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 T-C = 28.7 K and the magnetic field H from I to 800 mT. The magnetic structure of MnSi in zero magnetic field is a long-period spin spiral with the propagation vector along the [111] axis caused by the Dzialoshinskii-Moriya interaction. The single crystal was oriented in such a way that two axes [111] and [111] 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 PO has been set in four different directions of interest: parallel to the axes [110], [001], [111] 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. (C) 2004 Published by Elsevier B.V.