Magnetic-state-controlled proximity effect across high-T$_{C}$ superconductor/ferromagnetic interfaces.*

We have investigated the electronic density of states of a ferromagnet (F: a Co/Pt superlattice) in contact with a c-axis YBCO film. This was done by measuring the current-perpendicular-to-plane differential conductance across vertical junctions of area down to 6 $\mu $m$^{2}$, which were fabricated using optical lithography and ion etching. We have found salient features of the leakage of the superconducting order parameter into the F layer, such as a zero-bias conductance peak which can be modulated by the magnetic state of the ferromagnet. We discuss the possibility of triplet superconducting correlations induced in the F layer as the origin of this behavior.