Competing Antiferromagnetic-Ferromagnetic States in $d^7$ Kitaev Honeycomb Magnet

Abstract

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 $L{i}_{3}C{o}_{2}Sb{O}_6$, built of high-spin $d^7$ ($C{o}^{2+}$) 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 $C_{C}2/m$, below $T_N$ = 11 K at ${\mu }_{0}H$ = 0 T. The magnetic entropy recovered between $T$ = 2 K and 50 K is estimated to be 0.6Rln2, 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 $C2/m$ under an applied magnetic field of ${\mu }_{0}H$ $\approx$ 0.7 T at $T$ = 2 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 $L{i}_{3}C{o}_{2}Sb{O}_6$, 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.

16 pages, 8 figures