Hyperfine Interactions 27 (1986) 353-356 353

MOSSBAUER STUDIES OF NiToFeloSisB12 METGLASS : REENTRANT PROPERTIES GIRISH CHANDRA*, J.L. DORMANN, R. KRISHNAN and P. RENAUDIN

Laboratoire de Magn~tisme, CNRS, 92195 Meudon Principal Cedex, France

Amorphous metal FeloNi7oSi8B]2 has been studied by Moss- bauer spectroscopy without and with an weak applied field in the plane of ribbons. Reentrant properties have been confirmed with the existence of a canted state at low tem- perature. Studies with different values of the applied field have shown that a field near 50 Oe is sufficient to saturate the average direction of the ferromagnetism.

I. INTRODUCTION

Numerous amorphous metals show reentrant spin glass (RSG) behaviours : (MTxMT'I_x) 75PI6B6AI3 where MT = Fe or Co, MT' = Mn or

Ni /i/, FexNi80_xPl4B 6 /2/ and FexNi78_xSi9B13 /3/. However the na-

ture of the low temperature state remains unclear. The neutron scat- tering studies on (FexMnI_x~5PI6B6AI 3 /4/ have not shown the coexis-

tence of ferromagnetic (FM) and spin glass (SG) orders at low tempe- rature in disagreement with the Gabay-Toulouse Model. On the contra- ry, M6ssbauer studies on FexNi78_xSi9B13 /3/ have shown the existence

of canted state, i.e. a superposition of FM and SG ordering, like in FexAUl-x /5/.

In this context we have studied the amorphous metal FexNi80_xSi8Bl2 for x = 10.4 by MSssbauer spectroscopy without and

with weak field applied in the plane of ribbons. In this geometry, the demagnetizing field is very small or zero.

2. EXPERIMENTAL

Amorphous ribbons were prepared by the usual melt-spinning method /6/. M~ssbauer measurements have been performed with a con- ventional spectrometer in constant acceleration mode. 57Co in rhodium sources were used.

3. RESULTS AND DISCUSSION

M6ssbauer spectra were taken in the temperature range 1.4 K up to 300 K. Typical spectra without applied field are shown in the figure I. At low temperature the spectra show resolved six-line

~On leave from Tata Inst. of Fundamental Research, Bombay, 400005, India.

�9 J.C. Baltzer A.G., Scientific Publishin~ Company

354 Girish Chandra, et al., MOssbauer studies of NiToFesoSisB#2 metglass

AS,,. :.,.,~.,/,, ~,, < ~ i 331221 ~ . . , "' 2 ~ S " ' "*1'5"K"""'1 .....

I o ;, V I I I I I I ~._

Fig. i. Thermal variation of the M6ssbauer

spectra of Ni7OFeloS8BI2

magnetic hyperfine patterns but the spectral lines remain broad and assymetric as typical for similar magnetic amorphous systems. The method used (le Caer and Dubois) to analyse the spectra is descri- bed in ref. /7/. The intensities of the line 2 and 5 are approxima- tively twice weaker than usual value obtained from a random distri- bution of spins. They are nearly temperature independent. These fea- tures indicate a weak preferential orientation normal to the ribbon plane. The temperature variation of the average hyperfine field < H > and the half distribution width at half height AH are shown in the figure 2. We observe a weak decrease of AH with tempe- rature.Below IOO K, thermal variation of < H > is almost linear, that is quite unusual. This variation is similar to those observed in FexNi78_xSi9B13 for x = 5 and 6 /3/ and seems to indicate a

different behaviour of the local magnetic moment at high and low temperatures but the transition is not as sharp as for example was found for Au-Fe /5/. The smooth behaviours are confirmed by the low value of the reentrant temperature TRS G = 7 K determined by a.c.

Girish Chandra, et al., MOssbauer studies of Ni7oFeloSisB12 metglass 355

(H> (kOe}

100 "-t,,,, AH '~

50 100 T(K) \ I I i

Fig. 2. Thermal variation of the average hyperfine field < H > and the half distribution width at half height AH

susceptibility at a frequency v = 24 Hz /8/. It is well known that the S.G. properties are very sensitive

to applied field. To check the canted state by MSssbauer spectrosco- py, it seems more useful to use an applied field in the ribbon pl~ne. In this case, the demagnetizing field being zero the internal field is exactly known, equal to the applied field. The X measurements performed On Fe Ni Si B have shown an importan~'~&nsibility

x 80-x 8 12 to the applied field and the frequency /8/. Hence, it is better to use only weak field. We have performed M~ssbauer spectroscopy with different external magnetic fields in the ribbon plane.

% U') V �9 12

�9 2 0

II

§ ! T

100 200 H (Oe) I I

Fig. 3. Thermal variation of < sin2@ > with the applied field (see text).

356 Girish Chandra, et al., MOssbauer studies of NiToFeloSisB ~2 metglass

Unfortunatly, the maximum field available with our experiment was 300 Oe.

To eliminate the weak preferential orientation, the sample was annealed. The values of < sin20 > have been deduced of the intensi- ties of the line 2 and 5 and are represented in figure 3. @ is the angle between the directions of the spins and the y-rays. The ap- plied field direction is perpendicular to the y-ray direction, then sin2@ = 1 if the spin and field directions are aligned. For T > 12 K. a field of 150 Oe is sufficient to saturate the ferromagnetic system. At T = 4.2 K, with increasing the field, we observe a partial rota- tion of the magnetic moments in direction of the external field, but after 50 Oe, the rotation slightly evolves leading a < sin20 > value lesser than One. These results are a clear evidence of the existence of canted state at low temperature. In a first step, with increasing the field, the average direction of the ferromagnetism falls into the applied field direction. Spin directions seem to be unchanged by further increasing applied field. To destroy the canting high applied fields are necessary. Further experiments are necessary to define TRS G with accuracy. Presently the value deduced of our M~ssbauer experiments is near i0 • 2 K.

4. CONCLUSION

In this study, we have confirmed the reentrant properties of amorphous metal of FexNiy_xXy with y = 20 (X = metalloid) for cer-

tain x values. We have shown the existence of a canted state at low temperature, the average direction of ferromagnetism being satu- rated with a field near 50 Oe in the case of FelONi7oSi8BI2.

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