Spin-lattice coupling mediated multiferroicity in (ND4)2FeCl5D2O

TL;DR

This study uses neutron diffraction to reveal how spin-lattice interactions induce ferroelectricity in (ND4)2FeCl5D2O through complex magnetic phase transitions and cycloid distortions.

Contribution

It demonstrates the role of spin-lattice coupling in mediating multiferroicity in a molecular compound with quenched orbital angular momentum.

Findings

Identification of two magnetic transitions with associated ferroelectricity.

Observation of higher-order harmonics indicating strong spin-lattice coupling.

Evidence that electric polarization is mediated by spin-lattice interaction, similar to TbMnO3.

Abstract

We report a neutron diffraction study of the multiferroic mechanism in (ND4)2FeCl5D2O, a molecular compound that exhibits magnetically induced ferroelectricity. This material exhibits two successive magnetic transitions on cooling: a long-range order transition to an incommensurate (IC) collinear sinusoidal spin state at TN=7.3 K, followed by a second transition to an IC cycloidal spin state at TFE=6.8 K, the later of which is accompanied by spontaneous ferroelectric polarization. The cycloid structure is strongly distorted by spin-lattice coupling as evidenced by the observations of both odd and even higher-order harmonics associated with the cycloid wave vector, and a weak commensurate phase that coexists with the IC phase. The appearance of the 2nd-order harmonic coincides with the onset of the electric polarization, thereby providing unambiguous evidence that the induced electric polarization is mediated by the spin-lattice interaction. Our results for this system, in which the orbital angular momentum is expected to be quenched, are remarkably similar to those of the prototypical TbMnO3, in which the magnetoelectric effect is attributed to spin-orbit coupling.