Magnetoelectric coupling in the honeycomb antiferromagnet Co sub(4) Nb sub(2) O sub(9)

The magnetic structure and magnetoelectric effect have been investigated for single crystals of the antiferromagnet Co sub(4) Nb sub(2) O sub(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. Co super(2+) magnetic moments are found to be almost lying in the basal plane, which lowers the magnetic symmetry to C 2/c' with the propagation vector 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 Neel temperature T sub()Nwith 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 Co sub(4) Nb sub(2) O sub(9). Such a magnetoelectric effect can be ascribed to the reduction of symmetry caused by simple antiferromagnetic order in a honeycomb network.