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

alpha-RuCl3 is drawing much attention as a promising candidate for the Kitaev quantum spin liquid(1-8). 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 studies(5,6,9-15). The significance of the symmetric off-diagonal exchange interaction (referred to as the Gamma term) is another contentious question(16-18). 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 Gamma 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 alpha-RuCl3 as arising from the large Gamma term. We therefore provide a crucial foundation for understanding the interactions underpinning the exotic magnetic behaviours observed in alpha-RuCl3.