Nanostructural defects for effective flux pinning in high-quality Bi2Sr2CaCu2O8+x epitaxial thin films with high critical current density

Abstract

The dependence of the critical current density (Jc) and current–voltage (I–V) characteristics on the magnetic field H and temperature T were determined in c-axis-oriented epitaxial Bi2Sr2CaCu2O8+x (Bi2212) thin films prepared by metalorganic chemical vapor deposition. The films showed high Tc(R = 0) ≥ 83 K and high transport Jc > 1010 A m−2 at 10 K, but Jc decreased sharply when a magnetic field was applied in the c direction, except at low temperatures (≤20 K). The n-values (indices of the power-law I–V characteristics) decreased together with Jc, showing that the decrease is caused by thermally activated flux motion. To clarify the flux pinning mechanism operating in the films, nanostructural defects were investigated by transmission electron microscopy (TEM). Cross-sectional TEM images of the thin films revealed that the Bi2212 phase was twinned and that the twin boundary runs either parallel or perpendicular to the ab-plane depending on the region. We observed misfit dislocations at twin boundaries, and other dislocations associated with stacking faults parallel to the ab-plane, as well as anti-phase boundaries. Contrasts corresponding to dislocations parallel to the ab-plane and/or to the anti-phase boundaries were observed in plan-view TEM images, and nanoscale precipitates as observed in some YBa2Cu3O7−y (YBCO) films were almost entirely absent. The temperature dependence of low-field Jc, which was mostly unaffected by the thermally activated flux motion, approximately followed ∼(1−T/Tc)m(1+T/Tc)2 (m = 1.7–2.2). The temperature dependence with m ≈ 2, which can be explained by a simple theoretical model, was recently observed in YBCO thin films in which flux pinning was dominated by linear pins (partial dislocations surrounding stacking faults parallel to the ab-plane). From these results and theoretical considerations on the flux pinning force, we conclude that flux pinning in our Bi2212 films is mainly caused by dislocations parallel to the ab-plane and anti-phase boundaries.