LAUSR

The magnetic insulator $\ensuremath{\alpha}\ensuremath{-}{\mathrm{RuCl}}_{3}$ is a promising candidate to realize Kitaev interactions on a quasi-two-dimensional honeycomb lattice. We perform extensive susceptibility measurements on single crystals of $\ensuremath{\alpha}\ensuremath{-}{\mathrm{RuCl}}_{3}$, including angle dependence of the in-plane longitudinal and transverse susceptibilities, which reveal a unidirectional anisotropy within the honeycomb plane. By comparing the experimental results to a high-temperature expansion of a Kitaev-Heisenberg-$\mathrm{\ensuremath{\Gamma}}$ spin Hamiltonian with bond anisotropy, we find excellent agreement with the observed phase shift and periodicity of the angle-resolved susceptibilities. Within this model, we show that the pronounced difference between in-plane and out-of-plane susceptibilities as well as the finite transverse susceptibility are rooted in strong symmetric off-diagonal $\mathrm{\ensuremath{\Gamma}}$ spin exchange. The $\mathrm{\ensuremath{\Gamma}}$ couplings and relationships between other terms in the model Hamiltonian are quantified by extracting relevant Curie-Weiss intercepts from the experimental data.