  The excitation spectrum of clean ferromagnet-superconductor bilayers is
calculated within the framework of the self-consistent Bogoliubov-de Gennes
theory. Because of the proximity effect, the superconductor induces a gap in
the ferromagnet spectrum, for thin ferromagnetic layers. The effect depends
strongly on the exchange field in the ferromagnet. We find that as the
thickness of the ferromagnetic layer increases, the gap disappears, and that
its destruction arises from those quasiparticle excitations with wavevectors
mainly along the interface. We discuss the influence that the interface quality
and Fermi energy mismatch between the ferromagnet and superconductor have on
the calculated energy gap. We also evaluate the density of states in the
ferromagnet, and we find it in all cases consistent with the gap results.
