Magnetic Field Response in (UTh)Be₁₃

We report on recent magnetic field studies on U1-xThxBe13 with x less than or equal to 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) approximate to 0.019. In the critical concentration range x(cl) < x < x(c2) approximate to 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 approximate to x(c2) is rather unusual: A negative normal-state contribution, alpha(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, chi((3)), of UBe13 and U0.9Th0.1Be13. In contradiction to suggestions made previously by Aliev et al. [Aliev et al., Europhys. Lett. 32, 765 (1995)] for x=0.1, chi((3)) remains negative down to T approximate to 100 mK, the lowest temperature investigated