Vortex order-disorder transition in relaxation and field-sweep measurements

Time evolution of the induction profile across a Bi2Sr2CaCu2O8+delta crystal was magneto-optically recorded in two experiments: during ramping the external magnetic field or after a sudden application of the field. The results of these experiments reveal a front-like propagation or retreat of a transient disordered vortex state, respectively. Analysis of the front velocity shows that while the retreat is continuously decelerated, the propagation is accelerated, approaching a constant velocity when the front induction reaches the vortex order-disorder phase transition field B-od. The latter observation suggests a method for determining the value of B-od, eliminating transient state effects. (C) 2004 Elsevier B.V.. All rights reserved