Three-dimensional vortex pinning by nano-precipitates in a Sm-doped YBa2Cu3O7−x coated conductor

We report on the thickness and angular dependence of the critical current density Jc(H,theta), the irreversibility field Hirr, and the bulk pinning force Fp(H) of a metal-organic chemical vapour deposition (MOCVD) grown YBa2Cu3O7-x (YBCO) coated conductor, which contains ~17 vol% of ~10 nm sized (Y,Sm)2O3 precipitates with an average spacing of ~10-15 nm. Some surface porosity and amorphous second-phase particles on the scale of ~0.5-1 mum appear to reduce the current-carrying cross-section, which controls the magnitude of Jc but not the vortex pinning. We observed an enhanced Hirr~9 T at 77 K along the c-axis which, like the shape of Jc(H) and Fp(H), was independent of thickness as the sample was milled down to ~0.16 mum. Angular-dependent measurements of Jc 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 Hirr and the thickness-independent shape of Fp(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 10-15 nm is always much smaller than the film thickness.