Magnetic Ferroelectrics Bi,Pb-3$d$ Transition Metal Perovskites

Magnetic ferroelectrics attract much attention because of the possible application for the memory device and the fascinating coupling between magnetic and dielectric properties. A classical way to obtain a magnetic ferroelectric is to locate Bi$^{3+}$ or Pb$^{2+ }$ions and a magnetic transition metal ion on A and B sites of perovskite structure. The 6s$^{2}$ lone pair and the strong covalent character of Bi(Pb)-O bonds stabilize a noncentrosymmetric distorted structure. For example, BiFeO$_{3}$ and BiMnO$_{3}$ are established antiferromagnetic and ferromagnetic ferroelectrics, respectively. We have studied structure, magnetic and electric properties of Bi$M$O$_{3}$ with $M$=Cr, Co and Ni and PbVO$_{3}$ stabilized by high-pressure synthesis. BiCrO$_{3}$ is an antiferromagnetic ferroelectric with BiMnO$_{3}$ type structure. BiCoO$_{3}$ and PbVO$_{3}$ are found to have tetragonal PbTiO$_{3}$ type structures with expected polarizations of $\sim $100$\mu $C/cm$^{2}$. BiNiO$_{3 }$crystallizes in a triclinic structure where disproportionation into Bi$^{3+}$ and Bi$^{5+}$ takes place. We have also succeeded in preparing a designed ferromagnetic ferroelectric double provskite Bi$_{2}$NiMnO$_{6}$. In this compound, NaCl type ordering of Ni$^{2+}$ ($e_{g}^{1})$ and Mn$^{4+}$ ($t_{2g}^{3})$ leads to ferromagnetism with $T_{C}$=140 K.