Magnetic, specific heat and transport properties of U_1-xTh_xPt alloys

Previous studies on UPt samples indicated ferromagnetic ordering either at T_C = 27 K or at T_C = 19 K while in some samples there was evidence of the presence of both these transitions. These differences are thought to be associated with the presence of material of different crystal structure, viz. a monoclinic structure for the higher-T_C material (H phase) and an orthorhombic structure for the lower-T_C material (L phase). We have studied the crystal structure, magnetization, specific heat and transport properties of the solid solutions U_1-xTh_xPt, including a pure UPt sample. We observed that the substitution of Th for U in UPt stabilizes the L phase for all compositions. Ferromagnetism is observed for 0 ≤ x ≤ 0.4 and there are indications of the presence of short-range magnetic order for x ≥ 0.5, which vanishes slowly up to x ≈ 1. At low temperatures in the magnetically ordered region the magnetic and electronic contributions to the specific heat C_5f and electrical resistivity ρ_5f are described by expressions that contain the magnon energy-gap Δ. These also include the γ_5f(0)T and AT² terms to describe the C_5f and ρ_5f temperature dependences respectively. For L-phase UPt, the low-temperature fitting (below 10 K) yields Δ = 19(1) K and γ_5f(0) = 0.122(2) J K^-2/mol U, while the higher-temperature one (above 10 K) yields Δ = 35(2) K and γ_5f(hT) = 0.151(3) J K^-2/mol U. With Th substitution Δ decreases, but the γ_5f(0) value for both the magnetically ordered and non-ordered samples remains practically unchanged, being close to that of the higher-temperature value found for this compound. The magnetoresistance is negative and large in the ordered region and becomes slightly positive at low temperatures for the Th-rich alloys. The presence of the Kondo interaction is indicated in these alloys almost up to x ≈ 1.