Magnetoelectric coupling in the honeycomb antiferromagnet CoNbO

Khanh, N. D.*; Abe, Nobuyuki*; Sagayama, Hajime*; Nakao, Akiko*; Hanashima, Takayasu*; Kiyanagi, Ryoji  ; Tokunaga, Yusuke*; Arima, Takahisa*

The magnetic structure and magnetoelectric effect have been investigated for single crystals of the antiferromagnet CoNbO. 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 magnetic moments are found to be almost lying in the basal plane, which lowers the magnetic symmetry to with the propagation vector = 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 TN 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 CoNbO. Such a magnetoelectric effect can be ascribed to the reduction of symmetry caused by simple antiferromagnetic order in a honeycomb network.