Magnetic anisotropy and thermodynamic anomaly in the superconducting mixed state of UBe13 probed by static dc magnetization measurements

摘要

Static dc magnetization M(H) measurements were performed for a single crystal of UBe13 down to 0.1 K in magnetic fields up to 80 kOe for H parallel to < 100 > and H parallel to < 100 >. For both field directions, an unusual magnetic anomaly was observed at H-Mag* similar to 20-30 kOe in the superconducting mixed state. This anomaly was seen in the thermal-equilibrium-magnetization curves as well as the increasing-and decreasing-field processes of the raw magnetization curves, implying a change of the thermodynamic property at H-Mag*. Furthermore, the magnetic anisotropy of the superconducting diamagnetic response is found to be significant above H-Mag*, possibly associated with the anisotropy of the upper critical field H-c2. Considering the suppression of reduced upper critical field h* and the dramatic decrease of Maki parameter kappa(2) around 0.9T(c), the paramagnetic effect is present in UBe13. The paramagnetic effect in UBe13 could become anisotropic at low temperature below similar to T-c/2. An alternative explanation of the magnetic anisotropy is however that the anisotropy of the nonlinear susceptibility for the heavy electrons in the vortex cores becomes significant below similar to T-c/2.