The polarized neutron scattering in helimagnetic MnSi at low T reveals existence of a partially disordered chiral state at ambient pressure in the magnetic field applied along <111> axis below the first order transition to the non-chiral ferromagnetic state. This unexpected phenomenon is explained by the analysis of the spin-wave spectrum. We demonstrate that the square of the spin-wave gap becomes negative under magnetic field applied along <111> and <110> but not along the <100> direction. It is a result of competition between the spin-wave interaction and cubic anisotropy. This negative sign means an instability of the spin wave spectrum for the helix and leads to a destruction of the helical order, giving rise to the partially disordered state below the first order ferromagnetic transition.
@article{arxiv.0704.0683,
title = {Partially disordered state near ferromagnetic transition in MnSi},
author = {S. V. Maleyev S. V. Grigoriev},
journal= {arXiv preprint arXiv:0704.0683},
year = {2007}
}