Theory of magneto-optical effects in helical multiferroic materials via toroidal magnon excitation

Abstract

We investigate dynamical magnetoelectric effect owing to Nambu-Goldstone magnon resonance in helical multiferroic materials. Using the spin wave expansion, we analyzed magneto- and electroactive modes in the ${J}_{1}$-${J}_{2}$ Heisenberg model through the spin-current mechanism. Under external magnetic field, the Nambu-Goldstone magnons accompany dynamical toroidal moments, i.e., toroidal magnons, and their resonant absorption shows anomalous features such as nonreciprocal directional dichroism and natural circular dichroism. The estimation indicates that such effects are gigantic in helical magnets.