Phase Transitions in HoAlGa
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PHASE TRANSITIONS IN HoAlGa
M. DOUKOURE*, D. GIGNOUX* and F. SAYETAT** *Laboratoire Louis NMel, **Laboratoire de Cristallographie, C.N.R.S., 166 X, 38042 Grenoble Cedex, France ABSTRACT HoAlGa is hexagonal at room temperature. It undergoes two magnetic transitions succesively at TN = 32 K from a paramagnetic to a triangular antiferromagnetic state where the Ho moments lie in the basal plane and at Tt = 18 K in the course of which the moments rotate toward c giving rise to a colinear antiferromagnetic arrangement. X-ray experiments performed between 5 and 300 K allow to determine the crystal evolution through the two transitions. The hexagonal symmetry is not lowered through the transitions ;this result is compatible with the observed magnetic groups. The thermal expansion curves show a very anisotropic behaviour of the lattice parameters. The "c" parameter shrinks below TN and this anomaly is to be related to the magnetic order. Along a, a positive thermal anomaly appears below 70 K and this can be interpreted by crystal field effects. Stability of magnetic structures is discussed with regard to exchange interactions and magnetocrystalline anisotropy. INTRODUCTION All the RGa2 compounds crystallize in the hexagonal AIB2 type structure, which consists of alternating rare-earth (R) and gallium planes perpendicular to c [1]. This type of structure is very attractive because it favours rather sophisticated magnetic arrangements where the exchange interactions can be "frustrated" [2], [31, [4]. However, the magnetic structure results from a compromise between the exchange interaction and the strong magnetocrystalline anisotropy. The partial substitution of Ga by Al, giving rise to RAlxGa2-x alloys, preserves the hexagonal crystal structure over a large range of x [5], but greatly modifies the anisotropy. Furthermore, the RKKY type exchange interactions between rare-earth moments are strongly dependent on the distance between atoms and are likely to change from one alloy to another. The study of physical properties of substituted intermetallic rare earth alloys should be of great interest to understand the stability of the possible magnetic structures. HoAl xGa 2-x Intermetallic holmium alloys HoAlxGa2-x crystallizes in hexagonal AIB2 type structure [1] over a range of xo= 0 to 1.2 [5j. Thus, HoAlGa is hexagonal at room temperature (a = 4.4353 A, c = 3.5453 A, c/a = 0.800) [6] ; ýhe elementary cell contains one unit formula. The space group is P6/mmm [Dgh]. Holmium, the only atoms bearing magnetic moments are situated in the site la(O00) where the symmetry is 6/mmm[D6h] ; the environment of twelve gallium and aluminium, at z = 1/2 ands z = - /2, is constituted of two hexagons.
Ga and
Al are randomly* distributed on the same site 2d(1/3 1/3 1/2) and (2/3 1/3 1/2) where the symmetry is 6m2[D3h] (*as deduced from neutron diffraction experiments) [6]. Mat.
Res. soc.
Symp. Proc. Vol.
21 (1984) QElsevier Science Publishing Co.- Inc.
202
As already mentioned by Girgis et al. [5] the c/a ratio is drastically reduced fro
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