Bond-dependent interactions and ill-ordered state in the honeycomb cobaltate BaCo${}_2$(AsO${}_4$)${}_2$

Abstract

The ground state and Hamiltonian of the honeycomb lattice material BaCo${}_2$(AsO${}_4$)${}_2$ hosting magnetic Co${}^{2+}$, have been debated for decades. The recent proposal for anisotropic bond-dependent interactions in such honeycomb cobaltates has raised the prospect of revisiting its Hamiltonian in the context of Kitaev physics. To test this hypothesis, we have combined magnetization, ac-susceptibility and neutron scattering measurements on a BaCo${}_2$(AsO${}_4$)${}_2$ single-crystal, together with advanced modeling. Our experimental results highlight a collinear magnetic ground state with intrinsic disorder associated to an average incommensurate propagation vector. Monte Carlo simulations and linear spin wave calculations were performed to obtain a spin model compatible with this unusual ground state, the dispersion of magnetic excitations and a magnetization plateau under magnetic field. We thus show that bond-dependent anisotropic interactions, including Kitaev-like interactions, are necessary to account for the puzzling properties of this long-explored material, and are hence a general ingredient in the cobaltates.