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