Growth-induced columnar defects in YBa 2Cu 3O 7−x thin films grown on miscut

Abstract

Measurements of the critical current density in YBa 2Cu 3O 7− x (YBCO) thin films grown on deliberately miscut (1.6 °) LaAlO 3 substrates that also contain a small mosaic spread of subgrain orientations have shown an anomalous angular dependence of J c. A strong, magnetic field-dependent peak in J c is observed when the magnetic field is oriented between the c-axis (the film normal direction) and the a- b-plane; in this orientation flux pinning is normally not strong. Transmission electron microscopy (TEM) has been used to determine the defects which are responsible for the flux pinning related to the anomalous J c peak. [001] columnar defects have been observed to penetrate right through the YBCO films. The diameters of the columnar defect regions are ∼3–5 nm, which is near ideal for producing the observed anomalous pinning and high J c. Numerous stacking faults involving excess CuO layers have also been observed; these can also serve as effective flux pinners. Small-angle planar boundaries perpendicular to the substrate are rarely seen. Images of the YBCO-substrate interface reveal that some of the columnar growth defects initiate at substrate steps/dislocations. The YBCO unit cells grown on the upper and lower terraces of a substrate step have a relative shift of c/3[001]; thus, the CuO planes are broken within a region less than 10 nm from the substrate, presumably reducing the superconducting order parameter in this region and leading to pinned vortices situated on these sites. Thus, the columnar defects initiated from the interface and the stacking faults are useful for pinning flux through the entire film thickness.