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

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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).