Ferroics

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Overview

The basis of ferroics is to understand the large changes in physical characteristics that occur over a very narrow temperature range. The changes in physical characteristics occur when

take place around some critical temperature value, normally denoted by ${\displaystyle T_{c}}$. 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 ${\displaystyle T_{c}}$ it undergoes a spontaneous 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 ${\displaystyle T_{c}}$, a spontaneous

is induced along an axis of the unit cell. Although individual dipole moments can sometimes be small, the effect of ${\displaystyle 10^{24}}$ 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 ${\displaystyle \left(x,y,z\right)}$ can be mapped onto a lattice point ${\displaystyle \left(-x,-y,-z\right)}$.

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, ${\displaystyle T_{c}}$ 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

is induced within the bulk.

In recent years, multiferroics have been attracting increased interest. These materials exhibit more than one ferroic property simultaneously in a single phase. A fourth ferroic order termed ferrotoroidic order has also been proposed.^[1]

See also

• Piezoelectricity
• Pyroelectricity

References