Hybrid improper ferroelectricity: a mechanism for controllable polarization-magnetization coupling

First-principles calculations are presented for the layered perovskite Ca3Mn2O7. The results reveal a rich set of coupled structural, magnetic and polar domains in which oxygen octahedron rotations induce ferroelectricity, magnetoelectricity and weak-ferromagnetism. The key point is that the rotation distortion is a combination of two non-polar modes with different symmetries. We use the term “hybrid ” improper ferroelectricity to describe this phenomenon and discuss how control over magnetism is achieved through these functional antiferrodistortive octahedron rotations. The utility of multiferroics for low-power electronic de-vices stems from the possibility for electric-field control of magnetism at room temperature [1–4]. A challenge that has so far not been overcome is to identify a stable, single phase multiferroic material in which the magneti-zation can be deterministically switched 180◦. A large electrical polarization strongly coupled to the magneti-zation is generally thought to be a key requirement [4]. Magnetically-driven improper ferroelectrics, such a