Superconducting properties of (M _x /YBa ₂Cu ₃O _7-δy ) _N multilayer films with variable layer thickness x

The superconducting properties of (M _x /YBa ₂Cu ₃O _7-δy ) _N multilayer films were studied for varying layer thickness x. Different M phases were examined including green-phase Y ₂BaCuO ₅ (211), Y ₂O ₃, BaZrO ₃, CeO ₂, SmBa ₂Cu ₃O _7-δ (Sm123), brown-phase La ₂BaCuO ₅ (La211), and MgO. Multilayer (M _x /YBa ₂ Cu ₃O _7-δy ) _N structures were grown by pulsed laser deposition onto SrTiO ₃ or LaAlO ₃ single-crystal substrates by alternate ablation of separate YBa ₂Cu ₃O _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 BaZrO ₃ at 65 to 77 K, Y ₂O ₃ at 65 K, and CeO ₂ for H < 0.5 T. BaZrO ₃ 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 × 10 ¹¹ 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.