Abstract
Magnetism in cobalt compounds has an interesting facet due to the element’s non-negligible spin-orbit interplay. In recent years, several cobalt oxides with layered honeycomb crystal structures have been proposed to have the potential to realize the celebrated Kitaev model for quantum spin liquids, and becoming intensively studied as 3d-electron analogues of 5d iridates and 4d α-RuCl3. From a crystal grower and spectroscopist’s view point, I will present our study on two of these compounds: Na2Co2TeO6 (NCTO) and Na3Co2SbO6 (NCSO), which look similar at first sight. Our data suggest that subtle differences make the systems highly contrasting against each other: NCTO has a nominally C3-symmetric crystal structure, whereas NCSO has monoclinic stacking; NCTO is more antiferromagnetic than NCSO, probably due to further-neighbor interactions. At the end of the day, we believe that the two systems might not be very close to the idealized Kitaev model but are nonetheless highly interesting – the fact that subtle differences in the “initial” conditions of the crystal lattice lead to dramatically different physical properties indicates strong magnetic phase competition, which we see as a precursor to quantum spin liquids. Experimental hints at the realization of the latter will be discussed, along with remaining open issues.