Nonreciprocal magnon excitations by the Dzyaloshinskii-Moriya interaction on the basis of bond magnetic toroidal multipoles

TL;DR

This paper theoretically investigates how bond magnetic toroidal dipoles and multipoles induce nonreciprocal magnon excitations in noncentrosymmetric magnets, demonstrating effects like band shifts and valley splitting in specific lattice structures.

Contribution

It introduces the role of bond magnetic toroidal multipoles in enabling nonreciprocal magnon excitations and explores their effects in various magnetic lattice models.

Findings

Asymmetric magnon excitations linked to bond magnetic toroidal dipoles.

Magnetic toroidal multipoles cause band shifts and valley splitting.

Demonstrated effects in 1D and 3D noncentrosymmetric structures.

Abstract

The microscopic conditions for the emergence of nonreciprocal magnon excitations in noncentrosymmetric magnets are theoretically investigated. We show that asymmetric magnon excitations appear when a bond magnetic toroidal dipole becomes active, which is defined as a parallel component between the Dzyaloshinskii-Moriya vector and the averaged spin moments at the ends of the bonds. Depending on magnetic structures accompanying the bond magnetic toroidal dipoles, the higher-rank magnetic toroidal multipoles can also be activated in a magnetic cluster, which describes various angle dependences of asymmetric magnon excitations. We demonstrate a variety of asymmetric magnon excitations for two magnetic systems in the noncentrosymmetric lattice structures; a one-dimensional breathing chain under a uniform crystalline electric field and a three-dimensional layered breathing kagome structure. We show that a bottom shift of magnon bands occurs by the magnetic toroidal dipole and a valley splitting occurs by the magnetic toroidal octupole under magnetic orderings.