Magnetic susceptibility and spin dynamics of MnSi

Abstract

A Landau-type free energy serves as the basis for a phenomenological theory of the generalised susceptibility and spin dynamics associated with the helical spin-density wave phase of MnSi. The effect of including magnetoelastic interactions in the model leads to the prediction of nonanalytic wavevector dependence for the static susceptibility, as a result of elastic anisotropy. Spin-wave frequency dispersion relations are calculated, in the long-wavelength regime, for the spin-density-wave and paramagnetic phases in the presence of an applied magnetic field. The results show a variety of interesting behaviour including the appearance of gaps in the frequency at certain values of the wavevector, as a consequence of spin-orbit coupling effects and the incommensurability of the magnetic ordering. The imaginary part of the dynamic susceptibility matrix is also derived and comparisons are made with recently reported neutron scattering data, which exhibits anisotropic behaviour associated with a symmetry-breaking term in the free energy that determines the sense of the helicity of the spin-density wave.