Magnetotropic Response in Ruthenium Chloride.

We consider the exchange couplings present in an effective Hamiltonian of $\alpha$-RuCl$_3$, known as the K-$\Gamma$ model. This material has a honeycomb lattice, and is expected to be a representative of the Kitaev materials (which can realize the 2d Kitaev model). However, the behaviour of RuCl$_3$ shows that the exchange interactions of the material are not purely Kitaev-like, especially because it has an antiferromagnetic ordering at low temperatures and under low strengths of an externally applied magnetic field. Fitting the data obtained from the measurements of the magnetotropic coefficient (the thermodynamic coefficient associated with magnetic anisotropy), reported in Nature Physics 17, 240-244 (2020), we estimate the values of the exchange couplings of the effective Hamiltonian. The fits indicate that the Kitaev couplings are subdominant to the other exchange couplings.