Microstructures and Critical Current Properties for YBCO Films Prepared by No-Additional-Water Annealing of Pre-Cursor Films Depoisted Y, BaF_2 and Cu

摘要

YBa_2Cu_3O_7–x (YBCO) Films are prepared on SrTiO_3 substrates by no-additional-water annealing of precursor films which are deposited at room temperature by a co-evaporation technique using Y, BaF_2 and Cu as evaporation sources. In the annealing process, we intentionally do not introduce water vapor to the reaction chamber. Previous results have indicated that the YBCO films obtained by the present annealing process grow epitaxially throughout the entire film thickness, as seen in cross-sectional transmission electron microscope (TEM) images. The YBCO films have a high critical current density (J_c) of approximately 5 MA/cm~2 at 77 K in a self-field. When magnetic fields are applied perpendicular to the film surface, the Jc decrease is relatively large in the magnetic fields less than 1 T, despite of an enough high magnetic field of glass-liquid transition, BGL, as revealed by the pinning analyses. These results indicate that crystal defects, where a magnetic flux moves easily in a magnetic field applied perpendicular to the film surface, possibly exist in the YBCO films. Moreover, some pinning centers perpendicular to the film surface appear to exist in the YBCO films, on the basis of the result of the angle dependence of magnetic fields on Jc. We investigate the microstructures from the viewpoint of crystal defects. We found that a-axis grains might play a role inJc reduction in low and medium magnetic fields and that the pinning center for magnetic fluxes penetrated perpendicular to the film surface.