Cubic magnetic anisotropy in $B$20 magnets: Interplay of anisotropy and magnetic order in Fe$_{1-x}$Co$_{x}$Si

Abstract

The metallic systems MnSi and Fe$_{1-x}$Co$_{x}$Si are known to feature a generic magnetic phase diagram primarily determined by the isotropic exchange and Dzyaloshinskii-Moriya interactions. However, additional weaker anisotropies, lowest in the hierarchy of energy scales, play a crucial role: they determine the relative order of phases in the phase diagram and may even enable skyrmion stability far below the ordering temperature. Among cubic B20 helimagnets, the insulator Cu$_2$OSeO$_3$ is currently the only known example exhibiting a low-temperature, anisotropy-induced skyrmion pocket. In this manuscript, we present a systematic study of cubic magnetocrystalline anisotropy by means of angle-resolved SQUID magnetization measurements in MnSi and Fe$_{1-x}$Co$_{x}$Si ($0.08 \leq x \leq 0.70$) single crystals and provide quantitative values of the anisotropy constants. For Fe$_{1-x}$Co$_{x}$Si, the cubic anisotropy is found to be strongly dependent on the Co concentration $x$. In particular, for low Co concentrations ($x \sim 0.10$), the anisotropy is sufficiently strong to stabilize a low-temperature skyrmion lattice, in agreement with theoretical predictions. This finding suggests that Fe$_{1-x}$Co$_{x}$Si may represent the first chiral metallic system to exhibit a low-temperature skyrmion phase controllably stabilized by cubic anisotropy for specific directions of the magnetic field.