Noncollinear antiferromagnetic order in the buckled honeycomb lattice of magnetoelectric Co4Ta2O9 determined by single-crystal neutron diffraction

Co4Ta2O9 is a newly interested magnetoelectric material with strong coupling. It exhibits unusual magnetoelectric effects, including sign change and non-linearity, which cannot be understood without detailed knowledge of the magnetic structure.

Here, we report systematic neutron diffraction measurements and direction-dependent magnetic susceptibility measurements on Co4Ta2O9 single crystals [1]. Below 20.3 K, we find a long-range antiferromagnetic order in the alternating buckled and flat honeycomb layers of Co2+ ions stacked along the c-axis. Within experimental accuracy, we propose the magnetic moments lying in the ab-plane as a minimal model. They form a tilted antiferromagnetic structure with a tilt angle of about 14 degrees at 15 K in each buckled layer and collinear moments in each flat layer, directly evidenced by a finite (0, 0, 3) magnetic Bragg peak. The obtained magnetic space group is C2'/c, different from C2/c' previously reported by powder neutron diffraction and in the isostructural Co4Nb2O9. Our revised magnetic structure successfully explains the main features of the magnetoelectric tensor of Co4Ta2O9 within the framework of the spin-flop model.

[1] Sungkyun Choi et al., https://arxiv.org/abs/2007.06561 (2020).