Ferroelectric materials are those that exhibit spontaneous electric polarization below a certain temperature, T0, known as the Curie temperature. The polarization is reversible; that is, it can be “switched” by applying an external electric field. The most well-known ferroelectric material is Rochelle salt, discovered in the early 18th century. Other common examples include barium titanate and lead zirconate titanate. Ferroelectric materials have many applications, including in non-volatile memories (such as those used in computers), sensors, actuators, and transducers.
The term “ferroelectric” was first coined by J.Elliott in 1921 to describe the property of Rochelle salt below its Curie temperature . At temperatures above T0, the dipoles are randomly oriented and there is no net polarization. As the temperature is lowered through T0, the dipoles begin to align with each other and a spontaneous polarization develops along one direction, typically the c-axis of the crystal structure . This transition is second order; that is, there is no latent heat associated with it. The dielectric constant also changes at T0: above T0 it behaves like a normal insulator but below T0 it diverges as 1/T – this divergence arises from contributions from both electronic and ionic polarizabilities .
One of the most important properties of ferroelectrics is their ability to undergo “ferroelectric switching”: when an external electric field is applied, the direction of spontaneous polarization can be changed . This effect can be used to create non-volatile memories (NVRAMs), which store data even when power is turned off . NVRAMs based on ferroelectric materials have several advantages over other kinds of NVRAMs: they are fast (on the order of nanoseconds), they have high storage densities (on the order of gigabytes per square inch), and they are radiation-hard . Ferroelectric materials are also used in sensors and actuators. For example, piezoelectric sensors made from lead zirconate titanate can be used to measure pressure or acceleration , while ferroelectric actuators can be used for haptic feedback or microfluidic control . Last but not least, because of their large dielectric constants, ferroelectrics can also be used as transducers for ultrasonic waves , making them important for medical imaging and nondestructive testing applications.