Critical small-angle scattering of polarised neutrons in MnSi

The chiral fluctuations in the weak itinerant magnet MnSi were studied by means of small-angle neutron scattering with polarized neutrons. Due to the lack of a centre of symmetry, the magnetic moments are arranged along a left-handed spiral due to the Dzyaloshinskii–Moriya (DM) interaction. The experiments show that the incommensurate magnetic peaks evolve with increasing temperature into diffuse scattering that is mainly concentrated in a ring with the radius q c =0.039 A ̊ −1 centred at the position of the direct beam q=0. The scattering is fully polarised for Q || P 0 and depolarised for Q⊥ P 0 proving that the scattering is chiral and that the spiral has a unique handiness. From the data, we have determined the critical exponents of the magnetisation, the susceptibility and the correlation lengths of the chiral fluctuations. The results are discussed in the framework of the renormalisation theory.