Ferroelectricity is a physical property of a material whereby it exhibits a spontaneous electric polarization, the direction of which can be switched between equivalent states by the application of an external electric field ^[1]. In Classical electromagnetism, the polarization density (or electric polarization, or simply polarization) is the Vector field that expresses The term is used in analogy to ferromagnetism, in which a material exhibits a permanent magnetic moment. Ferromagnetism is the basic mechanism by which certain materials (such as Iron) form Permanent magnets and/or exhibit strong interactions with Magnets it In Physics, Astronomy, Chemistry, and Electrical engineering, the term magnetic moment of a system (such as a loop of Electric current Ferromagnetism was already known when ferroelectricity was discovered in 1920 in Rochelle Salt by Valasek^[2]. 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.

Ferroelectrics are key materials in microelectronics. Their excellent dielectric properties make them suitable for electronic components such as tunable capacitors and memory cells.

Contents 1 Polarization 2 Applications 3 Materials 4 See also 4.1 Physics 4.2 Lists 5 References 6 External links

Polarization

Most materials are polarized linearly with external electric field; nonlinearities are insignificant. This is called dielectric polarization (see figure). Some materials, known as paraelectric materials, demonstrate nonlinear polarization (see figure). The electric permittivity, corresponding to the slope of the polarization curve, is thereby a function of the applied electric field. Permittivity is a Physical quantity that describes how an Electric field affects and is affected by a Dielectric medium and is determined by the ability Ferroelectric materials are also nonlinear but thereto demonstrate, by definition, a spontaneous polarization (see figure). Commonly, materials demonstrate ferroelectricity only below a certain phase transition temperature, while being paraelectric above.

Applications

The nonlinear nature of ferroelectric materials can be used to make capacitors with tunable capacitance. Typically, a ferroelectric capacitor simply consists of a pair of electrodes sandwiching a layer of ferroelectric material. Ferroelectric capacitor is a Capacitor based on a ferroelectric material The permittivity of ferroelectrics is not only tunable but commonly also very high in absolute value, especially when close to the phase transition temperature. This fact makes ferroelectric capacitors small in size compared to dielectric (non-tunable) capacitors of similar capacitance.

The spontaneous polarization of ferroelectric materials implies a hysteresis effect which can be used as a memory function. A system with hysteresis can be summarised as a system that may be in any number of states independent of the inputs to the system Indeed, ferroelectric capacitors are used to make ferroelectric RAM^[3] for computers and RFID cards. Ferroelectric RAM ( FeRAM or FRAM) is a Random access memory similar in construction to DRAM but uses a Ferroelectric layer instead Radio-frequency identification ( RFID) is an automatic identification method relying on storing and remotely retrieving data using devices called RFID tags or 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 reliably^[4].

All ferroelectrics are required by symmetry considerations to be also piezoelectric and pyroelectric. The combined properties of memory, piezoelectricity, and pyroelectricity make ferroelectric capacitors very useful, e. Piezoelectricity is the ability of some materials (notably Crystals and certain Ceramics including bone to generate an Electric potential in response to Pyroelectricity is the ability of certain materials to generate an Electrical potential when they are heated or cooled g. for sensor applications. Ferroelectric capacitors are used in 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. As well, the electro-optic modulators that form the backbone of the Internet are made with ferroelectric materials. Electro-optic modulator (EOM is an optical device in which a signal -controlled element displaying Electro-optic effect is used to modulate a Beam of

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 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 ^[5].

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. Piezoelectricity is the ability of some materials (notably Crystals and certain Ceramics including bone to generate an Electric potential in response to The generation of current in response to a change in temperature is called pyroelectricity. Pyroelectricity is the ability of certain materials to generate an Electrical potential when they are heated or cooled

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. Barium titanate is an Oxide of Barium and Titanium with the Chemical formula BaTiO3 In Physics, the space surrounding an Electric charge or in the presence of a time-varying Magnetic field has a property called an electric field (that can In Physics, a force is whatever can cause an object with Mass to Accelerate. 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. Lead zirconate titanate ( 0 x Ceramic Perovskite material that shows a marked piezoelectric effect. 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.

In 1979 Swedish Sven Torbjörn Lagerwall discovered ferroelectric liquid crystals in collaboration with Noel Clark. The technology allows the building of flat-screen monitors. Mass production began in 1994 by Canon, who bought the licence.

