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

Direct local measurements of the characteristics of a mixed state indu ced by a magnetic field of a small ferromagnetic particle have been ca rried 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 breakin g of these half-loops into vortex-antivortex pairs. The observed pheno mena are consistent with the Josephson medium model. We obtained the t emperature dependence of the intergrain critical current density, whic h points to strong pair breaking effects at the grain boundaries. [S01 63-1829(98)01634-8].