Critical current density, lower critical field, and flux creep in Ag-sheathed high-J c tape of Bi-Pb-Sr-Ca-Cu-O

Abstract

Initial magnetization curve M(H) and magnetic relaxation M(t) were studied on the Ag-sheathed and highly c-axis oriented tape of Bi-Pb-Sr-Ca-Cu-O with transport Jct=5400 and 2400 A/cm2 (77 K, H=0). The Jc value, derived from M(H) curve based on Bean’s critical-state model, agrees fairly well with the Jct. The lower critical field Hc1 for H⊥c axis from M(H) curve increases with a slope of −0.2 Oe/K as temperature decreases from Tc, and it approaches the saturated value of 15 Oe at T∼0 K. The magnitude is much lower than that for the high-Tc superconductors La-Sr-Cu-O and Y-Ba-Cu-O. The creep rate and activation energy U0 are evaluated from M(t) based on the flux creep model. The results show a significant dependence on the measuring process and magnetic field H. The U0 at 5 K for H=500 Oe (H⊥c) is 0.4 eV for the in-field magnetization (IN) and 0.1 eV for the remanent magnetization (REM), which are larger than those for Bi-based high-Tc superconductors reported previously. The U0 for the IN shows a peak of 0.8 eV around 30 K and then decreases as temperature increases, while a peak in it is weakened for the REM. The U0 at H=30 Oe (H⊥c) for the IN is one order larger than that for 500 Oe and the peak temperature is shifted to the lower temperature (∼10 K).