Ferroelectric crystals often show several transition temperatures and domain structure hysteresis, much as do ferromagnetic crystals. In Materials science, a crystal is a Solid in which the constituent Atoms Molecules or Ions are packed in a regularly ordered repeating Transition temperature is the temperature at which material changes from one Crystal to another A system with hysteresis can be summarised as a system that may be in any number of states independent of the inputs to the system Ferromagnetism is the basic mechanism by which certain materials (such as Iron) form Permanent magnets and/or exhibit strong interactions with Magnets it The nature of the phase transition in some ferroelectric crystals is still not well understood. In Thermodynamics, phase transition or phase change is the transformation of a thermodynamic system from one phase to another

The ferroelectric effect also finds use in liquid crystal physics by incorporation of a chiral dopant into an achiral smectic C matrix. Liquid crystals are substances that exhibit a phase of matter that has properties between those of a conventional Liquid, and those of a Solid These liquid crystals exhibit the Clark-Lagerwall effect^[6] which effects a change in one bistable state to another upon switching of electric field direction.

See also

• Stabilizing ferroelectric materials
• FeRAM
• Relaxor ferroelectric

Physics

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

Lists

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

References

1. ^ M. Ferroelectric RAM ( FeRAM or FRAM) is a Random access memory similar in construction to DRAM but uses a Ferroelectric layer instead Paraelectricity is the ability of many materials (specifically ceramic crystals) to become polarized under an applied Electric field. Piezoelectricity is the ability of some materials (notably Crystals and certain Ceramics including bone to generate an Electric potential in response to Pyroelectricity is the ability of certain materials to generate an Electrical potential when they are heated or cooled Condensed matter physics is the field of Physics that deals with the macroscopic physical properties of Matter. Spintronics (a Neologism meaning "spin transport electronics" also known as magnetoelectronics is an Emerging technology which exploits the intrinsic The word ceramic is derived from the Greek word κεραμικός ( keramikos) Electrical phenomena are commonplace and unusual events that can be observed which illuminate the principles of the Physics of Electricity and are explained by them This and related pages aim to list all Wikipedia articles that are related to Physics. Alphabetization has been neglected in some parts of this article (the "b" section in particular Lines,A. Glass, Principles and applications of ferroelectrics and related materials (Clarendon Press, Oxford, 1979)
2. ^ J. Valasek, Piezoelectric and allied phenomena in Rochelle Salt Phys. Rev.15, 537 (1920),"Phys. Rev."17, 475 (1921)
3. ^ J. Physical Review (frequently abbreviated as Phys Rev) is one of the oldest and most-respected Scientific journals publishing research on all aspects of Physical Review (frequently abbreviated as Phys Rev) is one of the oldest and most-respected Scientific journals publishing research on all aspects of F. Scott, Ferroelectric Memories, Springer (2000)
4. ^ M. Dawber, K. M. Rabe, J. F. Scott, Physics of thin-film ferroelectric oxides Rev. Mod. Phys 77 1083 (2005). The Reviews of Modern Physics is a journal of the American Physical Society.
5. ^ R. Ramesh,N. A Spaldin, Nature Materials 6 21 (2007),W. Nature Materials is a monthly multi-disciplinary journal aimed at bringing together cutting-edge research across the entire spectrum of materials science. Eerenstein,N. D. Mathur,J. F. Scott, Nature 442 759 (2006),N. Nature, in the broadest sense is equivalent to the natural world, physical universe, material world or material universe. A. Spaldin, M. Fiebig, Science 309 5391 (2005),M. Science (from the Latin scientia, meaning " Knowledge " or "knowing" is the effort to discover, and increase human understanding Fiebig, J Phys. D, 38 R123 (2005)
6. ^ Noel A. Journal of Physics D Applied Physics, often referred to simply as Journal of Physics D or abbreviated as J Clark, Sven Torbjörn Lagerwall: Submicrosecond Bistable Electro-Optic Switching in Liquid Crystals, Appl. Phys. Lett. 36, 899 (1980)

External links

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

Applied Physics Letters is a weekly Peer-reviewed Scientific journal published by the American Institute of Physics devoted to the publication of new experimental

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