Reduction in the field-dependent microwave surface resistance in YBa(2)Cu(3)O(7-delta) with submicrometric BaZrO(3) inclusions as a function of BaZrO(3) concentration

In order to study the vortex pinning determined by artificially introduced pinning centers in the small-vortex displacement regime, we measured the microwave surface impedance at 47.7 GHz in the mixed state of YBa(2)Cu(3)O(7-delta) thin films, where submicrometric BaZrO(3) particles have been incorporated. As a function of the BaZrO(3) content, we observed that the absolute losses slightly decrease up to a BaZrO(3) content of 5%, and then increase. We found that the magnetic-field-induced losses behave differently in that they are not monotonic with increasing BaZrO(3) concentration. At small concentration (2.5%) the field-induced losses increase, but large reduction in the losses themselves, by factors up to 3, is observed upon further increasing the BaZrO3 concentration in the target up to 7%. Using measurements of both surface resistance and surface reactance, we estimate vortex pinning-related parameters. We found that BaZrO(3) inclusions introduce deep and steep pinning wells. In particular, the minimum height of the energy barrier for single vortices is raised. At larger BaZrO(3) content (5% and 7%) the phenomenon is at its maximum, but it is unclear whether it shows a saturation or not, leaving room for further improvements. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3056179]