Phase transitions in helicoidal ferromagnets with concentration fluctuations of local magnetization

A phenomenological description of phase transitions induced by fluctuations in helicoidal ferromagnets with concentration fluctuations has been developed. For this, the method of the Ginzburg–Landau functional was used and random variables that are equal to unity in the place occupied by the magnetic atom and zero otherwise are introduced. It is shown that, due to concentration effects, the local magnetization is preserved above the temperature of the magnetic transition $(T_S)$, fluctuations of the spin spiral arise, and skirmion states arise in a magnetic field. The disappearance of vortex states is associated with the suppression of local magnetization by thermodynamic fluctuations at temperature $T_S(>T_C)$. Theoretical results explain the reasons for the significant expansion of the temperature range of states of the skyrmion in a non-stoichiometric manganese monosilicide with manganese deficiency.