Field-induced reorientation of the spin helix in MnSi near Tc

The chiral spin fluctuations and the spiral structure of the single crystal MnSi has been studied by small angle polarized neutron scattering near T-c=29 K under applied field. A weak magnetic field applied along one of the < 111 > axes produces a single domain sample with the helix wave vector along the field. The 90 degrees reorientation of the spin spiral from the [111] axis to [110] axis is observed in the field range from 130 to 180 mT in the close vicinity to T-c. Further increase of the field above 180 mT restores the original orientation of the helix and leads to the induced ferromagnetic state at 350 mT. This observation clarifies the nature of the structural spin instability found in the H-T phase diagram of MnSi by other techniques. We explain this phenomenon as a result of competition of cubic anisotropy and the spin-wave Bose condensation provoked by the field perpendicular to the helix axis.