Vortex matter transition in Bi2Sr2CaCu2O8+y under tilted fields

Vortex phase diagram under tilted fields from the c axis in Bi2Sr2CaCu2O8+y is studied by local magnetization hysteresis measurements using Hall probes. When the field is applied at large angles from the c axis, an anomaly (H-p*) other than the well-known peak effect (H-p) is found at fields below H-p. The angular dependence of the field H-p* is nonmonotonic and clearly different from that of Hp and depends on the oxygen content of the crystal. The results suggest existence of a vortex matter transition under tilted fields. Possible mechanisms of the transition are discussed.