Field-dependent critical currents in thin Nb superconducting disks

Abstract

Magnetic hysteresis loops were measured on thin Nb superconducting disks using a SQUID-based technique; field-dependent critical currents have been observed in thin superconducting disks at low magnetic flux trapping densities (⩽ 4 G). Such phenomena were found to exist over a wide range of temperatures below the transition temperature. The observed field-dependent critical current densities are attributed to long-range interactions between trapped vortices in a superconducting 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 critical currents by fitting the measured magnetic hysteresis loops. Our experimental data were adequately explained by Kim's critical state model. Shielding currents and magnetic field patterns in thin superconducting disks were calculated.