Magnetic phase diagram in a HgBa2CuO4+delta single crystal

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 ne w mechanism controls the irreversibility up to T-c. The hysteresis loops me asurements (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 reg ion of the second peak show, at constant temperature, that effective activa tion 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 t he hysteresis loops. (C) 1999 Elsevier Science B.V. All rights reserved.