User Contributed Dictionary
Adjective
- of, or relating to the permanent electrical polarization of a crystalline dielectric in an electric field
Noun
- A ferroelectric material
Extensive Definition
expert-portal Science
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
. 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. 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.
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.
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 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.
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 .
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 which effects a change in one bistable
state to another upon switching of electric field direction.
Physics
References
External links
ferroelectric in German: Ferroelektrikum
ferroelectric in French: Ferroélectricité
ferroelectric in Japanese: 強誘電体
ferroelectric in Polish: Ferroelektryk
ferroelectric in Russian: Сегнетоэлектрик
ferroelectric in Slovenian: Feroelektrik
ferroelectric in Ukrainian:
Сегнетоелектрики