Study of the Spin-Wave Dynamics of Amorphous Ferromagnets and Helimagnets with the Dzyaloshinskii‒Moriya Interaction

The “inclined” dynamics method developed for studying spin waves in ferromagnets was proposed and implemented at the Leningrad Nuclear Physics Institute in Gatchina in the mid-1980s. The method is based on the analysis of the left-right asymmetry in magnetic scattering of polarized neutrons, which occurs when the magnetization direction in a sample is inclined with respect to the wave vector of the incident beam. Neutron scattering by spin waves is concentrated inside a cone bounded by a critical (cutoff) angle equal to the magnon-to-neutron mass ratio. By measuring this ratio, the magnon mass and the spin wave stiffness in a ferromagnetic material are found. The experiments on measuring the spin wave stiffness in helimagnets with the Dzyaloshinskii–Moriya interaction by the method of small-angle polarized neutron scattering show the validity of quadratic dispersion law shifted in the field direction by the value of the spiral wave vector: _q=A(𝐪-𝐤)^2-gμ_B(H-H_C2) . The spin wave stiffness has been measured as a function of temperature for binary and quasi-binary compounds: MnSi, Mn_1-x Fe x Si, FeGe, Mn_1-x Fe x Ge, Fe_1-x Co x Si, Cu 2 OSeO 3 .