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 nontrivial magnetic phases, like the skyrmion lattice in cubic 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 ${\mathrm{Mn}}_{0.9}{\mathrm{Fe}}_{0.1}\mathrm{Si}$ deep within its field-polarized ferromagnetic state. We observe nonreciprocal spin waves with a parabolic dispersion that shifts linearly with magnetic field. Crucially, the spin-wave stiffness is highly anisotropic, with values of 17.2(5) meV ${\text{Å}}^2$ parallel to the applied field and 9.3(3) meV ${\text{Å}}^2$ perpendicular to it. While the applied field fundamentally lowers the systems symmetry from cubic to uniaxial, we demonstrate that the induced spin-wave anisotropy is significantly large, which is not captured within the simple theoretical approximations widely applied up to date.

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• Ferromagnetism
• Magnons
• Spin dynamics
• Spin waves
• Ferromagnets
• Helimagnets
• Metals
• Neutron scattering