Magnetic behaviour of Fe80−xVxB12Si8 metallic glasses

j R journal magnetiofsm and magnetic

N

materials

ELSEVIER

Journal of Magnetism and Magnetic Materials 163 (1996) 345-348

Magnetic behaviour of Fes0_xVxB12Si8 metallic glasses H. Lassri a,*, A. Habiballah b, E.H. Sayouty b, R. Krishnan c a Laboratoire de Physique des Mat£riaux et de Micro-dlectronique, Universit£ Hassan 11, Facult£ des Sciences Ain Chock, B.P. 5366, Route d'El Jadida, Ion-8, Casablanca, Morocco b Laboratoire de Physique Nucl£aire, Universit£ Hassan I1, Facult£ des Sciences Ain Chock, B.P. 5366, Route d'El Jadida, km-8, Casablanca, Morocco c Laboratoire de Magn&isme et d'Optique de l'Universit£ de Versailles, CNRS, URA 1531, 92195 Meudon, France Received 12 December 1995; revised 8 February 1996

Abstract We have studied the magnetization of melt spun amorphous Feso_xVx B12Si 8 alloys with 0 _
1. Introduction Amorphous alloys of the transition metal-metalloid type have been the subject of intense research in various laboratories [1-3]. In particular Fe-based alloys produced by melt spinning techniques hold promise for several applications such as magnetic shields, power and electronic transformers and recording heads. Amorphous alloys present random magnetic anisotropy (RMA) due to topological disorder. The amorphous ferromagnet has no global direction of the anisotropy which randomly fluctuates from one magnetic atom to another [4]. The effect of an applied magnetic field in R M A systems was first studied by Chudnovsky et al. [4-7].

* Corresponding author.

This model enables one to analyze the magnetic behaviour of the alloys presenting RMA. We have undertaken a detailed investigation of the effect of substitution of V for Fe and results on magnetization studies of amorphous Fes0_xVxB12Si8 alloys are reported.

2. Experimental methods Amorphous Fes0_xVxB]2Si 8 ribbons with 0 < x < 15, were obtained in an inert atmosphere of Ar. The ribbon samples were about 30 txm thick with different widths varying from about 3 to 10 mm. X-ray diffraction was used to verify the amorphous structure. The exact chemical composition of the samples was determined by electron probe micro-

0304-8853/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S 0 3 0 4 - 8 8 5 3 ( 9 6 ) 0 0 3 4 3 - 5

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H. Lassri et al. / Journal of Magnetism and Magnetic Materials 163 (1996) 345-348

analysis. High-field magnetization of amorphous alloys at 4.2 K was measured in applied fields up to 150 kOe (at Service National des Champs Intenses at Grenoble). Vibration sample magnetometry was used to measure for fields up to 18 kOe. The Curie temperature Tc was determined from the evolution o f the magnetization, in a weak field, versus temperature in the range 300 to 1000 K.

~

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o 28

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3. Results a n d d i s c u s s i o n

I

I 4

I 8

I o 12

65 16

20

X(V) 3.1. M a g n e t i c s t u d i e s

Fig. 2. The V concentration dependences of JFe-Fe and A.

The technical saturation is attained for H = 10 kOe. The average magnetic m o m e n t ( / x ) of amorphous Feao_xVxB12Si8 alloys shows a monotonic decrease with increasing V concentration (Fig. 1). The Curie temperatures Tc of amorphous Fe80_xVxB12Si 8 alloys were determined from the thermomagnetic data. The values of Tc thus obtained are shown in Fig. 1 as a function of V concentration. The decrease in Tc is due to the diminishing Fe content. The strong descent o f Tc with x for Fes0_xVxB12Si s alloys can be mainly attributed to the decrease of the coordination number and the iron moment. F r o m mean field theory one can write Z C = 2ZFeFeJFe_FeSFe(SFe + I ) / 3 k B ,

JFe-Fe is

were SFe is the spin of iron,

2 1.7 t.4

[

5 0 --

• 0

t

I

(1) the exchange

f

700

580 •

460 ff

• 0 0

340 0

0.8

o

0

the coordination number. One can hence write ZFeFe = 12(80 - x ) / 1 0 0 .

(2)

The JVe-Fe values obtained are plotted as a function of x in Fig. 2. The value of JFe-Fe is about 73 × 10 -23 J for x = 0 and increases with increasing x. Therefore the decrease of iron magnetic moment (/XFe) and the increase of JF~-Fe with an increase of V content is attributable to hybridization effects and /XFe is decreased by the charge transfer phenomenon arising from B and Si [8]. The exchange constant A can be obtained from the Curie temperature using mean field theory [9,10]. a = C S F e k B T c / 4 ( 1 + SFe ) rFeFe ,

(3)

where C is the iron concentration, and the interatomic distance rFeFe is taken as 2.5 A. Knowing all the parameters we can now calculate A and one finds that A decreases from 37 × 10 .8 to 11 × 10 .8 e r g / c m when x is increased from 0 to 15 (Fig. 2). 3.2. R a n d o m a n i s o t r o p y s t u d i e s

O

.~ I.I

0.5

integral, k B is the Boltzmann constant and ZFeFe is

220

I

F

I

(

4

8

12

16

I00

20

XfV) Fig. 1. Variation of/x and Tc with V content for Fes0_xVxBa2Sis amorphous alloys.

