A-phase origin in B20 helimagnets
Abstract
A-phase origin in cubic helimagnets (M n S i,F e G e etc) is explained. It is shown that its upper bound is a result of the spin-wave instability at H>8/3=HA2 where H are the magnetic field perpendicular to the helix axis k and the spin-wave gap respectively. The last appears due to the spin-wave interaction if one takes into account that the Dzyaloshinskii-Moriya interaction acts between different spins. The infra-red divergences (IRD) in the 1/S series for the magnetic energy at H HA2 are responsible for the lower A-phase boundary HA1. It is shown that the A-phase exists at all T<TC but if T TC it is very narrow and can not be observed. However in the critical region just below TC its width increasing strongly. Preliminary estimations demonstrate semi-quantitative agreement with the existing experimental data. The existence of the spin-wave gap is a crucial point of our consideration. In Appendix A we present its derivation in more transparent form than in previous publication.