Vortex solid-solid phase transition in an untwinned (formula presented) crystal

Local magnetic measurements vs temperature in an untwinned (Formula presented) crystal reveal an abrupt increase in the local magnetization at a field-dependent temperature, well below the melting line. At the same field and temperature a pronounced kink is observed in the local magnetization vs field curves. The line (Formula presented) describing the locations of these anomalies in the field-temperature phase diagram divides the vortex solid phase into two regions characterized by weak and strong pinning. A recently developed model describing the vortex solid-solid disorder-induced phase transition explains quantitatively the observed behavior of (Formula presented) From this behavior we infer that the microscopic origin of pinning in (Formula presented) is fluctuations in the charge-carrier mean free path.