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Ferroics

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 Tc. Above this critical temperature, the crystal is in a nonferroic state and exists with no notable physical characteristics. Upon cooling the material down below Tc 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 Tc a spontaneous dipole moment is induced along an axis of the unit cell. Although individual dipole moments can sometimes be small, the effect of 1024 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 $\left (x,y,z \right )$ can be mapped onto a lattice point $\left ( -x,-y,-z \right )$.

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, Tc 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.

A series of other ferroic materials exist, such as piezoelectrics and pyroelectrics, however in general their properties are linked to the three types of ferroic materials described.