Porosity induced magnetic granularity in epitaxial YBa2Cu3O7 thin films

We present a study of the interrelation between porosity and magnetic granularity in epitaxial YBa2Cu3O7 thin films. The main signature of the magnetic granularity is the shift in the hysteresis loops of the maximum magnetization to positive applied magnetic fields. The existing pores promote the formation of constrictions where the capability to carry current is reduced and the sample behaves as a granular material with two critical current densities, a local J(c)(G) and a percolating J(c)(PERC). We show that only J(c)(PERC) depends on porosity and it exhibits a clear correlation with normal state resistivity. On its side, J(c)(G) reflects the local current density of the films and it is not modified by the presence of holes. The geometrical origin of the magnetic granularity is confirmed by observing the same phenomenology in thin films where porosity has been artificially induced.