Magnetic anisotropy and thermodynamic anomaly in the superconducting mixed state of UBe_(13) probed by static dc magnetization measurements

摘要

Static dc magnetization M(H) measurements were performed for a single crystal of UBe_(13) down to 0.1 K in magnetic fields up to 80 kOe for H‖(100) and H‖〈110〉. For both field directions, an unusual magnetic anomaly was observed at H_(Mag)~* ～ 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 κ_2 around 0.9T_c, the paramagnetic effect is present in UBe_(13). The paramagnetic effect in UBe_(13) could become anisotropic at low temperature below ～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 ～T_c/2.