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 interme tallics 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 t emperature range from 4 up to 450 K. All these compounds crystallize i n the C15-type structure (cubic Laves phase structure). By comparing t he nonmagnetic Y-based compounds we could show that there is an enhanc ed 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). Belo w 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 anisotro pic crystal field. The thermal expansion of the orthorhombic TmCu2, Gd Cu2 and YCu2 compounds has also been investigated for comparison. The influence of the crystal field on the thermal expansion in TmCu2 in th e paramagnetic range (TmCu2 orders magnetically at T(N) = 6.3 K) has b een determined by comparing the thermal expansion of the nonmagnetic Y Cu2 with that of TmCu2. The data thus obtained are compared with a the oretical model. GdCu2, for which the influence of the crystal field ca n be neglected, has been investigated in order to study the influence of the exchange interaction in the magnetically ordered state (below 4 2 K).