho-B alloys at 6 K.
7599 J. Appl. Phys. 73 (II), 1 June 1993 0021-8979f 93/I 17599-03$06.00 @ 1993 American Institute of Physics 7599 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:
155.33.120.167 On: Fri, 19 Dec 2014 13:19:01
Ho (xl
FIG. 2. The Ho concentration dependence of the magnetization at 6 K.
is attributed on the one hand to the increased filling up of 3d bands of Fe by the sp electrons from B since now the relative concentration of B with respect to Fe increases, and on the other hand, to the hybridization of the 5d and 3d orbitals. There could also be some effect arising from the s electrons from Ho.
The temperature dependence of the magnetization of the samples was studied at H= 1 T and some typical results are shown in Fig. 3. For the samples with x > 4, the mag- netization shows a decrease as the temperature is decreased and the magnetization compensation is seen clearly for x > 13. The compensation temperature is found to increase with increasing Ho concentration as to be expected.
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