Non-reciprocal spin excitations across the skyrmion-paramagnetic phase transition in MnSi

The magnetic excitations of the skyrmion lattice in MnSi comprise a multitude of individual modes, which are non-reciprocal and thereby propagate unidirectionally. We report inelastic neutron scattering experiments for temperatures near and above the skyrmion-paramagnetic phase transition in the chiral magnet MnSi tracking the evolution from the skyrmion lattice towards the high-temperature paramagnetic state. Within the resolution of the triple-axis measurements the excitations vary smoothly across the skyrmion-paramagnetic boundary, and, the quasi-elastic paramagnetic signal under applied field retains the non-reciprocal character seen in the skyrmion phase even far above the critical temperature. Using a resolution-convolution our results are consistent with linear spin-wave theory.

💡 Research Summary

In this work the authors investigate how the non‑reciprocal (i.e., direction‑dependent) spin‑wave excitations evolve when the chiral magnet MnSi is driven from its skyrmion‑lattice phase into the high‑temperature paramagnetic regime. MnSi is a prototypical itinerant helimagnet that, below the Curie temperature (T_c ≈ 29 K), exhibits a sequence of magnetic phases: helical, conical, a small‑pocket skyrmion lattice (SkX), and finally a field‑polarised ferromagnet. The lack of inversion symmetry (space group P2₁3) together with a sizable Dzyaloshinskii‑Moriya (DM) interaction gives rise to magnon bands that are strongly back‑folded into the magnetic Brillouin zone and, crucially, to non‑reciprocal dispersions: magnons created with momentum +q have a different energy (and spectral weight) than those with –q. Such asymmetry is of great interest for magnonic devices that require unidirectional propagation.

To probe these excitations beyond the zero‑momentum limit of microwave techniques, the authors performed inelastic neutron scattering (INS) on the cold‑triple‑axis spectrometer IN12 at the Institut Laue‑Langevin. A high‑quality single crystal (≈15 g, cylinder,