Modifying the Current Distribution of Grain Boundaries in YBCO Films

Abstract

The current distribution in thin superconducting films can be influenced by magnetic surroundings. A soft magnet put parallel to a thin film edge can reduce or prevent flux entry and therefore stabilise Meissner screening currents in the film. This is particularly interesting for the investigation of currents across grain boundaries in high temperature superconductors, where the critical current density strongly depends on the flux which penetrates into the grain boundary. Furthermore, using special magnetic arrangements, asymmetric flux and current distributions can be obtained. An increase of the grain boundary critical current density is obtained in the Meissner state, compared to flux penetrated states. We show, that flux penetration into grain boundaries can be suppressed up to a certain external field H*, which depends on the temperature, allowing larger intergranular current densities. All investigations are done by magneto-optical imaging and the inversion of Biot and Savart.