Local magnetic relaxation measurements in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ 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 $U_{\mathrm{pl}}$, which decreases with field, drops below the elastic (collective) creep activation energy, $U_{\mathrm{el}}$, which increases with field. A crossover in flux dynamics from elastic to plastic creep is shown to be the origin of the fishtail in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$.

• Received 23 April 1996

DOI:https://doi.org/10.1103/PhysRevLett.77.1596