CROSSOVER FROM THE FIRST-ORDER VORTEX PHASE-TRANSITION TO THE PEAK EFFECT IN BI2SR2CACU2OY HAVING DIFFERENT OXYGEN CONTENTS

We have measured the magnetization and the heat capacity of over-doped (OVD) and optimally doped (OPD) Bi2Sr2CaCu2Oy single crystals in the mixed state under magnetic fields applied along the c-axis. In both sa mples, a distinct magnetization step is observed in the magnetization curve at high temperatures and is found to change to a kink at low tem peratures. At still lower temperatures, the peak effect dominates the magnetization curve and the kink vanishes. The origin of the step and the kink in the magnetization curve is considered to be the first-orde r phase transition in a quasi-three-dimensional vortex system although the heat capacity shows little anomalies at the transition because th e latent heat is small. The first-order phase transition line of each sample is drawn on the H-T plane and it is found that the slope of the line of the OVD sample is steeper than that of the OPD sample. The en tropy jump at the transition of the OVD sample is estimated to be larg er than that of the OPD one at all temperatures and magnetic fields st udied. This behavior is contradictory to the simple vortex-lattice mel ting picture.