THERMAL EXPANSION ANOMALIES AT SPIN-REORIENTATION IN SmFe : THE ROLE OF THE MAGNETIC TEXTURE

Temperature dependency ofthe thermal expansion coefficient (TEC) was studied at 4.2- 300 K in SmFe2 ferromagnetic compound having spin-reorientation temperature near 190 K. We have found a strong effect of the magnetic texture on TEC of this high mag- netostrictive compound at the spin-reorientation phase transition from [1 1] to [1 10] easy direction. The substantial role in the texture formation is expected to belong to atoms of hydrogen that are always present in a small amount in the compounds of such type.


INTRODUCTION
According to the Akulov's rule of even effects, the anisotropic linear magnetostriction (A) does not contribute to the thermal expansion of a polycrystalline polydomain specimen. That is, in the first approximation, a rotation of the magnetization, for example at spin-reorientation transition, does not give rise to any volume magnetostriction. However, when a magnetic texture is created in a specimen with high A value, a deformation due to magnetostriction may be observed by thermal expansion of the specimen with pseudocubic structure. We have found a strong effect of the magnetic texture on the thermal expansion of high * Corresponding author. 1] to 0] easy direction (ED) .
The substantial role in the texture formation is expected to belong to atoms of hydrogen that are always present in a small amount in the compounds of such type. Since magnetostriction constants A111 and A100 are negative (see Table I), the lattice is compressed along the crystallographic axis, along which the spontaneous magnetization vector Ms is aligned. Such a spontaneous rhombohedral magnetoelastic lattice deformation starts the hydrogen ordering Gaviko et aL, 1990;. This in turn results in a sharp increase of the rhombohedral distortion and the occurrence of a local uniaxial magnetic anisotropy, which is added to the initial cubic anisotropy. The ED of the induced magnetic anisotropy is determined by the direction of the magnetoelastic deformation that can be changed by varying the direction of the applied magnetic field at the thermomagnetic treatment. Without thermomagnetic treatment the hydrogenation does not result in a macroscopic anisotropy (Korolyov et al., 1993). In the present paper, the thermal expansion in SmFe2 compound with the magnetic texture has been studied. We observed both compression and an elongation in the same specimen with the magnetic texture at the spin-reorientation transition temperature. EXPERIMENTAL Polycrystalline samples were obtained by melting constituents from 99.9% Sm and 99.99% Fe in an induction furnace in a purified argon atmosphere and then anneled for 70 h at 1000 K. After anneling, the amount of secondary phases was not more than 3%. Two types of samples have been investigated: bulk samples and samples prepared by powder metallurgical technology. A thermomagnetie treatment of the specimens was done along the axis of the cylindrical specimen and perpendicular to it. Thus, we may have the same specimen with a magnetic texture in the first case for thermomagnetic treatment by magnetic field along the axis of the cylindrical specimen (TMTA). In the second case the thermomagnetic treatment was perpendicular to the axis of the cylindrical specimen (TMTP). The thermal expansion coefficient (TEC) was measured along the axis of the cylindrical specimen using a capacitance dilatometer. The error in measurements of TEC at all temperatures was less than 5 10 -K -1 at the specimen length of about 30 mm.

RESULTS AND DISCUSSION
Temperature dependencies of TEC, a(T), were studied in the temperature range 4.2-300 K in SmFe2 ferromagnetic compound having spinreorientation phase transition near 190 K and the Curie temperature near 660 K. The temperature dependences of the TECs of SmFe2 with TMTA and TMTP magnetic texture are shown in Fig. 1. It was found the occurrence of an additional contribution to TEC Oextra(T) relative to the non-magnetic state, which may be described by the Debye-Grfineise function Oqattice(T) as

Oextra (T) a(T) Oqattic e (T).
(1) As illustrated in Fig. 1, this extra contribution to Otextra(T) has the shape of a smoothed 6-function and may be both positive and negative. This phenomenon is observed both in the bulk and in the powdered specimens. The location ofthe 6-type anomaly is inside the spin-reorientation range. The results are completely reproducible when the thermomagnetic prehistory is repeated carefully. We observed a temperature hysteresis for these anomalies. It is about 2 K, concluding from the position of the extremum values of axtra(T). These data favor the view that the phase transition is a first order type one . We believe that the observed 6-type anomalies of the TEC are caused by the magnetic texture of the specimens. The obtained results are understood in the model of domain structure changing and the formation of a new magnetic alignment after spin-reorientation (Korolyov  Mushnikov et al., 1991). The substantial role in the texture formation is expected to belong to atoms of hydrogen, which occupy e-type (AB3-type) interstitial sites . The phenomenon of anomalies of TEC can be explained as follows. For a domain the elongation observed in the direction of (ill, /2, f13) can be expressed as 2 2 02fl2 (6L/L) =(3/2)A100(cfll 2 + a2/ 2 + 3 + 3/111 (O102/1fl2 "+-O203/2/3 q-O301fl3/1), where (al, a2, a3) are the direction cosines of internal magnetization.
So, from these data Oextra is negative (Oextra < 0) in the first case when the observation is along the [1 1] axis, and in the second case Oextra is positive (Oextra>0). These simple model calculations will allow an understanding of the role of the magnetic texture in the a(T) dependencies of the investigated specimens of SmFe2.

CONCLUSION
The spontaneous deformation of the high magnetostrictive polycrystalline specimens at the spin-reorientation phase transition will be determined by the magnetic texture. Because of this the sign of the additional contribution to TEC of SmFe2 compound may be positive or negative according to the direction of the magnetic texture and the direction of the observation.