We report spherical neutron polarimetry (SNP) and discuss the spin-flip scattering cross sections as well as the chiral fraction $\eta$ close to the helimagnetic transition in MnSi. For our study we have developed a miniaturized SNP device that allows fast data collection when used in small angle scattering geometry with an area detector. Critical spin-flip scattering is found to be governed by chiral paramagnons that soften on a sphere in momentum space. Carefully accounting for the incoherent spin-flip background, we find that the resulting chiral fraction $\eta$ decreases gradually above the helimagnetic transition reflecting a strongly renormalized chiral correlation length with a temperature dependence in excellent quantitative agreement with the Brazovskii theory for a fluctuation-induced first order transition.

• Received 6 March 2014

DOI:https://doi.org/10.1103/PhysRevB.89.180408