A close correlation between nanostructure formations and the thickness dependence of the critical current density in pure and BaSnO_3-added GdBa_2Cu_3O_(7-δ) films

摘要

A close correlation between the nanostructure formations and the thickness dependence of the infield critical current density (J_c) in GdBa_2Cu_3O_(7-δ) (GdBCO) films is reported. Pure and 2wt. ％ BaSnO_3 (BSO)-added GdBCO films with film thicknesses (d) ranging from 0.2 μm to 1.5 μm were deposited on SrTiO_3 single-crystalline substrates by using a pulsed laser deposition technique. Magnetization data measured at 77 K with the magnetic field applied parallel to the c-axis of the films showed the general trend of decreasing in-field J_c with increasing residual film thickness. The two special inversions, however, were observed at d ～0.6 μm, for which the J_c's of both the pure and BSO-added GdBCO films were larger than those of the films with d ～ 0.4 μm. A sequential ion-milling process and scanning electron microscopy studies were employed to examine the microstructural evolution in the 1.5- μm-thick GdBCO films. For the pure GdBCO films, nanosized dislocations were observed to start growing at intersections of perpendicularly connected a-axis-oriented grains in a residual film thickness (t) of ～ 0.6 μm and the growth persisted to the top surface of the 1.5-μm-thick GdBCO film. For the BSO-added GdBCO films, the density of BSO nanorods was estimated to be decreased versus t with an increase found at t of ～ 0.6 μm A reason for this inversions of J_c might be the nanostructure formations at t ～0.6 μm, which were proved to serve as effective pinning centers.