Time dependence of the magnetization in Bi2Sr2CaCu2O8+z

The Maley procedure has been applied to obtain U_eff(j) of a Bi-n = 2 single crystal for B < 4 T//c-axis and T < 20 K from the time dependence of the non-equilibrium magnetization. Analyzing the resulting U_eff (j) within the collective creep model it is found that for large current densities, flux creep of elastic small (<λ) flux bundles consisting of quasi 2-dimensional pancake vortices occurs. However, at large j better agreement is found with the logarithmic potential model. From the logarithmic U_eff(j) dependence j_c in the absence of thermal activation has been derived. It has been found that j > 10¹⁰ Am^-2 for all fields investigated. These high values of j_c indicate that the pancake vortices are pinned independently and that oxygen vacancies in the CuO₂ planes are not the only pinning centers.