Ferromagnetism in the multiferroic alloy systems BiFeO₃-BaTiO₃ and BiFeO₃-SrTiO₃: Intrinsic or extrinsic?

Abstract

Among the different bulk alloys of the room temperature multiferroic compound BiFeO₃, alloying with ATiO₃ (A = Ba, Sr, Pb) is interesting as they have been reported to induce ferromagnetism under certain conditions. While this makes them interesting as potential multiferroic magnetoelectric materials, there is a lack of clarity on the origin of ferromagnetism in these systems. We have performed a detailed magneto-structural analysis on two alloys, namely, BiFeO₃-BaTiO₃ and BiFeO₃-SrTiO₃, and found the systems' propensity for the formation of ferrimagnetic hexaferrite (BaFe₁₂O₁₉ and SrFe₁₂O₁₉) phases. Though insignificantly small to the extent of being undetectable by x-ray powder diffraction, we prove that the ferromagnetic character of the specimen is entirely due to spontaneous precipitation of the hexaferrite grains. While our results suggest that care should be exercised before attributing ferromagnetism in such alloy systems as intrinsic to the perovskite phase, the propensity for the spontaneous precipitation of the hexaferrite phase in these multiferroic alloy systems is encouraging as it provides an opportunity for designing self-grown ferroelectric-ferromagnetic composites with good magnetoelectric coupling.