VORTEX-LATTICE PHASE-TRANSITIONS IN BI2SR2CACU2O8 CRYSTALS WITH DIFFERENT OXYGEN STOICHIOMETRY

The vortex-lattice phase transitions in Bi2Sr2CaCu2O8 crystals with va rious oxygen stoichiometry are studied using local magnetization measu rements. Three new findings are reported: The first-order-phase transi tion line at elevated temperatures shifts upward for more isotropic ov erdoped samples. At lower temperatures another sharp transition is obs erved that results in enhanced bulk pinning in the second magnetizatio n peak region. The two lines merge at a multicritical point at interme diate temperatures forming apparently a continuous phase transition li ne that is anisotropy dependent.