Superconducting Properties of (Mx/YBa2Cu3O7-δy)N Multilayer Films with Variable Layer Thickness x.

The superconducting properties of (M_x/YBa₂Cu₃O_7-δy)_N multilayer films were studied for varying layer thickness x. Different M phases were examined including green-phase Y₂BaCuO₅ (211), Y₂O₃, BaZrO₃, CeO₂, SmBa₂Cu₃O_7-δ (Sm123), brown-phase La₂BaCuO₅ (La211), and MgO. Multilayer (M_x/YBa₂Cu₃O_7-δy)N structures were grown by pulsed laser deposition onto SrTiO₃ or LaAlO₃ single-crystal substrates by alternate ablation of separate YBa_2-Cu₃O_7-δ (123) and M targets, at temperatures of 750°C to 790°C. The x layer thickness was varied from 0.1 nm to 4.5 nm, and the y 123 layer thickness was kept constant within a given range of 10 to 25 nm. Different M phase and x layer thicknesses caused large variations of the microstructural and superconducting properties, including superconducting transition (T_c), critical current density as a function of applied magnetic field J_c(H), self-field J_c(77 K), and nanoparticle layer coverage. Strong flux-pinning enhancement up to 1 to 3x was observed to occur for M additions of 211 and BaZrO₃ at 65 to 77 K, Y2O₃ at 65 K, and CeO₂ for H < 0.5 T. BaZrO₃ had a noticeably different epitaxy forming smaller size nanoparticles ~8 nm with 3 to 4x higher areal surface particle densities than other M phases, reaching 5 x 10¹¹ nanoparticles cm^-2. To optimize flux pinning and J_c (65 to 77 K, H = 2 to 3 T), the M layer thickness had to be reduced below a critical value that correlated with a nanoparticle surface coverage <15% by area. Unusual effects were observed for poor pinning materials including Sm123 and La211, where properties such as self-field J_c unexpectedly increased with increasing x layer thickness. [ABSTRACT FROM AUTHOR]