Polarized inelastic neutron scattering of nonreciprocal spin waves in MnSi

Abstract

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 spinflip scattering processes is concentrated on single branches for wavevector transfer parallel to the helix axis as inferred from well-defined peaks in the neutron spectra. In contrast, for wavevector 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 spinresolved spectral weight displays a pronounced non-reciprocity that implies a distinct non-reciprocal response in the limit of vanishing uniform magnetization at zero magnetic field. This is a pre-print of our paper [1] at http://dx.

Observation of Magnon Polarization

“…It can differentiate magnon creation and annihilation processes [15], which relates to the magnon polarization because the total spin of magnon plus neutron must be conserved in the scattering process. The symmetry of magnetic fluctuations [16] and non-reciprocal magnons [17] can also be directly investigated. More recently the "chiral terms" [18] were used to measure chiral magnetic order [19] and excitations in paramagnetic [20] and chiral phases [21].…”

Confined dipole and exchange spin waves in a bulk chiral magnet with Dzyaloshinskii-Moriya interaction

“…Magnon band structures are nontrivial in chiral magnets because of bulk Dzyaloshinskii-Moriya interaction (DMI) [1][2][3][4][5][6]. As a consequence of the asymmetric exchange interaction, bulk DMI introduces non-reciprocity for the spin waves [5,[7][8][9][10][11][12][13]. Therefore chiral magnets can serve as non-reciprocal microwave devices [9].…”

Topological magnon band structure of emergent Landau levels in a skyrmion lattice

“…In addition, spectra for momentum transfers parallel to the skyrmion tubes were recorded (Fig. 4), which display the characteristics of conventional nonreciprocal magnons akin those observed in the topologically trivial phases ( 21 – 25 , 33 , 34 ). The scattering geometries were chosen carefully to further distinguish nuclear and magnetic scattering ( 17 ).…”

“…Here, we used polarized inelastic neutron scattering to determine the predicted dispersion and orbital motion of magnons in the skyrmion lattice of MnSi. Neutron and microwave studies of spin excitations in MnSi have been reported for the topologically trivial magnetic phases, namely the helical, conical, and spin-polarized states ( 21 – 25 ) as well as the paramagnetic regime ( 26 – 31 ). This work has revealed, across the entire Brillouin zone, well-defined dispersive, nonreciprocal spin waves and spin fluctuations in the ordered states and the paramagnetic regime, respectively.…”