Nonreciprocity of spin waves by chiral fluctuations induced in the conical state

Nonreciprocity in chiral magnets is promising for applications in chiral spintronics. Spin-wave nonreciprocity happens when applying an external magnetic field to a chiral magnet while a stream of particles parallel to the field flows through it. Here, we found through micromagnetic simulations that, in the absence of lattice chirality, the excitation spectra of spin waves in the conical magnetic state depend on the relative orientations of the spin-polarized current and the external magnetic field applied along the magnetic helix axis. When both are in the same direction, the spin component of the magnet along the helical axis increases with time while the chiral index decreases. Statistical analysis shows that the spin fluctuations are anisotropic, where the fluctuations along the helical axis have chiral characters as multi-modal, high-frequency, and low-intensity. We give an intuitive interpretation of these observations from the point of view of symmetry breaking and explore the factors affecting the nonreciprocity of the spin-wave spectrum, guiding experimental observations.