Fluctuation-Induced Phase Transitions and Skyrmions in Strongly Correlated Fe_1 – xCo_xSi with a Disturbed Crystal B20-Type Structure

Abstract

The concentration and temperature transitions in strongly correlated Fe_1 – xCo_xSi alloys with the Dzyaloshinski–Moriya (DM) interaction are considered in terms of the spin-fluctuation theory taking into account the results of LDA + U + SO calculations of the density of electronic states. It is shown that the concentration order–order transitions with change in the sign of the left spin chirality (at x < 0.2) to the right that (at x ≤ 0.2x < 0.65, and again to the left chirality at x ≥ 0.65 are related to the change in the sign of the parameter of the mode–mode interaction. It is also shown that, in the composition range of Fe_1 – xCo_xSi with 0.2 ≤ x < 0.65, the first-order transitions extended in temperature arise, accompanied by the appearance of an intermediate temperature range of the spin short-range order with a noncompensated local magnetization and the DM interaction. In an external magnetic field, in this temperature range at 0.2 ≤ x < 0.65, skyrmion microstructures arise. At x = 0.65, the mode–mode interaction parameter becomes be zero, the long-range order becomes ferromagnetic (since the experiment demonstrates the compensation of the DM interaction), and a temperature second-order transition appears instead of the extended first-order transition. The (h–T) diagrams of magnetic states of Fe_1 – xCo_xSi have been constructed; they agree with the experiment.