Vortex matter transition in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+y}$ under tilted fields

Vortex phase diagram under tilted fields from the c axis in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+y}$ 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 $H_p$ 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.