Evolution of vortex phase diagram with oxygen-doping in Bi2Sr2CaCu2O8+y single crystals

We studied the vortex phase diagram in Bi2Sr2CaCu2O8+y with oxygen-doping by using micro-Hall-probes. The field Bs, where the magnetization steps occur, increases with oxygen concentrations. In addition to this, we found that oxygen doping makes the entropy change smaller. The magnetization steps were completely suppressed by further doping, while the magnetization peak was observed at high temperatures, whose temperature dependence is similar to Bs(T). Moreover, the peak field Bp at low temperatures shows pronounced re-entrant behavior in highly overdoped samples. All these features in the evolution of phase diagram can be explained by taking account of disorders as well as the decrease in the anisotropy due to oxygen-doping.