Bond-dependent interactions and ill-ordered state in the honeycomb cobaltate BaCo 2 ( AsO 4 ) 2

The ground state and Hamiltonian of the honeycomb lattice material <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:msub> <a:mi>BaCo</a:mi> <a:mn>2</a:mn> </a:msub> </a:math> ( <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:msub> <b:mi>AsO</b:mi> <b:mn>4</b:mn> </b:msub> </b:math> ) <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:msub> <c:mrow/> <c:mn>2</c:mn> </c:msub> </c:math> hosting magnetic <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:msup> <d:mrow> <d:mi>Co</d:mi> </d:mrow> <d:mrow> <d:mn>2</d:mn> <d:mo>+</d:mo> </d:mrow> </d:msup> </d:math> 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 <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"> <e:msub> <e:mi>BaCo</e:mi> <e:mn>2</e:mn> </e:msub> </e:math> ( <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"> <f:msub> <f:mi>AsO</f:mi> <f:mn>4</f:mn> </f:msub> </f:math> ) <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"> <g:msub> <g:mrow/> <g:mn>2</g:mn> </g:msub> </g:math> single crystal, together with advanced modeling. Our experimental results highlight a collinear magnetic ground state with intrinsic disorder associated with 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.