Cubic magnetic anisotropy in B 20 magnets: Interplay of anisotropy and magnetic order in Fe 1 − x Co x Si

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 ≤ x ≤ 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 . From a theoretical point of view, for low Co concentrations ( x ∼ 0.10 ) , the anisotropy is expected to be sufficiently strong to stabilize a low-temperature skyrmion lattice. Therefore, 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.