In physics, ferroics is the study of ferromagnets, ferroelectrics, and ferroelastics.
Phase transitions The basis of ferroics is to understand large changes in physical characteristics that can occur over a narrow temperature range. Changes in physical characteristics occur when phase transitions take place around some critical temperature value, normally denoted by T c {\displaystyle T_{c}} . Above this critical temperature, the crystal is in a nonferroic state and does not exhibit ferroic characteristics. Falling below T c {\displaystyle T_{c}} it undergoes a spontaneous phase transition. Such a transition typically results in a small deviation from the nonferroic crystal structure, but with a different shape of the unit cell, the point symmetry of the material is reduced. This breaking of symmetry allows the formation of the ferroic phase. Lowering the temperature below T c {\displaystyle T_{c}} includes a spontaneous dipole moment along an axis of the unit cell. Although individual dipole moments can sometimes be small, the effect of 10 24 {\displaystyle 10^{24}} unit cells gives rise to a significant electric field. Ferroelectrics cannot exist in a centrosymmetric crystal. A centrosymmetric crystal is one where lattice point ( x , y , z ) {\displaystyle \left(x,y,z\right)} can be mapped onto lattice point ( − x , − y , − z ) {\displaystyle \left(-x,-y,-z\right)} . 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, T c {\displaystyle T_{c}} is 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, inducing a strain within the material.
Variants Multiferroic materials exhibit more than one ferroic property simultaneously in a single phase. A fourth ferroic order termed ferrotoroidic order has been proposed. Many ferroic phase transitions have been studied under non-equilibrium conditions. Under energy disspative condition, quantum system or event colloid particles may shows ferrioic-like order which can be regarded as a nonequilibrium analogue to ferroic material. For example, chemically reactive colloidal particles have been observed to exhibit criticality and long-range order, collectively referred to as ferrochemical order.
See also Piezoelectricity Pyroelectricity
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