Magnetic relaxation and critical current density limited by flux creep in Bi1.6Pb0.4Sr1.6Ca2Cu2.8Ox (Tc=115 K) and YBa2Cu3O7−x (Tc=92 K)

Abstract

We have investigated the magnetization behavior of ${\mathrm{Bi}}_{1.6}$ ${\mathrm{Pb}}_{0.4}$ ${\mathrm{Sr}}_{1.6}$ ${\mathrm{Ca}}_2$ ${\mathrm{Cu}}_{2.8}$ ${\mathrm{O}}_{\mathit{x}}$ (Bi 2:2:2:3) and ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{x}}$ (Y-Ba-Cu-O) as a function of field, temperature, and time. We find a logarithmic time decay in both zero-field-cooled magnetization and remanent magnetization, consistent with the Anderson-Kim flux-creep model with activation energies of $U_0$/k=700 K (0.06 eV) for Bi 2:2:2:3 and $U_0$/k=1700 K (0.15 eV) for Y-Ba-Cu-O in an applied field of 1 kOe. Measured activation energies remain essentially temperature independent for T/$T_c$ less than 0.6 while $J_c$(T) decays rapidly as a result of the relatively small $U_0$.