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 critical value necessary to induce long-range magnetic order in the Co d-electron 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 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 TmCo₂ 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 TmCo₂ as well as for the pseudobinary Tm_1-xGd_xCo₂ compounds. At elevated temperatures the physical properties of all these compounds are dominated by spin fluctuations.