Cubic magnets with Dzyaloshinskii-Moriya interaction at low T

Ground state and spin-wave spectrum of cubic magnets with the Dzyaloshinskii-Moriya interaction (DMI) such as MnSi and FeGe are studied theoretically. The linear spin-wave theory is developed. The spin-wave spectrum depends strongly on the magnetic field and strongly anisotropic. It is a result of incommensurate magnetic structure when the DMI breaks the total spin conservation law. The spin-wave spectrum is unstable at infinitesimal perpendicular field. If the gap $\Delta$ is introduced the spectrum becomes stable. The gap appears due to cubic anisotropy and the spin-wave interaction considered in the Hartree-Fock approximation. Peculiar properties of the ESR and neutron scattering in the helical magnets are considered and possibilities of corresponding experimental studies are discussed.