Magnetic coupling in amorphous Fe80−x R x B12Si8(R=Er, Gd) alloys
R. Krishnan, H. Lassri, and R. J. Radwanski
Citation: Applied Physics Letters 61, 354 (1992); doi: 10.1063/1.107935 View online: http://dx.doi.org/10.1063/1.107935
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Magnetic coupling in amorphous FesoJ3,B12Si8(R = r, Gd) alloys
R. Krishnan and H. Lassria)
Laboratoire de MagnSme et Mat&-iaux Magnetiques, C.N R.S. 92195 Meudon, France R. J. Radwanski
Van der Waals-Zeeman Laboratorium der Universiteit van Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam, The Netherlands
(Received 9 March 1992; accepted for publication 4 May 1992)
We have prepared amorphous Fe-Er-B-Si and Fe-Gd-B-Si alloys’ and magnetization studies in fields up to 35 T at 4.2 K have been carried out on some selected samples. The results have been analyzed on the existing model proposed by Verhoef et al. for crystalline Fe and Co compounds with rare earths. The magnetic moments of Fe, Er, and Gd, as well as other parameters such as the intersublattice molecular-field coefficient (n,), the strength of the exchange interaction etc., have been determined.
Amorphous alloys based on rare earth metals with strong spin orbit ( LS ) coupling present random anisotropy behavior due to topological disorder and are of interest from fundamental and applications points of view.tm3 In the alloys with heavy rare earths as in crystalline ones the magnetization of the 3d transition and the rare earth met- als are aligned antiparallel to each other. We have reported recently on the magnetic and Mlissbauer studies under ex- ternal magnetic fields, on amorphous Fe-Er-B-Si alloy~.~
From these studies the magnetic moment of the Er ion was determined to be 8 PB at 4 K. In the magnetization curves presented in Ref. 4, (see Fig. 2) a strong increase has been observed for the alloy with x= 11, for fields close to 13 T.
For 15% Er this phenomenon occurs already at a much lower field of about 4 T. The origin of this phenomenon was not clear and we therefore wanted to investigate the properties at higher fields and also to compare the behavior with that of alloys based on Gd which is interesting be- cause, as is well known, it has no spin orbit coupling. We describe in this work our magnetization studies in fields up to 35 T. We have applied to these amorphous alloys, a model recently developed by Verhoef et al5 which allows us to determine the various magnetic parameters such as, the intersublattice molecular tield coefficient and the strength of exchange interactions etc. from the high field magnetization of the intermetallic compounds. In the treatment of this model the sample is free to rotate in the applied field so that the magnetization of the sublattice with stronger anisotropy gets aligned in the field. In our studies two types of experiments were made ( 1) small bits of amorphous samples were taken which were free to orient themselves in the field and (2) samples which were rigidly fixed. Both measurements gave identical results indicating in the materials with random anisotropy this is not signif- icant. Let us mention that a similar model had been pro- posed earlier for rare earth iron garnets also.6 After the model of Ref. 5 and as veritied also experimentally, for alloys with relatively high net moment, the applied critical fields necessary to break the antiferromagnetic coupling in such alloys would be much higher than the maximum
‘)Also at Laboratoire de Physique des MatCriaux, Facultk des Sciences, BP 1014 Rabat, Morocco.
available field of 35 T. So we have concentrated in this work on alloys with appropriate rate earth metal concen- trations, in order to have small resultant magnetization, namely the alloys with the rare earth content in the range
11-15.
The amorphous Fe,,-,R,B,,S& alloys where R=Er and Gd were prepared by the usual melt spinning tech- nique under an inert atmosphere. We chose the alloys with x = 11 and 15 for Er and 15 for Gd. Smaller values of x for Gd are not interesting from the present experiments for the reasons mentioned in the introduction. The amorphous state was verified by x-ray diffraction and the exact com- position was determined by electron probe micro-analysis.
The high field magnetization measurements up to 35 T were performed at 4.2 K in the high field installation of the University of Amsterdam.’
