Coupling of Magnetization and Structural Distortions in Multiferroic BiFeO₃: an Ab Initio Density Functional Theory Study

The coupling between magnetism and structural distortions in BiFeO₃(BFO) is investigated using density functional theory by considering the spin--orbit effect. Computational results show that the resulting magnetization M is
rotated by reversal of sense of rotation of the oxygen octahedra in the double cell. The resulting magnetization is determined by the antiferrodistortive (AFD) distortions and ferroelectric (FE) displacements. This work clarifies the previous view that magnetism is only coupled with, and determined by, FE displacements. The excellent ferroelectricity is attributed significantly to the anomaly of Born effective charge of Bi, which is caused by the stereochemically active long pair of Bi 6s.