Complex Magnetic Incommensurability and Electronic Charge Transfer Through the Ferroelectric Transition in Multiferroic Co3TeO6
Wen-Hsien Li1*, Chi-Hung Lee1, Chin-Wei Wang2, Yang Zhao3,4, Jeffrey W. Lynn3, A. Brooks Harris5, Kirrily Rule6, Hung-Duen Yang7, Helmuth Berger8
1Department of Physics, National Central University, Jhongli 32001, Taiwan
2Neutron Group, National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan
3NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA
4Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, USA
5Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, PA 19104, USA
6Bragg Institute, Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW 2234, Australia
7Department of Physics and Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan
8Institute of Physics of Complex Matter, EPFL, Lausanne, Switzerland
* presenting author:Wen-Hsien Li,
Many novel physical behaviors have recently been identified in the M3TeO6-class of metal tellurates, where M is a first-row transition metal. In particular, the monoclinic cobalt tellurate Co3TeO6 has been characterized as a type-II multiferroic, where the order parameters of electrical polarization and spontaneous magnetization are closely coupled. In this study we focus on new aspects of this interesting but complicated multiferroic, including the magnetic structure, charge density, and coupling to the ferroelectric order. Polarized and unpolarized neutron diffractions have been carried out to investigate the nature of the magnetic structures and transitions in monoclinic Co3TeO6. As the temperature is lowered: below TM1 = 26 K long range order develops, which is fully incommensurate (ICM) in all three crystallographic directions in the crystal. Below TM2 = 19.5 K additional commensurate magnetic peaks develop, consistent with the Γ4 irreducible representation, along with a splitting of the ICM peaks along the h direction which indicates that there are two separate sets of magnetic modulation vectors. Below TM3 = 18 K, this small additional magnetic incommensurability disappears, ferroelectricity develops, a commensurate Γ3 irreducible representation appears, and the k component of the ICM wave vector disappears. Synchrotron x-ray diffraction measurements demonstrate that there is a significant shift of the electronic charge distribution from the Te ions at the crystallographic 8f sites to the neighboring Co and O ions. These results, together with the unusually small electric polarization, its strong magnetic field dependence, and the negative thermal expansion in all three lattice parameters, suggest this material is an antiferroelectric. Below TM4 = 15 K the k component of the ICM structure reappears, along with second-order ICM Bragg peaks, which polarized neutron data demonstrate are magnetic in origin. The existence of these new peaks indicates that a small canting with a net magnetization has developed.

Keywords: Multiferroic, Neutron diffraction