Magnetically-induced ferroelectricity in the (ND4)2[FeCl5(D2O)] molecular compound

TL;DR

This study reveals magnetically-induced ferroelectricity in a molecular compound, (NH4)2[FeCl5(H2O)], through detailed neutron diffraction analysis showing a cycloidal magnetic structure that explains the multiferroic behavior.

Contribution

It provides the first detailed magnetic and structural analysis of improper ferroelectricity in a molecular multiferroic, demonstrating the inverse Dzyaloshinskii-Moriya mechanism in this context.

Findings

Magnetic structure is cycloidal with moments in the ac-plane.

Ferroelectricity is explained by the inverse Dzyaloshinskii-Moriya mechanism.

Order-disorder transition of ammonium ions observed.

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 2K 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.