Collective effects in vortex movements in type-II superconductors observed by a method for the registration of individual vortices

A method for the registration of Abrikosov vortices in type-II superconductors has been employed for studying magnetic-flux propagation through a superconducting film. This method is based on measurements of the vortex-induced magnetoresistance in normal-metal microprobes placed in the immediate vicinity of a superconductor surface. The sensitivity of such probes provides a registration of individual vortex movements in a submicrometer area of the superconductor. With use of this technique, it is demonstrated that both the penetration of magnetic flux into the superconductor and its withdrawal are accomplished not by individual vortex movements but by the hopping of vortex bundles. The size of the hopping bundles appears to be much larger than the correlation volume in the vortex lattice. A description of flux penetration based on the critical-state model, which assumes a smooth gradient of the vortex concentration across the sample, has been found to break down on the microscopic scale.