Thickness dependence of Jc for YBCO thin films prepared by large-area pulsed laser deposition onCeO2-buffered sapphire substrates

Abstract

We evaluated for the first time the thickness dependence of the critical current density(Jc) of micrometrethick YBCO films on CeO2-buffered sapphire. YBCO films were successfully grown in microcrack-free form up to athickness of by large-area pulsed laser deposition.Jc was found to decrease exponentially with YBCO thickness. Results suggest that the reduction inJc with film thickness can be attributed to an evolving film microstructure as afunction of thickness, as well as a corresponding change in the defect structuresresponsible for flux pinning. It was observed that film porosity and roughnessincreased with film thickness due to the growth and encroachment of theBaY2O4 phase. To clarify the flux pinning mechanism, we measured the angular dependence ofJc for films of different thicknesses and correlated this with the defect structure as revealed from theetch pit method and atomic force microscopy (AFM) observations. An unusually prominentJc peak was observed when , which is due to correlated extended defects parallel to thec-axis of YBCO. Examination of the film microstructure revealed two defect types that give rise tothe Jc peak: linear defects in the form of screw and edge dislocations, and planar defects possiblyin the form of stacking faults. The density of linear defects decreased with film thicknesswhereas that of the planar defects increased considerably. From the behaviour ofJc with film thickness, these results suggest that linear defects may be more effective pinningcentres than planar defects, or that an overabundance of planar defects may offset the increase ofJc for thick films.