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.