  We propose a new process of magnon-assisted Andreev reflection at a
ferromagnetic metal - superconductor interface, which consists of the
simultaneous injection of a Cooper pair from the superconductor and the
emission of a magnon inside the ferromagnet. At low temperature this process
represents an additional channel for sub-gap transport across an FS interface,
which lifts restrictions on the current resulting from the necessity to match
spin-polarized current in the ferromagnet with spin-less current in the
superconductor. For a junction between a superconductor and a ferromagnet with
an arbitrary degree of polarization, the inelastic magnon-assisted Andreev
reflection process would manifest itself as a nonlinear addition to the $I(V)$
characteristics which is asymmetric with respect to the sign of the bias
voltage and is related to the density of states of magnons in the ferromagnet.
Expressions for the subgap $I(V)$ characteristics are given for arbitrary
interfacial quality whilst the limiting cases of uniformly transparent and
disordered interfaces are discussed in detail.
