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) increases 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 decrease of the activation energy is probably indicative of a plastic-type creep of 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 HgBa2CuO4+delta exhibits similarities with that observed in Bi2Sr2CaCu2O8+delta and YBa2Cu3O6.95 compounds. [S0163-1829(99)04018-7].

The irreversibility magnetic field H-irr(T) decreases exponentially in the temperature range 20 less than or equal to T less than or equal to 88 K. At T* approximate to 88 K, the H-irr(T) curve changes slope abruptly and a new mechanism controls the irreversibility up to T-c. The hysteresis loops measurements (H parallel to c) show a second peak for 5 less than or equal to T less than or equal to 50 K. Relaxation measurements performed in the region of the second peak show, at constant temperature, that effective activation energy U-o(H) first increases up to H-sp'(T) and then decreases for H > H-sp'(T). H-sp'(T) is located lower than the field H-sp(T) deduced from the hysteresis loops. (C) 1999 Elsevier Science B.V. All rights reserved.