Ferroelectricity
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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]. The term 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 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.
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[edit] 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. 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.
[edit] 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. 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. Indeed, ferroelectric capacitors are used to make ferroelectric RAM[3] 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 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.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.
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].
[edit] 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.
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. 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 effect[6] which effects a change in one bistable state to another upon switching of electric field direction.
[edit] See also
- Stabilizing ferroelectric materials
- FeRAM
- Relaxor ferroelectric
[edit] Physics
[edit] Lists
[edit] References
- ^ M. Lines,A. Glass, Principles and applications of ferroelectrics and related materials (Clarendon Press, Oxford, 1979)
- ^ J. Valasek, Piezoelectric and allied phenomena in Rochelle Salt Phys. Rev.15, 537 (1920),"Phys. Rev."17, 475 (1921)
- ^ J.F. Scott, Ferroelectric Memories, Springer (2000)
- ^ M. Dawber, K.M. Rabe, J.F. Scott, Physics of thin-film ferroelectric oxides Rev. Mod. Phys 77 1083 (2005).
- ^ 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)
- ^ Noel A. Clark, Sven Torbjörn Lagerwall: Submicrosecond Bistable Electro-Optic Switching in Liquid Crystals, Appl. Phys. Lett. 36, 899 (1980)
