Magnetic field dependence of chiral fluctuations in MnSi

Polarized neutrons were used to determine the antisymmetric part of the magnetic dynamical susceptibility χ(Q,ω) in the weak ferromagnet MnSi. We find that above the Curie temperature T_c the spin fluctuations are incommensurate with the chemical lattice and that χ(Q,ω) depends on the polarization of the incident neutron beam P_i like (Q·P_i) where Q is the scattering vector. Because the crystal structure of MnSi is non-centrosymmetric we interpret these results in terms of the antisymmetric Dzyaloshinskii-Moriya interaction which yields a non-zero contribution to the antisymmetric part of χ(Q,ω). Additional measurements performed below T_c show that in zero field the spin excitations in MnSi have also in the ordered phase a chirality. With increasing external magnetic field B, the magnetic excitations evolve from helical to ferromagnetic-like. In the field-induced phase where the magnetic moments are aligned, magnon annihilation and creation are observed for ±P _i//B, respectively.