Defects and self-organized vortices in melt-textured YBa sub(2)Cu sub(3)O sub(7-x)

This paper presents the structural as well as the magnetic properties of high-quality YBa sub(2)Cu sub(3)O sub(7-x) melt-textured material, before and after 2 GeV Au ion irradiation. The irradiation affects a surface layer of about 10% of the whole sample thickness. The core of the paper consists in the investigation of the behaviour of the critical current densities J sub(c) and the pinning forces F sub(p) as a function of the magnetic field. The pinning forces are analyzed using a modified Dew-Hughes model. Particular emphasis is given to the influence of different kinds of defects, evaluated at 65 and 75 K, before and after irradiation. The low-current regime is also investigated by means of the irreversibility line evaluation. It is shown that at higher temperature the main properties of the irradiated samples are determined by the competition and/or correlation between surface columnar defects and intrinsic defects, distributed inside the full volume. The vortices trapped in the surface layer by the columnar defects seem to contribute to organize the matching phenomena between the defect lattice and the vortex lattice, at fields near and above the dose-equivalent field.