title: Magnetically-induced ferroelectricity in the (ND4)2[FeCl5(D2O)] molecular compound.
abstract: The number of magnetoelectric multiferroic materials reported to date is scarce, as magnetic structures that break inversion symmetry and induce an improper ferroelectric polarization typically arise through subtle competition between different magnetic interactions. The (NH4)2[FeCl5(H2O)] compound is a rare case where such improper ferroelectricity has been observed in a molecular material. We have used single crystal and powder neutron diffraction to obtain detailed solutions for the crystal and magnetic structures of (NH4)2[FeCl5(H2O)], from which we determined the mechanism of multiferroicity. From the crystal structure analysis, we observed an order-disorder phase transition related to the ordering of the ammonium counterion. We have determined the magnetic structure below TN, at 2 K and zero magnetic field, which corresponds to a cycloidal spin arrangement with magnetic moments contained in the ac-plane, propagating parallel to the c-axis. The observed ferroelectricity can be explained, from the obtained magnetic structure, via the inverse Dzyaloshinskii-Moriya mechanism.
authors.affiliation: Instituto de Ciencia de Materiales de Aragón (ICMA), CSIC - Universidad de Zaragoza, 50009 Zaragoza, Spain.|nstitut Laue-Langevin, 38042 Grenoble Cedex 9, France.
authors.forename: José
authors.initials: J
authors.lastname: Alberto Rodríguez-Velamazán
authors.affiliation: nstitut Laue-Langevin, 38042 Grenoble Cedex 9, France.
authors.forename: Óscar
authors.initials: Ó
authors.lastname: Fabelo
authors.affiliation: Instituto de Ciencia de Materiales de Aragón (ICMA), CSIC - Universidad de Zaragoza, 50009 Zaragoza, Spain.
authors.forename: Ángel
authors.initials: Á
authors.lastname: Millán
authors.affiliation: Instituto de Ciencia de Materiales de Aragón (ICMA), CSIC - Universidad de Zaragoza, 50009 Zaragoza, Spain.
authors.forename: Javier
authors.initials: J
authors.lastname: Campo
authors.affiliation: Univ. Oxford, Dept Phys, Clarendon Lab, Oxford OX1 3PU, England.
authors.forename: Roger D
authors.initials: RD
authors.lastname: Johnson
authors.affiliation: nstitut Laue-Langevin, 38042 Grenoble Cedex 9, France.
authors.forename: Laurent
authors.initials: L
authors.lastname: Chapon