Study of trapped flux in a superconducting thin film-observation by scanning SQUID microscope and simulation

Flux trapping in superconducting devices, such as Josephson circuits and SQUIDs is a major cause for degradation of device performance. Intentionally made holes and moats in superconducting films were previously found effective in overcoming the effects of flux trapping. Despite the need for a design rule for the arrangement of holes or moats, comparison between experiment and theory of the flux trapping locations has not yet been discussed. We therefore studied the locations of trapped fluxes in a superconducting thin film cooled at a rate of 0.02 K/sec for various external magnetic flux densities from 1 μT to 3 μT by using a scanning SQUID microscope. Trapped fluxes were observed at positions where holes were prepared in the superconducting film and also at superconducting locations outside the holes. Trapped fluxes outside the holes were orderly arranged regardless of magnetic flux density during cooling. These locations were then compared with those determined by simulations based on a model that considers both the surface barrier effect introduced by Bean and Livingston and the interaction among fluxes. The simulation shows that the potential wells appear below the transition temperature, corresponding to the trapped fluxes outside holes