Enhancement of flux pinning in YBa₂Cu₃O_7-δ films via nano-scale modifications of substrate surfaces

We have acquired positive results in controlled study to investigate the effects of substrate surface modification on the growth-induced flux-pinning nanostructures in YBCO films. Second-phase nano-scale iridium, (MgO, BaZrO3), and (CeO2, BaTiO3) particles were applied to single crystal and biaxially textured metal substrate surfaces using dc-magnetron sputtering, solution- and suspension-based techniques, respectively. YBCO films were then grown on these and unaffected control substrate surfaces. A combination of methods was used to document the superconducting and structural properties. Critical current levels of YBCO films grown on the modified substrates were measured over a wide range of field and temperature by transport, magnetization, and magneto optical imaging. Initial results have shown systematic enhancement in the critical current densities (Jc). Cross-sectional TEM analysis reveals nanoparticle induced defects and second-phase precipitates in proximity to surface nanoparticles. Details of the field-orientation dependence of Jc are found to be related to the nanoparticle species. Nanoparticles applied by using suspension-based techniques revealed correlated c-axis pinning and improved in-field Jc performance, while for sputtered and solution processed nanoparticles, results show a more uniform dependence of Jc over all orientations of magnetic field, along with improved irreversibility behavior. Analysis of the scaling behavior of pinning force density, Fp, indicates consistency of the pinning mechanism with respect to temperature and provides a powerful tool for predicting Jc at arbitrary B,T.