Hysteresis, optical properties

Crystals with one of the ten polar point-group symmetries (Ci, C2, Cs, C2V, C4, C4V, C3, C3v, C(, Cgv) are called polar crystals. They display spontaneous polarization and form a family of ferroelectric materials. The main properties of ferroelectric materials include relatively high dielectric permittivity, ferroelectric-paraelectric phase transition that occurs at a certain temperature called the Curie temperature, piezoelectric effect, pyroelectric effect, nonlinear optic property - the ability to multiply frequencies, ferroelectric hysteresis loop, and electrostrictive, electro-optic and other properties [16, 388],... 

Electrooptic Properties, The electrooptic properties of the PLZT materials are intimately related to their ferroelectric properties. Consequently, varying the ferroelectric polarization with an electric field such as in a hysteresis loop, produces a change in the optical properties of the ceramic. In addition, the magnitude of the observed electrooptic effect is dependent on both the strength and direction of the electric field,... 

Cooperativity is one of the most appealing and elusive facets of the spin-crossover phenomenon. It is a main aspect because discontinuity in the magnetic and optical properties along with thermal hysteresis confer to these systems potential memory effect. Nevertheless, because most of the spin-crossover systems are discrete in nature, cooperativity stems from assemblies of molecules held together by nonco-valent interactions and, consequently, difficult to control. 

Observations between crossed polarizers of the effects of electric fields on the optical properties of butyl p-anisylidene-pf-amino-cinnamate (BA AC) have demonstrated the existence of a domain pattern which, in contrast to the cigar-shaped patterns of p-azoxyanisole, is circular. These circular domains have their optic axis parallel to the applied field, while in p-azoxyanisole the optic axis is essentially perpendicular to the field. Measurement of the polarization as a function of applied field in nematic BAAC has yielded hysteresis loops similar to those found in ferroelectric crystals. This evidence for a spontaneous polarization indicates that in a material such as BAAC with a dipole moment essentially along the molecular axis the molecules are predominantly oriented in one direction within the domains. 

Like solid ferroelectrics, the ferroelectric liquid crystals, particularly the FLCPs, show a pyroelectric effect and a piezoelectric effect and are capable of switching polarization direction (dielectric hysteresis). Moreover, they can switch propagating or reflected polarized light. Finally, the polar symmetry of the phase leads to nonlinear optical properties of the FLCPs such as second-harmonic generation, the Pockels effect, and the Kerr effect. These physical properties of the ferroelectric LC polymers are discussed in the following sections. 

During the study of electro-optical properties, a well-known hysteresis phenomenon is observed in PDLC films (Drzaic 1988). Hysteresis has been a problem, which must be tackled for perfect practical applications as a display material. It has been found that the transmittance values obtained at various voltages during scan-down cycle do not follow the same path as that of the scan-up cycle. A measure of hysteresis is given by the voltage width at half of maximum transmittance (AVso). The transmittance lags behind and may trace any of the three paths as shown in Fig. 7.7 ... 

Barakat NAM, Woo KD, Kanjwal MA, Choi KE, Khd MS, Kim HY (2008) Surface plasmon resonances, optical properties, and electrical conductivity thermal hysteresis of silver nanofibers produced by the electrospinning technique. Langmuir 24 11982-11987... 

The thus obtained high-density Mn-Zn ferrite was investigated in detail from the view of physical and mechanical properties, that is, the relationships between the composition of metals (a,) ) and <5 the magnetic properties such as temperature and frequency dependence of initial permeability, magnetic hysteresis loss and disaccommodation and the mechanical properties such as modulus of elasticity, hardness, strength, and workability. Figures 3.13(a) and (b) show the optical micrographs of the samples prepared by the processes depicted in Fig. 3.12(a) and (b), respectively. The density of the sample shown in Fig. 3.13(a) reached up to 99.8 per cent of the theoretical value, whereas the sample shown in Fig. 3.13(b) which was prepared without a densification process, has many voids. 

---