  Two-dimensional honeycomb ferromagnets offer the unprecedented opportunity to
study interactions between collective modes that in standard bulk ferromagnets
do not cross paths. Indeed, they harbor an optical spin-wave branch, i.e. a
spin wave which disperses weakly near the Brillouin zone center. When doped
with free carriers, they also host the typical gapless plasmonic mode of 2D
itinerant electron/hole systems. When the plasmon branch meets the optical
spin-wave branch, energy and momentum matching occurs, paving the way for
interactions between the charge and spin sector. In this Letter we present a
microscopic theory of such plasmon-magnon interactions, which is based on a
double random phase approximation. We comment on the possibility to unveil this
physics in recently isolated 2D honeycomb magnets such as ${\rm Cr}_2{\rm
Ge}_2{\rm Te}_6$.
