We report high-precision measurements of the upper critical magnetic field of UBe13 as determined from the behavior of magnetic forces and torques acting on our samples. Our results suggest the presence of two sharp features or kinks in the upper critical field as a function of temperature in both single-crystal and polycrystalline samples. We discuss these results in light of recent theoretical and experimental work on the coexistence of heavy-fermion superconductivity with magnetic and/or additional superconducting states. © 1992 The American Physical Society.

We present measurements of the upper critical magnetic field Hc2(T) determined from the dc magnetization of polycrystalline samples of U(1-x)ThxBe13 with x=0 and x=0.0331 and a single crystal of pure UBe13. We find changes in slope in the Hc2 vs T phase diagrams of both polycrystalline samples which suggests entrance into a second superconducting phase at about Tc2. A degradation of the superconducting critical field is observed in the single crystal which may be related to the absence of second-phase behavior in this sample. Our results are discussed in light of recent experimental and theoretical treatments of second-phase behavior in heavy-fermion superconductors. © 1988 The American Physical Society.

We have measrued the D.C. magnetization of the heavy fermion superconductor UBe13 in both the superconducting and normal state to high magnetic fields. We find an exponential time dependence for the relaxation of the mixed-state magnetization which supports the mechanism of flux jump. τ is both temperature and field dependent and UBe13 behaves differently from similar experiments in conventional type II superconductors in this respect. Other substantial difference between the two include observation of a paramagnetic background and evidence for anisotropy effects in the single crystal of UBe13 studied. These results will be discussed in light of recent theoretical and experimental work on heavy fermion systems. © 1987 The Japan Society of Applied Physics.

We present measurements of the low temperature dc magnetization of a single crystal of U0.9688Th0.0312Be13 as a function of magnetic field for two orientations of the crystal with respect to the field. A qualitative difference in the behavior of the amplitude of the magnetic hysteresis in the superconducting state may be interpreted as evidence for the existence of two superconducting phases, at least one of which is anisotropic. © 1990.