Experimental investigation of a local mixed state induced by a small ferromagnetic particle in Y-Ba-Cu-O films: Extremely low energy barrier for formation of vortex-antivortex pairs

Abstract

Direct local measurements of the characteristics of a mixed state induced by a magnetic field of a small ferromagnetic particle have been carried out in thin film Y-Ba-Cu-O samples, using a scanning Hall probe. The results provide evidence for an extremely low energy barrier for entry of vortex half-loops into high quality films and further breaking of these half-loops into vortex-antivortex pairs. The observed phenomena are consistent with the Josephson medium model. We obtained the temperature dependence of the intergrain critical current density, which points to strong pair breaking effects at the grain boundaries.