Nonreciprocity of spin waves in the conical helix state

Abstract

Significance While spin waves in conventional ferromagnets are well understood, those in more complicated magnets hosting spatially textured magnetization, such as helical/conical and skyrmion/hedgehog configurations, are less explored. We investigate experimentally and theoretically the spin waves of the conical spin helix in a chiral magnet with the help of Brillouin light scattering. We discovered that the dynamics of the conical spin helix is characterized by a distinct nonreciprocity with spin waves preferentially propagating in a specific direction controlled by the orientation of the magnetic field. Our work therefore demonstrates that all the spin phases of bulk chiral magnets allow, in principle, the realization of a switchable spin wave diode functionality. Nonreciprocity emerges in nature and in artificial objects from various physical origins, being widely utilized in contemporary technologies as exemplified by diode elements in electronics. While most of the nonreciprocal phenomena are realized by employing interfaces where the inversion symmetry is trivially lifted, nonreciprocal transport of photons, electrons, magnons, and possibly phonons also emerge in bulk crystals with broken space inversion and time reversal symmetries. Among them, directional propagation of bulk magnons (i.e., quanta of spin wave excitation) is attracting much attention nowadays for its potentially large nonreciprocity suitable for spintronic and spin-caloritronic applications. Here, we demonstrate nonreciprocal propagation of spin waves for the conical spin helix state in Cu2OSeO3 due to a combination of dipole and Dzyaloshinskii–Moriya interactions. The observed nonreciprocal spin dispersion smoothly connects to the hitherto known magnetochiral nonreciprocity in the field-induced collinear spin state; thus, all the spin phases show diode characteristics in this chiral insulator.