FIELD-DEPENDENT CRITICAL CURRENTS IN THIN NB SUPERCONDUCTING DISKS

Magnetic hysteresis loops were measured on thin Nb superconducting dis ks using a SQUID-based technique; field-dependent critical currents ha ve been observed in thin superconducting disks at low magnetic flux tr apping densities (greater-than-or-equal-to 4 G). Such phenomena were f ound to exist over a wide range of temperature below the transition te mperature. The observed field-dependent critical current densities are attributed to long-range interactions between trapped vortices in a s uperconducting film whose thickness is much less than or comparable to the penetration depth. In the framework of Kim's critical state model , we developed a numerical method to calculate the field-dependent cri tical currents by fitting the measured magnetic hysteresis loops. Our experimental data were adequately explained by Kim's critical state mo del. Shielding currents and magnetic field patterns in thin supercondu cting disks were calculated.