In this chapter, we are shortly reviewing some problems of electromagnetic and acoustic wave propagation and radiation in the magnets with helicoidal spin structure. We show the band structure of the coupled wave spectrum in the materials. The band gap width depends on the spiral angle (or, equivalently, on external magnetic field value). Interaction of spin and electromagnetic waves leads to opening the gap in spin-electromagnetic dispersion. This gap leads to opacity window in reflection spectrum of spiral magnet plate. The opacity window closes at phase transition into collinear ferromagnetic state and reaches a maximum at simple spiral state. At the frequencies near band gap boundaries, the rotation of polarization plane of propagating electromagnetic wave is observed. Account of interaction of spin and electromagnetic waves with acoustic subsystem leads to opening the gap in spin-acoustic spectrum. This gap leads to some features in electromagnetic reflectance spectrum and to rotation of acoustic wave polarization plane, i.e. to acoustic Faraday effect. We also show the possibility of acoustic and electromagnetic wave radiation by helicoidal magnets at phase transition into collinear ferromagnetic state. Some features of electromagnetic waves generation by spiral magnets placed in homogeneous magnetic field with harmonical time-dependence are also discussed.
Part of the book: Magnetic Materials