Flux creep in a polycrystallineBa0.6K0.4BiO3superconductor

Magnetization curves have been measured as a function of temperature for a polycrystalline sample of ${\mathrm{Ba}}_{0.6}$ ${\mathrm{K}}_{0.4}$ ${\mathrm{BiO}}_3$. These measurements reveal significant reversibility at modest fields for all temperatures studied. The irreversibility line ($H^{\mathrm{*}}$,$T^{\mathrm{*}}$) is determined for this material and shows that $H^{\mathrm{*}}$ is less than 0.8 T at all temperatures. The logarithmic magnetic relaxation rate A=dM/d ln(t) has been studied for this material, as a function of both field and temperature. Analysis using the flux-creep model reveals the average pinning energy $U_0$ to be 0.08±0.02 eV. This extremely small pinning energy is used to explain the small critical current, its strong dependence on field, and the significantly reversible behavior in this material.