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

We investigate strongly correlated solid solutions of Fe_1 – xCo_xSi with a disrupted B20 cubic structure. Within the framework of spin-fluctuation theory and a model of the density of electronic states derived from first-principles calculations using the generalized gradient approximation with the inclusion of strong Coulomb correlations (GGA + U), temperature transitions in Fe_1 – xCo_xSi alloys (illustrated with the examples of x = 0.2 and 0.3) with Dzyaloshinskii–Moriya interaction are examined. In the considered compounds, a “blurred” temperature-dependent first-order magnetic phase transition occurs, which is accompanied by a change in the sign of the intermode-coupling parameter in the Ginzburg–Landau functional. During such a transition, skyrmion A phases emerge within limited temperature ranges and external magnetic fields, beyond which experimentally observable fluctuations of the magnetic moments of spin spirals occur. The (h–T) diagrams are plotted, indicating regions of long-range order, and fluctuational and skyrmion phases. Fe_1 – xCo_xSi phases with x = 0.2 and 0.3 align well with experimental observations.