Multiferroics are materials with more than one ferroic property – namely, ferroelectricity, ferroelasticity and ferromagnetism. Ferroic materials exhibit uniform, yet differently oriented three-dimensional regions called domains. In multiferroic materials with two ferroic properties, the ferroic properties and their domains, may be either rigidly coupled or completely independent. Both of these cases have their merits.
Using two prototypical examples, we show that in materials with three or more ferroic orders, unusual combinations of couplings and domain patterns are possible. One such material is Dy0.7Tb0.3FeO3. Here, an antiferromagnetic order (L) and a ferromagnetic order (M) induce an electric polarization (P) and a trilinear coupling contribution M · L · P to the free energy. We show that this coupling enables the transfer a domain pattern in M to P , and vice versa, while erasing it in the source state. The third order parameter L acts as “memory buffer” and mediates the transfer.
Taking ErMnO3 as second example, we demonstrate that multiferroics with separately emerging magnetic and electric order can have a magnetoelectric coupling even though the magnetoelectric effect itself is symmetry-forbidden. We reveal that the structural distortions that induce the ferroelectric polarization also break the balance of the competing superexchange contributions. By doing so the structural distortion links the polar and the magnetic order.
In both cases, the multi-order interactions lead to atypical effects on the basis of domains and domain walls. These examples demonstrate the significance of exploring multiferroics beyond the coexistence of ferroelectric and magnetic order.
Institut des Molécules et Matériaux du Mans, UMR 6283 CNRS, Le Mans Université, Le Mans