Anisotropy of spin waves in the field-polarized phase of Fe-doped MnSi

TL;DR

This study uses inelastic neutron scattering to reveal highly anisotropic spin-wave stiffness in Fe-doped MnSi's field-polarized phase, challenging existing theoretical models of its magnetic interactions.

Contribution

It provides the first detailed measurement of spin-wave anisotropy in Fe-doped MnSi, highlighting the need to revise current theoretical frameworks.

Findings

Spin waves are non-reciprocal with a linear field shift.

Spin-wave stiffness is highly anisotropic: 14.7 vs 7.6 meV·Å².

Anisotropy contradicts standard models for MnSi.

Abstract

Chiral magnetic textures, such as skyrmions, are of great interest to the condensed matter community due to their novel transport properties. The stabilization of topologically non-trivial magnetic phases, like the skyrmion lattice in MnSi, is governed by underlying magnetic interactions which can be probed via measurements of spin-wave excitations. Here, we report high-resolution inelastic neutron scattering (INS) measurements of the spin waves in Fe-doped Mn$_{0.9}$Fe$_{0.1}$Si deep within its field-polarized ferromagnetic state. We observe non-reciprocal spin waves with a parabolic dispersion that shifts linearly with magnetic field. Crucially, the spin-wave stiffness is highly anisotropic, with values of 14.7 meV $\rm{\mathring{A}}$$^2$ parallel to the applied field and 7.6 meV $\rm{\mathring{A}}$$^2$ perpendicular to it. This pronounced anisotropy in a cubic material is inconsistent with standard theoretical models for MnSi and indicates a necessity to revise our theoretical understanding.