Figure 1 shows the high held dependence of the mag- netization (expressed in emu/g) at 4.2 K for Fes,.+R,B,,Sis alloys for x=1 1% and 15%. Figure 2 shows the result for 15% Gd alloy. In all the cases one can see that for field higher than a critical value the magneti- zation rises steeply and linearly. The linear portion when extended to lower fields passes through the origin only for Er = 15% and not for other cases. The reason for this is not clear at present. The above results could be analyzed in terms of the model of Ref. 5 as described below;
The molecular field coefficient nnr where R and T stands for the rare earth and transition metals (in the present case R- Er, Gd and T=Fe) respectively is ob- tained as proposed by the model for intermetallic com- pounds5 from the slope,
dA4
z= hlT) -I. (1)
The critical field qtit where the straight part of the mag- netization curve starts is given by the product of the re- sultant alloy moment (M=Mr--MR) and IIRT,
B~it=n&f~-MR~. (2)
At sufficiently high fields the second critical field is ex- pected when the R and T moments are aligned parallel and one should observe a plateau. This second critical field is given by the relation
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1201--==---
I TABLE I. Magnetic moment and other parameters at 4.2 K.
. Xdl .
.
B(T)
FIG. 1. The field dependence of magnetization for Fe,,-,Er,Bt,Sis with x=11% and 15% at 4.2 K.
Knowing nsr experimentally it is possible then to calculate the molecular field arising from T sub network magnetiza- tion (Bn) acting on the R and which is given by the rela- tion Bn=nnr.& and also the exchange field strength &, acting on the rare earth using the relation Be,==BR/yR where 3/R = 2 (gn - 1) /gu and where gn stands for the spec- troscopic splitting factor of the rare earth metal. Therefore, one finds Yod= 1 and ynr= l/3 taking the respective gR values.
Let us now apply the above model to extract the var- ious parameters. The magnetic moment of Er was obtained as explained in Ref. 4. The Fe moment will not be per- turbed by Er for small concentrations of the order of 5%.
Therefore by analyzing the magnetization of the alloys with low Er contents and assuming the Fe moment as measured on Er free alloy the Er moment was determined to be 8~~ Then using this value for Er the moment of the Fe ion has been calculated from the magnetization values measured for the alloys rich in Er at fields less than 5 T.
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FIG. 2. The field dependence of magnetization for Fe,,-xGdxB,,Si, with 4R. Krishnan, H. La&, and J. Teillet, J. Magn. Magn. Mat. 98, 155
x= 15% at 4.2 K. (1991).
~Alloy nR-Fc JR-me
Sample (pB) (T/pa) B,(T) B,(T) et(T) g”(T) (lo-23 J)
Er 11% 0.4 32.3 41.3 124 13 70 25
Er 15% 0.1 36.2 39.8 119.5 3.6 83 16.8
Gd 15% 0.13 123 145 145 15.5 274 19
Similar operation was also done for Gd based alloys and we obtained the moment of Gd=7 ps. Table I shows the values of the alloy moments at 4 K.
The various parameters then could be calculated from the relations described above and the values are also shown in Table I. It is seen that nsr is 123 T//LB for Gd which is very much higher than that for Er which is in the range 32-36 T/pB for the two compositions studies. It can be seen from the Table I that the calculated values of flit agree with the experimental ones. And in the case of Er alloys, the qrit increases strongly from 3.6 to 13 T as the concentration decreases from 15% to 11% and therefore for alloy compositions with lower Er content, say 5% for example, (and hence higher moment), @ ’ would be much higher than 35 T and hence cannot be observed by us. We have also calculated Bc2fit for the alloys and as shown in Table I the values are very large particularly for Gd. The BR and Be, values for Er are comparable to those reported for intermetallic Er,Fe,,-JvIn,C compounds with x=3 and 4 by de Boer et aI.’
It is also possible to calculate the exchange interaction strength JR-Fe using the relation,’
Be, =ZR-PJR-P~&~~B, (4)
where ZRmFe denotes the number of nearest neighbors of Fe for R atom and other symbols have the usual meaning. In order to calculate JR-Fe, we need to know ZR-Fe. This has to be model dependent and we have assumed as other authors have done previously,g’10 that ZRqe= 12 corrected to the Fe concentration. Under this condition JR-Fe can be calcu- lated and the values are shown in Table I. These values are in general agreement with those published for similar amorphous alloys. lo
In conclusion we have analyzed the high field magne- tization data on Er and Gd containing amorphous alloys, using the model developed for crystalline intermetallic compounds. We have calculated the various magnetic pa- rameters of fundamental importance and some of them are in general agreement with the values published for similar alloys in the literature.
Some measurements were made with fields up to 15 T at Service National des Champs Intenses, C.N.R.S. Greno- ble. Part of this work was performed under The European Commission Action CEAM 3.
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355 Appl. Phys. Lett., Vol. 61, No. 3, 20 July 1992 Krishnan, Lassri, and Radwanski 355
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356 Appt. Phys. Lett., Vol. 61, No. 3, 20 July 1992 Krishnan, Lassri, and Radwanski 356
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