Multifunctional, phase separated, BaTiO₃+CoFe₂O ₄ cap buffer layers for improved flux-pinning in YBa₂ Cu₃O_7-δ based coated conductors

Phase separated, epitaxial, nanostructured film comprised of BaTiO ₃ (BTO) and CoFe₂O₄ (CFO) composite has been developed as a potential multifunctional cap buffer layer for improved flux-pinning in YBa₂Cu₃O_7-δ (YBCO) films. All films were deposited by pulsed laser deposition on SrTiO₃ (STO) (100) single crystal substrates. The CFO fraction and growth temperature were identified as key factors for determining the areal number density and mean diameter of the CFO nanocolumns. Compared to the reference sample grown on a pure BTO cap layer, the YBCO films grown on BTO+CFO cap layers show a remarkable improvement in isotropic flux-pinning and, consequently, J_c over the entire field and angle ranges. Transmission electron microscopy analysis confirmed the presence of a very defective YBCO layer containing a high density of randomly distributed defects at the interface area, induced by nanostructural modulation on the surface of the BTO+CFO composite cap layer.