PLASTIC VORTEX CREEP IN YBA2CU3O7-X CRYSTALS

Local magnetic relaxation measurements in YBa2Cu3O7-x crystals show ev idence for plastic vortex creep associated with the motion of dislocat ions in the vortex lattice. This creep mechanism governs the vortex dy namics in a wide range of temperatures and fields below the melting li ne and above the field corresponding to the peak in the ''fishtail'' m agnetization. In this range the activation energy U-pl, which decrease s with field, drops below the elastic (collective) creep activation en ergy, U-el, which increases with field. A crossover in flux dynamics f rom elastic to plastic creep is shown to be the origin of the fishtail in YBa2CU3O7-x.