The approach to saturation in the magnetization can be described in the following two ways according to Chudnovsky et al. For an applied field H > Hco, the field dependence is expected to follow an H -2 law, whereas when H < / / c o , the dependence is best described by an H - 1 / 2 law, where Hco = 2 A / M o R 2 is a crossover field separating the H - 1/2 and H -2 saturation regimes. The parameter R a corresponds to the distance over which the local anisot-

H. Lassri et al. / Journal of Magnetism and Magnetic Materials 163 (1996) 345-348

ropy axes are correlated (short range structural order). Usually R a is of the order of several atomic distances. In the large field regime ( H > Hco) the RMA model gives

( M o - M ) / M o = ~ ( H r / H ) a,

(4)

where, M 0 is the magnetization extrapolated value to H~ and H r is the random local anisotropy field and is related to the local anisotropy energy K L by the relation

H r = 2 K L / M o.

(5)

This magnetization law does not have any dependence on A and R a. In amorphous materials with large exchange and R a of atomic scale, the crossover field //co can be reached experimentally. The values of Hco and H r can be determined by plotting M as function of H -2 (Fig. 3). We obtained the random anisotropy field H r from the slope and from Eq. (5) we also obtained the anisotropy constant K L. The data points align well in the high field regime H > Hco. But one observes a deviation from the linear dependence in the intermediate regime. This field region is known as the crossover field. From the Hco mentioned above, it is possible to calculate the important structural parameter Ra, which is the distance over which the local anisotropy axes are correlated. From Chudnovsky's theory [7] we can write Ra

=

[ 2 A / M o H c o ] 1/2. 195

I Hco

-

(6)

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I

I

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I

I

I

60

120

180

240

300

T (K) Fig. 4. Temperature dependence of the local random anisotropy.

Hco is found to be 51 kOe and using this value of Hco in Eq. (6), we calculate R a = 10.5 A. One finds in transition metal rich alloys, generally, that R a is about 10 A. The length R a is found to be practically independent of temperature. The anisotropy directions are assumed to be randomly distributed beyond the characteristic length scale R a where atomic short range order takes place. The temperature dependence of the anisotropy constant of Fe79.sV0.sB]2Si8, presented in Fig. 4 shows reductions in K e with increasing temperature from 4 K to room temperature. The K L values of the alloys studied are larger than that of elemental Fe ( K = 4.8 × 105 e r g / c m 3 at 300 K). This fact implies that the Fe orbital moment is incompletely quenched in the alloys. It is likely that a small but appreciable orbital moment of the relatively large Fe moment of the site is mainly responsible for the Fe sublattice anisotropy [11]. For H < Hco, the RMA model gives

189 o4.2~ x 79

183 &

177

120 K 160 K ,i, 180 K

_

II

•

171

•

165

t

0

I

•

I

250t~

I

0.28 0.56 0.84 1.12 1.4

(Mo-M)/Mo=[A/(2MoR~H)]

1/2

(7)

The values of the ferromagnetic correlation length Rf can be determined by plotting M as function of H - 1 / 2 (Fig. 5). The data points align well in the low field regime H < / / c o . Experimental data show that Rf increases with increasing temperature (Fig. 6). This behaviour of Rf can be understood in terms of the temperature dependence of variables on which Rf depends according to

H-2( I0-3kOe -2) Fig. 3. Determination of the parameters //co and H r by plotting M as a function of H -2.

R f = R a / h 2, where A = (2/15)I/ZKLR2a/A.

(8)

The dimensionless parameter A plays an important

348

H. Lassri et al. / Journal of Magnetism and Magnetic Materials 163 (1996) 345-348

200

a ferromagnet system with high exchange and a weak anisotropy.

190

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H-m(kOe m ) Fig. 5. Determination of the parameters //co and Rf by plotting M as a function of H -1/2.

role in distinguishing between the cases of strong anisotropy (A > 1) and weak anisotropy (A < 1). It is found that in our alloys A < 1, which corresponds to 185 -

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I

I•

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120

180

240

172 ,< ¢d

159 146 133 120 0

300

T (K) Fig. 6. Temperature dependence of the ferromagnetic correlation length.

4. Conclusion In conclusion, we have analyzed high field magnetization curves of the series of amorphous alloys Fes0_xVxB12Si s in the framework of the Chudnovsky model. The results show several features (crossover field, length over which the local axes show a correlation, ferromagnetic correlation length) all consistent with each other and in agreement with theoretical predictions. The anisotropy studies show that these alloys are weak anisotropy ferromagnets.

References [1] K.V. Ran, Stainback, H.H. Liebermann and L. Barton, J. Appl. Phys. 53 (1982) 7795. [2] E.M.T. Velu, P. Rougier and R. Krishnan, J. Magn. Magn. Mater. 54-57 (1986) 265. [3] R. Krishnan, K. Le Dang, V.R.V. Ramanan and P. Veillet, J. Magn. Magn. Mater. 54-57 (1986) 263. [4] E.M. Chudnovsky and R.A. Serota, J. Phys. C 16 (1983) 4181. [5] E.M. Chudnovsky, W.M. Saslow and R.A. Serota, Phys. Rev. B 33 (1986) 251. [6] E.M. Chudnovsky, J. Appl. Phys. 64 (1988) 5770. [7] E.M. Chudnovsky, J. Magn. Magn. Mater. 79 (1989) 127. [8] F.E. Luborsky, in: Ferromagnetic Materials, ed. E.P. Wohlfarth, Vol. 1 (North-Holland, Amsterdam, 1980) p. 451. [9] N. Hassanain, H. Lassri, R. Krishnan and A. Berrada, J. Magn. Magn. Mater. 146 (1995) 315. [10] N. Hassanain, H. Lassri, R. Krishnan and A. Berrada, J. Magn. Magn. Mater. 146 (1995) 37. [11] M.M. Abd-E1 Aal, J. Magn. Magn. Mater. 131 (1994) 148.