A-phase origin in B20 helimagnets

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 $H_\perp>\sqrt{8/3}\Delta=H_{A2}$ where $H_\perp$ are the magnetic field perpendicular to the helix axis $\m k$ and $\Delta$ 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 $H_\perp\to H_{A2}$ are responsible for the lower A-phase boundary $H_{A1}$. It is shown that the A-phase exists at all $T<T_C$ but if $T\ll T_C$ it is very narrow and can not be observed. However in the critical region just below $T_C$ 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.