Spin-Wave and Electromagnon Dispersions in Multiferroic MnWO4 as Observed by Neutron Spectroscopy: Isotropic Heisenberg Exchange versus Anisotropic Dzyaloshinskii-Moriya Interaction

TL;DR

This study uses neutron spectroscopy to explore magnetic excitations in multiferroic MnWO4, revealing both spin waves modeled by Heisenberg exchange and electromagnons linked to Dzyaloshinskii-Moriya interaction, highlighting complex magnetoelectric coupling.

Contribution

It provides detailed experimental evidence of electromagnons and their relation to exchange interactions in MnWO4, emphasizing the role of anisotropic Dzyaloshinskii-Moriya exchange.

Findings

Identification of low-energy electromagnons at 0.07 and 0.45 meV

Spin-wave gap measured at 0.61 meV

Electromagnons persist across magnetic phases

Abstract

High resolution inelastic neutron scattering reveals that the elementary magnetic excitations in multiferroic MnWO4 consist of low energy dispersive electromagnons in addition to the well-known spin-wave excitations. The latter can well be modeled by a Heisenberg Hamiltonian with magnetic exchange coupling extending to the 12th nearest neighbor. They exhibit a spin-wave gap of 0.61(1) meV. Two electromagnon branches appear at lower energies of 0.07(1) meV and 0.45(1) meV at the zone center. They reflect the dynamic magnetoelectric coupling and persist in both, the collinear magnetic and paraelectric AF1 phase, and the spin spiral ferroelectric AF2 phase. These excitations are associated with the Dzyaloshinskii-Moriya exchange interaction, which is significant due to the rather large spin-orbit coupling.