Developments of Multiferroic

Multiferroic magnetoelectrics are materials that are both ferromagnetic and ferroelectric in the same phase. For example, nickel boracite, Ni₃B₇O₁₃I. However, very few exist naturally or have been synthesized in the lab. It was proposed by N.A. Hill that the transition metal d electrons, which are essential for magnetism, reduce the tendency for off-center ferroelectric distortion.
Considering the origin of ferromagnetism, it is useful to treat the interaction as shifting the energy of the 3d band for electrons with one spin direction relative to the band for electrons with the opposite spin direction. In the stoner theory, the fundamental driving force for ferromagnetism is the exchange energy, which is minimized if all of the electrons have the same spin. However, transfering electrons from the lowest band states (occupied equally with up and down electrons) to band states of higher energy will increase the band energy. And this band energy prevents simple metals from being ferromagnetic. Depending on the 3d and 4s band structures and the Fermi energy level, some metals are ferromagnetic, such as, Fe(3d⁶4s²), Co(3d⁷4s²) and Ni(3d⁸4s²), some are not ferromagnetic, like Cu(3d¹⁰4s¹) and Zn(3d¹⁰4s²).
For ferroelectricity, it is a matter of two competing forces. One is the short-range repulsion between adjacent electron clouds, which favor the nonferroelectric symmetric structure. The opposing force is the additional bonding considerations, which might stabilize the ferroelectric phase. The short-range repulsions dominate at high temperature, resulting in the symmetric, unpolarized state. As the temperature is decreased, the stabilizing force become stronger and the polarized state becomes stable.
In 1961, G.T.Rado and VJ Folen first observed the magnetoelectric effect in Cr₂O₃