Multiferroics - Mechanisms For Ferroelectricity in Multiferroics

Mechanisms For Ferroelectricity in Multiferroics

A necessary but not sufficient condition for the appearance of spontaneous electric polarization is the absence of inversion symmetry. We can distinguish between proper and improper ferroelectric (FE). The difference lies in the driving force (the primary order parameter) that leads to ferroelectricity: In the case of proper FE, the primary order parameter is the ferroelectric distortion. One example of proper FE is BaTiO₃ where a covalent bonding between the transition metal and the oxygen happens to allow a polar state. In usual perovskite-based ferroelectrics like BaTiO₃, the ferroelectric distortion occurs due to the displacement of B-site cation (Ti) with respect to the oxygen octahedral cage. Here the transition metal ion (Ti in BaTiO₃ ) requires an empty “d” shell since the ferroelectric displacement occurs due to the hopping of electrons between Ti “d” and O “p” atoms.

For technological applications it's highly desirable to combine ferroelectric and ferromagnetic order within one material, but it has become clear that usual displacive ferroelectric order, e.g. like in BaTiO₃, cannot coexist together with magnetic order. Whereas the latter requires at least partially filled d-shells (e.g.-orbitals) providing a non-zero magnetic moment, usual displacive ferroelectricity requires empty d-shells. Therefore new ferroelectric driving mechanisms must be present for electric and magnetic ferroic order to occur simultaneously. A variety of driving mechanism is described below.