single crystals

A class of antiferromagnetic honeycomb lattices compounds, A₄B₂O₉ (A = Co, Fe, Mn; B = Nb, Ta), have been explored owing to the occurrence of linear magnetoelectricity. We observe a highly nonlinear magnetoelectric effect on single crystals of Co₄Ta₂O₉ (CTO), distinctive from the linear behavior in the isostructural Co₄Nb₂O₉. Ferroelectricity emerges primarily along the [110] direction under magnetic fields, with the onset of antiferromagnetic order at T_N = 20.5 K. For in-plane magnetic field, a spin-flop occurs at H_C ≈ 0.3 T, above which the ferroelectric polarization gradually becomes negative and reaches a broad minimum. Upon increasing magnetic field further, the polarization crosses zero and increases continuously to ~60 μC/m² at 9 T. In contrast, the polarization for a magnetic field perpendicular to the hexagonal plane increases monotonously and reaches ~80 μC/m² at 9 T. This observation of a strongly nonlinear magnetoelectricity suggests that two types of inequivalent Co²⁺ sublattices generate magnetic field-dependent ferroelectric polarization with opposite signs. These results motivate fundamental and applied research on the intriguing magnetoelectric characteristics of these honeycomb lattice materials.