Hyperfine field studies of (FeRu)90Zr10 amorphous alloys

ELSENIER

Journal of Magnetism and Magnetic Materials 140-144 (1995) 301-302

Hyperfine field studies of ( FeRu) ,Zr,, amorphous alloys

P.L. Paulose a, V. Nagarajan a**, R. Krishnan b, J. Voiron ‘, H. Lassri b, R. Nagarajan a, R. Vijayaraghavan a

’ Tata institute of Fundamental Research, ikmky-5, India

’ Laboratoire de MagnPtisme, C.N.R.S., 92195 Meudon, France

’ Laboratoire L. Niel, C.N.R.S. 166 X38042-Grenoble, France

Abstract

High field magnetization and “Fe MSssbauer spectroscopy studies in amorphous FexRugO-rZr10 are presented. The hyperfine field distribution has been interpreted in terms of rise in antiferromagnetic interactions with Ru substitution.

Analysis of high-field magnetization supports this view further.

There is a great deal of interest in the study of amor- phous Fe-Zr alloys. The Zr-rich alloys in this system superconduct, while the Fe-rich exhibit complex magnetic ordering with invar characteristics. Widely varying de- scriptions have been employed in literature to explain the observed magnetic behavior [l]. We had reported earlier the magnetic phase diagram of FexRuQO-xZr,O amorphous alloys from low-field magnetization studies and have pointed out that Ru-induced change in magnetic properties is quite different from other transition metal substitutions in Fe,Zr,, [2]. We have now carried out “7Fe ‘Mijssbauer spectroscopy studies in this system to get a deeper insight into Fe moments and their exchange interactions. Also, we present high-field magnetization data of this system.

Amorphous alloys of Fe,Ru90-sZr,0 (90 2 x 2 70) were prepared by the melt spinning technique [2]. 57Fe Ltissbauer spectroscopy was done in a constant accelera- tion spectrometer using “Co in an Rh matrix as source.

Magnetization up to 14 T was measured using an extrac- tion technique.

Fe&rlo is ferromagnetic with a Curie temperature (Tc) of 230 K. Ru substitution destroys ferromagnetism very rapidly and the system shows spin-glass-like behavior for 3 at% of Ru (Fig. 1). The spin-glass tail persists up to 20 at% of Ru (the maximum Ru in the present study).

Magnetization at 4.2 K as a function of applied magnetic field is shown in Fig. 2 for alloys with x = 93, 87 and 85, which lie in the reentrant and spin-glass like regions of the phase diagram. Saturation was not observed for any of the

l

Corresponding author. Fax: No. t91-22.21521fO; email: na- garaja@tifrvax.bitnct.

alloys and there is a very large high-field slope. We found that the approach to saturation (in the range 2-14 T) can be best described by an expression of the type:

M(H) = XhfH + M,(l - cyIP5) , (1)

where ~,,r, M, and a are constants. xhr corresponds to high-field susceptibility. The term aIf-‘.’ corresponds to some type of local anisotropy. Both ~,,r and (r are found to increase with the Ru concentration. If Ru introduces antiferromagnetic @FM) interactions in the system, one wou!d expec! !he high-field susceptibility to increase. The AFM exchange could pin the neighboring ferromagnetic spins and act as a source of tocal anisotropy. The coeff~- cient LY can be taken as a direct measure of this anisotropy and hence the monotonic increase of it with the Ru con- centration.

The Miissbauer spectra of the alloys at 4.2 K is shown in Fig. 3. All concentrations including the spin glass ones show hyperfine split spectra. The broad spectra were anal-

Fig. 1. Magnetic phase diagram of (FeRu),Zr,, and average Fe Ehr-

0304-8853/!X/$O9.50 Q 1995 Elcevier Scknce B.V. All rights reserved

SSDI 0304-8853(94)01335-7

302 P.L. Paulose et 01. /Journal of Magnetism and Magnetic Malerials 140-144 (1995) 301-302

0 2 4 6 8 10 12 14

Applied Field (T)

Fig. 2. High-field magnetization at 4.2 K. Continuous lines are fit to Eq. (1) in text.

ysed using a modified Window method 131. Typical hyper- fine field distributions for alloys with x = 90, 89, 85, 75 and 70 are showr! in Fig. 4. A bimodal type distribution is clearly observable. The peak at high-field loses in intensity and shifts to lower fields with increasing Ru concentration (indicated by dotted lines in Fig. 4). However, the low-field peak exhibits a different behavior; it shifts to higher fields initially before shifting to lower fields (shown by dashed tines in Fig. 4) while gaining in intensity monotonically.

Though dilution of the Fe moment by Ru can shift the peak to lower fields, we observe that the nonmonotonous variation of the low-field peak position with the Fe con- centration correlates well with the spin freezing tempera- tures in the phase diagram. This strongly suggests that the low-field peak arises from Fe spins experiencing AFM exchange. The average H,, as a function of the Fe concern tradon is shown in Fig. 1. We find that average Hhf varies linearly with X, but the slope changes in the vicinity of the multicritical point (MCP) in the phase diagram. It is not

1.00 0.9a 0.96

43 -4 Cl 4 0

Velocity -(mm/s) Fig. 3. “Fe Miissbauer spectra at 4.2 K.

"0 10 20 30 40

Hyperfine Field (T) Fig. 4. Fe byperfine tieid distribution at 4.2 K.

clear whether this behavior implies a change of short range order in passing from the reentrant to the spin glass like state. The monotonous decrease in H,, clearly points to dilution of the Fe moment with Ru. However, there is a sizable Fe moment c:en at 20 at% Ru substitution (very much away from the MCP) as evidenced by the large Fe hyperfiie field.

The saturation moment ( ~~1 was obtained from extrap olation of magnetization to l/H = 0. The scaling of aver- age H,, with K gives a value of 15 T/p., for x = 90,89 and 87, in agreement with the value quoted for the Fe-Zr system [l]. But this ratio increases rapidly (> 40 T//.+, for x = 70) as the Ru concentration increases. Clearly the ps obtained from the high-field extrapolation does not reflect the actual moment on Fe at high Ru concentrations.

This again confirms the existence of antiferromagnetic interactions in (FeRu),Zr,, that can lead to canting of moments. A detailed study of H,, as a function of temper- ature in Fe,,RulZr,, shows evidence for a canting transi- tion below T, [4].

Acknowledgement: We thank SK. Paghdar for his help in Miissbauer measurements.

References

[ll D.H. Ryan, J.M.D. Coey, E. Batalla, Z, Altounian and J.0, Sttim-Olsen, Phys. Rev. B 35 (1987) 8630, and references therein-

[2] V. Nagarajan, P.L. Paulose and R. Vijayaraghavan, J. de Phys.

49 (1988) cs-1135.

[3] B. Window, J. Phys. E 4 (1971) 401.

t4] V. Nagarajan and P.L. Paulose, to be published.