Origin of Multiferroism of Ion Doped at Different Sites: Bi4Ti3O12 Bulk and Nanoparticles

Abstract

Based on a microscopic model and the Green's function theory, the temperature, size, magnetic field, and ion doping dependence of the magnetic, electric, and dielectric properties in Fe‐and Nd‐doped ferroelectric Bi4Ti3O12 bulk and nanoparticles is investigated. The multiferroism can be achieved in two ways. At once the magnetism appears by substitution of the nonmagnetic Ti ion with Fe or other transition metal ions in bulk Bi4Ti3O12. Otherwise, the simultaneous existence of Fe and Fe gives rise to a superexchange mechanism responsible for ferromagnetic interaction. Moreover, multiferroism is possible also in pure Bi4Ti3O12 nanoparticles. Due to surface effects and oxygen vacancies leading to different valence states on the surface of Ti or Ti with spin value , there appears weak magnetism, which can be improved by Fe ion doping. It is demonstrated that doping with rare earth ions, for example Nd, at the Bi site is able to improve the ferroelectric, multiferroic, and dielectric properties.