Plastic vortex creep in YBa2Cu3O7-x crystals

Local magnetic relaxation measurements in YBa2Cu3O7-x crystals show evidence for plastic vortex creep associated with the motion of dislocations in the vortex lattice. This creep mechanism governs the vortex dynamics in a wide range of temperatures and fields below the melting line and above the field corresponding to the peak in the ''fishtail'' magnetization. In this range the activation energy U-pl, which decreases with field, drops below the elastic (collective) creep activation energy, U-el, which increases with field. A crossover in flux dynamics from elastic to plastic creep is shown to be the origin of the fishtail in YBa2CU3O7-x.