Interplay Between as Grown Defects and Heavy Ion Induced Defects in YBCO Films

The dominating pinning mechanism in YBa 2 Cu 3 O 7-x thin film has been investigated by means of resistivity and ac susceptibility measurements and by structural characterizations. The aim of the work is to determine optimal pinning conditions in order to prevent the field-induced drop of the critical current density. The Clem and Sanchez model is used to extract critical current values from the susceptibility data. In this framework, the role of columnar-like defects is extensively studied. The formation of columnar pins was promoted by a suitable modification in the preparation process. In addition, columnar tracks of different density were induced by Au-ion irradiation. Analysis of the critical current dependence on the magnetic field reveals that linearly-correlated defects are effective in improving the vortex lattice stability in "matching regions" of field and temperature.