Multiferroics exhibiting the coexistence and a possible coupling of ferromagnetic and ferroelectric order are attracting widespread interest in terms of academic interests and possible applications. However, room-temperature single-phase multiferroics with soft ferromagnetic and displacive ferroelectric properties are still rare owing to the contradiction in the origin of ferromagnetism and ferroelectricity. In this study, we demonstrated that sizable ferromagnetic properties are induced in the ferroelectric bismuth ferrite-barium titanate system simply by introducing Co ions into the A-site. It is noted that all modified compositions exhibit well-saturated magnetic hysteresis loops at room temperature. Especially, 70Bi_0.95Co_0.05FeO₃-30Ba_0.95Co_0.05TiO₃ manifests noticeable ferroelectric and ferromagnetic properties; the spontaneous polarization and the saturation magnetization are 42 μC/㎠ and 3.6 emu/g, respectively. We expect that our methodology will be widely used in the development of perovskite-structured multiferroic oxides.

Recently, many lead-free piezoelectric materials have been investigated for the replacement of existing Pb-based piezoelectric ceramics because of globally increasing environmental interest. There has been remarkable improvement in piezoelectric properties of some lead-free ceramics such as (Bi, Na) Ti03-(Bi, K) Ti03-BaTiO3, (Na, K) Nb03-LiSbO3, and so on. However, no one still has comparable piezoelectric properties to lead-based materials. Therefore, new lead-free piezoelectric ceramics are required. BiFeO3 has a rhombohedrally distorted perovskite structure at room temperature and a very high Curie temperature (Tc=1,100 K). And a very large electric polarization of 50~60 uC/cm2 has been reported both in epitaxial thin film and single crystal BiFeO3. Therefore, a high piezoelectric effect is expected also in a BiFeO3 ceramics. The recent research activities on BiFeO3 or BiFeO3-based solid solutions are reviewed in this article.