Ferroelectric effect
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In physics, the ferroelectric effect is an electrical phenomenon whereby certain ionic crystals and piezoelectric polymers may exhibit a spontaneous dipole moment, which can be reversed by the application of an electric field. The term ferroelectricity is used in analogy to 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.
Another important ferroelectric material is lead zirconate titanate.
Ferroelectric crystals often show several Curie points and domain structure hysteresis, much as do ferromagnetic crystals. By analogy to magnetic core memory, this hysteresis can be used to store information in ferroelectric RAM, which has ferroelectric capacitors as memory cells. 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 used as the dielectric material in capacitors. They also often have unusually large nonlinear optical coefficients.
Older publications used the term electret for ferroelectric materials.
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