Correlation between ferroic orders in the polar magnet system RFeWO₆

Multiferroics are materials with more than one ferroic order. We are interested, in particular, in compounds with coexisting magnetic and ferroelectric orders the so called magnetoelectric multiferroics. The coupling between the orders is technologically relevant in applications like MERAMS but it is not completely understood. They are classified according to their coupling strength of the orders from independent origins (type I) to the strong coupling limit (type II) 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 but the EP emerges only below MO. One such system is RFeWO₆ (R: smaller rare earths) which crystallizes in the aeschynite structure. The EP sets in at the MO temperature around 15-20 K depending on R. Although ferroelectric and magnetic properties have been determined, detailed magnetic and crystal structure investigations to determine the origin and coupling of the ferroic orders are lacking. In this work we utilize neutron scattering techniques, detailed 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.