Pinning, thermally activated depinning and their importance for tuning the nanoprecipitate size and density in high J c YBa 2Cu 3O 7−x films

Abstract

YBa 2Cu 3O 7− x (Y123) films with quantitatively controlled artificial nanoprecipitate pinning centers were grown by pulsed laser deposition (PLD) and characterized by transport over wide temperature ( T) and magnetic field ( H) ranges and by transmission electron microscopy (TEM). The critical current density J c was found to be determined by the interplay of strong vortex pinning and thermally activated depinning (TAD), which together produced a non-monotonic dependence of J c on c-axis pin spacing d c . At low T and H, J c increased with decreasing d c , reaching the very high J c ∼ 48 MA/cm 2 ∼20% of the depairing current density J d at 10 K, self-field and d c ∼ 10 nm, but at higher T and H when TAD effects become significant, J c was optimized at larger d c because longer vortex segments confined between nanoprecipitates are less prone to thermal fluctuations. We conclude that precipitates should extend at least several coherence lengths along vortices in order to produce irreversibility fields H irr (77 K) greater than 7 T and maximum bulk pinning forces F p ,max(77 K) greater than 7–8 GN/m 3 (values appropriate for H parallel to the c-axis). Our results show that there is no universal pin array that optimizes J c at all T and H.