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

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(100) and H(110). For both field directions, an unusual magnetic anomaly was observed at HMag∗∼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 HMag∗. Furthermore, the magnetic anisotropy of the superconducting diamagnetic response is found to be significant above HMag∗, possibly associated with the anisotropy of the upper critical field Hc2. Considering the suppression of reduced upper critical field h∗ and the dramatic decrease of Maki parameter κ2 around 0.9Tc, the paramagnetic effect is present in UBe13. The paramagnetic effect in UBe13 could become anisotropic at low temperature below ∼Tc/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 ∼Tc/2.