Surface magnon polaritons in insulating ferromagnets in out-of-plane configuration

Abstract

We predicted two surface magnon polaritons in an insulating ferromagnet (FM) in the out of plane configuration, where the saturated magnetization and external field are vertical to the FM surface. They are virtual surface magnon polaritons without magnetostatic limit and consist of two branch-waves in the FM. They are different surface hybrid-polarization polaritons, whose electromagnetic fields do not monotonously decay with the distance away from the surface due to the out-sync or interference between the two branch-waves. One of them is a ghost surface polariton (GSP) whose electromagnetic fields not only attenuate but also oscillate with the distance (x) away from the surface, and the other is a hybrid-polarization surface polariton (HSP). The two surface polaritons are situated on different segments of a continual dispersion curve. The external magnetic field can lead to the transition or switch between the two polariton-states and linearly changes the surface-polariton frequency. The threshold values of external magnetic field are finite for the existence of the two surface polaritons. The energy-flux density (S) of the surface polaritons always is positive and has its maximum and minimum in the FM. For the GSP, S exhibits a seral interferent fringes on the plane normal to the propagating direction and the fringes rapidly decay with x in the FM.