Polaritons in rare-earth magnet-nonmagnetic superlattices

We study the spectrum of magnetic polaritons that propagate in binary superlattices made up of a rare-earth material intercalated by a non-magnetic layer. We consider both the spiral and the cone states of the rare-earth constituent, apart from its simple ferromagnetic (antiferromagnetic) alignment due to the nearest (next-nearest) exchange coupling. We derive the polariton's dispersion relation within the framework of the effective medium approach, since this is very suitable to explain experimental observations of excitations in the far infrared. Numerical results are presented to illustrate the polariton's spectrum, with emphasis to the non-reciprocity aspect presented in some of them.