MAGNETIC FIELD INDUCED UNIDIRECTIONAL ANISOTROPY IN ANTIFERROMAGNETS WITH FERROMAGNETIC IMPURITIES

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MAGNETIC FIELD INDUCED UNIDIRECTIONAL ANISOTROPY IN ANTIFERROMAGNETS WITH

FERROMAGNETIC IMPURITIES

L. Pál, T. Tarnóczi, G. Zimmer

To cite this version:

L. Pál, T. Tarnóczi, G. Zimmer. MAGNETIC FIELD INDUCED UNIDIRECTIONAL

ANISOTROPY IN ANTIFERROMAGNETS WITH FERROMAGNETIC IMPURITIES. Journal de

Physique Colloques, 1971, 32 (C1), pp.C1-107-C1-109. �10.1051/jphyscol:1971133�. �jpa-00214363�

JOURNAL DE PHYSIQUE Colloque C 1, supplkment au no 2-3, Tome 32, Fkvrier-Mars 1971, page C 1 - 107

MAGNETIC FIELD INDUCED UNIDIRECTIONAL ANISOTROPY IN ANTIFERROMAGNET S WITH FERROMAGNETIC IMPURITIES

L. PAL, T. TARNOCZI and G. ZIMMER Central Research Institute for Physics, Budapest, Hungary

Rksumk. - Une redistribution des impuretks ferromagnetiques entre les sous-rbseaux est attendue dans les cristaux antiferromagnetiques au cas oh I'on applique un fort champ magnetique pendant le recuit fait a une tempkrature elevee mais plus basse que la tempkrature de Nkel. La redistribution aboutit a un faible ferromagnktisme qui est orienteprefk- rentiellement dans la direction du champ appliqd. Dans un alliage Mn-Ni presque kquiatomique avec quelques pour-cent d'excks de Ni, le recuit magnetique a pour resultat un faible ferromagnetisme, tandis qu'un traitement thermique en I'absence du champ magnktique n'affecte pas les propriktks magnktiques. L'aimantation des Cchantillons trempes est fortement anisotrope, les plus petites valeurs &ant mesurees dans la direction opposee B la direction du champ applique.

Abstract. - The ferromagnetic impurities in antiferromagnetic crystals are expected to redistribute between the sublattices if a strong magnetic field is applied during the annealing at high but below the Nee1 temperature. The redis- tribution results in a weak ferromagnetism oriented preferentially in the direction of the applied field. In a nearly equia- tomic Mn-Ni alloy with a few per cent of excess Ni the magnetic annealing results in a weak ferron~agnetism indeed, while the heat treatment in the absence of a magnetic field does not affect the magnetic properties. The magnetization of the quenched specimens is found to be highly anisotropic with the smallest value measured in the direction opposite to that of the applied field.

A few years ago, the investigation of the nearly equiatomic Mn-Ni alloys [I] drew our attention to a very surprising anomaly of the magnetic properties, namely it was found that the magnetic annealing of the samples with a few per cent of excess Ni content resulted in a weak spontaneous magnetization with unidirectional anisotropy, if the annealing was carried out a t high but below the N6el temperature. The aim of this paper is to summarize the experimental results of recent investigations and to put forward a possible explanation of the observed anomalies.

The Mn-Ni alloys near the equiatomic-concentration are of an ordered tetragonal CuAu-I-type structure

1 8-phase 1 below 7000C and of an ordered CsC1- type structure I P-phase I above this temperature [2].

The P-phase transforms almost completely into a disordered f. c. c. structure I y-phase 1 at a temperature somewhat below 900OC. The 0 ~t P transformation was found to be a diffusionless martensitic process.

The 6-phase is antiferromagnetic with a simple structure already described in [3]. Since the effective exchange field is zero at the Ni sites in the exactly stoichiometric alloys the magnetic moment of the Ni atoms is assumed to be zero and this is in a good agreement with the neutron diffraction measure- ments [4]. The antiferromagnetic-paramagnetic trans- formation occurs as a result of the 6 -+ p transition.

The temperature dependence of the susceptibility in the equiatomic and Mn-rich Mn-Ni alloys is shown in figure 1. The susceptibility is almost constant in both cases up to 700 OC but it shows a sharp increase at the temperature of the 8 + p phase transition. The curves measured on heating and on cooling are the same except for the temperature range where the first-order 6 e P phase transition takes place.

Quite another behaviour is observed in the samples with a few per cent of excess Ni. If they are cooled down slowly in a magnetic field of several kOe, the room temperature susceptibility becomes three-four times higher than before the heat treatment. It is

TEMPERATURE

FIG. 1. - Temperature dependence ^of the susceptibility ^{in the} equiatomic and Mn-rich Mn-Ni alloys.

important to note here that the heat treatment in the absence of a magnetic field does not lead to any subs- tantial changes of the original susceptibility.

