Anisotropic susceptibilities in the honeycomb Kitaev system $\hat{I}\pm$-RuCl${}_3$

Abstract

The magnetic insulator $\hat{I}\pm$-RuCl${}_3$ is a promising candidate to realize Kitaev interactions on a quasi-2D honeycomb lattice. We perform extensive susceptibility measurements on single crystals of $\hat{I}\pm$-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-$\hat{I}“$ 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 $\hat{I}“$ spin exchange. The $\hat{I}“$ couplings and relationships between other terms in the model Hamiltonian are quantified by extracting relevant Curie-Weiss intercepts from the experimental data.