Multiferroism in Iron-Based Oxyfluoride Perovskites.

Multiferroic materials with simultaneous magnetic and ferroelectric ordering that persist above room temperature are rare. Using first-principles density functional theory calculations, we demonstrate fluorination of oxygen-deficient AA'Fe2O5 perovskites, where A and A' are cations with +3 and +2 oxidation states, respectively, and have a layered ordering, as an effective strategy to obtain room-temperature multiferroics. We show that by controlling the size of the A and A' cations, it is possible to stabilize a noncentrosymmetric phase arising due to the hybrid improper ferroelectricity mechanism, with polarization as high as 13 μC/cm2. The fluorination also stabilizes Fe in +3 oxidation state, which results in superexchange interactions that are strong enough to sustain magnetic order well above room temperature. We also show the presence of a magnetoelectric coupling wherein the switching mode that reverses the direction of the spontaneous polarization also affects the strength of the magnetic interactions. The results show that low-temperature fluorination of anion-deficient perovskites with layered cation ordering can be an effective approach to design new multiferroics.