${\mathrm{Co}}_4{\mathrm{Ta}}_2{\mathrm{O}}_9$ exhibits a three-dimensional magnetic lattice based on the buckled honeycomb motif. It shows unusual magnetoelectric effects, including the sign change and nonlinearity. These effects cannot be understood without the detailed knowledge of the magnetic structure. Herein, we report neutron diffraction and direction-dependent magnetic susceptibility measurements on ${\mathrm{Co}}_4{\mathrm{Ta}}_2{\mathrm{O}}_9$ single crystals. Below 20.3 K, we find a long-range antiferromagnetic order in the alternating buckled and flat honeycomb layers of ${\mathrm{Co}}^{2+}$ ions stacked along the $\mathit{c}$ axis. Within experimental accuracy, the magnetic moments lie in the $\mathit{ab}$ plane. They form a canted antiferromagnetic structure with a tilt angle of $\sim {14}^{\circ }$ at 15 K in the buckled layers, while the magnetic moments in each flat layer are collinear. This is directly evidenced by a finite (0, 0, 3) magnetic Bragg peak intensity, which would be absent in the collinear magnetic order. The magnetic space group is $C{2}^{\prime }/c$. It is different from the previously reported $C2/c^{\prime }$ group, also found in the isostructural ${\mathrm{Co}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$. The revised magnetic structure successfully explains the major features of the magnetoelectric tensor of ${\mathrm{Co}}_4{\mathrm{Ta}}_2{\mathrm{O}}_9$ within the framework of the spin-flop model.

• Received 28 August 2020
• Accepted 13 November 2020

DOI:https://doi.org/10.1103/PhysRevB.102.214404