Ferroics

Ferroic domairts (or domairts in domain twin) differ in the orientation of sporrla-neous electric polarization, spontaneous magnetization or spontaneorrs strain in case of so-called primary ferroics. Magnitude of the sporrtaneoirs qirarrtity is typically the same in both domains btrilding domain twin, tensor components are different (i.e. vector components for polarization and magnetization and syrranetrical second-order tensor components for mechanical strain). Situation is more complicated for 

Ferroics Switchable quantity or material property Switching force Example 

Ferroelectrics Spontaneous polarization Electric field BaTiOs 

Ferromagnetics Spontaneous magnetization Magnetic field Fe304 

Ferrobielectrics Dielectric susceptibility Electric field SrTiOsf ) 

Ferroelastic Spontaneous strain Mechanical stress CaAljSijOg 

Ferrobielastic Elastic compliance Mechanical stress a-quartz 

Ferroelastoelectric Piezoelectric coefficients Electric field and mechanical stress NH4CI 

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Ferromagnetic and ferroelectric materials are only two examples of a wider group that contains domains built up from switchable units. Such solids, which are called ferroic materials, exhibit domain boundaries in the normal state. These include ferroelastic crystals whose domain structure can be switched by the application of mechanical stress. In all such materials, domain walls act as planar defects running throughout the solid. 

Newnham, R.E., Trolier-Mckinstry, S. and Giniewicz, T.R. (1993) Piezoelectric, pyroelectric and ferroic crystals. Journal of Materials Education, 15, 189-223. 

Several possible ferroic phenomena become evident from equation (6.60), depending upon the dominance of particular terms. In a material which has a large value of spontaneous polarization, other terms become unimportant and the free energy in an electric field is governed by the expression... 

Dominance of the other terms gives rise to similar expressions for other secondary ferroics. 

At high temperatures, ferroelectric materials transform to the paraelectric state (where dipoles are randomly oriented), ferromagnetic materials to the paramagnetic state, and ferroelastic materials to the twin-free normal state. The transitions are characterized through order parameters (Rao Rao, 1978). These order parameters are characteristic properties parametrized in such a way that the resulting quantity is unity for the ferroic state at a temperature sufficiently below the transition temperature, and is zero in the nonferroic phase beyond the transition temperature. Polarization, magnetization and strain are the proper order parameters for the ferroelectric. 

Figure 6.54 Diagram illustrating several types of order parameters involved in proper and improper ferroics. (After Newnham Cross, 1981.)...

A hexagonal representation of proper and improper primary ferroics as proposed by Newnham Cross (1981) is given in Fig. 6.54. The order parameter for proper ferroics appears on the diagonals of the hexagon, while the sides of the hexagon represent improper ferroics. They indicate the cross-coupled origin of ferroic phenomena. An improper primary ferroic in this classification is distinguished from a true secondary... 

Secondary ferroics. SrXi03 (which is an incipient ferroelectric) and NaNb03 (which is an antiferroelectric) may be considered to be ferrobielectric. Xhe dielectric anisotropy in antiferroelectric domains can give rise to high values of induced polarization that are orientationally different in different domains. Xhus, they give rise to domain rearrangement under applied fields. Quartz is a classic example of... 

Composite ferroics. Ingenious experiments have been performed with composites made from a ferroic and another material (Newnham Cross, 1981 Lynn et al, 1981 Rittenmeyer et al, 1982 Safari et al, 1982). For example, in a piezoelectric like PZT, the piezoelectric voltage coefficient g can be defined for a given direction (say Z = 33) thus, 33 = where d and k stand for piezoelectric coefficient... 

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