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 $B_{\mathrm{on}}$. The same field marks a change in the field profiles across the sample, from profiles dominated by geometrical barriers below $B_{\mathrm{on}}$ to Bean-like profiles above it. The temperature dependence of $B_{\mathrm{on}}$ and the flux distribution above and below $B_{\mathrm{on}}$ imply a disorder-induced transition at $B_{\mathrm{on}}$ from a relatively ordered vortex lattice to a highly disordered, entangled vortex solid. Local magnetic relaxation measurements above $B_{\mathrm{on}}$ show evidence for plastic vortex creep associated with the motion of dislocations in the entangled vortex structure.

• Received 26 March 1997

DOI:https://doi.org/10.1103/PhysRevLett.79.2542