Control of the critical current density inYBa2Cu3O7−δfilms by means of intergrain and intragrain correlated defects

Abstract

The aim of this work is to investigate the mechanisms controlling the current-carrying capability of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ thin films. A comparison between the magnetic properties of a film with intrinsic grain-boundary defects and two films crossed by columnar defects with different densities is presented. Such properties have been studied by means of ac susceptibility measurements, resistivity measurements, and structural characterizations. The Clem and Sanchez model [Phys. Rev. B 50, 9355 (1994)] is used to extract critical current values from the susceptibility data. In the virgin film, correlated grain-boundary defects were created among islands with homogeneous size, by means of the appropriate modifications in the growth process. Columnar defects were produced through 0.25-GeV Au-ion irradiation. The central issue concerns the investigation of the plateaulike features characterizing the log-log field dependence of the critical current density, the analysis of the ${J}_{c}$ temperature dependence, and of the irreversibility line. An analytical expression of ${J}_{c}$ vs B is given in order to compare the main issues with the experimental data. This model suggests that the intergrain pinning dominates in the high-current/low-temperature regime through a network of frustrated Josephson junctions, while the intragrain pinning is effective near the irreversibility line.