Ferromagnetic-ferroelectric properties

By analogy with ferromagnetism, ferroelectricity is the property by which a crystal has a permanent electric dipole moment which can be reversed by the application of an electric field. The perovskite BaTiOs, in which the BaO layers are compressed and the Ti02 layers stretched (see Fig. 13.1), is a ferroelectric... 

CURIE POINT (or Curie Temperature). Ferromagnetic materials lose their permanent or spontaneous magnetization above a critical temperature (different for different substances). This critical temperature is called the Curie point. Similarly, ferroelectric materials lose their spontaneous polarization above a critical temperature. For some such materials, this lemperaLure is called the "upper Curie point." for there is also a "lower Curie point." below which the ferroelectric property disappears. See also Ferromagnetism. 

If we can combine ferroelectrically polarizable guest molecules with PCPMs, then we can obtain new types of multiferroic materials, where ferroelectricity and ferromagnetism coexist. Such multifunctional materials have recently aroused increasing interest from the viewpoint of the development of new materials with very large magnetoelectric effects. [Mn3(HCOO)6] EtOH is considered to be the first ferroelectric PCP.182 Because the desolvated compound [Mn3(HCOO)6] shows a very small and almost temperature-independent dielectric constant, irrespective of the direction of the electric field,183 the observed ferroelectric property is considered to come mainly from the guest EtOH molecules. 

We are also interested in magnetoelectrics which form an important subgroup of multiferroics. Magnetoelectrics exhibit, simultaneously, ferromagnetic and ferroelectric properties, the combination of which is expected to revolutionize computers. A memory element combining both properties would permit an electric write operation combined with a magnetic read, eliminating the need... 

Von Hippel, Arthur Robert (1898-2003) reported the ferroelectric properties of BaTiOj in 1946. Since then over a hundred pure materials and many more mixed crystal systems that are ferroelectric have been found. He was on the faculty of MIT from 1936 until he retued in 1964. His starting salary was only 3,500 per year and he is reported to have sold textbooks to pay for medical bills for his children. The Materials Research Society has named one of its major awards in recognition of von Hippel s contribution to dielectrics, semiconductors, ferromagnetics, and ferroelectrics. ... 

Despite the apparent similarity of equations describing thermodynamic properties of ferromagnets, ferroelectrics and ferroelastics, the nature of the interactions defining the transition temperature Tc and the values of coefficients a and B are essentially different for these ferroics. For example, in ferromagnets the Tc value is defined by the exchange interactions between magnetic spins. At the same time. 

The crystal structure of a solid can influence the properties of a material, for example, the structure must be noncentric for a material to demonstrate antiferromagnetic, ferromagnetic, ferroelectric, or piezoelectric behavior. Rapid cooling of a sample from high temperature and/or high pressure can quench in a structure that is not stable at room temperature or atmospheric pressure. High-pressure oxide polymorphs, which are more dense, have been studied to model the earth s interior. Furthermore, unique crystal structure characteristics of a material can allow stmcture-property variation, for example, insertion compound formation in layered materials. 

It is far beyond the scope of this chapter to review the electronic structures and properties of all metal oxides, or even all of the important metal oxide stmc-ture types. Instead, this section covers some featnres of one stmctural family, perovskite, in some detail. In doing so, it is hoped that the important concepts will be illnstrated in snch a way that they can be widely appUed. Of course, the choice of the perovskite stmctnre as an illnstrative example is not a random choice. The perovskite family of componnds is very extensive, encompassing most of the periodic table. Fnrthermore, perovskites exhibit nearly every type of interesting electronic or magnetic behavior seen in oxides (ferromagnetism, ferroelectricity, piezoelectricity, nonlinear optical behavior, metaUic condnctivity, snpercondnct-ivity, colossal magnetoresistance, ionic conductivity, photoluminescence, etc.). One important property that is not readily found among perovskites, transparent conductivity, is the focus of Section 6.7. 

These materials have the ilMnOs R = Sc or small, rare earth cation) stoichiometry,and have been erroneously referred to as hexagonal perovskites. The compounds do not exhibit the perovskite structure. The Mn cations are not octahedrally coordinated, rather the cation is surrounded by five oxide anions in a trigonal prismatic coordination environment. Also the R cations are not 12-coordinate, as would be the case in a perovskite, but are in seven-fold coordination. The materials are multi-ferroic, with anti-ferromagnetic and ferroelectric properties.The nature of the polarity and therefore the ferroelectric behaviour was only recently described. Careful structural studies indicated that although the dipole moments are attributable to the R-O bonds and not the Mn-O bonds, the R-cations are not directly responsible for the ferroelectric behaviour. The noncentrosymmetry is attributable to the tilting of the MnOs polyhedra, which in conjunction with the dipole moments in the R-O bonds results in ferroelectric behaviour. Thus the ferroelectric behaviour in these materials is termed improper " and occurs by a much different mechanism than BaTiOs or even BiFeOs. 

In this chapter piezoelectric crystals and polymers ferroelectric and ferromagnetic solids resistance of metals shock-induced electrical polarization electrochemistry elastic-plastic physical properties. 

In Science, every concept, question, conclusion, experimental result, method, theory or relationship is always open to reexamination. Molecules do exist Nevertheless, there are serious questions about precise definition. Some of these questions lie at the foundations of modem physics, and some involve states of aggregation or extreme conditions such as intense radiation fields or the region of the continuum. There are some molecular properties that are definable only within limits, for example, the geometrical stmcture of non-rigid molecules, properties consistent with the uncertainty principle, or those limited by the negleet of quantum-field, relativistic or other effects. And there are properties which depend specifically on a state of aggregation, such as superconductivity, ferroelectric (and anti), ferromagnetic (and anti), superfluidity, excitons. polarons, etc. Thus, any molecular definition may need to be extended in a more complex situation. 

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. 

The highly symmetrical perovskite structure is very common for MM X3 type compounds. Some of the many hundreds of such compounds are listed in Table 5.6. Because of their important ferroelectric, ferromagnetic, and superconducting properties, many compounds have been synthesized varying the ratios of metals to optimize desired properties. New compounds are reported frequently. We will discuss structures related to perovskite in Sections 5.3.5, 5.3.6, 5.3.7, 5.4.11, 5.4.12, and 5.4.13. 

Later, the evolution of the electronic industry initiated the development of an immense variety of materials and devises based, essentially, on the properties of semiconductor, dielectric, ferromagnetic, superconductor, and ferroelectric materials. 

---