Topological magnons driven by the Dzyaloshinskii-Moriya interaction in the centrosymmetric ferromagnet Mn$_5$Ge$_3$

TL;DR

This paper demonstrates that the ferromagnet Mn$_5$Ge$_3$ hosts topological Dirac magnons with a tunable gap, verified through theory and neutron scattering, revealing a new platform for exploring topological magnons in centrosymmetric materials.

Contribution

It provides the first experimental and theoretical evidence of gapped topological Dirac magnons in a centrosymmetric ferromagnet, with tunability via magnetic field orientation.

Findings

Mn$_5$Ge$_3$ hosts gapped Dirac magnons.

The magnon gap can be closed by rotating the magnetization.

The topological character of magnons can be controlled experimentally.

Abstract

The phase of the quantum-mechanical wave function can encode a topological structure with wide-ranging physical consequences, such as anomalous transport effects and the existence of edge states robust against perturbations. While this has been exhaustively demonstrated for electrons, properties associated with the elementary quasiparticles in magnetic materials are still underexplored. Here, we show theoretically and via inelastic neutron scattering experiments that the bulk ferromagnet Mn$_5$Ge$_3$ hosts gapped topological Dirac magnons. Although inversion symmetry prohibits a net Dzyaloshinskii-Moriya interaction in the unit cell, it is locally allowed and is responsible for the gap opening in the magnon spectrum. This gap is predicted and experimentally verified to close by rotating the magnetization away from the $c$-axis with an applied magnetic field. Hence, Mn$_5$Ge$_3$ realizes a gapped Dirac magnon material in three dimensions. Its tunability by chemical doping or by thin film nanostructuring defines an exciting new platform to explore and design topological magnons. More generally, our experimental route to verify and control the topological character of the magnons is applicable to bulk centrosymmetric hexagonal materials, which calls for systematic investigation.