Pseudospin exchange interactions in d(7) cobalt compounds: Possible realization of the Kitaev model

The current efforts to find the materials hosting Kitaev model physics have been focused on Mott insulators of d(5) pseudospin-1/2 ions Ir4+ and Ru3+ with t(2g)(5) (S = 1/2, L = 1) electronic configuration. Here we propose that the Kitaev model can be realized in materials based on d(7) ions with t(2g)(5) e(g)(2) (S = 3/2, L = 1) configuration such as Co2+, which also host the pseudospin-1/2 magnetism. Considering possible exchange processes, we have derived the d(7) pseudospin-1/2 interactions in 90 degrees bonding geometry. The obtained Hamiltonian comprises the bond-directional Kitaev K and isotropic Heisenberg J interactions as in the case of d(5) ions. However, we find that the presence of additional spin-active e(g) electrons radically changes the balance between Kitaev and Heisenberg couplings. Most remarkably, we show that the exchange processes involving e(g) spins are highly sensitive to whether the system is in Mott (U < Delta) or charge-transfer (U > Delta) insulating regime. In the latter case, to which many cobalt compounds do actually belong, the antiferromagnetic Heisenberg coupling J is strongly suppressed and spin-liquid phase can be stabilized. The results suggest cobalt-based materials as promising candidates for the realization of the Kitaev model.