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