LOCAL MAGNETIC STRUCTURE IN REENTRANT FERROMAGNET Au 19 % Fe 2 % Sn BY 57Fe AND 119Sn MÖSSBAUER EFFECT

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LOCAL MAGNETIC STRUCTURE IN REENTRANT

FERROMAGNET Au 19 % Fe 2 % Sn BY 57Fe AND

119Sn MÖSSBAUER EFFECT

A. Aït-Bahammou, C. Meyer, F. Hartmann-Boutron, Y. Gros, I. Campbell

To cite this version:

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JOURNAL DE PHYSIQUE

Colloque C8, Supp16ment au no 12, Tome 49, d6cembre 1988

LOCAL MAGNETIC STRUCTURE IN REENTRANT FERROMAGNET

Au 19

%

Fe

2 %

Sn BY 5 7 ~ e

AND '19sn MOSSBAUER EFFECT

-

A. Kit-Bahammou (I), C. Meyer (I), F. Hartmann-Boutron (I), Y. Gros (I) and I. A. Campbell (2)

( I ) Laboratoire de Spectrome'trie Physique (asgocie' au CNRS), Universite' Grenoble I, B.P. 87, 38402 Saint- Martin d'H2res Cedex, France

(2) Laboratoire de Physique des Solides (associe' au CNRS), Universite' Paris-Sud, 91405 Orsay Cedez, France Abstract. - Comparison of 5 7 ~ e and 'l9sn hyperfine fields indicates that in the canted phase of & 19 % Fe 2 % Sn, the transverse components of the magnetic moments are locally parallel. This confirms previous results obtained with 67F'e and l g 7 ~ u by Abd-Elmeguid, Micklitz, Brand and Keune.

In 1981 Gabay and Toulouse [I] predicted that reentrant ferromagnets exhibit, first a ferromagnetic. transition a t T,, and then, at Tf

<

T,,

a second transition to a canted.mixed ph&e with spin glass ordering of the transverse components of the magnetic moments. In 1982, in-field Mossbauer experiments [2-31 con- firmed the appearance of canting at low temperature but they did not yield information on the magnetic structure of the transverse components. In order to get more details, recent experiments use two isotopes, magnetic 5 7 ~ e and a diamagnetic atom. In [4] Abd- Elmeguid et al. studied

&

16.8 % Fe with 5 7 ~ e and l g 7 ~ u . They found that the thermal variations of the intrinsic h.f. field on 5 7 ~ e and of the transferred h.f. field on lg7Au ,are proportional: in particular both fields exhibit an anomalous increase below a certain temperature

Tf.

With the help of a simplified model, Abd-Elmeguid et al. deduced from their results that the transverse moments in the canted phase don't have a spin glass structure but are locally parallel, contrary to the predictions of [I]. In this paper we present experiments on 5 7 ~ e and l19sn in

Au

19 % Fe 2 % Sn which confirm the conclusions of [4]. The model of [4] consists in assuming that the tranferred h.f. field on a

_-

diamagnetic atom is only due to its n iron first neighbours (this is an approximation as shown, [5] Fig. 4, by the h.f, field distribution P (H) in

Au

3 % Fe 1 % Sn in which 69

%

of Sn atoms have no Fe nearest neighbour). When n = 0 : H = 0. When 1

5

n

5

12, for a l19sn atom with n Fe neighbours:

where Oz is the magnetization direction in the ferromagnetic phase and ht the transverse component in the canted phase.

Pn

being the probability of hav- ing n neighbours

,

one gets an average

modulus:

In the ferromagnetic phase, hZi = h, and ht, = OVi :

in which c = iron concentration (c = 19 %).

In the canted phase, two situations may occur. If the transverse components are parallel: hZi = h, and

hti = htVi :

Sn and Fe h.f. fields are proportional

VT.

If the transverse components have a 2D spin glass ordering

Whence:

Sn and Fe hf fields are not proportional at L.T. Note that our equation contains (1

-

Po) / f i instead of (l/n)

in reference 141.

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C8 - 1158 JOURNAL DE PHYSIQUE

Let us now consider the extrapolated values a t 0 K -20 -16 -12 -8 -4 o 4 8 12 16 20

of the H.T. and L.T. parts of the curve H ( T )

.

In the _10.2% locally parallel case

while in the spin glass case

1

-

Po

Fig. 2. - 'l9sn Mossbauer spectra of

&

I

19 % Fe 2 % Sn.

1

-

Po

= [I+

-

(R:

-

1 ) ] =

R2.

A

According to the Mossbauer results presented in fig- ures 1-3,

H,S"

and

H,F"

are proportional

VT,

with Tp=

123 K and Z=42

K. At

0 K, H , " L T = ~ o ~ kOe, H S T = 238 kOe whence

RI

= 1.29 and

R2

= 1.13. On the other hand ~ : + = 9 4 kOe, H$=68 kOe, whence a ra- tio 1.38 in reasonable agreement with R1 i.e. with the

local parallelism assumption, in view of the approxi- mate character of the model.

Note that these results also agree with small angle neutron scattering measurements [6].

Fig. 1. - 5 7 ~ e Mossbauer spectra of = 4 ~ 19 % Fe 2 % Sn.

3 . FWHk m m j s

0 50 100 TIKI

Fi

.

3. - Thermal variations of the h.f. fields on 6 7 ~ e and "'Sn. They lead t o a transition temperature Tp=123 K

in agreement with the break in slope of the 6 7 ~ e high T

linewidth (insert).

[ I ] Gabay, M . et al., Phys.

Rev. Lett.

47 (1981) 201. [2] Varret, F .

et

al., Phys.

Rev.

B

26 (1982) 5282. [3] Lauer, J .

et

al., Phys.

Rev. Lett.

48 (1982) 1850. [4] Abd-Elmeguid, A. A.

et

al. Phys.

Rev.

B 33

(1986) 7833.

[5] Kit-Bahammou, A.

et

al., this conference.