Scattering of exchange spin waves from a helimagnetic layer sandwiched between two semi-infinite ferromagnetic media

We have calculated the scattering (reflection and transmission) coefficients of linear exchange spin waves normally incident upon a helimagnetic layer sandwiched between two semi-infinite ferromagnetic media. Our calculations show that, despite the helimagnetic order induced in the layer by the Dzyaloshinskii-Moriya interaction (DMI), the scattering is reciprocal and insensitive to the presence of the helimagnetic order in the layer. This comes as a result of the disappearance of the DMI from the boundary conditions in the considered geometry under the small-amplitude approximation and from the specific form of the nonreciprocity of the spin-wave dispersion relation in the helimagnetic material. We show that the helimagnetic layer's interfaces act as a system of two semicrossed polarizers for the circularly polarized spin waves incident from the ferromagnetic media. This results from the ellipticity of the magnetic precession induced by the easy-plane anisotropy in the helimagnetic layer. Our calculations also reveal the importance of evanescent solutions to correctly describe the spin-wave scattering in samples with elliptical precession. Our findings will aid development of magnonic devices containing helimagnetic constituents.