First-principles exploration of ferromagnetic and ferroelectric double-perovskite transition-metal oxides

Possible ferromagnetic and ferroelectric phases are explored for bismuth transition-metal oxides with double-perovskite structure A2BB′O6A2BB′O6 on the basis of first-principles calculations within the local spin-density approximation (LSDA) and generalized gradient approximation (GGA). It is found that a lattice instability of the cubic to a non-centrosymmetric phase always happens in the all cases of lead and bismuth perovskite oxides with the 3d transition-metal ions at the B site. Placing bismuth ion at the A site in the double-perovskite structure, several sets of the 3d transition-metal ions are selected according to their total valence sum and the Goodenough–Kanamori rule for the superexchange coupling. Ferromagnetic solutions are actually obtained both within LSDA and GGA for Bi2CrFeO6Bi2CrFeO6, Bi2MnNiO6Bi2MnNiO6 and Bi2CrCuO6Bi2CrCuO6. For non-centrosymmetric monoclinic Bi2MnNiO6Bi2MnNiO6, the ferromagnetic and ferroelectric phase has the spin magnetic moment of 5μB5μB and the electric polarization of 28μC/cm2.