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

Abstract

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 T C=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 [ 1 1 1 ¯ ] were set in a plane perpendicular to the incident beam. This particular geometry allows one to observe four major diffraction peaks at ± q 1 and ± q 2 along the axes and four minor peaks at q=± q 1± q 2 explained as a result of double scattering. The magnetic field H along the polarisation vector P 0 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 I p = I ( + P 0 ) + I ( - P 0 ) , the “polarization” P A = [ I ( + P 0 ) - I ( - P 0 ) ] / I p and position q p of the peaks were studied as a function of the temperature and magnetic field.