Dynamic Evolutions of Flux Distributions in a Superconductor by a Pulsed Current*

The dynamics of vortices in a superconductor has long been investigated because of its practical importance for the application of the superconducting magnets. The direct imaging of flux-density distributions must be powerful for the understandings of the dynamics of vortices. However, observations of the time-evolution of the flux-density distributions have been scarcely reported. From this viewpoint, it is still unclear how the static state of vortices collapses into the flow state. Hence, we observed the changes of flux–density distributions in a superconductor by a pulsed current with the magneto-optical microscopy. The flux-density distributions in a NbN film were measured up to 10000 frames per second. We investigated for different initial flux distributions in a comparative manner; the field-cooled state, the remanent state, the zero-field-cooled (ZFC) state. In the remanent state and the ZFC state, local reconfiguration of vortices occurred even below the critical current, which was qualitatively explained within the critical state model. However, some deviations between experiments and the theory were observed in the ZFC state, indicating that the current flows in the sample even far below the critical current.