  The magnetic excitations in multiferroic TbMnO3 have been studied by
inelastic neutron scattering in the spiral and sinusoidally ordered phases. At
the incommensurate magnetic zone center of the spiral phase, we find three
low-lying magnons whose character has been fully determined using
neutron-polarization analysis. The excitation at the lowest energy is the
sliding mode of the spiral, and two modes at 1.1 and 2.5meV correspond to
rotations of the spiral rotation plane. These latter modes are expected to
couple to the electric polarization. The 2.5meV-mode is in perfect agreement
with recent infra-red-spectroscopy data giving strong support to its
interpretation as an hybridized phonon-magnon excitation.
