Co4Ta2O9 exhibits a three-dimensional magnetic lattice based on the buckled honeycomb motif. It shows unusual magnetoelectric effects, including the sign change and nonlinearity. 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∘ 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.
Bibliographical noteFunding Information:
We thank X. Xu for the assistance with the traveling solvent floating zone sample growth, and Professor G. Schütz for supporting the susceptibility measurements using the MPMS magnetometry. S.C. thanks R. Coldea for sharing the software tools for visualizing single-crystal time-of-flight neutron diffraction data. This work was supported by the NSF under Grant No. DMR-1609935. S.C. was also supported by the international postdoctoral scholarship at Max Planck Institute for Solid State Research in Germany in the initial stage of this project. Sample growth efforts (J.W.K. and S.W.C.) were supported by the DOE under Grant No. DOE: DE-FG02-07ER46382. S.P. acknowledges financial support from the China Scholarship Council (File No. 201808330123). The work at Yonsei University was supported by the National Research Foundation of Korea (NRF) Grants No. NRF-2017R1A5A1014862 (SRC program: vdWMRC center), No. NRF-2018R1C1B6006859, and No. NRF-2019R1A2C2002601.
© 2020 American Physical Society.
All Science Journal Classification (ASJC) codes
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics