Low-temperature crystal and magnetic structures of the magnetoelectric material Fe4Nb2O9

Abstract

$\mathrm{F}{\mathrm{e}}_{4}\mathrm{N}{\mathrm{b}}_{2}{\mathrm{O}}_{9}$ was recently reported to be a new magnetoelectric material with two distinct dielectric anomalies located at ${T}_{N}\ensuremath{\approx}90\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for an antiferromagnetic transition and ${T}_{\mathrm{str}}\ensuremath{\approx}77\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ of unknown origin, respectively. By analyzing low-temperature neutron-powder-diffraction data, here we determined its magnetic structure below ${T}_{N}$ and uncovered the origin of the second dielectric anomaly as a structural phase transition across ${T}_{\mathrm{str}}$. In the antiferromagnetically ordered state below ${T}_{N}$, both Fe1 and Fe2 magnetic moments lying within the weakly and strongly buckled honeycomb layers are arranged in a fashion that the three nearest neighbors are directed oppositely. Upon cooling below ${T}_{\mathrm{str}}$, the symmetry of crystal structure is lowered from trigonal $P\ensuremath{-}3c1$ to monoclinic $C2/c$, in which a weak sliding of the metal octahedral planes introduces a monoclinic distortion of $\ensuremath{\sim}1.{7}^{\ensuremath{\circ}}$. The magnetic structure is preserved in the low-temperature monoclinic phase, and the Fe magnetic moment increases from $2.1(1){\ensuremath{\mu}}_{B}$ at 95 K to $3.83(4){\ensuremath{\mu}}_{B}$ at 10 K assuming an equal moment configuration at Fe1 and Fe2 sites. The magnetic point group and linear magnetoelectric tensor at each temperature region are determined. From a symmetry-related tensor analysis, the microscopic origins of the magnetoelectric effects between ${T}_{N}$ and ${T}_{\mathrm{str}}$ are proved to be spin-current and $d\ensuremath{-}p$ hybridization mechanisms.