Polarized inelastic neutron scattering of nonreciprocal spin waves in MnSi

We report spin-polarized inelastic neutron scattering of the dynamical structure factor of the conical magnetic helix in the cubic chiral magnet MnSi. We find that the spectral weight of spin-flip scattering processes is concentrated on single branches for wave-vector transfer parallel to the helix axis as inferred from well-defined peaks in the neutron spectra. In contrast, for wave-vector transfers perpendicular to the helix the spectral weight is distributed among different branches of the magnon band structure as reflected in broader features of the spectra. Taking into account the effects of instrumental resolution, our experimental results are in excellent quantitative agreement with parameter-free theoretical predictions. Whereas the dispersion of the spin waves in MnSi appears to be approximately reciprocal at low energies and small applied fields, the associated spin-resolved spectral weight displays a pronounced nonreciprocity that implies a distinct nonreciprocal response in the limit of vanishing uniform magnetization at zero magnetic field.