We examine the effect of CuO intergrowths on the superconductivity in epitaxial La_{2/3}Ca_{1/3}MnO_3/YBa_2Cu_3O_{7-delta} (LCMO/YBCO) thin-film heterostructures. Scanning transmission electron microscopy on bilayer LCMO/YBCO thin films revealed doub
le CuO-chain intergrowths which form regions with the 247 lattice structure in the YBCO layer. These nanoscale 247 regions do not appear in x-ray diffraction, but can physically account for the reduced critical temperature Tc of bilayer thin films relative to unilayer films with the same YBCO thickness, at least down to ~25 nm. We attribute the CuO intergrowths to the bilayer heteroepitaxial mismatch and the Tc reduction to the generally lower Tc seen in bulk 247 samples. These epitaxially-induced CuO intergrowths provide a microstructural mechanism for the attenuation of superconductivity in LCMO/YBCO heterostructures.
