High field magnetization measurements in FexSi1−x amorphous alloys

Solid State Communications, Vol. 25, pp. 555—559, 1978. Pergainon Press. Printed in Great Britain

HIGH FIELD MAGNETIZATION MEASUREMENTS IN FExSI1_x AMORPHOUS ALLOYS D. Bloch

Laboratoire de Magnétisire - CNRS - B. P. 166 38042 - GRENOBLE-CEDEX (Franth) Ph. Mangin, G. Marchal and Chr. Janot Laboratoire de Physique du Solide Uñiversit~ de Nancy - I - C. 0. no 140 54037 - NANCY-CEDEX (France)

(Received 21 november 1977 by E.F. Bertaut) Magnetization in Fe~Si

1_~amorphous alloys was measured at high magnetic field up to 150 k0e. The experimental data sh~ the existence of critical conditions for the occurence of magnetic order whose first stage is the appearance of giant moments in the 0.35 < x <0.50 composition range. At larger iron concentration (x 0.5) a fdrronagnet behaviour arises from a percolation process.

L

IntroductIon

50 mg of amorphous alloys. High magnetic fields up to 150 k0e were available thanks to a water couled copper coil in “La Service National des (lamps Intenses” in Grenoble (France). Experi-

In a previous paper 1 were reported ~gnetization measurements carried out in metic fields up to 20 k0e on Fex Si1~ amorphous

mental data were corrected for diamagnetism from kapta-i foil and plexiglass holder. The applied field was always m~ichlarger than the demagnetization field. Typical magnetization curves are presented on figure 1 and 2. Arrott plots (72 = f (Him) given on figure 3 for various alloy composition, deviate strongly from the parallel linear behaviour that could be expected 8,from except homogeneous perhaps at weak high itinerant iron concentraferrotion (x > 0.6). magnet Near the critical composition (x 0.35 or x 0J40), the high field surceptibility is not strongly temperature dependent and a nonmagnetic contribution x H can be extracted from o. The resulting data a - x H (fig. 4) can be fitted by Brillouin functions only after subtraction of a so—called “impurity” contribution a~which is slightly temperature dependant, saturates under weak magnetic field (H < S k0e) and is roughly the same whatever the alloy composition ; a~has been attributed to magnetic rich iron clusters associated with elaboration and corresponds to less than 3 ~ atoms. The remaining contribution f (H/T) a - x H can be analysed in term of giant moments 7 4B for x 0.35 and 15 UB for x 0.4. For alloy composition corresponding to x near or larger than 0.5, 0 144 emuig. Fe and then, at least 20 % atoms are in a magnetic state. As a consequence most of the iron atoms are not located in isolated giant moments any more but in large clusters which result from percolation. This is consistent with ~5ssbauer spectroscopy data 5 giving evidence of a hyperfine field on some of the iron atoms only for x> 0.5. Moreover, as pictured on figure 5 the quantity Am o(50 k0e) - a(5 k0e) has a maximum value in the 0.’+S < x < 0.5 composition range which, consequently, corresponds to the state of the largest smell cluster nuither.

alloys, corresponding to a large composition range (0.3 < x < 0.7). The main feature was the evidence of a critical composition for the occur-ence of magnetic coupling between iron atons near x 0.4, and the necessity of performing measurements at hi~er field to separate localized and itinerant effects. This is the purpose of the present paper and information collected here will be similar to the one conNi~ Pi~xother 3 andamorphous Nix (P B)i-x must be 2, cerning systems4. : ItFex Gei_x pointed out that the appearance of ferronagnetism in these systems cannot be described in terms of a unic model : (1) in Fe~Gei_x is weak itineraHt like with rather strong superposed inhomogeneity effects 2, (ii) in Nix Pi-x , x 0.82 is a critical composition, with the existence of magnetic clusters when x < 0.82 and weak itinerant ferromagnetism when x > 0.82 3. (iii) x ~ 0.75 is a critical concentration for the occurence of giant moments in Ni~ (P B)i_~. For x> 0.75, sons Ni atoms have only Ni nearest neighbours which are coupled together in 5 to 7 p B magnetic moments. The Fex Silx and Fe~Gel-x amorphous alloys can be obtained in a large composition range by the vapour quenching technique. This is not the case for Nix ~1-x or Nix (P B)i_x which are detained by electrodeposition. Moreover, magnetization measurements in Fex Sil-x amorphous alloys can be fruitfully compared to data collected in crystallized material (table I) and to I’fôssbauer spectroscopy results 5.

2. ExperImental Results The samples were made of iron and silicon deposited on kapton foils ~ and then rolled to obtain small cylinders (6 nnn long and 2 to 4 mm diameter) that were put in plexiglass spedmen holders ; each sample contained about 10 to

555

556

MAGNETIZATION MEASUREMENT IN FeSi

1~ALLOYS

C [emu /gFe]

a [emu/gFe]

Feo.5Sio.5

Vol. 25, No. 8

Feo.55Sio.45 42K

~

4O~~~~99K — - —

—

1 3 7K

—

:

: —

H(kOe)

10 0

50

100

Fig. 1

2

A2~

50

Feo.4Sio.6 50

H[kOe]

100 Fig. ~.C

150

100

150

Fig. lB Typical magnetization curves for some Fe~ Si1~ alloys.

- — 4o,~—~

—

0

H(kOe)

3C 0

150

Fig. 1A 12 a {emu/gFe]

7 8K

~

X ~o.aO

~

x=O.45

~

::::

20 ~ 50

Fig. 2

100

Magnetization data at T Fe~ Sii_x alloys.

