Critical fluctuations in MnSi near TC:: A polarized neutron scattering study -: art. no. 134420

Results of polarized neutron small angle scattering in MnSi, a cubic itinerant magnet, near T-c=28.8 K are presented and analyzed. The diffuse scattering intensity looks like half-moons oriented along the incident neutron polarization. The sum of the intensities for two opposite polarizations form an anisotropic ring with weak spots, which below T-c transform into the Bragg peaks originating from the helical structure. These results are in semiquantitative agreement with the mean-field calculations based on the Bak-Jensen model that takes into account the hierarchy of the interactions: the exchange interaction, the isotropic Dzyaloshinskii-Moriya (DM) interaction and the weak anisotropic exchange (AE) interaction. The DM interaction is responsible for the scattering intensity concentrated in the half-moons. The AE interaction provides the anisotropy so that the correlation length diverges along [111] only. The corresponding critical exponent is nu=0.62(1). The exponent of the Bragg intensity due to the helical structure at T < T-c is 2 beta=0.44(1) where beta is the exponent of the helix magnetization.