Imaging Current Percolation and Ac Losses in Artificially Granular YBCO Thin Films

The magnetic effects of granularity in YBCO tapes are investigated using model granular systems. Numerical simulations of the field profile, current distribution and AC susceptibility in simple granular configurations have been carried out under a critical-state, magnetic energy minimization approach. Experimentally, an artificially patterned "multigranular" YBCO epitaxial thin film has been fabricated and used to investigate the geometry-related magnetic effects of granularity. High-resolution Hall probe microscopy measurements have allowed us imaging the evolution of the magnetization distribution with the applied field, and deduce the circulation of inter- and intragranular critical-currents. AC-susceptibility measurements up to high driving-fields evidenced that AC losses arise mainly from dissipation at the artificial "grain boundary" network.