MÖSSBAUER STUDY OF DILUTE GOLD-IRON ALLOYS IN SPIN GLASS REGIME

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MÖSSBAUER STUDY OF DILUTE GOLD-IRON

ALLOYS IN SPIN GLASS REGIME

G. Chandra, J. Ray

To cite this version:

G. Chandra,

J. Ray.

MÖSSBAUER STUDY OF DILUTE GOLD-IRON ALLOYS IN

JOURNAL DE PHYSIQUE Colloque C6, supplément au n° 8, Tome 39, août 1978, page C6-914

MOSSBAUER STUDY OF DILUTE GOLD-IRON ALLOYS IN SPIN GLASS REGIME

G. Chandra and J . Ray

Tata Institute of Fundamental Research, Homi Bhabha Road, Bombay - 400 005, India

Résumé.- Nous étudions ici le spectre d'absorption Mossbauer des alliages AuFe dont la concentration en " F e varie de 0,49 à 0,76 at.% à 4,2 K dans des champs magnétiques extérieurs allant jusqu'à 60 KOe. La distribution des champs magnétiques hyperfins dans ces alliages a été obtenue en utili-sant la distribution Gaussienne des interactions d'échange, qui a été établie dans la théorie des verres de spin.

Abstract.- The Mossbauer absorption spectra of AuFe alloys with 57Fe concentration of 0.49 and 0.76 at.% at 4.2 K in external magnetic fields upto 60 KOe are reported. The distribution of magne-tic hyperfine fields in these alloys have been obtained by using Gaussian distribution of exchange interactions assumed in the theory of spin glasses.

1. INTRODUCTION.- The properties of spin glasses ha-ve been inha-vestigated intensiha-vely in recent years both experimentally and theoretically /l/. Number of Mossbauer spectroscopic studies /2,3/ of AuFe alloys have supported the concept of spin freezing in ran-dom orientations at the temperatures determined from low field ac magnetic susceptibility measurements /4/. The spin freezing temperature increases from 5 K to 28 K as Fe concentration increases from 0.5 to 8 at.%. Number of theoretical formalisms have been proposed /1,5/ to understand this new magnetic phase. The RKKY type of interaction between randomly dis-tributed magnetic spins results in a distribution of exchange interactions. One of the theories of spin glasses /6/ assumes a Gaussian distribution of ex-change interaction, the simple and the most random function possible. This should result in a similar distribution of magnetic hyperfine fields at the impurity nuclei.

2. RESULTS.- The Mossbauer absorption spectra of AuFe alloys with Fe concentration of 0.49 and 0.76 at.% were obtained at 4.2 K against a 50 mCi source of 5^Co in a Rh matrix. The. spectra were obtained with transverse external magnetic fields upto 60K0e. The alloy foils were 10 urn thick and cold rolled. No heat treatment was given. The spin glass transition temperature of these alloys are expected to be abo-ve 6 K. The obserabo-ved spectra are shown in figure 1 and figure 2. The spectra are least-squares fitted with a Gaussian distribution of magnetic hyperfine fields. The zero velocity in these spectra is defi-ned with respect to the centre of gravity of a pure

iron absorption spectrum against the same source.

C T H „ , | l % '.' 0 762 300 0

/

Fig. 1 : Mossbauer absorption spectra of AuFe alloy with 0.762 at.% Fe at 4.2 K'and 300 K. C : Fe con-centration, T : temperature (K), H : externally applied magnetic field (kG) e x

The Lorenzian shape and line width of 0.38 mm/s ob-tained from room temperature spectrum were used for the analysis. The x2 lies between 300 and 400 for 256 data points. Due to scatter of data points, no attempts to obtain a distribution of isomer shift and electric quadrupole interaction was made. The

result of the analysis is shown in figure 3 and the

tized.

mean hyperfine fields are shown in table I.

The Gaussian distribution of magnetic hyper-

fine field indicates a similar distribution of ex-

-k.ww-.

,

change interactions experienced by randomly distri-

:

+,.*

*.L*-

. .

.,.T

.-

buted Fe spins as both are proportional to thermal

10.2%

q

0.104 4.2 0

_{average of spins. This supports the assumptions of}

I

-.

Gaussian distribution of exchange field in various

theories of spin glasses 161.

-. -. . . . -a k *,

-Table I

The mean hyperfine fields at 4.2 K for different

iron concentrations in different external magnetic

fields.

Fe conc.

External magnetic

Mean hyperfine

(at

field

field

(kG)

(kG)

VELOCITY (mrns/s J

0.762

60

184

Fig.

absorption

alloy

Acknowledgement.- One of the authors (GC) wishes to

with 0.494 at.%Fe. Symbols are the same as

acknowledge the hospitality of MEssbauer Group,

in figure 1

.

AERE, Harwell, U.K. where these measurements were

carried out.

References

004

-

/I/ Fischer, K.H., Physica

86-88

B

C, Part I1

I

...

(1977) 813

a

,A

A

;"?,

/2/ Window, B., Phys. Rev.

(1972) 2013

/3/ Violet, C.E. and Borg, R.J., Phys. Rev.

149

o m ( 0 ) (1

966) 540

O 140 200 t60 1 x 1 190 250 120 8 0 240

141 Cannella,.V. and Mydosh, J.A., Phys. Rev.

H ( ~ G I

(1972) 4220

/51 Sherrington, D., A I P Conf. Proc. No 29

(1976) 224

Fig. 3

Magnetic hyperfine field distribution obtai-

161 Edwards,S.F. and ~nderson,~.~.,

J. phys. 5

ned from fitting the spectra.

(1975) 965

a) &Fe

alloy with 0.762 at.%Fe

b) &Fe

alloy with 0.494 at.%Fe