Magnetoelectric (ME) multiferroics, exhibiting both ferromagnetic and ferroelectric behavior in a single phase, have generated considerable intense research interest due to its potential applications. A major focus in ME multiferroic studies is recently on perovskite-structured oxides (ABO₃) thanks to the discovery of a bismuth ferrite (BiFeO₃, BF), which is the only one working at room temperature. However, the state of the art is still far from the goal due to a paradox originating from the atomic configurations of perovskite oxides; i.e., ferroelectric behavior of perovskite oxides is coming from inversion symmetry breaking by d⁰-ness, while ferromagnetic behavior of that results from an exchange interaction by the existence of unpaired electrons in d orbitals. Alongside, the practical use of BF is hard due to secondary phases and leakage current.

BiFeO₃-BaTiO₃ (BF-BT) shows multiferroicity well above the ambient temperature like as BF and strengthen the ferroelectric performances. However, BF-BT is antiferromagnetic.

One attempts to change antiferromagnetic to ferromagnetic by A site engineering, so we substituted part of A site cations with Co²⁺. A cobalt ion at A-site and an iron ion at B-site form 90⁰ with oxygen ion in between. This meets 90⁰ super-exchange interaction by Goodenough-Kanamori rule. It is compatible with lone-pair ferroelectricity of BF-BT, therefore ferroelectric-ferromagnetic multiferroic operating at the room temperature was discovered.