Magnetic phase diagram in a HgBa2CuO4+δ 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 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.