MAGNETIC STUDIES ON THE FIELD-DRIVEN TRANSITION FROM DECOUPLED TO COUPLED PANCAKE VORTEX PHASE IN BI2SR2CACU2O8+DELTA WITH COLUMNAR DEFECTS

The reversible (M-rev) and the irreversible magnetization of Bi2Sr2CaC u2O8+delta single crystals with columnar defects were measured as a fu nction of field and temperature. In contrast to the conventional InB d ependence of M-rev observed in the unirradiated sample, the field depe ndence of M-rev for the irradiated samples exhibits an anomalous dip, corresponding to the expulsion of vortices from the sample. We show,th at the coupling transition, which, to our knowledge;. has not been con sidered previously in analyzing the magnetization data, is : very impo rtant to understand the anomalous behavior of M-rev. From the measurem ent, ae obtain the almost temperature-independent transition line B-cp (T)similar to B-Phi/3, where B-Phi is a matching field. This is in goo d accordance with the recent computer simulation. The anomalous peak e ffect is observed at the same field, B similar to B-Phi/3, in Bose-gla ss (BG) regime, which accompanies the reentrant behavior of BG melting line.