Gd substitutions in the TmCo₂Laves phase: the onset of long-range magnetic order in the itinerant subsystem

We found that in TmCo 2 the molecular field does not exceed the critical value necessary to induce long-range magnetic order in the Co d-electron subsystem, which is in contrast to the other heavy RCo 2 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 Co d subsystem. The onset of the magnetic order in the d-electron subsystem can most clearly be seen from thermal expansion measurements. There is an increasing positive volume anomaly with increasing Gd content below T c . The estimated magnetostriction constant lambda 111 of TmCo 2 is -4.1*10 -3 , in agreement with the single-ion model. Pronounced discontinuities at T c are characteristic for the transport properties (resistivity and thermopower) for TmCo 2 as well as for the pseudobinary Tm 1-x Gd x Co 2 compounds. At elevated temperatures the physical properties of all these compounds are dominated by spin fluctuations.