Magnetic-field-induced change of magnetoelectric coupling in the hybrid multiferroic (ND4)(2)[FeCl5 center dot D2O].
In this paper, we elucidate the changes of magnetoelectric coupling mechanism in different zones of the rich magnetic field-temperature (B-T) phase diagram of the molecular multiferroic (NH4)(2)[FeCl5 center dot H2O], which represents one of the rare cases where improper ferroelectricity has been observed in a hybrid material. We have recently proposed a mechanism of multiferroicity in zero magnetic field in the deuterated form of this material from a detailed determination of its crystal and magnetic structures. The proposed magnetic structure at zero magnetic field corresponds to a cycloidal spin arrangement that gives rise to a ferroelectric polarization through the spin current mechanism induced via the inverse Dzyaloshinskii-Moriya interaction. In this paper, we present a single-crystal neutron diffraction study under external magnetic field, aimed at elucidating the evolution of the magnetic structure under applied magnetic field, and determine the mechanism of magnetoelectric coupling, which allows us to describe an unprecedented change from spin current to spin-dependent p-d hybridization mechanism.