The chiral spin fluctuations in single crystal of MnSi have been studied above critical temperature by small angle diffraction with polarized neutrons. The results are in quantitative agreement with mean-field calculations based on the Bak-Jensen model that takes into account the hierarchy of the interactions: exchange interaction, Dzyaloshinskii-Moriya interaction and weak anisotropic exchange interaction. In the vicinity of TC, with magnetic field applied along one of the 3-fold cubic axes, a 90o-reorientation of the spiral wave vector is observed, from the [111] axis to the [1-10] axis. This reorientation under applied field is interpreted in terms of theory developed recently for cubic magnets without the centre of symmetry, that are characterised by the presence of Dzyaloshinskii-Moriya interaction [S.V. Maleyev, PRB 73 (2006) 174402]. In particular, the theory predicts the presence of a spin wave gap (2 Hin2/2), stabilizing the spiral structure with the wavevector k perpendicular to the field direction for an infinitely small magnetic field. We speculate that an applied pressure may decrease the gap, thus, driving the magnetic system to quantum phase transition observed in MnSi experimentally [C. Pfleiderer, et al, Nature 414, 427 (2001)]
PNPI, Gatchina