Magnons and plasmons are different collective modes,
involving
the spin and charge degrees of freedom, respectively. Formation of
hybrid plasmon–magnon polaritons in heterostructures of plasmonic
and magnetic systems faces two challenges, the small interaction of
the electromagnetic field of the plasmon with the spins, and the energy
mismatch, as in most systems plasmons have energies orders of magnitude
larger than those of magnons. We show that graphene plasmons form
polaritons with the magnons of two-dimensional ferromagnetic insulators,
placed up to to half a micrometer apart, with Rabi splittings in the
range of 100 GHz (dramatically larger than cavity magnonics). This
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 coupling can be modulated
both electrically and mechanically, and we propose a ferromagnetic
resonance experiment implemented with a two-dimensional ferromagnet
driven by graphene plasmons.