  Recent experimental studies have indicated novel superconducting proximity
effects in thin-film heterostructures comprising ferromagnetic manganites and
superconducting cuprates. To look for such effects microscopically, we
performed scanning tunneling spectroscopy on
La$_{2/3}$Ca$_{1/3}$MnO$_3$/YBa$_2$Cu$_3$O$_{7-\delta}$ (LCMO/YBCO) bilayer
thin films. $\emph{c}$-axis oriented films of varying thickness were grown on
SrTiO$_{3}$ substrates using pulsed laser-ablated deposition.
Heteroepitaxiality of the films was confirmed by cross-sectional transmission
electron microscopy. Tunneling spectra were measured at 4.2 K, and analyzed for
signatures of a pairing gap on the LCMO layer. For bilayer samples with LCMO
thickness down to 5nm, asymmetric conductance spectra characteristic of
single-layer LCMO films were observed, showing no clear gap structures. These
observations are consistent with a very short-range proximity effect involving
spin-singlet pairs, and difficult to reconcile with longer-range proximity
scenarios involving spin-triplet pairs.
