The influence of the magnetic state on the thermal expansion in 1:2 rare earth intermetallic compounds

The attempt is made to demonstrate on some selected rare earth intermetallics the influence of the magnetic state on the thermal expansion. Using the X-ray powder diffraction method we investigated the thermal expansion of some selected non-magnetic compounds (YAl₂, YNi₂ and YCo₂) and some magnetic RE (rare earth) - cobalt compounds (RCo₂) in the temperature range from 4 up to 450 K. All these compounds crystallize in the C15-type structure (cubic Laves phase structure). By comparing the nonmagnetic Y-based compounds we could show that there is an enhanced contribution of the 3d electrons to the thermal expansion in YCo₂. In the magnetic RCo₂ compounds the induced 3d magnetism gives rise to large volume anomalies at the magnetic ordering temperature T_c. Below T_c there is in addition a distortion of the cubic unit cell due to the interaction of the magnetically ordered RE ions with the anisotropic crystal field. The thermal expansion of the orthorhombic TmCu₂, GdCu₂ and YCu₂ compounds has also been investigated for comparison. The influence of the crystal field on the thermal expansion in TmCu₂ in the paramagnetic range (TmCu₂ orders magnetically at T_N = 6.3 K) has been determined by comparing the thermal expansion of the nonmagnetic YCu₂ with that of TmCu₂. The data thus obtained are compared with a theoretical model. GdCu₂, for which the influence of the crystal field can be neglected, has been investigated in order to study the influence of the exchange interaction in the magnetically ordered state (below 42 K).