Nano-engineered defect structures in Ce- and Ho-doped metal-organic chemical vapor deposited YBa₂Cu₃O_6+δ films: Correlation of structure and chemistry with flux pinning performance

This study reports on the fabrication of metal-organic chemical vapor deposited (MOCVD) YBa₂Cu₃O_6+δ (YBCO) films doped with varying amounts of Ce and Ho and the characterization of their electrical, microstructural, and chemical properties. The films are prepared by vapor phase deposition of a Y-Ba-Cu precursor mix containing controlled amounts of Ce and Ho onto buffered metal strip templates. The comprehensive characterization of these films by critical current measurement, transmission electron microscopy, x-ray diffraction, Raman microspectroscopy, and x-ray absorption spectroscopy provides detailed information about the structure/chemistry/performance relationships and how they vary with varying amounts of Ce and Ho in the YBCO films. The microstructure exhibited by both the Ce-doped and the Ho-doped films contains a high density of crystal basal-plane aligned, fluoritelike precipitates within the YBCO matrix. For optimally doped samples, the influence of these nanocrystalline phases on the flux pinning properties manifests itself as a significant improvement in the critical current density (J_c) for magnetic field orientations that approach being parallel to the ab planes of the YBCO, while no appreciable change is observed in either self-field J_c or applied-field J_c performance in the vicinity of field orientations parallel to the YBCO c-axis. The Ce is almost exclusively concentrated in the fluoritelike nanoprecipitates, while the Ho incorporates into both the planar arrays of nanoprecipitates and the superconducting matrix, where it substitutes for Y in the YBCO lattice. The present findings for Ce and Ho doping are in interesting contrast with our prior findings for Zr-doped MOCVD films due to the fact that the Zr-doped films exhibit columnar precipitate arrays that produce a substantial improvement in J_c for magnetic field orientations parallel to the YBCO c-axis, while no appreciable change is observed in either self-field or applied-field J _c performance for field orientations parallel to the ab planes. These results suggest that improvement of the in-field J_c of YBCO films over an extended range of field angles could be achieved by doping strategies that simultaneously produce both ab-plane aligned and c-axis aligned precipitate arrays.