Electromagnon in multiferroic materials with Dzyaloshinsky-Moriya-interaction-induced helical spin structures

摘要

We investigate the electro-active magnon, the so-called "electromagnon", which is an excitation process in a multiferroic material with a long-period helical spin structure that is induced by a Dzyaloshinsky-Moriya (DM) interaction. We clarify the selection rule and the resonance frequency of electromagnetic wave absorption due to an electromagnon excitation process via a spin current mechanism. Under an external magnetic field, the magnon carries a dynamical toroidalmoment T~d = ΔP × ΔM, where ΔP and ΔM are the fluctuations of the electric polarization and the magnetization, respectively, and is thus named as the "toroidalmagnon." The toroidalmagnon shows non-reciprocal directional dichroism (NDD); i.e., an absorption intensity that depends on the sign of the electromagnetic wave's propagation direction. In helical magnets induced by the DM interaction, the intensity of the NDD effects can be larger than the absorption intensity of the electromagnon. Our results indicate that the observation of the NDD is a crucial test to identify the coupling between the electric polarization and the helical spin structure, even when the electromagnon absorption itself cannot be observed due to its small intensity.