In physics, the ferroelectric effect is an electrical phenomenon whereby certain ionic crystals may exhibit a spontaneous dipole moment. The term ferroelectricity refers to the similarity with ferromagnetism, in which a material exhibits a permanent magnetic moment.
There are two main types of ferroelectrics: displacive and order-disorder. The effect in barium titanate, a typical ferroelectric of the displacive type, is due to a polarization catastrophe , in which, if an ion is displaced from equilibrium slightly, the force from the local electric fields due to the ions in the crystal increase faster than the elastic restoring forces. This leads to an asymmetrical shift in the equilibrium ion positions and hence to a permanent dipole moment. In an order-disorder ferroelectric, there is a dipole moment in each unit cell, but at high temperatures they are pointing in random directions. Upon lowering the temperature and going through the phase transition, the dipoles order, all pointing in the same direction within a domain.
Ferroelectric crystals often show several Curie points and domain structure hysteresis , much as do ferromagnetic crystals. The nature of the phase transition in some ferroelectric crystals is still not well understood.
Ferroelectrics often have very large dielectric constants, and thus are often found in capacitors. They also often have unusually large nonlinear optical coefficients.
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