Magnetic field dependence of the critical current and the flux pinning mechanism inYBa2Cu3O6+xfilms doped withBaZrO3

Abstract

Dependence of the critical current on applied magnetic field ${j}_{c}({B}_{a})$ is investigated in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+x}$ (YBCO) films prepared by pulsed laser deposition from pure YBCO target and from those doped with 2.9 or $9.0\phantom{\rule{0.3em}{0ex}}\mathrm{wt}.\phantom{\rule{0.2em}{0ex}}%$ ${\mathrm{BaZrO}}_{3}$. All targets were sintered from nanophase powders made by a citrate-gel process. Transmission electron microscopy images show distributions of ${\mathrm{BaZrO}}_{3}$ particles with typical diameters of $5--10\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ in the doped films. In comparison with the pure YBCO film the value of ${j}_{c}(0)$ is slightly smaller in the film deposited from the target doped with $2.9\phantom{\rule{0.3em}{0ex}}\mathrm{wt}.\phantom{\rule{0.2em}{0ex}}%$ ${\mathrm{BaZrO}}_{3}$ and only half in case of $9.0\phantom{\rule{0.3em}{0ex}}\mathrm{wt}.\phantom{\rule{0.2em}{0ex}}%$ doping. Two interesting phenomena in the behavior of ${j}_{c}({B}_{a})$ are observed for the doped films. First, the low-field plateau of ${j}_{c}$ is significantly lengthened and second, in fields above the accommodation field ${B}^{*}$ marking the end of this plateau the rate of decrease of ${j}_{c}$ is strongly reduced. These results are well fitted with a critical state model of superconductors taking into account the self field effects at low applied magnetic fields.