Magneto-elastic interaction in cubic helimagnets withB20 structure

`The magneto-elastic interaction in cubic helimagnets with B20 symmetry is considered. It is shown that this interaction is responsible for a negative contribution to the square of the spin-wave gap Delta and it alone appears to disrupt the assumed helical structure. It is suggested that competition between the positive part of Delta(2)(I), which stems from magnon-magnon interaction, and its negative magneto-elastic part leads to the quantum phase transition observed at high pressure in MnSi and FeGe. This transition has to occur when Delta(2)-> 0. For MnSi it was shown using rough estimations that at ambient pressure both parts Delta(I) and vertical bar Delta(ME)vertical bar are comparable with the experimentally observed gap. The magneto-elastic interaction is responsible for 2k modulation of the lattice where k is the helix wavevector and contributes to the magnetic anisotropy. Properties of the magnetic state above the quantum phase transition are also discussed. Experimental observation of the lattice modulation by x-ray and neutron scattering allows the determination of the strength of the anisotropic part of the magneto-elastic interaction responsible for the above phenomena and the lattice helicity.