Trapped Flux and Critical Currents in Superconducting Thin-Film Rings

Critical persistent currents have been measured in thin-film tin rings by a mechanical method which utilizes the magnetic moment due to trapped flux in such rings. In addition, this technique yields a measurement of the penetration depth for critical persistent currents in thin films. Currents slightly less than critical have been shown to be truly persistent for periods of more than 10 hours in films whose thickness is less than 5% of the penetration depth. It is found that for tin films less than 700 Å thick, current densities greater than 10⁶ amp/cm² can be readily achieved within a degree of the transition temperature.