New material design strategies to realize strong coupling in multiferroics and beyond

Perovskite ABO$_3$ oxides display an amazing variety of phenomena that can be altered by subtle changes in the chemistry and internal structure, making them a favorite class of materials to explore the rational design of novel properties. In this talk I will review a recent advance in which rotations and tilts of the BO$_6$ octahedra give rise to a novel form of ferroelectricity. Octahedral rotations strongly influence other structural, magnetic, orbital, and electronic degrees of freedom in perovskites and related materials. Thus, I want to discuss the idea that octahedral rotation-driven ferroelectricity has the potential to robustly control emergent phenomena with an applied electric field. As one example, I will show from first principles how these ``functional'' octahedral rotations simultaneously induce ferroelectricity, magnetoelectricity, and weak ferromagnetism in a class of naturally occurring Ruddlesden-Popper (RP) (ABO$_3$)$_2$(AO) layered perovskites and discuss the challenges to realize electric field switching of magnetism in these RP and in (ABO$_3$)/(A'BO$_3$) perovskite superlattice novel multiferroics. \\[4pt] N. A. Benedek and C. J. Fennie, {\it Phys. Rev. Lett} {\bf 106}, 107204, 2011;\\[0pt] J. M. Rondinelli and C. J. Fennie, arXiv 2011; N. A. Benedek and C. J. Fennie, arXiv 2011.