Easy-plane multi-𝐪 magnetic ground state of Na_3Co_2SbO_6

Na_3Co_2SbO_6 is a potential Kitaev magnet with a monoclinic layered crystal structure. Recent investigations of the C_3-symmetric sister compound Na_2Co_2TeO_6 have uncovered a unique triple-𝐪 magnetic ground state, as opposed to a single-𝐪 (zigzag) one, prompting us to examine the influence of the reduced structural symmetry of Na_3Co_2SbO_6 on its ground state. Neutron diffraction data obtained on a twin-free crystal reveal that the ground state remains a multi-𝐪 state, despite the system's strong in-plane anisotropy. This robustness of multi-𝐪 orders suggests that they are driven by a common mechanism in the honeycomb cobaltates, such as higher-order magnetic interactions. Spin-polarized neutron diffraction results show that the ordered moments are entirely in-plane, with each staggered component orthogonal to the propagating wave vector. The inferred ground state favors a so-called XXZ easy-plane anisotropic starting point for the microscopic model over a Kitaev one, and features unequal ordered moments reduced by strong quantum fluctuations.