Characteristic properties of doped Bi2212 single crystals studied by magneto optical measurement

Local J_c distribution under H || c was carefully investigated for lightly Pb-doped Bi2212 single crystals having uniform microstructure in order to clarify essential effect of Pb-doping on intra-grain J_c properties. An abrupt change of internal field gradient was observed at a constant field, H^*, for each Bi(Pb)2212 single crystal below 25 K. J_c is apparently higher above H^*. The H^* was observed at several hundred Oe which was much lower than the second peak field, H_pk, in the magnetization hysteresis loops. This suggested that dimensionality of vortex changed at H^* from 3D to quasi-2D state with an increase of internal magnetic field, possibly due to an inhomogeneous distribution of lead ions in the crystal. Higher J_c above H^* and temperature dependent H_pk indicated generation of field-induced pinning sites in the present system. Doping effects of cobalt for heavily Pb-doped Bi2212 single crystals on the flux pinning properties were also studied. In-plane J_c was found to become isotropic with increasing the Co-doping level.