Mechanisms of weak thickness dependence of the critical current density in strong-pinningex situmetal–organic-deposition-route YBa₂Cu₃O7−xcoated conductors

We report on the thickness dependence of the superconducting characteristics including critical current I c , critical current density J c , transition temperature T c , irreversibility field H irr , bulk pinning force plot F p ( H ), and normal state resistivity curve ρ( T ) measured after successive ion milling of ∼1 µm thick high- I c YBa 2 Cu 3 O 7− x films made by an ex situ metal–organic deposition process on Ni–W rolling-assisted biaxially textured substrates (RABiTS TM ). In contrast to many recent data, mostly on in situ pulsed laser deposition (PLD) films, which show strong depression of J c with increasing film thickness t , our films exhibit only a weak dependence of J c on t . The two better textured samples had full cross-section average J c,avg (77 K, 0 T) ∼4 MA cm −2 near the buffer layer interface and ∼3 MA cm −2 at full thickness, despite significant current blocking due to ∼30% porosity in the film. Taking account of the thickness dependence of the porosity, we estimate that the local, vortex-pinning current density is essentially independent of thickness, while accounting for the additional current-blocking effects of grain boundaries leads to local, vortex-pinning J c values well above 5 MA cm −2 . Such high local J c values are produced by strong three-dimensional vortex pinning which subdivides vortex lines into weakly coupled segments much shorter than the film thickness.