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 Bi
4Ti
3O
12 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
...