Magnetic field dependence of chiral fluctuations in MnSi

Polarized neutrons were used to determine the antisymmetric part of the magnetic dynamical susceptibility chi(Q,omega) 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 chi(Q,omega) depends on the polarization of the incident neutron beam P-1 like (Q (.) P-1) 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 chi(Q,omega). 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-1//B, respectively. (C) 2003 Elsevier B.V. All rights reserved.