Field dependence of non-reciprocal magnons in chiral MnSi

TL;DR

This study investigates how non-reciprocal magnons in chiral MnSi evolve under different magnetic fields, revealing asymmetric magnon spectra in the polarized phase and symmetric spectra in the helimagnetic phase, aligning with theoretical predictions.

Contribution

It provides a detailed experimental analysis of magnon spectra in MnSi across magnetic phases, confirming theoretical models with quantitative neutron scattering data.

Findings

Asymmetric magnon spectra in the field-polarized phase.

Symmetric magnon spectra in the helimagnetic phase.

Excellent agreement with low-energy cubic chiral magnet theory.

Abstract

Spin waves in chiral magnetic materials are strongly influenced by the Dzyaloshinskii-Moriya interaction resulting in intriguing phenomena like non-reciprocal magnon propagation and magnetochiral dichroism. Here, we study the non-reciprocal magnon spectrum of the archetypical chiral magnet MnSi and its evolution as a function of magnetic field covering the field-polarized and conical helix phase. Using inelastic neutron scattering, the magnon energies and their spectral weights are determined quantitatively after deconvolution with the instrumental resolution. In the field-polarized phase the imaginary part of the dynamical susceptibility $\chi''(\varepsilon, {\bf q})$ is shown to be asymmetric with respect to wavevectors ${\bf q}$ longitudinal to the applied magnetic field ${\bf H}$, which is a hallmark of chiral magnetism. In the helimagnetic phase, $\chi''(\varepsilon, {\bf q})$ becomes increasingly symmetric with decreasing ${\bf H}$ due to the formation of helimagnon bands and the activation of additional spinflip and non-spinflip scattering channels. The neutron spectra are in excellent quantitative agreement with the low-energy theory of cubic chiral magnets with a single fitting parameter being the damping rate of spin waves.