Deposition and characterization of YBa₂Cu₃O _7-δ/LaMnO₃/MgO/TiN heterostructures on Cu metal substrates for development of coated conductors

In this paper a novel buffer layer architecture consisting of LaMnO ₃/MgO/TiN is proposed as a suitable structural and chemical template for the epitaxial growth of high-transition temperature (T_c) superconductors on Cu metal surfaces. Using techniques such as high-energy electron diffraction and scanning transmission electron microscopy, we present in situ and ex situ analyses of the buffer-layer and superconductor growth with focus on structural properties of the interfaces formed. While MgO is a good barrier to oxygen diffusion, we find that MgO alone is not a suitable buffer layer due to rapid Cu diffusion. Further, growth of MgO with a single epitaxy can be hindered by the presence of impurities such as S, which form strongly bonded superstructures on the metal surface. With the addition of a TiN layer as a barrier to Cu diffusion, oxide formation is suppressed, interfaces are clean, and a single cube-on-cube epitaxy is observed. While the Cu/TiN and TiN/MgO interfaces are rough, the MgO and LaMnO₃ layers planarize the material, leading to growth of smooth YBa₂Cu₃O _7-δ (YBCO). Residual strain in the YBCO film is 0.25% or less and does not lead to apparent cracking. The superconducting properties of the samples were investigated by electrical transport and magnetization measurements. For the first time, high critical current density (J_c) values are reported for YBCO films grown on (001) single-crystal and {100}〈100〉 textured Cu surfaces without intervening metal coatings. J_c on single crystal-like substrates is as high as 3.5 MA/cm ². Reduced J_c of approximately 1 MA/cm² on rolled Cu tapes is limited by damage to the tape surface during the rolling process.