Direct observation of the exchange anisotropy in the helimagnetic insulator Cu\(_2\)OSeO\(_3\)

The helical magnetic structures of cubic chiral systems are well-explained by the competition among Heisenberg 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 Cu\(_2\)OSeO\(_3\), which features tilted conical and disordered skyrmion states for a specific orientation of the applied magnetic field (\(\mu_0 \vec{\mathrm{H}} \parallel [001]\)). In this study, we exploit transmission resonant x-ray scattering (\(t-\)REXS) in vector magnetic fields to directly quantify the strength of the AEI in Cu\(_2\)OSeO\(_3\), 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.