Competing antiferromagnetic-ferromagnetic states in a d⁷ Kitaev honeycomb magnet

The Kitaev model is a rare example of an analytically solvable and physically instantiable Hamiltonian yielding a topological quantum spin liquid ground state. Here we report signatures of Kitaev spin liquid physics in the honeycomb magnet Li₃Co₂SbO₆, built of high-spin $d^7$ (Co²⁺) ions, in contrast to the more typical low-spin $d^5$ electron configurations in the presence of large spin-orbit coupling. Neutron powder diffraction measurements, heat capacity, and magnetization studies support the development of a long-range antiferromagnetic order space group of $C_C\text{2/}m$, below $T_N\hspace{0.17em}\text{=}\hspace{0.17em}\text{11 K}$ at ${\mathrm{micro}}_{\text{0}}H\hspace{0.17em}\text{=}\hspace{0.17em}\text{0 T}$. The magnetic entropy recovered between $T\hspace{0.17em}\text{=}\hspace{0.17em}\text{2}$ and 50 K is estimated to be $\text{0.6}R\text{ln2}$, in good agreement with the value expected for systems close to a Kitaev quantum spin liquid state. The temperature-dependent magnetic order parameter demonstrates a $\beta$ value of 0.19(3), consistent with XY anisotropy and in-plane ordering, with Ising-like interactions between layers. Further, we observe a spin-flop-driven crossover to ferromagnetic order with space group of $C\text{2/}m$ under an applied magnetic field of ${\mathrm{micro}}_{\text{0}}H\hspace{0.17em}\approx \hspace{0.17em}\text{0.7 T}$ at $\text{T}\hspace{0.17em}\text{=}\hspace{0.17em}\text{2}\hspace{0.17em}\text{K}$. Magnetic structure analysis demonstrates these magnetic states are competing at finite applied magnetic fields even below the spin-flop transition. Both the $d^7$ compass model, a quantitative comparison of the specific heat of Li₃Co₂SbO₆, and related honeycomb cobaltates to the anisotropic Kitaev model further support proximity to a Kitaev spin liquid state. This material demonstrates the rich playground of high-spin $d^7$ systems for spin liquid candidates and complements known $d^5$ Ir- and Ru-based materials.