Ferroelectric

Ferroelectricity is a physical property of a material whereby it exhibits a spontaneous electric dipole moment, the direction of which can be switched between equivalent states by the application of an external electric field.M. Lines,A. Glass, Principles and applications of ferroelectrics and related materials (Clarendon Press, Oxford, 1979). Ferroelectrics are key materials in microelectronics. Their excellent dielectric properties make them suitable for electronic components such as capacitors, filters etc.

Ferroelectrics should not be confused with electrets, in which the polarization state is only metastable.

History

The term ferroelectricity is used in analogy to ferromagnetism, in which a material exhibits a permanent magnetic moment. Ferromagnetism was already known when ferroelectricity was discovered in 1920 in Rochelle Salt by ValasekJ. Valasek, Piezoelectric and allied phenomena in Rochelle Salt Phys. Rev.15, 537 (1920),"Phys. Rev."17, 475 (1921). Thus, the prefix "ferro", meaning iron, was used to describe the property despite the fact that most ferroelectric materials do not have iron in their lattice.

Applications

Placing a ferroelectric material between two conductive plates creates a ferroelectric capacitor. Ferroelectric capacitors exhibit nonlinear dielectric properties and often have very high dielectric constants, especially when close to their phase transition temperature. The fact that the internal electric dipoles can be forced to change their direction by the application of an external voltage gives rise to hysteresis in the "polarization vs voltage" property of the capacitor. In this case, polarization is defined as the total charge stored on the plates of the capacitor divided by the area of the plates. The term hysteresis refers to the property that the polarization displays a memory effect and indeed ferroelectric capacitors are used to make ferroelectric RAMJ.F. Scott, Ferroelectric Memories, Springer (2000) for computers and RFID cards. These applications are usually based on thin films of ferroelectric materials as this allows the high coercive field required to switch the polarization to be achieved with a moderate voltage, though a side effect of this is that a great deal of attention needs to be paid to the interfaces, electrodes and sample quality for devices to work reliablyM. Dawber, K.M. Rabe, J.F. Scott, Physics of thin-film ferroelectric oxides Rev. Mod. Phys 77 1083 (2005)..

All ferroelectrics are required by symmetry considerations to be also piezoelectric and pyroelectric. The combined properties of memory, piezoelectricity, and pyroelectricity make ferroelectric capacitors some of the most useful technological devices in modern society. Ferroelectric capacitors are at the heart of medical ultrasound machines (the capacitors generate and then listen for the ultrasound "ping" used to image the internal organs of a body), high quality infrared cameras (the infrared image is projected onto a two dimensional array of ferroelectric capacitors capable of detecting temperature differences as small as millionths of a degree Celsius), fire sensors, sonar, vibration sensors, and even fuel injectors on diesel engines. Engineers use the high dielectric constants of ferroelectric materials to concentrate large values of capacitance into small volumes, resulting in the very tiny surface mount capacitor. Without the space savings allowed by surface mount capacitors, compact laptop computers and cell phones simply would not be possible. As well, the electro-optic modulators that form the backbone of the Internet are made with ferroelectric materials.

One new idea of recent interest is the ferroelectric tunnel junction (FTJ) in which a contact made up by nanometer-thick ferroelectric film placed between metal electrodes. The thickness of the ferroelectric layer is thin enough to allow tunneling of electrons. The piezoelectric and interface effects as well as the depolarization field may lead to a giant electroresistance (GER) switching effect.

Another hot topic at the moment is Multiferroics, where researchers are looking for ways to couple magnetic and ferroelectric ordering within a material or heterostructure; there are several recent reviews on this topic R. Ramesh,N.A Spaldin, Nature Materials 6 21 (2007),W. Eerenstein,N.D. Mathur,J.F. Scott, Nature 442 759 (2006),N.A. Spaldin, M. Fiebig, Science 309 5391 (2005),M. Fiebig, J Phys. D, 38 R123 (2005)

Materials

The internal electric dipoles of a ferroelectric material are physically tied to the material lattice so anything that changes the physical lattice will change the strength of the dipoles and cause a current to flow into or out of the capacitor even without the presence of an external voltage across the capacitor. Two stimuli that will change the lattice dimensions of a material are force and temperature. The generation of a current in response to the application of a force to a capacitor is called piezoelectricity. The generation of current in response to a change in temperature is called pyroelectricity.

Ferroelectric phase transitons are often characterized as either displacive and order-disorder, though often phase transitions will have behaviour that contains elements of both behaviours. In barium titanate, a typical ferroelectric of the displacive type, the transition can be understood in terms of 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 increases faster than the elastic-restoring forces. This leads to an asymmetrical shift in the equilibrium ion positions and hence to a permanent dipole moment. The ionic displacement in barium titanate concerns the relative position of the titanium ion within the oxygen octahedral cage.In lead titanate,another key ferroelectric material, although the structure is rather similar to barium titanate the driving force for ferroelectricity is more complex with interactions between the lead and oxygen ions also playing an important role. 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.

An important ferroelectric material for applications is lead zirconate titanate(PZT), which is part of the solid solution formed between ferroelectric lead titanate and anti-ferroelectric lead zirconate. Different compositions are used for different applications, for memory application PZT closer in composition to lead titanate is preferred, whereas piezoelectric applications make use of the diverging piezoelectric coefficients associated with the morphotropic phase boundary that is found close to 50/50 composition.

Ferroelectric crystals often show several transition temperatures and domain structure hysteresis, much as do ferromagnetic crystals. The nature of the phase transition in some ferroelectric crystals is still not well understood.

The ferroelectric effect also finds use in liquid crystal physics by incorporation of a chiral dopant into an achiral smectic C matrix. These liquid crystals exhibit the Clark-Lagerwall effectNoel A. Clark, Sven Torbjörn Lagerwall: Submicrosecond Bistable Electro-Optic Switching in Liquid Crystals, Appl. Phys. Lett. 36, 899 (1980) which effects a change in one bistable state to another upon switching of electric field direction.

See also

• Stabilizing ferroelectric materials
• FeRAM

Physics

• Paraelectricity
• Piezoelectricity
• Pyroelectricity
• Condensed matter physics
• Spintronics
• Ceramics

Lists

• Examples of electrical phenomena
• List of physics topics
• List of electronics topics

References

External links

• A useful starter on ferroelectrics
• A ferroelectrics research group at Stony Brook University

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