Effects of microstructure on flux pinning in epitaxial ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$ films

The role of microstructure on flux pinning in c-axis-oriented epitaxial ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{\mathit{x}}$ films grown on ${\mathrm{LaAlO}}_3$ and ${\mathrm{SrTiO}}_3$ has been studied. For a magnetic field parallel to the Cu-O planes, the resistivity and critical current density ${\mathit{J}}_{\mathit{c}}$ have been measured as a function of the angle θ between the applied field and the direction of the transport current. In addition to a Lorentz-force-independent resistivity, the Lorentz-force-dependent component showed several broad deviations from ${\sin }^2$θ when a field was aligned parallel to certain microstructural features which vary with the film thickness and substrate material. These features were identified by transmission-electron-microscopy analysis. For films of 5000 Å thickness on ${\mathrm{LaAlO}}_3$, resistivity dips were observed for a field applied parallel to the substrate twin boundaries or along misoriented a-axis grains. In thinner films of 900 Å thickness also on ${\mathrm{LaA}1\mathrm{O}}_3$ in which a-axis grains were negligible, we observed dips corresponding to the orientation of substrate twin boundaries only. For thin films on ${\mathrm{SrTiO}}_3$ in which substrate twins are absent, resistivity dips corresponding to the direction of twin boundaries in the film and, perhaps, interfacial dislocations were observed. Overall, such dips decreased with increasing transport current density and became negligible in ${\mathit{J}}_{\mathit{c}}$ measurements.