Magnetic-ion-induced displacive electric polarization in FeO5 bipyramidal units of (Ba, Sr) Fe12O19 hexaferrites

Electric polarization in conventional ferroelectric oxides usually involves nonmagnetic transition-metal ions with an empty d shell. Here we unravel a new mechanism for local electric dipoles based on magnetic Fe³⁺ (3d⁵) ion violating the d⁰ rule. The competition between the long-range Coulomb interaction and short-range Pauli repulsion in a FeO₅ bipyramid with proper lattice parameters would favor an off-center displacement of Fe³⁺ that induces a local electric dipole. The manipulation of this kind of non-d⁰ electric dipoles opens up a new route for generating unconventional dielectrics, ferroelectrics, and multiferroics . As a prototype example, we show that the non-d⁰ electric dipoles in ferrimagnetic hexaferrites (Ba,Sr)Fe₁₂O₁₉ lead to a new family of magnetic quantum paraelectrics.