Ferroelectric Hysteresis Research Paper

2.1.8 Dielectric hysteresis

Generally, the individual dipoles are randomly oriented in the space. Surface charge, a measure of the macroscopic spontaneous polarization, PS is created by, all dipoles oriented in the same sense, in pyroelectric material. In ferroelectrics , a special case of polar materials , spontaneous polarization PS possesses at least two equilibrium states; by an electric field , the direction of the spontaneous polarization vector may be switched between those orientations. All ferroelectric materials must be pyroelectric and all pyroelectric materials must be piezoelectric for crystal symmetry. From a high-temperature non ferroelectric (or paraelectric) phase into a low temperature ferroelectric

Several phase transitions into successive ferroelectric phases occurs in ferroelectrics, like barium titanate, BaTiO3. The transition into a ferroelectric phase usually leads to strong anomalies in the dielectric, elastic, thermal and other properties of the material [1,16,17], [1,5,6]and is accompanied by changes in the dimensions of the crystal unit cell. The associated strain is called the spontaneous strain,xS. It represents the relative difference in the dimensions of the ferroelectric and paraelectric unit cells. Fig. 2.5 illustrates Some of the changes that can occur in a ferroelectric material that transforms from a paraelectric cubic into a ferroelectric tetragonal phase . polarization reversal (or switching) by an electric field is the most important, characteristic of ferroelectric materials .The natural state of a ferroelectric material is a multidomain state. Domain walls will be reduced (in ceramics) or completely removed in (crystals ) by the application of an electric field . The occurrence of the ferroelectric hysteresis loop switching in ferroelectric materials is the

For a ferroelectric material like PZT, however, P is itself a function of E as shown in figure 2.6 in this figure, the hysteresis effect, familiar from ferromagnetic, is quite evident. In this region, the field is not strong enough to switch domains with unfavorable direction of polarization. The polarization of domains with unfavorable direction of polarization will start to switch along directions crystallographic ally as close as possible to the direction of the field when the field is increased, causing rapid increase in the measured charge density . The dipoles become increasing aligned with the field and the polarization will follow the `initial curve' shown in Fig.2.6 if an initially unpolarized sample of PZT is subjected to an increasing electric field at a temperature slightly below its Curie point, When the field has increased beyond a certain value, because the dipoles are then all aligned with the field there will be no further increase in polarization. The material is then said to have reached its saturation polarization Ps*. If the field is now reduced to zero, the dipoles become less strongly aligned, since in the absence of an external field they're bound to certain preferred directions within the individual crystallites. They do not, however, return to their original alignment (i.e. the alignment before the field was applied) since there are several preferred directions