Three-dimensional vortex pinning by nano-precipitates in a Sm-doped YBa₂Cu₃O_7-x coated conductor

We report on the thickness and angular dependence of the critical current density J_c(H,), the irreversibility field H^irr, and the bulk pinning force F_p(H) of a metal-organic chemical vapour deposition (MOCVD) grown YBa₂Cu₃O_7-x (YBCO) coated conductor, which contains ∼17vol% of ∼10nm sized (Y,Sm) ₂O₃ precipitates with an average spacing of ∼10-15nm. Some surface porosity and amorphous second-phase particles on the scale of ∼0.5-1νm appear to reduce the current-carrying cross-section, which controls the magnitude of J_c but not the vortex pinning. We observed an enhanced H^irr∼9 T at 77K along the c-axis which, like the shape of J_c(H) and F_p(H), was independent of thickness as the sample was milled down to ∼0.16νm. Angular-dependent measurements of J_c showed the usual excess vortex pinning along the c-axis and along the ab-plane, but with a background that could only be fitted with an unusually small anisotropy parameter of 3, which, like the high H^irr and the thickness-independent shape of F_p(H), we ascribe to strong vortex pinning centre interactions. Together, these measurements show very different behaviour from most pulsed-laser-deposited films, which exhibit strong thickness-dependent properties. We ascribe the present different results to the dense array of small, insulating precipitates, which act as strong pinning centres and produce strong three-dimensional (3D) vortex pinning, because their separation of 10-15nm is always much smaller than the film thickness.