Ferroics
Encyclopedia
Ferroics is the generic name given to the study of ferromagnets, ferroelectrics, and ferroelastics. Essentially the basis of this study is to understand the large changes in physical characteristics that occur in a very short temperature range. The changes in physical characteristics occur when phase transitions take place around some critical temperature value, normally denoted by 
. Above this critical temperature, the crystal is in a nonferroic state and exists with no notable physical characteristics. Upon cooling the material down below 
it undergoes a spontaneous phase transition
. These phase transitions result in only small deviations from the nonferroic crystal structure, but in altering the shape of the unit cell the point symmetry of the material is reduced. This breaking of symmetry is physically what allows the formation of the ferroic phase.
In ferroelectrics, upon lowering the temperature below
a spontaneous dipole moment
is induced along an axis of the unit cell. Although individual dipole moments can sometimes be small, the effect of
unit cells gives rise to an electric field that over the bulk substance that is not insignificant. An important point about ferroelectrics is that they cannot exist in a centrosymmetric crystal. A centrosymmetric crystal is one where a lattice point 
can be mapped onto a lattice point 
.
Ferromagnets is a term that most people are familiar with, and as with ferroelastics, the spontaneous magnetisation of a ferromagnet can be attributed to a breaking of point symmetry in going from the parramagnetic to the ferromagnetic phase. Here,
is normally known as the Curie Temperature.
In ferroelastic crystals, in going from the nonferroic (or prototypic phase) to the ferroic phase, a spontaneous strain is induced. An example of a ferroelastic phase transition
is when the crystal structure spontaneously changes from a tetragonal structure (a square prism shape) to a monoclinic structure (a general parallelepiped
). Here the shapes of the unit cell before and after the phase transition
are different, and hence a strain
is induced within the bulk.
In recent years a new class of ferroic materials gathered high attention. This so called multiferroics
exhibit more than a single ferroic property simultaneously in a single phase.
. Above this critical temperature, the crystal is in a nonferroic state and exists with no notable physical characteristics. Upon cooling the material down below it undergoes a spontaneous phase transitionPhase transition
A phase transition is the transformation of a thermodynamic system from one phase or state of matter to another.A phase of a thermodynamic system and the states of matter have uniform physical properties....
. These phase transitions result in only small deviations from the nonferroic crystal structure, but in altering the shape of the unit cell the point symmetry of the material is reduced. This breaking of symmetry is physically what allows the formation of the ferroic phase.
In ferroelectrics, upon lowering the temperature below
a spontaneous dipole momentPolarity
In physics, polarity is a description of an attribute, typically a binary attribute , or a vector . For example:* An electric charge has a polarity of either positive or negative....
is induced along an axis of the unit cell. Although individual dipole moments can sometimes be small, the effect of
unit cells gives rise to an electric field that over the bulk substance that is not insignificant. An important point about ferroelectrics is that they cannot exist in a centrosymmetric crystal. A centrosymmetric crystal is one where a lattice point can be mapped onto a lattice point .Ferromagnets is a term that most people are familiar with, and as with ferroelastics, the spontaneous magnetisation of a ferromagnet can be attributed to a breaking of point symmetry in going from the parramagnetic to the ferromagnetic phase. Here,
is normally known as the Curie Temperature.In ferroelastic crystals, in going from the nonferroic (or prototypic phase) to the ferroic phase, a spontaneous strain is induced. An example of a ferroelastic phase transition
Phase transition
A phase transition is the transformation of a thermodynamic system from one phase or state of matter to another.A phase of a thermodynamic system and the states of matter have uniform physical properties....
is when the crystal structure spontaneously changes from a tetragonal structure (a square prism shape) to a monoclinic structure (a general parallelepiped
Parallelepiped
In geometry, a parallelepiped is a three-dimensional figure formed by six parallelograms. By analogy, it relates to a parallelogram just as a cube relates to a square. In Euclidean geometry, its definition encompasses all four concepts...
). Here the shapes of the unit cell before and after the phase transition
Phase transition
A phase transition is the transformation of a thermodynamic system from one phase or state of matter to another.A phase of a thermodynamic system and the states of matter have uniform physical properties....
are different, and hence a strain
Strain (materials science)
In continuum mechanics, the infinitesimal strain theory, sometimes called small deformation theory, small displacement theory, or small displacement-gradient theory, deals with infinitesimal deformations of a continuum body...
is induced within the bulk.
In recent years a new class of ferroic materials gathered high attention. This so called multiferroics
Multiferroics
Multiferroics have been formally defined as materials that exhibit more than one primary ferroic order parameter simultaneously . The four basic primary ferroic order parameters are ferromagnetism, ferroelectricity, ferroelasticity and ferrotoroidicity, the latter still being under debate...
exhibit more than a single ferroic property simultaneously in a single phase.
See also
- Ferroelectric effect
- FerromagnetismFerromagnetismFerromagnetism is the basic mechanism by which certain materials form permanent magnets, or are attracted to magnets. In physics, several different types of magnetism are distinguished...
- PiezoelectricityPiezoelectricityPiezoelectricity is the charge which accumulates in certain solid materials in response to applied mechanical stress. The word piezoelectricity means electricity resulting from pressure...
- PyroelectricityPyroelectricityPyroelectricity is the ability of certain materials to generate a temporary voltage when they are heated or cooled. The change in temperature modifies the positions of the atoms slightly within the crystal structure, such that the polarization of the material changes. This polarization change...
- FerroelasticityFerroelasticityFerroelasticity is a phenomenon in which a material may exhibit a spontaneous strain. In ferroics, ferroelasticity is the mechanical equivalent of ferroelectricity and ferromagnetism. When a stress is applied to a ferroelastic material, a phase change will occur in the material from one phase to an...
- MultiferroicsMultiferroicsMultiferroics have been formally defined as materials that exhibit more than one primary ferroic order parameter simultaneously . The four basic primary ferroic order parameters are ferromagnetism, ferroelectricity, ferroelasticity and ferrotoroidicity, the latter still being under debate...