Flux pinning regimes in Hg-1201

We report on measurements of the time dependent irreversible magnetization due to flux creep in the high-Tc superconductor HgBa2CuO4 (Hg-1201). Close attention is paid to the low-field and low-temperature region of the mixed state where the flux dynamics differs significantly from that found at higher temperature and field strength. Due to low-vortex density and only small thermal perturbations flux creep in this pinning regime is dominated by the motion of individually pinned flux lines. The observed time dependent decay of the magnetization departs clearly from a simple logarithmic time law. As is commonly accepted, the deviation results from dependence of the mean activation energy upon current density which is nonlinear and takes into account that during creep relaxation the effective pinning barrier depends on the actual distribution of vortices. We analyzed the relaxational behavior in Hg-1201 in order to specify the nonlinear relationship between activation energy and current density. Our experimental findings fit in best with predictions given by the collective pinning theory, i.e. the current dependent activation barrier follows a power-law behavior. On going to higher temperatures, this behavior first changes only moderately and in a continuous manner. In the vicinity of a characteristic temperature T∗, however, a uniform description is basically questioned by strong changes in vortex dynamics, which are associated with a crossover between pinning regimes of different type.