Journal of Magnetism and Magnetic Materials 140-144 (1995) 355-356
High fieid magnetisation studies in WiiE rare Hdl hid amorphous ribbons
R. Krishnan *, H, Lassri, L. Driouch
Laboratoire de Magn.&.sme et Mat&iaux Magnbiques, CNRS, 92195 Met&n, France
Abstract
We have carried out magnetisation studies at fields up to 20 T of melt spun amorphous Fe,,-,Y,Ho,B, alloys. The approach to saturation for H 5 11 T has been analysed on the basis of Chudnovsky’s model, and the exchanee cnnctant;
random anisotropy etc. h?ve bee:: cz!cu!a:;iL ‘.‘;E LW aisu ex[ractecl the local anisotropy from tbe coercivity.
1. Introduction
Amorphous alloys based on rare earth metals wi.h strong spin-orbit coupling present random anisotropy. This arises from the topological disorder which causes the local symmetry axes to be randomly oriented. These materials are of fundamental interest [1,2]. Several important mag- netic parameters, such as local anisotropy constant, ex- change field, can be obtained by analysing the approach to magnetic saturation on the basis of the model proposed by Chudnovsky et al. [3,4]. From the above model we can obtain a number of equations which are as follows. For applied fields less than the exchange field Ho, the mag netisation shows a linear dependence on He'/'. From the slope one can deduce the field H, using the equation
Wo-M)/Mo=&i~~ (1)
where
H, = H,4/H,: . (2)
The anisotropy field H, and the local random anisotropy constant are related as
H, = 2K,/M,. (31
From the same laodel one can also relate H,, and the exchange constant A by the expression
H,, = 2A/M,R,2, (4)
where R, is the length over which the local axes show a correlation. The exchange constant can be obtained from the mean field model proposed by Hasegawa [5] and from
l Corresponding author. Fax: +33-l-45075822; email: krish- [email protected].
the Curie temperature using the relation proposed by Heiman et al [6].
We have recently reported the magnetic behaviour of amorphous Fe,z-XYXHo,B,, alloys under high magnetic fields up to 35 T [7]. For instance, for x = 0, the antiferro- magnetic coupling between Ho and Fe subnetwork breaks down at a critical field of 25 T. This critical field decmases with the addition of Y and is only 11 T for x = 10. In this paper we will discuss the magnetization behaviour for fields less than 11 T, where the antiferromagnetic coupling is still stable. We will focus our attention to the problem of approach to sahuation and extract some parameters of fundamental interest as described earlier.
2. Experimental details
The samples were prepared by melt spinning the alloy in an inert atmosphere. The samples, ahout 2 mm in width and about 30 Frn thick, were X-ray amorphous. The exact composition was determined by electron probe microanaly- sis. Tbe magnetisation was measured at 4 K up to 10 T.
The Af-H li~lps were also taken with a vibrating sample magnetometer with a maximum field of 2 T.
3, Resulls and discus&~
First let us discuss the results for H = 2 T. As ex- pected, the alloy magnetisation decreases with an increase in Y concentration. Fig. 1 shows the H-'j2 dependence of M fcr three compositions. The slight deviation from the linear dependence observed for the higher field region arises from the instability of antifemmagnetic coupling as we had mentioned earlier [‘I]. Table 1 shows lhe various parameters obtained from the analysis of the data using lhe models described. It is Seen that for the alloy with x = 0, the random local anisotropy constant is 0.9 X 10’
03&l-8853/95/$09.50 Q 1995 Elsevier Science B.V. All rights reserved SSDI 0304-8853(94)00889-2
35% R at d/Jcwd ofA4agnetism and h4agnetic Materials 140-W fNP5) 355-356
1
c -X=4.6
H-l/2 rice of the maguetisation M at 4 K for
@es with x = 0,4.6 and 10.
t ore hfth of what we found for similar Er concentration [8]. It that from EL IIt is interesting to note that both
Becker [9] have developed a theory which
(5)
Hers from appach to saturation at 4 K
Y H, A 4, 4 K,
(xl (~~/ @d (10-a erg/ (T) (T) (lO’erg/
Cd Cm’)
0. 453.2 17343 29.4 12.5 4.3 0.9 4.6 273.0 2412 22.3 16.25 5.7 0.8 10.0 161.2 49166 1.5.6 19.4 13.8 1.1
Table 2
Y (xl KL (10’ erg/cm31
0.00 41 0.6
4.6 124 0.9
10.0 730 1.0
where d =R,, defined earlier. We assumed R, = 1 nm, which was determined experimentally on similar alloys [lo]. We studied the M-H loops of the samples at 6 K, under fields up to 1.8 T using a vibrating sample magne- tometer. From the experimental H, values we calculated the random local anisotropy constant and the results are given in Table 2. The calculated values of KL agree fairly well with those obtained fron: the analysis above (Table
1).
Achowledgements: We acknowledge with thanks the high field measurements carried out at van der Waals- Zeeman Laboratorium and thank Mrs. Y. Dumond for the electron probe micro analysis. This work was performed under European project CEAM.
References
[l] R.W. Cochrane, R. ilarris and M.J. Zuckenuaua, Phys. Rep.
48 (1978) 1.
[2] S.G. Corn&on and D.J. Sellmeyer, Phys. Rev. I3 30 (1984) ]3] E Chudnovsky and R.A. Serota, J. Phys. C 16 (1983)
4181.
[4] E.M. Chudnovsky, W.M. Saslow and R.A. Serota, Phys.
Rev. B 33 (1986) 251.
[5] R. Hasegawa, J. Appl. Phys. 45 (197413109.
[ti] N. Heimann, K Lee, R. Potter and S. Kirkpatrick, J. Appl.
Phys. 47, (19763 2634.
[7] R. Krishnau, H. Lassri, L. Driouch, F.E. Kayzel and J.J.M.
France, J. Magu. Magu. Mater. 1310994) L 297.
181 H. Lassri and R. Krishnaa, J. Magu. Maga. Mater. U-107 (1992) 157.
[9] R. Atben and JJ. Becker, J. Appl. Phys. 49 (1978) 3.
[RI] J. Diximier, H. Lassri and R. Krishnaa, to be published.
