Ferromagnetic Kitaev interaction and the origin of large magnetic anisotropy in α-RuCl3

α-RuCl3 is drawing much attention as a promising candidate for the Kitaev quantum spin liquid1–8. However, despite intensive research efforts, controversy remains about the form of the basic interactions governing the physics of this material. Even the sign of the Kitaev interaction (the bond-dependent anisotropic interaction responsible for Kitaev physics) is still under debate, with conflicting results from theoretical and experimental studies5,6,9–15. The significance of the symmetric off-diagonal exchange interaction (referred to as the Γ term) is another contentious question16–18. Here, we present resonant elastic X-ray scattering data that provide unambiguous experimental constraints to the two leading terms in the magnetic interaction Hamiltonian. We show that the Kitaev interaction (K) is ferromagnetic, and that the Γ term is antiferromagnetic and comparable in size to the Kitaev interaction. Our findings also provide a natural explanation for the large anisotropy of the magnetic susceptibility in α-RuCl3 as arising from the large Γ term. We therefore provide a crucial foundation for understanding the interactions underpinning the exotic magnetic behaviours observed in α-RuCl3. α-RuCl3 is a promising candidate for realizing the Kitaev quantum spin liquid, but the physics governing its magnetic behaviour remain elusive. Resonant elastic X-ray scattering data now set unambiguous constraints on the leading terms in the Hamiltonian.