  Hexagonal HoMnO3, a two-dimensional Heisenberg antiferromagnet, has been
studied via inelastic neutron scattering. A simple Heisenberg model with a
single-ion anisotropy describes most features of the spin-wave dispersion
curves. However, there is shown to be a renormalization of the magnon energies
located at around 11 meV. Since both the magnon-magnon interaction and
magnon-phonon coupling can affect the renormalization in a noncollinear magnet,
we have accounted for both of these couplings by using a Heisenberg XXZ model
with 1=S expansions [1] and the Einstein site phonon model [13], respectively.
This quantitative analysis leads to the conclusion that the renormalization
effect primarily originates from the magnon-phonon coupling, while the
spontaneous magnon decay due to the magnon-magnon interaction is suppressed by
strong two-ion anisotropy.
