O 7-d (Sm123), brown-phase La 2 BaCuO 5 (La211), and MgO. Multilayer (M x /YBa 2 Cu 3 O 7-dy ) N structures were grown by pulsed laser deposition onto SrTiO 3 or LaAlO 3 single-crystal substrates by alternate ablation of separate YBa 2-Cu 3 O 7-d (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 2 O 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.

Time-resolved magneto-optical studies are performed on MgB2 thin film samples grown by in situ pulsed laser deposition and in situ reactive deposition technique. The latter reveal dendritic avalanche-free flux penetration. The kinetic roughening of magnetic flux penetration is studied for applied ac current. Dynamic scaling laws determined for both static field and ac current are consistent with the directed percolation depinning model, placing the vortex dynamics in MgB2 in the same universality class as YBCO and Nb.