  We contrast perturbative expansions of ferromagnet / superconductor
interfaces in two geometries: (i) a point contact geometry where a single weak
link connects a 3D ferromagnet to a 3D superconductor and (ii) an atomic
thickness geometry with an infinite planar interface connecting a quasi-2D
ferromagnet to a quasi-2D superconductor. Perturbation theories are rather
different in the two approaches but they both break down at order $t^4$ ($t$ is
the tunnel amplitude). The regimes of strong ferromagnets are in a qualitative
agreement in both geometries. The regime of weak ferromagnets exists only for
the atomic thickness geometry and is related to Andreev bound states due to
lateral confinement in the superconductor.
