First-principles exploration of multiferroic oxides with double-perovskite structure

Multiferroics have attracted much attention recently because of their novel properties. There are a few known as ferromagnetic and ferroelectric materials, particularly with perovskite-type crystal structure. Ferroelectrics should be insulating and likely ionic. Furthermore, it is widely recognized that covalent bonds between the cation and anion orbitals are crucial to realize atomic displacements to a noncentrosymmetric structure. As for magnetism, most of magnetic perovskite oxides usually have an antiferromagnetic order (mostly frustrating) due to a superexchange coupling. According to the Kanamori-Goodenough rule for the superexchange coupling, certain combinations of the transition-metals ions ($d^3$-$d^5$ and $d^3$-$d^8$ configurations) may possibly give a ferromagnetic coupling by the 180$^{\circ}$ superexchange mechanism. In this study, we explore possible co-existence of spontaneous electric polarization and ferromagnetic ordering from first principles, by focusing bismuth double-perovskite oxides Bi$_2$$BB'$O$_6$ ($B, B'$ = 3$d$ ions) as target materials. Ferromagnetic and ferrimagnetic solutions are obtained for cubic Bi$_2$MnNiO$_6$, Bi$_2$CrFeO$_6$ and Bi$_2$CrCuO$_6$ with nearly gapped electronic structure. Quite recently, Bi$_2$MnNiO$_6$ has been successfully synthesized by a high-pressure technique and revealed multiferroic properties. Possible multiferroic properties of Bi$_2$MnNiO$_6$ with the observed monoclinic structure are investigated in detail.