  Magnons and plasmons are two very different types of collective modes, acting
on the spin and charge degrees of freedom, respectively. At first sight, the
formation of hybrid plasmon-magnon polaritons in heterostructures of plasmonic
and magnetic systems would face two challenges, the small mutual interaction,
via Zeeman coupling of the electromagnetic field of the plasmon with the spins,
and the energy mismatch, as in most systems plasmons have energies in the eV
range, orders of magnitude larger than magnons. Here we show that graphene
plasmons form polaritons with the magnons of two-dimensional ferrromagnetic
insulators, placed up to to half a micron apart, with Rabi couplings in the
range of 100 GHz (dramatically larger than cavity QED magnonics). This strong
coupling is facilitated both by the small energy of graphene plasmons and the
cooperative super-radiant nature of the plasmon-magnon coupling afforded by
phase matching. We show that the Rabi coupling can be modulated both
electrically and mechanically and we propose a attenuated total internal
reflection experiment to implement ferromagnetic resonance experiments on 2D
ferromagnets driven by plasmon excitation.
