Mechanisms of weak thickness dependence of the critical current density in strong-pinning ex situ metal-organic-deposition-route YBa₂Cu ₃O_7-x coated 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 νm thick high-I_c YBa₂Cu₃O _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.