Disorder-induced transition to entangled vortex solid in Nd-Ce-Cu-O crystal

Local magnetic measurements in a highly anisotropic Nd-Ce-Cu-O crystal reveal a sharp onset of an anomalous magnetization peak at a temperature-dependent field B_on. The same field marks a change in the field profiles across the sample, from profiles dominated by geometrical barriers below B_on to Bean-like profiles above it. The temperature dependence of B_on and the flux distribution above and below B_on imply a disorder-induced transition at B_on from a relatively ordered vortex lattice to a highly disordered, entangled vortex solid. Local magnetic relaxation measurements above B_on show evidence for plastic vortex creep associated with the motion of dislocations in the entangled vortex structure.