The magnetic structure and magnetoelectric effect have been investigated for single crystals of the antiferromagnet ${\mathrm{Co}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$. Single-crystal neutron diffraction and magnetic susceptibility measurement have revealed that the magnetic structure is different from a collinear arrangement with spin parallel to the trigonal axis as proposed previously. ${\mathrm{Co}}^{2+}$ magnetic moments are found to be almost lying in the basal plane, which lowers the magnetic symmetry to $C2/c^{\prime }$ with the propagation vector $\mathit{k}=0$. Associated with the magnetic phase transition, a sharp anomaly in the dielectric constant and displacement current indicate the appearance of the magnetoelectric below Néel temperature $T_N$ with a large coupling constant up to 30 ps/m. The existence of off-diagonal components in a magnetoelectric tensor indicate the formation of ferrotoroidic order in ${\mathrm{Co}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$. Such a magnetoelectric effect can be ascribed to the reduction of symmetry caused by simple antiferromagnetic order in a honeycomb network.

• Received 2 June 2015
• Revised 8 January 2016

DOI:https://doi.org/10.1103/PhysRevB.93.075117