Magnetic field response in (Uth) Be₁₃

We report on recent magnetic field studies on U_1-x Th_xBe₁₃ with x ≤ 0.1. These new results are related to previous data published by Kromer et al. [F. Kromer et al., Phys. Rev. B 62, 12477 (2000)]. The new magnetization measurements confirm the existence of an anomaly found earlier through calorimetric and dilatometric measurements at T_L 〈 T_c for x 〈 X_c1 ≈ 0.019. In the critical concentration range X_c1 〈 X 〈 X_c2 ≈ 0.0455, the critical-field curves for the mean-field type of phase transition that occur at T_c1 and T_c2 (at zero field) appear to merge as T → 0. This interesting fact which has been inferred already from the previous measurements could be confirmed for a sample with x = 0.038. The normal state for Th concentrations x ≈ x_c2 is rather unusual: A negative normal-state contribution, α_n, is observed in the thermal expansion coefficient as well as the occurrence of a positive peak when superconductivity is suppressed by a magnetic field. Both effects are studied in detail and a comparison to specific heat results is given. Finally, we have investigated the non-linear contribution to the magnetization, X⁽³⁾, of UBe₁₃ and U_0.9 Th_0.1 Be₁₃. In contradiction to suggestions made previously by Aliev et al. [Alieu et al., Europhys. Lett. 32, 765 (1995)] for x = 0.1, X⁽³⁾ remains negative down to T ≈ 100 mK, the lowest temperature investigated.