Vortex phase transitions in Bi2Sr2CaCu2O8+y in fields nearly parallel and perpendicular to the CuO2 plane

The angular dependence of vortex phase transitions is studied in Bi2Sr2CaCu 2O8+y through detailed magnetization measurements using micro-Hall probes. including the two limiting cases, H almost parallel (theta = 0 degrees) and perpendicular (theta = 90 degrees) to the CuO2 plane. The crossing-lattice s ground state is found to be taken over by a homogeneously tilted vortex l attice state at small theta (< 10 degrees). At larger theta close to 90 deg rees. the linear in-plane field dependence of the vortex phase transition f ields breaks down, suggesting the presence of another phase transition. At the transition point, the spacing of pancake vortices becomes nearly equal to the spacing of Josephson vortices along the plane, suggesting a matching effect as a possible origin for this transition. Several other anomalies i n the hysteresis curve are also found. We discuss possible origins of these new anomalies. (C) 2001 Elsevier Science B.V. All rights reserved.