Mechanisms of weak thickness dependence of the critical current density in strong-pinning ex situ metal–organic-deposition-route YBa2Cu3O7− x coated conductors (original) (raw)

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 the 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 ). Contrary 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 (77K,0T) ~ 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. which levels off above a critical thickness t c ~ 1 µm [2-9]. Such a thickness dependence is suggestive of the transition from the 2 dimensional (2D) pinning of rigid vortex lines in films thinner than the longitudinal pinning correlation length l c to the 3-dimensional (3D) pinning of deformable vortices at t > t c [14]. It was recently pointed out [15] that t c can indeed approach a few µm if the collective pinning model incorporates a multi-scale pinning potential appropriate for the strong-pinning second phase precipitates, pores, and correlated defects found in CCs. Strong-pinning defects have a pin interaction range, r p much greater than the coherence length ξ and produce large plastic deformations of vortices, rather than the small elastic deformations produced by weak, point pins. This strong-pinning model predicts a crossover thickness t c as large as 1-2 µm, in agreement with the observed J c (t) dependence of many PLD films [2-8] and qualitatively consistent with many recent studies of the angular dependence of 3 J c in CCs, which also reveal much evidence for correlated pinning along the c-axis in PLD films . This multiscale pinning model also predicts the t -1/2 thickness dependence of J c (t), but the magnitudes of J c and t c can be very dependent on the specific pinning microstructure and thus on the film growth process.