Accelerated magnetic relaxation of transient disordered vortex states in Bi2Sr2CaCu2O8+δ

Time relaxation of the local persistent current in a Bi2Sr2CaCu2O8+δ crystal was magneto-optically recorded after suddenly exposing the crystal to a constant magnetic field between 140 and 840 G. A remarkable behavior of the relaxation is revealed for fields below the order–disorder vortex phase transition: The relaxation starts at a slow rate, it then accelerates for a short period of time, after which a slow relaxation rate is resumed. We show that this acceleration marks a local transition of a transient disordered vortex state to a thermodynamically favored quasi-ordered vortex phase. The slow relaxation rates, before and after the transition, reflect ordinary thermally activated flux creep process in the disordered and quasi-ordered vortex phases, respectively.