Magnetic hysteresis of Re-doped HBCCO polycrystals

We have performed a study of the magnetic hysteresis of (Hg1-xRex)Ba2Ca2Cu3 O8+delta samples with two "extreme" Re-substitutions in a temperature inter val close to T-c. We observe a strong asymmetry of the magnetization loops that can be associated with grains surface barriers or reversible magnetiza tion. We also determined the temperature dependence of the first penetratio n field and the irreversibility field of the grains, and the field dependen ce of the magnetization curve width. The results suggest that the optimum p inning level is located at 0.09<x<0.24.