FERROELECTRIC MATERIALS, HYSTERESIS CURVE AND SPONTANEOUS POLARIZATION
In ferroelectric materials, the polarization P does not vary linearly with electric field E.
Hysteresis curve: The plot of P versus E in which the material is polarized in one direction and then in opposite direction is called the hysteresis curve of the specimen.
When a piece of ferroelectric material initially unpolarized, is subjected to a gradually increasing electric field, the polarization P varies with E.
Remanent polarization or retentivity or remanence: When E reduces to zero, the material still remained polarized and P has a certain value.
The polarization remaining in the material when the polarization field is reduced to zero, is called the remanent polarization. The power of a material of retaining this polarization is called the retentivity or remanence of the material. Therefore, the retentivity or remanence of a material is a measure of the polarization remaining (residual polarization) in the material when the electric field is totally removed.
Spontaneous Polarization: A material is said to be spontaneously polarized when electric field E is zero but polarization P is not zero. This phenomenon is called spontaneous (by its own) polarization.
Coercivity: The coercivity of a material is a measure of the strength of the reverse polarising field E required to wipe out the remanent polarization of the specimen.