CRITICAL CURRENT AND THE SPATIAL-DISTRIBUTION OF MAGNETIC-FLUX IN SUPERCONDUCTING THIN-FILMS

Visual observations of the magnetic flux profile have been performed in TmBa2Cu3O7-x superconducting thin films using magnetic garnet films covering the superconductor as a sensor of magnetic field. The observed changes of domain structure in the garnet film during magnetic field penetration measure the normal-flux density gradient along the film plane. The same samples are used in order to measure the critical current density J(c) by means of a standard four-probe technique. A comparison between J(c) and the value of the normal-flux density gradient suggests that the critical current is sustained primarily by the normal gradient in the in-plane component of flux density. It confirms that a conventional analysis ignoring flux-line curvature cannot be applied to the superconducting thin films.