EVOLUTION OF VORTEX PHASE-DIAGRAM WITH OXYGEN-DOPING IN BI2SR2CACU2O8-CRYSTALS(Y SINGLE)

We studied the vortex phase diagram in Bi2Sr2CaCu2O8+y with oxygen-dop ing by using micro-Hall-probes. The field B-s, where the magnetization steps occur, increases with oxygen concentrations. In addition to thi s, we found that oxygen doping makes the entropy change smaller. The m agnetization steps were completely suppressed by further doping, while the magnetization peak was observed at high temperatures, whose tempe rature dependence is similar to B-s(T). Moreover, the peak field B-p a t low temperatures shows pronounced re-entrant behavior in highly over doped samples. All these features in the evolution of phase diagram ca n be explained by taking account of disorders as well as the decrease in the anisotropy due to oxygen-doping. (C) 1998 Elsevier Science B.V. All rights reserved.