Magneto-optical investigation of flux penetration in a superconducting ring

Using advanced magneto-optics, the field and current distributions in superconducting thin film (formula presented) 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.