Magnetization measurements on high-quality untwinned YBa2Cu3Oy, single crystals are performed in magnetic fields up to 30 T using a Hall probe magnetometry. We find an anomalous second peak in the magnetization hysteresis curve, which is different from the previously reported "fish tail". This behavior is interpreted in terms of the field-driven transition from the vortex lattice or the Bragg glass to the disordered vortex glass phases with increasing magnetic field. We discuss, furthermore, the influence of disorder of the system on the transition.
- Vortex lattice melting transition
- Vortex phase diagram