Complex Magnetic Incommensurability and Electronic Charge Transfer through the Ferroelectric Transition in Multiferroic Co3TeO6
Chi-Hung Lee, Chin-Wei Wang, Yang Zhao, Wen-Hsien Li, Jeffrey W. Lynn, A. Brooks Harris, Kirrily Rule, Hung-Duen Yang, Helmuth Berger
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 they 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. Finally, searches were performed for commensurate peaks for the form (0, 1/2, 1/4) as was inferred from powder diffraction data of Ivanov, et al. No evidence of such peaks was found, indicating that these peaks are not present in Co3TeO^6^.