Nonreciprocal Directional Dichroism and Toroidalmagnons in Helical Magnets

Abstract

We investigate a dynamical magnetoelectric effect due to a magnetic resonance\nin helical spin structures through the coupling between magnetization and\nelectric polarization via a spin current mechanism. We show that the magnon has\nboth the dynamical magnetic moment $\\Delta M^\\omega$ and the electric moment\n$\\Delta P^\\omega$ ($\\perp \\Delta M^\\omega$), i.e., a dynamical toroidal moment,\nunder external magnetic fields, and thus it is named the {\\em toroidalmagnon}.\nThe toroidalmagnon exists in most conical spin structures owing to the\ngenerality of the spin current mechanism. In the absorption of electromagnetic\nwaves, the toroidalmagnon excitation process generally induces a nonreciprocal\ndirectional dichroism as a consequence of an interference of the magnetic and\nelectric responses.\n