TEMPERATURE-DEPENDENCE OF PEAK EFFECTS IN OXYGEN-DOPED AND IODINE-INTERCALATED BI2SR2CACU2O8+Y

We studied the peak effects in high pressure oxygen annealed Bi2Sr2CaC u2O8+y (HPO-Bi2212) and iodine-intercalated Bi2Sr2CaCu2O8+y (I-Bi2212) single crystals by local magnetic measurements using micro Hall probe s. In contrast to optimum-doped Bi2212, the peak fields (B-p) rapidly decrease with increasing temperature above around 30 K in HPO-Bi2212. Since the shape of the peak and temperature dependence of B-p in HPO-B i2212 are similar to those in I-Bi2212, the anomalous temperature depe ndent peak effects in both materials may have a common origin. Such pe ak effects can be explained by an enhanced critical current due to fie ld-induced pinning centers and reduction of it by a possible phase-tra nsition in the vortex system.