Pinning, thermally activated depinning and their importance for tuning the nanoprecipitate size and density in high Jc YBa2Cu3O7−x films

YBa2Cu3O7−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 Jc 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 Jc on c-axis pin spacing dc. At low T and H, Jc increased with decreasing dc, reaching the very high Jc ∼ 48 MA/cm2 ∼20% of the depairing current density Jd at 10 K, self-field and dc ∼ 10 nm, but at higher T and H when TAD effects become significant, Jc was optimized at larger dc 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 Hirr(77 K) greater than 7 T and maximum bulk pinning forces Fp,max(77 K) greater than 7–8 GN/m3 (values appropriate for H parallel to the c-axis). Our results show that there is no universal pin array that optimizes Jc at all T and H.