Thickness dependence ofJ_cfor YBCO thin films prepared by large-area pulsed laser deposition on CeO₂-buffered sapphire substrates

Abstract

We evaluated for the first time the thickness dependence of the critical current density (Jc) of micrometre thick YBCO films on CeO2-buffered sapphire. YBCO films were successfully grown in microcrack-free form up to a thickness of by large-area pulsed laser deposition. Jc was found to decrease exponentially with YBCO thickness. Results suggest that the reduction in Jc with film thickness can be attributed to an evolving film microstructure as a function of thickness, as well as a corresponding change in the defect structures responsible for flux pinning. It was observed that film porosity and roughness increased with film thickness due to the growth and encroachment of the BaY2O4 phase. To clarify the flux pinning mechanism, we measured the angular dependence of Jc for films of different thicknesses and correlated this with the defect structure as revealed from the etch pit method and atomic force microscopy (AFM) observations. An unusually prominent Jc peak was observed when , which is due to correlated extended defects parallel to the c-axis of YBCO. Examination of the film microstructure revealed two defect types that give rise to the Jc peak: linear defects in the form of screw and edge dislocations, and planar defects possibly in the form of stacking faults. The density of linear defects decreased with film thickness whereas that of the planar defects increased considerably. From the behaviour of Jc with film thickness, these results suggest that linear defects may be more effective pinning centres than planar defects, or that an overabundance of planar defects may offset the increase of Jc for thick films.

Flux-pinning mechanisms in high current density superconducting YBa₂Cu₃O₇ films

“…So far many studies have been made on the flux-pinning mechanisms of epitaxial (RE)Ba 2 Cu 3 0 7 thin films (RE = Y, Nd, Sm, Gd, Dy, Ho, ---; abbreviated as (RE)BCO), in which high Je values have been observed [1][2][3][4][5][6][7][8][9]. In early days Hylton and Beasley theoretically pointed out that high J 0 observed in YBCO thin films is due to high density of point defects [1].…”

“…Nonetheless, nntil very recently the origin of naturally-formed "natural pins", which become the primary pinning centers in most (RE)BCO films, were not clear, and we were not able to answer a simple question, "Why high Jc is observed in epitaxial (RE)BCO thin films?". We prepared epitaxial YBCO thin films with the c-axis normal to the film surface on LaA10 3 (100) single crystals and CeOrbuffered sapphire (a-Al 2 0 3 ) substrates by PLD methods [8,9,13]. We used two different systems, one of which is a standard system with target-to-substrate distance D = 60-70 mm and another is a large-area deposition system with D = 112-142 mm.…”

“…To investigate the origin of the Jc decline with film thickness, we measured Jc(H, 8) for YBCO films with various thicknesses [9,28]. Typical examples are shown in Fig.…”

Flux-pinning mechanisms in high current density superconducting YBa₂Cu₃O₇ films

“…So far many studies have been made on the flux-pinning mechanisms of epitaxial (RE)Ba 2 Cu 3 O 7 thin films (RE = Y, Nd, Sm, Gd, Dy, Ho, ---; abbreviated as (RE)BCO), in which high J c values have been observed [1][2][3][4][5][6][7][8][9]. In early days Hylton and Beasley theoretically pointed out that high J c observed in YBCO thin films is due to high density of point defects [1].…”

“…Nonetheless, until very recently the origin of naturally-formed "natural pins", which become the primary pinning centers in most (RE)BCO films, were not clear, and we were not able to answer a simple question, "Why high J c is observed in epitaxial (RE)BCO thin films?". [8,9,13]. We used two different systems, one of which is a standard system with target-to-substrate distance D = 60-70 mm and another is a large-area deposition system with D = 112-142 mm.…”