GD SUBSTITUTIONS IN THE TMCO2 LAVES PHASE - THE ONSET OF LONG-RANGE MAGNETIC ORDER IN THE ITINERANT SUBSYSTEM

We found that in TmCo2 the molecular field does not exceed the critica l value necessary to induce long-range magnetic order in the Co d-elec tron subsystem, which is in contrast to the other heavy RCo2 compounds . Below T(C) = 3.8 K a first-order magnetic phase transition at 3.4 K exists, which is due to a rearrangement of the Tm 4f magnetic moments. When substituting Tm by Gd long-range magnetic order appears in the C o d subsystem. The onset of the magnetic order in the d-electron subsy stem can most clearly be seen from thermal expansion measurements. The re is an increasing positive volume anomaly with increasing Gd content below T(C). The estimated magnetostriction constant lambda111 of TmCo 2 is -4.1 x 10(-3), in agreement with the single-ion model. Pronounced discontinuities at T(C) are characteristic for the transport properti es (resistivity and thermopower) for TmCo2 as well as for the pseudobi nary Tm1-xGdxCo2 compounds. At elevated temperatures the physical prop erties of all these compounds are dominated by spin fluctuations.