  We report on high-resolution differential conductance experiments on
nanoscale superconductor/ferromagnet tunnel junctions with ultra-thin oxide
tunnel barriers. We observe subgap conductance features which are symmetric
with respect to bias, and shift according to the Zeeman energy with an applied
magnetic field. These features can be explained by resonant transport via
Andreev bound states induced by spin-active scattering at the interface. From
the energy and the Zeeman shift of the bound states, both the magnitude and
sign of the spin-dependent interfacial phase shifts between spin-up and
spin-down electrons can be determined. These results contribute to the
microscopic insight into the triplet proximity effect at spin-active
interfaces.
