Using advanced magneto-optics, the field and current distributions in superconducting thin film YBa2⁢Cu3⁢O7 rings in an externally applied magnetic field are studied experimentally. The observations are in close agreement with numerical calculations. During the initial flux penetration and field reversal a highly nonuniform current distribution is observed. In particular, concentric counterrotating current loops occur during field reversal. We explore implications of these results for the determination of critical currents and penetration fields from bulk magnetization measurements.

In a recent paper [1], it has been shown from transport measurements that artificial reversible pinning can be created by the application of a magnetic tape on the surface of a thin superconducting film. We study the influence of a periodic array of magnetic stripes, generated from a signal pre-recorded on a magnetic tape, on the pinning of vortices. With our high resolution magneto-optical technique [2], we visualize in two dimensions the penetration of the flux in a thin YBa2Cu3O7-δ film partially covered by such a magnetic tape. In particular, we investigate the evolution of flux entry in the superconducting strip under and next to the magnetic tape. We present results obtained for various temperatures and various applied fields.

We have investigated magnetic flux relaxation at low temperatures in a 90 nm epitaxial coevaporated film on YBa2Cu3O7−δ superconductor. This film is characterized by a high critical current Jc of 3.5 × 1011Am−2 at 4.2 K, and 1.3 × 1010Am−2 at 77 K. Although Jc depends strongly on the applied magnetic field B the magnetic relaxation is only weakly dependent on B. Also the magnetic relaxation does not vanish at zero temperature, indicating that besides a thermally activated process there is another intrinsic process working.