Flux Creep Relaxation in Single Crystal Hg-1201

Magnetization measurement on a single crystal of the high-T-c cuprate HgBa2CuO4 (Hg-1201) are performed for the two field orientations B parallel to c and B parallel to(a,b). Besides zero-field-cooling (zfc) - field-cooling (fc) and magnetization studies, the flux creep relaxation is determined. The zfc curves exhibit pronounced step-like anomalies in the mixed state above the lower critical field B-c1. They are interpreted as those points in the phase diagram where the penetrating flux fronts meet at the center of the sample. In order to obtain more information about the flux pinning mechanism flux creep relaxation experiments are performed at different temperatures in magnetic fields of 0.15 and 2 T. The resulting super current dependence on the mean activation energy is analysed according to the collective pinning theory which predicts U proportional to ((j/j(c))(-mu) -1). The mu-values for B parallel to c are 0.5 at higher temperatures and 2 at 5, and they vary between 1.1 and 1.5 for B parallel to(a,b). The observed behaviour is quite different from that found for powder samples.