  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 double 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.
