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| '''Ferroics''' is the generic name given to the study of [[ferromagnets]], [[ferroelectrics]], and [[ferroelastics]]. The basis of this study is to understand the large changes in physical characteristics that occur over a very narrow temperature range. The changes in physical characteristics occur when [[phase transitions]] take place around some critical temperature value, normally denoted by <math>T_c</math>. Above this critical temperature, the crystal is in a nonferroic state and does not exhibit the physical characteristic of interest. Upon cooling the material down below <math>T_c</math> it undergoes a spontaneous [[phase transition]]. Such a [[phase transition]] typically results in only a small deviation 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.
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| In ferroelectrics, upon lowering the temperature below <math>T_c</math>, a spontaneous [[Polarity|dipole moment]] is induced along an axis of the unit cell. Although individual dipole moments can sometimes be small, the effect of <math>10^{24}</math> 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 <math>\left (x,y,z \right )</math> can be mapped onto a lattice point <math>\left ( -x,-y,-z \right )</math>.
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| Ferromagnets is a term that most people are familiar with, and, as with ferroelastics, the spontaneous magnetization of a ferromagnet can be attributed to a breaking of point symmetry in switching from the paramagnetic to the ferromagnetic phase. In this case, <math>T_c</math> is normally known as the Curie Temperature.
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| 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 (materials science)|strain]] is induced within the bulk.
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| In recent years a new class of ferroic materials has been attracting increased interest. These [[multiferroics]] exhibit more than one ferroic property simultaneously in a single phase.
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| ==See also==
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| * [[Ferroelectric effect]]
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| * [[Ferromagnetism]]
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| * [[Piezoelectricity]]
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| * [[Pyroelectricity]]
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| * [[Ferroelasticity]]
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| * [[Multiferroics]]
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| [[Category:Condensed matter physics]]
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| [[Category:Magnetic ordering]]
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