Journal of hidgnetism and Magnetic Materials 140-144 @95) 337-338
ELSEVIER
Random anisotropy studies in amorphous Co-Tb ribbons
N. Hassanain a, A. Berrada ai * , H. Lassri ‘, R. Krishnan b
a Laboratoire de Physique des Mat&iawr, Facult6 des Sciences, Rabat, Morocco b Labororoire de MagntQsme et Matt%aur MagtGtiques, CNRS, 92195 Me&n, France
’ Laboratoire de Physique des Mart%aux et de Micrklectronique, Facult6 dcs Sciences, Ain Chok, Casablanca, Morocco Abstract
Amorphous Co,-Jh, ribbons with x = 0.45,0.55 and 0.65 have been prepared by melt spinning technique. Magnetiza- tion measurements were carried out at 4.2 K, under magnetic fields up to 150 kOe. The Co moment is found to be very small and the Th moment is 7.15~~ at 4.2 K, which iudicates a speromagnetic spin structure. Using Chudnovsky’s theory we have extracted some fundamental magnetic parameters.
1. Introduction
This ~LFS’ Gkussc5 iile na~urr of rhe magnenc order present at low temperature in rare-earth-transition-metal
@E-TM) amorphous alloys. This type of alloys shows noncollinear magnetic structure, which is caused by the competition between exchange interactions and by the large local random crystal fields acting on RE magnetic ions [l].
Rare-earth metals with spin-orbit moment are known to give rise to large random magnetic anisotropy (RMAI in amorphous alloys [2] and some models describe this phe- nomenon well [3]. These models allow us to analyze the magnetic behaviour of the alloys presenting RMA and have been used by several authors to interpret their results [4,S]. The magnetic structure depends on the parameter A, =KL(R,)2/A, where R, is the spatial correlation of the easy axes, K, is the local anisotropy constant and A the exchange constant. The RMA field H, is expressed as
H, = 2&./M,, (1)
where M,, is the saturation magnetization.
The crossover field H, can be written as
H,=2A/(M@f). 12)
It is possible to calculate A by combining the models proposed by Hasegawa [6] and Heiman et al. [7]. Due to
the lack of space we only give the relation [S]
x/,vc
A = 4(& + I)$.-, ’ (3)
* Corresponding author.
where rTb-.,,, is the inter atomic distance and is - 3.5 % J, the total angular momentum of Tb, x the concentra- tlon and 1, is (he Curie temperature.
2. Experimental details
The amorphous Tb,Co,-, alloys with x = 0.45, 0.55 and 0.65 were prepared by melt spinniig technique. The amorphous state was checked by X-ray diff+action. The magnetization was measured at 4.2 K under applied fields up 150 kOe.
3. Results and discussion
The field dependence of the magnetization at 4.2 K for different alloys was similar, and Fig. I shows the results
for the alloy with x = 0.65. It can be seen that even at 150 kOe, the saturation is not yet ccmplete. So the saturated magnetic moment M, was caklilated at H,, using Hm2
sb 1Oa
H(kOc) l&l
Fig. 1. The field dependence of magnetization a1 4.2 K for the alloy wl.,5’lb,.,,~
0304~8853/9S/$O9.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0304-8853(94)00902-3
338 N. Hossanain et al./Jaurnul of Magnetism andMagnetic Materials 140-144 @95~ 337-338 dependence. The magnetization of the alloy with x = 0.65
can be considered to arise only from the lb atoms. Under
_. -
mts assumprion, with an experimeniai vaiue of the ahoy moment or 4.+n, we find a Th moment p?a = 7.15pn which is smaller than 9pn, theoretically expected for the ground state. This indicates that the Th spin structure is not collinear. Knowing all parameters in - Eq. (31, A was calculated and is found to decrease from 6.8 X 1o-a to 4.9 x lo-* erg cm-’ when x increased from 0.45 to 0.65.
The approach to saturation in the magnetization can be described in two ways. For applied ftelds H> I&,, the field dependence follows H -‘. In the field regime H <
H,, the approach to saturation follows IT-‘.‘, where one can wtite
SM MO-M
-=
J%
-+
MO (4)
where the saturation field
Hs = HP/HA. (5)
However with some algebraic manipulation, as we de- scribe below, it is possible to take into account H, (coher- ent anisotropy field) and obtain
Eq. (4) can be rewritten as
not only H, but also
= -&H+H,) =B(H+H,), (6)
where 3 = 225/H,. So by plotting (SM/M,J2 as func- tion of H, one can obtain H, from the slope of B, and H, from the intercept [8,9]. Fig. 2 shows such as a plot for x = 0.65. The data points align well for this field regime.
But one observes a deviation from the linear dependence in the intermediate regime. This field regime is known as the crossover field H,, which is found to be close to 70 kOe. Using this value in Eq. (2), we calculated R, = 2.8 A.
The coherent anisotropy field H, is found to be about 3 kOe. This field can transform the correlated speromagnetic
Fig. 2.
04 I * . I I
0 a 10 60 w IW
H (kOe) Tke H dependence of
co0.3STb0.6S at 4.2 K
the w/M,)-2 for the
state into a ferromagnetic domain structure. Finally know- ing H, and H, and using Eq. (5), H, was calcutated.
From H,, with the help of Eq, (11, K, was calculated to be 8.2 X 10’ erg cm-4
Acknowledgement: We gratefully acknowledge high field magnetization measurements performed at Service National des Champs Intenses, CNRS, Grenoble.
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