Electromagnetic spin waves in crystals with a simple helical magnetic structure

The spectrum of electromagnetic spin waves propagating along the helical axis is obtained for a magnet with a simple helical magnetic structure (HMS) caused by the Lifshitz invariant. It is shown that unlike an easy-plane ferromagnet having no HMS, the equilibrium HMS offers a possibility of resonance interaction between spin and electromagnetic waves. The dependences of the wave vector frequency for oscillations with (+) and (−) circular polarizations are found to be essentially different, thus making it possible to determine the type (+) or (−) of helicity of the equilibrium HMS. The propagation of the electromagnetic waves through a plate with HMS is considered. The reflection coefficient of the resonance frequency region is close to unity but oscillates considerably beyond this region.