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 Bon. The same field marks a change in the field profiles across the sample, from profiles dominated by geometrical barriers below Bon to Bean-like profiles above it. The temperature dependence of Bon and the flux distribution above and below Bon imply a disorder-induced transition at Bon from a relatively ordered vortex lattice to a highly disordered, entangled vortex solid. Local magnetic relaxation measurements above Bon show evidence for plastic vortex creep associated with the motion of dislocations in the entangled vortex structure. [S0031-9007(97)04113-6]