Magnetic field dependence of the critical current and the flux pinning mechanism in YBa2Cu3O6+x films doped with BaZrO3

Dependence of the critical current on applied magnetic field j(c)(B-a) is investigated in YBa2Cu3O6+x (YBCO) films prepared by pulsed laser deposition from pure YBCO target and from those doped with 2.9 or 9.0 wt. % BaZrO3. All targets were sintered from nanophase powders made by a citrate-gel process. Transmission electron microscopy images show distributions of BaZrO3 particles with typical diameters of 5-10 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 wt. % BaZrO3 and only half in case of 9.0 wt. % 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.