150

H(kOe)

4.2 K for

Vol.

25, No. 8

MAGNETIZATION MEASUREMENT IN FeSi

1~ALLOYS

557 2

Feo.4sSio.55

02 1~uem/9F.]2

1300

Feo.sSio.s

02 [uemigFe]

3000

4.2K 4.2K

900 2OO~

(/.76K 2 5.7K

57.5K 4.3K

~

.118K

99K

55K 191.5K

137K 181K

233K

50 0

1000

8 3.5K

100

— H

~

I

0

I

5

224.5K K

[kO. ______ i uen~F.j

[uemig~e]

0

10 0

Fig. 3A

1

2 Fig. 33C

4

6

02{uem/gFe]2 4 2K

eo.65Sio.35

02 [uem/gFeJ2

F

4.2K

17500 3000

90.5K

1 5 OK

~‘~~iiiiiiiiiii”— i_ 1000C

2000

2 46K 1 9 8.5K

7500

-

Feo.e Sio.4 H

0

I

[

kOe 1 uem/gFe

j

______________________________________ 1

Fig. 3B

50000

Fig. 3 : Arrot from xplots = 0.145for to composition x = 0.65. ranging

I

1

Fig. 3D

H I 0

r uem/gFe kOe

2

J,

558

MAGNETIZATION MEASUREMENT IN Fe Sil_~ALL0YS

Vol. 25, No. 8

x

Feo.35Sio.65

[.mu/QF.]

Xii

~a-

6 4.2 K

:

~

4

35.5K

2

2 •~K

—

— ~ _________________________ •. Up

~ —~16O~~ S244K

0.4

Fig. 5

1

0.5

0.7 x

0.6

moments. Variation of contribution from giant

—

40

(emulg Fe

)

1H[kO~j 0

50

10

a-

100

150

XH[ernu/gFe]

Feo.4Sio.6

30

20 76K

9~.2K

10

_

b a

112K

55K

0.3

0.4

,x

0. 5

1 0

Fig. 14

~

50

I

1 00

—.284.5K

150

Magnetization near the critical concentration after extraction of the non-magnetic contribution,

3.

conciusion

In Fex Si~~amorphous alloys, magnetic coupling occurs in the 0.35 < x < 0.5 composition range and corresponds tc the appearance of giant moments whose percolation at higher iron concentration (x > 0.5) results in a ferromagnetic behaviour.

Fig. 6 : Comparison of experimental data (a) to randem model prediction (b) Using a Jaccarino-Walker model 9 and a1° Dense Random of hard structure in which ironpacking and silicon are sphere statistically distributed among the 12 nearest neighbours of a given iron atom, magnetism would occur for iron atoms having at least n~ 6 iron atoms out of 12 n n (fig. 6), with a magnetic moment of about 1 PB• This 1 ~B value is justified by the evolution of the magnetic moment on iron atonE in crystallized compounds as pictured on fig. 7 which shc~s that this magnetic moment essentially results from a molecular field due to nearest neighbours. The appearance of a 1 PB magnetic moment corresponds to the cr1-

Vol. 25, No. 8

MAGNETIZATION MEASUREMENT IN FexSiLxALLOYS

I~( ~1~)

tical molecular field H~. Hc:wever, the hypothesis of a perfectly disordered structure can be questioned. Indeed in crystallized Fe-Si systems, silicon atoms are always prevented from being nearest neighbours and the situation is similar in Co~P1-x amorphous material 11, Conversely, the very different magnetic behaviours of amorphous and crystallized Feo•uSio•u compounds along with the existence of amorphous alloys

~ ~s Fe

1

2

Fe1

~ /

(Fe5 S i3)/

/

1 (F e3S I)

,.—

0 Fe~ 1

I/

in a large concentration range favours the random model. As the experimental data shc~that the magnetization near the critical condition is

Fe..~,,

smaller than expected from the random model, it might be necessary to introduce a partial chemical order parameter or a slight difference be~een Si - Fe and Fe - Fe atomic distances in nearest neighbour shell.

I HmpI Hmolcr Fig. 7

559

Magnetic moment on iron atoms in function of the molecular field.

References 1. NARCHAL G., MANGIN Ph., PIECUCH M., JANOT Chr. and HUBSCH J., Journal of Physics F, 7, L 165 (1977). 2. ENDOH Y., YAMADA K., BEILLE J., BLOCH D., ENDO H., TAMURA K., and FUKTJHINA J., Solid State Con2mmications, 18, 735 (1976). 3. BERRADAA., GAIJFIER F., LAPTIERRE M. F., B. LOEGEL B., PANISSOD P. and ROBERT C., Solid State Convmszications, 21, 671 (1977). 4. AMAMOU A. and DURAND J., Communications on Physics, 1, 191 (1976). 5. MARCHAL G., MANGIN Ph., PIECUCH M. and JANOT Qir., Journal de Physique, C 6, 763 (1976). 6. SHINJO T., NAKANURA Y., SHIKAZONO N., Journal of Physical Society of Japan, 18, 797 (1963). 7. PAOLETTI A. and PASSARI R., Nuovo cirnento, 32, 114149 (19614). 8. ELWARDS D. M. and WOLFARTH E. P., Proceedings of the Royal Society of London, A 303, 127 (1968). 9. JACCARINO V. and WALKER L. R., Physical Review Letters, 15, 258 (1965). 10. MANGB~Ph., MARCHAL G., ROLMACQ B. and JANOT Chr., Philosophical Magazine (accepted). 11. SADOCJ. F., These, Orsay (1976).