Direct observation of exchange anisotropy in the helimagnetic insulator Cu2OSeO3

The helical magnetic structures of cubic chiral systems are well explained by the competition among Heisen- berg exchange, Dzyaloshinskii-Moriya interaction, cubic anisotropy, and anisotropic exchange interaction (AEI). Recently, the role of the latter has been argued theoretically to be crucial for the low-temperature phase diagram of the cubic chiral magnet Cu₂OSeO₃, which features tilted conical and disordered skyrmion states for a specific orientation of the applied magnetic field (μ₀H⃗ ∥ [001]). In this study, we exploit transmission resonant x-ray scattering in vector magnetic fields to directly quantify the strength of the AEI in Cu₂OSeO₃ and measure its temperature dependence. We find that the AEI continuously increases below 50 K, resulting in a conical spiral pitch variation of 10% in the (001) plane. Our results contribute to establishing the interaction space that supports tilted cone and low-temperature skyrmion state formation, facilitating the goals for both a quantitative description and eventual design of the diverse spiral states existing amongst chiral magnets.