Fundamentals of Ferroelectric Materials

Ferroelectricity is defined as the property of a material, with two characteristics, i.e. (i) spontaneous polarization is present and (ii) it is reversible when subjected to external electric fields [1]. The property was first observed in Rochelle salt and is named so because of its analogy to ferromagnetism, which is a magnetic property of a material that has a permanent magnetic moment [2, 3]. Other similarities include hysteresis loop, Curie temperature (TC), domains, and so on. The prefix, ferro, meaning iron (Fe), was used at that time because of the presence of the element in the magnetic materials. However, ferroelectricity has nothing to do with Fe. Even though some ferroelectric materials contain Fe, it is not the originating factor. Generally, as a material is polarized by an external electric field, the induced polarization (P) is linearly proportional to the magnitude of the applied external electric field (E), which is known as dielectric polarization. Above the Curie temperature TC, ferroelectric materials are at a paraelectric state. In this case, a nonlinear polarization is present versus an external electric field. As a result, electric permittivity, according to the slope of the polarization curve, is not a constant. At the ferroelectric state, besides the nonlinearity, a spontaneous nonzero polarization was present, as the applied field (E) is zero. Because the spontaneous polarization can be reversed by a sufficiently strong electric field, it is dependent on the currently applied electric field and the history as well, thus leading to the presence of the hysteresis loop. The electric dipoles in a ferroelectric material are coupled to the crystal lattice of the material, so that the variation in lattice could change the strength of the dipoles, i.e. the strength of the spontaneous polarization. The change in the spontaneous polarization, in turn, leads to a change in the surface charge, which causes