8.2 Historical Perspective

Magnetoelectric effect was first observed by Rontgen in 1888 and by Pierre Curie in 1894 in two independent studies. Rontgen found that a dielectric when moved in an electric field, became magnetized and conversely it became polarized when moved in a magnetic field. In contrast, Curie pointed out the magnetoelectric effect based on symmetry considerations. The term magnetoelectric was first used by Debye in 1926.

The first material with magneto-electric switching was Cr₂O₃ with small magnitudes of induced polarization and magnetization. Subsequently the research was carried on various materials and it is now established that more than 80 compounds including Ti₂O₃, GaFeO₃, boracites, phosphates showed magnetoelectric effect.

The first ever discovered multiferroic material that was simultaneously ferroelectric and ferromagnetic, was nickel iodine boracite, Ni₃B₇O₁₃I. Subsequently many studies were made on various boracite compounds. However, most of them had quite complex crystal structures and materials were not very useful from technological viewpoint.

This was followed by studies on mixed perovskites, essentially solid solutions of two perovskite oxide compounds. Russian scientists took the lead in these investigations where they replaced some of the d⁰ type cations in the ferroelectric perovskite oxides with magnetic dⁿ type elements in order to induce magnetic ordering. One of first such compounds to be discovered was a solid solution of Pb (Fe_2/3W_1/3)O₃  and Pb (Mg_1/2W_1/2)O₃. In this compound, ferroelectricity was caused by diamagnetic Mg and W atoms while magnetic ordering is caused by Fe³⁺ ions. Some other candidates were lead based Fe or Co doped tungstates or tantalates which showed ferroelectricity and antiferromagnetic ordering. However, most of these materials had either very low Curie temperatures or Neel temperature which prevented further research on these.

Subsequently, the research focus was on other perovskite materials which are either manganites or ferrites and have been more promising than previously research materials and will be discussed later in this module.