Kitaev interactions in the Co honeycomb antiferromagnets Na$_3$Co$_2$SbO$_6$ and Na$_2$Co$_2$TeO$_6$

TL;DR

This study investigates spin dynamics in cobalt honeycomb compounds Na$_2$Co$_2$TeO$_6$ and Na$_3$Co$_2$SbO$_6$, providing evidence for Kitaev interactions through neutron scattering and spin wave analysis, revealing complex magnetic couplings.

Contribution

First experimental demonstration of Kitaev interactions in cobalt-based honeycomb materials using neutron scattering and theoretical modeling.

Findings

Kitaev interactions dominate the magnetic spectra.

Additional Heisenberg and off-diagonal exchanges lead to zigzag magnetic order.

Evidence of Kitaev-type coupling despite weaker spin-orbit coupling in cobalt.

Abstract

Co$^{2+}$ ions in an octahedral crystal field, stabilise a j$_{eff}$ = 1/2 ground state with an orbital degree of freedom and have been recently put forward for realising Kitaev interactions, a prediction we have tested by investigating spin dynamics in two cobalt honeycomb lattice compounds, Na$_2$Co$_2$TeO$_6$ and Na$_3$Co$_2$SbO$_6$, using inelastic neutron scattering. We used linear spin wave theory to show that the magnetic spectra can be reproduced with a spin Hamiltonian including a dominant Kitaev nearest-neighbour interaction, weaker Heisenberg interactions up to the third neighbour and bond-dependent off-diagonal exchange interactions. Beyond the Kitaev interaction that alone would induce a quantum spin liquid state, the presence of these additional couplings is responsible for the zigzag-type long-range magnetic ordering observed at low temperature in both compounds. These results provide evidence for the realization of Kitaev-type coupling in cobalt-based materials, despite hosting a weaker spin-orbit coupling than their 4d and 5d counterparts.