Skyrmions and Fluctuations of Spin Spirals in Strongly Correlated Fe1–хCoхSi with Noncentrosymmetric Cubic Structure

Strongly correlated Fe1–хCoxSi solid solutions with broken B20-type cubic structure are studied. Within the framework of the spin-fluctuation theory and in the model of the density of electronic states, arising from first-principles calculations within the framework of the generalized gradient approximation taking into account strong Coulomb correlations (GGA+U) temperature transitions are considered in strongly correlated Fe1–хCoxSi alloys (for example, x = 0.2, 0.3) with the Dzyaloshinskii–Moriya (DM) interaction. It is shown that in the compositions under consideration, a first-order magnetic phase transition, which is prolonged in temperature, occurs, during which the sign of the intermode coupling parameter in the Ginzburg–Landau functional changes. It is found that such a transition results in the formation of skyrmion A-phases in limited ranges of temperatures and external magnetic fields, beyond which the experimentally observed fluctuations of spin spirals are realized. The constructed (h–Т)-diagrams (which indicate the range of long-range order, fluctuation and skyrmion phases) of Fe1–хCoxSi at x = 0.2 and 0.3 are consistent with the experiment.