Magnetic structures of the polar magnet system RFeWO₆ and its coupling to ferroelectricity

Multiferroics exhibit more than one ferroic order and intense research is dedicated in particular to compounds with coexisting magnetic and ferroelectric orders, the so called magnetoelectric multiferroics. The coupling between the orders is relevant in technological applications like MERAMS or MESO logic. However, the coupling mechanisms are still debated. One way to describe is to classify them according to their coupling strength of the orders from independent origins (type I, eg. BiFeO₃) to the strong coupling case (type II, eg. TbMnO₃) wherein the electric polarization (EP) emerges via symmetry-breaking by the magnetic ordering (MO). Recently a new class of multiferroics, the polar magnets, is described to have a polar structure in the paramagnetic phase like type I, but the EP emerges only below MO like type II. One such system is RFeWO₆ (R: smaller rare earths) which crystallizes in the aeschynite structure with a charge ordering of Fe³⁺ and W⁺⁶. The EP sets in at the MO temperature around 15-20 K depending on R. Although bulk properties have been determined, detailed magnetic and crystal structure investigations are lacking, which are essential to determine the origin and coupling of the ferroic orders. In this work we utilize neutron scattering techniques, bulk property measurements and theoretical modeling to address these issues for several members. Our results indicate that the EP emerges from multiple origins governed by the underlying magnetic interactions.