  The breakdown of magnons, the quasiparticles of magnetic systems, has rarely
been seen. By using an inelastic neutron scattering technique we report the
observation of spontaneous magnon decay in multiferroic LuMnO$_3$, a simple
two-dimensional Heisenberg triangular lattice antiferromagnet, with large spin,
S = 2. The origin of this rare phenomenon lies in the non-vanishing cubic
interaction between magnons in the spin Hamiltonian arising from the
noncollinear 120$^o$ spin structure. We observed all three key features of the
nonlinear effects as theoretically predicted: a roton-like minimum, a flat
mode, and a linewidth broadening, in our inelastic neutron scattering
measurements of single crystal LuMnO$_3$. Our results show that quasiparticles
in a system hitherto thought of as "classical" can indeed break down.
