Multiferroic Polar Metals.

Ca3Ru2O7 undergoes a second-order magnetic phase transition to AFM-a (ferromagnetic bilayers antiferromagnetically stack along c-axis with magnetic easy axis along a) at TN $=$ 56 K, followed by a concomitant first-order structural and magnetic phase transition to an AFM-b (antiferromagnetic with magnetic easy axis along b) at TS $=$ 48 K. For T\textless 30 K, a quasi-two-dimensional (2D) metallic state exists due to the survival of small non-nested Fermi pockets. With a proper magnetic field applied along b-axis, an additional phase of canted-AFM is induced. Here we propose a new strategy to tune the polar metal Ca3Ru2O7 into insulating state by chemical doping. In the meantime, the superexchange interaction is significantly weakened to allow the existence of a weak ferromagnetic state. Combined with its robust polar nature, we offer an experimental demonstration of a new multiferroic material. The mechanism is further discussed in the framework of hybrid improper ferroelectricity proposed by Benedek and Fennie. This new strategy proposed here may be utilized as a general approach for new multiferroics starting from a material on the verge of the Mott insulating. Here we will discuss our comprehensive magnetization and magnetostriction, and magnetic field dependent SHG study on this material