In order to find out at which temperature the pro- cess begins to bring about an increase in the suscepti- bility, we measured the susceptibility curves on heating, then on slow cooling in the presence of a magnetic field of 10 kOe. The results of these measurements are seen in figure 2. The susceptibility measured on cooling begins to increase very rapidly at about 600 OC and the net increase at room temperature is roughly proportional to the excess Ni concentration. The sus- ceptibility measured after the magnetic heat treat- ment shows a pronounced anisotropy. The maximum value was found in the direction of the field and the minimum in that opposite t o the field. Figure 3 shows this anisotropic behaviour a t room temperature on a specimen with 4.9 atomic per cent of excess Ni. For comparison the curve measured on a specimen annea- led in the absence of a magnetic field is also shown.

.In order to get some information about the nature

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971133

TEMPERATURE

FIG. 2. - Susceptibility versus temperature curves for alloys of different excess Ni contents.

ANGLE MEASURED FROM

THE

FIG. 3. - Angular dependence of the room temperature sus- ceptibility induced by magnetic annealing.

of the process resulting in an increase of the suscep- tibility, the kinetics of the susceptibility variation was measured for 20 hours on a specimen quenched from 700

to 517

in a magnetic field of 10 kOe.

Figure 4 shows the variation of the susceptibility with time in a specimen with 4.9 per cent of excess Ni content. The activation energy of the process was evaluated as 1.8 eV from the susceptibility versus time curves measured at different temperatures from

ANNEALING TIME IN WURS

FIG. 4. -Variation of the susceptibility with the annealing time in a specimen with 4.9 per cent of excess Ni content.

480 to 517

This value of the activation energy indicates the increase in susceptibility to result from an atomic diffusion process.

The observed anomalies can be explained by assu- ming a non-equal distribution of the excess Ni atoms in the Mn planes between the sublattices. Before the magnetic annealing the excess Ni atoms are expected to be randomly distributed and so the average net magnetization of the crystal must be zero. The magne- tic field applied during the heat treatment may bring about a non-equal distribution of the excess Ni atoms between the sublattices and this will produce a net magnetization.

where 31, is the antiferromagnetic unit vector, ha is the unit vector of the annealing magnetic field Ha, while h is that of the measuring field, p(T) is the effec- tive magnetic moment due to a single excess Ni atom, Ta is the annealing temperature, N is the total number of the Mn sites in the crystal, and c = (n, + n,)/N is the concentration of the excess Ni. Assuming a uni- form distribution for the orientations of the crystallites in a polycrystalline sample, the average magnetization can be expressed by

which clearly shows that the average magnetization is anisotropic. This type of anisotropy was observed in our experiments as seen in figure 3. However, the susceptibility has a rather large isotropic part, as seen in figure 3, which cannot be explained by the magnetic properties of the Mn planes only.

It is known that the nearly equiatomic Mn-Ni

alloys with a few per cent of excess Ni may contain

some traces of a f. c. c. phase with a higher than ave-

rage Ni concentration and this phase could be ferro-

magnetic under certain conditions [2]. The magneti-

zation of these small ferromagnetic particles is oriented

randomly when the heat treatment is carried out

in the absence of a magnetic field and thus the average

magnetization is expected to be zero even in a strong

magnetic field applied in the measurement, because

the orientation of the magnetization vectors is fixed

by a large bounding force resulting from the coupling

MAGNETIC FIELD INDUCED UNIDIRECTIONAL ANISOTROPY IN ANTIFERROMAGNETS C 1 - 109

between the antiferromagnetic matrix and the ferro- than those of the smaller ones. One may conclude magnetic particles. When the annealing is carried that the onset of the isotropic magnetization has its out in a magnetic field then the ferromagnetic particles origin in the appearance of the ferromagnetic f. c. c.

are expected to have larger dimensions than in the phase. This is supported by the fact that the isotropic previous case and, naturally, the magnetic moments part of the magnetization disappears at a well defined of the larger particles may rotate much more freely temperature below the N6el point.

References

[I] PAL (L.), K R ~ N (E.), KADLR ((G.), SZAB~) (P.) and ¹³¹ KASPER ^(J. S.) and KOUVEL ^(J. S.), J. P ~ Y s . Chem.

TARN~CZI (T.), J . Appl. Phys., 1968, 39, 538. ^Solids, 1959, 11, 231.

[2] HANSEN (M.), Constitution of Binary Alloys. McGraw- i41 K R ~ N (E.1, NAGY (E.), NAGY (I.), PAL (L.) and

Hill, New York, 1958, p. 939. SZABO (P.), J. Phys. Chem. Solids, 1968, 29, 101.