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.
@article{arxiv.1007.1003,
title = {Hybrid Improper Ferroelectricity: A Mechanism for Controllable Magnetization-Polarization Coupling},
author = {Nicole A. Benedek and Craig J. Fennie},
journal= {arXiv preprint arXiv:1007.1003},
year = {2010}
}