single crystals*

Strongly correlated materials with multiple order parameters provide unique insights into the fundamental interactions in condensed matter systems and present opportunities for innovative technological applications . 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. 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.