Relative pinning strength of twin boundaries and outgrowths in YBa₂Cu₃O₇ thin films and superlattices

YBa₂Cu₃O₇ films and superlattices displaying various structural properties were synthesized. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy characterization reveal that the defect densities can be changed over a large range of magnitude. In particular, by controlling the strain relaxation in the material, superlattices without twin boundaries as well as highly twinned ones are deposited ac susceptibility measurements are performed on many samples. Using a recent procedure based on flux-creep models, the relaxation rate of the vortices as a function of temperature is plotted in the region of large creep. The qualitative correlation between the defects densities (outgrowths, twins) and the pinning properties provides a description of the pinning landscape. Twin boundaries seem to play a preponderant role in vortex pinning. Intersections between the twin planes are believed to be the main pinning sites when present in our samples.