Island growth of Y₂BaCuO₅ nanoparticles in (211_{∼1.5 nm}/123_{∼10 nm})×N composite multilayer structures to enhance flux pinning of YBa₂Cu₃O_7−δ films

A controlled introduction of second-phase Y₂BaCuO₅ (211) nanoparticles into YBa₂Cu₃O_7−δ (123) thin films was achieved for the first time for the purpose of increasing flux pinning. The island-growth mode of 211 on 123 was utilized to obtain an area particle density >10¹¹ cm^-2 of 211 thick-disk-shaped nanoparticles in individual layers. Composite layered structures of (211_y nanoparticles/123_z)×N were deposited by pulsed laser deposition on LaAlO₃ substrates, with N bilayers = 24 to 100, y thickness = 1 to 2 nm, and z thickness = 6 to 15 nm (assuming continuous layer coverage). With 211 addition, the critical current densities at 77 K were higher at magnetic fields as low as 0.1 T and increased as much as approximately 300% at 1.5 T. The superconducting transition temperature was reduced by approximately 2 to 4 K for 211 volume fraction <20%. Reinitiation of 123 growth after every 211 layer resulted in a smooth and flat surface finish on the films and also greatly reduced surface particulate formation especially in thicker films (∼ μm).