Results of polarized neutron small angle scattering in MnSi, a cubic itinerant magnet, near $T_c=28.8\mathrm{K}$ 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 $\nu =0.62\left(1\right)$. The exponent of the Bragg intensity due to the helical structure at $T<T_c$ is $2\beta =0.44\left(1\right)$ where $\beta$ is the exponent of the helix magnetization.

• Received 16 August 2005

DOI:https://doi.org/10.1103/PhysRevB.72.134420