Magnetization studies of amorphous Fe80 − xTmxB20 alloys

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ELSEVIER Journal of Magnetism and Magnetic Materials 153 (1996) 132-134

~ 1 ~ Journal of magnetism ,444 and

magnutlc materials

Magnetization studies of amorphous Fes0_xTm xB20 alloys

M. Slimani a,b, M. Hamedoun ^{a ,} H. Lassri c, S. Sayouri ^{a, * ,}R. Krishnan d

a Laboratoire de Physique du Solide, D~partement de Physique, Facult~ des Sciences, B.P. 1796, Fbs Atlas, Morocco b Laboratoire de Chimie du Solide E.N.S., Ben souda F~s, Morocco

c Laboratoire de Physique des Mat~riaux et de Micro~lectronique, Facult~ des Sciences, Ain chok, Casablanca, Morocco a Laboratoire de Magn~tisme et Mat~riaux Magn~tiques, C.N.R.S., 1 Place A. Briand, F-92195 Meudon, France

Received 21 December 1994; revised 21 July 1995

Abstract

We have carried out magnetic studies at fields up to 18 kOe of melt- spun amorphous Fes0_xTmxB20 alloys.

Magnetization and Curie temperatures were investigated. We have extracted the anisotropy constant from the coercivity.

1. Introduction

In the last few years, studies of amorphous alloys based on rare earth metals have become very intense [1-4] because of their potentially useful physical properties in technological applications, and the fun- damentally very important random magnetic anisotropy and magnetic exchange. In order to study the influence of the addition of Tm on the various magnetic properties of amorphous F e - B alloys, we prepared some amorphous alloys in the system Fes0_xTmxB20 (0 < x < 15).

2. Experimental

The amorphous Fes0_xTmxB20 alloys (0 < x <

15) in the form of ribbons were prepared by the usual melt-spinning technique in an inert argon at- mosphere. The amorphous structure was charac- terised by X-ray diffraction using Co K s radiation.

The compositions of the alloys were determined

* Corresponding author.

by electron probe analysis. The magnetic moment was measured using a vibrating sample magnetome- ter (VSM) with a maximum applied field of 18 kOe, and in the temperature range 4 - 3 0 0 K. The Curie temperature T~ was determined from the evolution of the magnetic moment in a weak field (100 Oe) and in the temperature range 300 < T < 800 K.

3. Results and discussion

For all the samples studied, magnetic saturation could be obtained with H < 1.8 T at all temperatures. Fig. 1 shows some typical results at 10 K.

At the same temperature, 10 K, the magnetic moment of the alloy, /Za, decreases with increasing Tm concentration, indicating the antiparallel cou- pling between the Fe and Tm moments (Fig. 2). An extrapolation shows that at x = 20 a magnetic com- pensation would occur.

The magnetic moment of Tm (/ZTm) was calculated from the alloy moment as follows [5]. The alloy

m o m e n t ]£a can be written as

SSDI 0304-8853(95)005 18-8

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M. Slimani et al. / Journal of Magnetism and Magnetic Materials 153 (1996) 132-134 133

140 x=4 :

120

100

7r60 . . = .

40 _ : _= x=16

2O 0 } L @ ~ lb I} ~, ~5 18

H ( k ~ )

Fig. 1. Field dependence of the magnetization M in F e - T m - B alloys at 10 K for different concentrations.

For low Tm concentrations ( x < 4), the Fe moment is not perturbed. So taking the value of/xr~ ⁼ 2.06/z B obtained from the alloy with x = 0, and substituting in Eq. (1), it is possible to determine /~Tm for x = 4.

The calculated moment at 10 K is found to be 6.2/z B, which is smaller than the theoretical value of 7/z B, suggesting a conical spin structure of Tm. This phenomenon is the result of the strong random anisotropy o f Tm and the antiferromagnetic JFe-Tm

interaction which normally lead to a sperimagnetic structure [6]. N o w assuming /"['Tm = 6"2/XB for other compositions, the calculated /ZFe is found to be practically the same, namely ( 2 . 0 4 _ 0.02)/zB, and hence independent of Tm concentration in the range x = 0 - 1 5 . This consistency can be explained by the small hybridization effects arising from Tm. //'Fe is

80(:

60C

,_u 2O0

, ¥

~, ~ ~ 1'0 1~ i~

X (%)

5o

3o~

%

16 10

Fig. 3. Tm concentration dependences of the critical temperature T c and the exchange constant A at I0 K.

reduced by the charge transfer phenomenon arising from B, as has been observed in F e - B alloys.

The variation o f the Curie temperature T~ with x is also somewhat similar to that of the magnetic moment (Fig. 3). The decrease in T c could be caused by the weakening of F e - F e interaction and the re- sults are characteristic of an antiferromagnetic inter- action between Tm and Fe atoms which is well known.

Alben and Becker [7] have developed a theory which relates the coercivity to the fundamental pa- rameters and one can write

1 K4oc d 6

He 20 A3Ms '

where d is the length over which the local axes show a correlation. We have assumed d = 1 nm, which is normally used in the literature [8]. The exchange constant A can be obtained from the mean field model proposed by Hasegawa [9] and from the Curie

, 1 0 K

t5 • 300 IK

)

~.~

Oq 5 ~0 15 20 25 30

x¢1,)

Fig. 2. Tm concentration dependence of the alloy magnetic mo- ment /z a at 10 and 3 0 0 K.

tOO(

sO(

6O(

20(

2 2 6 8 /O 12 14 ~6

X (%)

Fig. 4. Concentration dependence of the coercivity H c at 10 K.

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134 M. Slirnani et al./Journal of Magnetism and Magnetic Materials 153 (1996) 132-134

temperature using the relation proposed by Heiman et al. [10]. We have the following relation:

A = CSFe K B T c / 4 ( S F e + 1)rFe_Fe,

where C ( = 8 0 - x) is the concentration of Fe in at%, SFe is the spin of Fe and rze_Fe is the average distance between the Fe atoms.

We found that the exchange constant A decreases from 37 × 10 -8 to 17 × 10 -8 when x is increased from 0 to 15.

From the experimental H e and M S values (Fig. 4) we have calculated the random local anisotropy constant KIo~; the results are given in Table 1. The local magnetic anisotropy in amorphous transition metal alloys was studied by a perturbation treatment on the basis of a semi-empirical self-consistent Hartree- Fock calculation [11]. The found value of Klo c is of the order of 107 e r g / c m 3. Recently, we have studied

Table 1

A M s Kio c

(10-8 erg/cm) (emu/g) (106 erg/cm 3)

0 36.97 190 7.97

4 32.67 140.43 8.29

8 26.12 99.27 9.29

12 20.62 57.78 11.82

15 17.30 36.92 12.61

the magnetic properties of amorphous F e 7 2 _ x Y x -

HosB20 alloys [12]. Using Chudnovsky's model, we have determined Klo c (assuming d = 1 nm). The calculated values agree fairly well with those obtained from the experimental H c values using the model of Alben and Becker [7].

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