Microscopic Study of c-axis Proximity Effect in Cuprate-Manganite Heterostructures

Recent studies have reported long-ranged proximity effect in epitaxial thin-film heterostructures of ferromagnetic manganites and superconducting cuprates, with possible origins in novel spin-triplet correlations [1]. A key evidence for this effect is the suppression of the superconducting $T_c$ observed in multilayer films of La$_{2/3}$Ca$_{1/3}$MnO$_3$/YBa$_2$Cu$_3$O$_{7-\delta}$ (LCMO/YBCO). However, scanning tunnelling spectroscopy on $c$-axis LCMO/YBCO bilayers have not seen direct evidence for proximity-induced pairing down to 5nm LCMO thickness [2]. We re-examine the $T_c$ suppression by performing atomically-resolved transmission electron microscopy and resistivity measurements on $c$-axis YBCO/LCMO films grown by pulsed laser deposition, and relating the microstructure in YBCO with the layer thickness and $T_c$. The microscopy revealed double CuO-chain intergrowths forming non-stoichiometric YBCO-247 regions that do not appear in x-ray diffraction, but can be related to the $T_c$ suppression. We attribute these intergrowths to heteroepitaxial strain, by comparing all the lattice parameters and symmetries involved. [1] Z. Sefrioui \emph{et al.}, PRB 67, 214511 (2003); C. Visani \emph{et al}, Nat. Phys. 8, 539 (2012). [2]I. Fridman \emph{et al}, PRB 84, 104522 (2011).