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

TL;DR

This study reveals the detailed noncollinear antiferromagnetic structure of Co4Ta2O9 using neutron diffraction, explaining its unusual magnetoelectric effects and providing insights into its magnetic ordering and symmetry.

Contribution

The paper determines the magnetic structure of Co4Ta2O9, showing a canted antiferromagnetic order with a revised magnetic space group, advancing understanding of its magnetoelectric properties.

Findings

Long-range antiferromagnetic order below 20.3 K.

Magnetic moments lie in the ab plane with a ~14° tilt.

Magnetic space group identified as C2'/c, different from previous reports.

Abstract

Co4Ta2O9 exhibits a three-dimensional magnetic lattice based on the buckled honeycomb motif. It shows unusual magnetoelectric effects, including the sign change and non-linearity. These effects cannot be understood without the detailed knowledge of the magnetic structure. Herein, we report neutron diffraction and direction-dependent magnetic susceptibility measurements on Co4Ta2O9 single crystals. 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, the magnetic moments lie in the ab plane. They form a canted antiferromagnetic structure with a tilt angle of ~ 14 degrees at 15 K in the buckled layers, while the magnetic moments in each flat layer are collinear. This is directly evidenced by a finite (0, 0, 3) magnetic Bragg peak intensity, which would be absent in the collinear magnetic order. The magnetic space group is C2'/c. It is different from the previously reported C2/c' group, also found in the isostructural Co4Nb2O9. The revised magnetic structure successfully explains the major features of the magnetoelectric tensor of Co4Ta2O9 within the framework of the spin-flop model.