Magnetic relaxation measurements in the region of the second magnetizationpeak in a HgBa2CuO4+delta single crystal

The temperature and magnetic-field dependence of activation energy U-o(T,H) for creep has been studied in an HgBa2CuO4+delta single crystal below and above the "fishtail" peak H-sp(T). At constant temperature, U-o(H) increase s up to H-sp'(T) and then for H>H-sp'(T) it decreases. H-sp'(T) is located lower than the field H-sp(T) deduced from the hysteresis loops. The decreas e of the activation energy is probably indicative of a plastic-type creep o f the vortex lattice. We demonstrate that the overall behavior of U-o(T,H) at the second magnetization peak line H-sp(T) in the phase diagram of HgBa2 CuO4+delta exhibits similarities with that observed in Bi2Sr2CaCu2O8+delta and YBa2Cu3O6.95 compounds. [S0163-1829(99)04018-7].