Improper ferroelectricity at antiferromagnetic domain walls of perovskite oxides

First-principles calculations are performed on magnetic multidomain structures in the SmFeO₃ rare-earth orthoferrite compound. We focus on the magnetic symmetry breaking at (001)-oriented antiphase domain walls, treating magnetism in the simplest (collinear) approximation without any relativistic (spin-orbit coupling) effects. We found that the number of FeO₂ layers inside the domains determines the electrical nature of the whole system: multidomains with odd number of layers are paraelectric, while multidomains with even number of layers possess an electric polarization aligned along b axis and a resulting multiferroic Pmc2₁ ground state. Our ab initio data and model for ferroelectricity induced by spin order reveal that this polarization is of the improper type and originates from an exchange striction mechanism that drives a polar displacement of the oxygen ions located at the magnetic domain walls. Additional calculations ratify that this effect is general among magnetic perovskites with an orthorhombic SmFeO₃-like structure.