RELATION BETWEEN THE PEAK EFFECT AND THE MAGNETIZATION STEP IN BI2SR2CACU2O8+Y

Local magnetization (B-l-H-a) as a function of magnetic field (H-a) wa s measured by using micro Hall probes in high quality Bi2Sr2CaCu2O8+y, single crystals with different oxygen content. In an as-grown sample, magnetization step which is the sign of a first order phase transitio n in the vortex system is observed. The step field, B-s, increases wit h decreasing temperature and saturates at 300 Oe where the peak effect occurs at lower temperatures. As oxygen content is increased, the mag netization step becomes smaller. Instead, an anomalous temperature dep endent peak effect (ATDPE) appears on a line similar to B-s(T) line. T he reduction of magnetization step and the appearance of ATDPE with in creasing oxygen contents can be understood by considering the increase of the inhomogeneity of samples rather than the decrease in the aniso tropy.