Metastable vortex states in YBa2Cu3O7-delta crystal

Magnetization measurements in an untwinned YBa2Cu3O7-delta crystal reveal a range of temperatures and fields for which the magnetization exhibits unus ual history-dependent behavior. This range envelopes the transition line, B -ss(T), separating between the vortex quasiordored and disordered phases. T he observed history effects indicate that a disordered (quasiordered) vorte x state can exist as a metastable state below (above) the B-ss(T) line. The fields, defining the phase transition and the borders of the metastability region, are correlated to sharp features in the magnetization loop.