The effect of the magnetic field on the spiral spin structure in MnSi studied by polarized SANS

The magnetic structure of a single crystal MnSi has been studied by small angle diffraction with polarised neutrons within the temperature range from 10 K to TC=28.7 K and the magnetic field H from 1 to 800 mT. The magnetic structure of MnSi in zero magnetic field is a long-period spin spiral with the propagation vector along the [1 1 1] axis caused by the Dzialoshinskii-Moriya interaction. The single crystal was oriented in such a way that two axes [1 1 1] and [111¯] were set in a plane perpendicular to the incident beam. This particular geometry allows one to observe four major diffraction peaks at ±q1 and ±q2 along the axes and four minor peaks at q=±q1±q2 explained as a result of double scattering. The magnetic field H along the polarisation vector P0 has been set in four different directions of interest: parallel to the axes [1 1 0], [0 0 1], [1 1 1] and perpendicular to the last one. The intensity Ip=I(+P0)+I(-P0), the “polarization” PA=[I(+P0)-I(-P0)]/Ip and position qp of the peaks were studied as a function of the temperature and magnetic field.