Spontaneous ferroelectric devices

In the operation of ferroelectric liquid crystal devices, the applied electric field couples directly to the spontaneous polarisation Ps and response times depend on the magnitude E Ps. Depending on the electronic structure (magnitude and direction of the dipole moment as well as position and polarity of the chiral species) and ordering of the molecules P can vary over several orders of magnitude (3 to 1.2 x 10 ), giving response times in the range 1-100 ps. 

There is considerable interest in developing new types of magnetic materials, with a particular hope that ferroelectric solids and polymers can be constructed— materials having spontaneous electric polarization that can be reversed by an electric field. Such materials could lead to new low-cost memory devices for computers. The fine control of dispersed magnetic nanostructures will take the storage and tunability of magnetic media to new levels, and novel tunneling microscopy approaches allow measurement of microscopic hysteresis effects in iron nanowires. 

The ferroelectric effect is an electrical phenomenon. Parhcular materials, including the ternary oxides (Ba,Sr)Ti03, Pb(Zr,Ti)03 and (Bi,La)Ti03, exhibit a spontaneous dipole moment which can be switched between equivalent states by an external electric held. Ferroelectric thin hlms are of importance for the production of nonvolahle ferroelectric random access memory devices (FeRAM) °. Two possibilities to synthesize such mixed metal oxides are given by the CVD and ALD methods. Table 10 shows the preparation methods of such materials synthesized from metal enolates recently. 

The spontaneous molecular polarization of ferroelectric liquid crystals, arising from their structure when constrained in small cell gaps, results in unique features that can be exploited in display devices. A low electric field of only a few volts can switch the ferroelectric liquid crystal between two equally stable states with opposing polarization directions. This is commonly referred to as bistability. In contrast, nematic displays generally require the electric field to maintain the ON state. The power required to run ferroelectric liquid crystal displays is consequently much less than that required for a nematic display. Since active switching is used in both directions, ferroelectric liquid crystals can switch hundreds of times faster than a... 

An important aspect of the enhanced intrinsic response of ferroelectrics is anisotropy, the direction dependence of properties. By symmetry, ferroelectric crystals are anisotropic with respect to dielectric, mechanical, and electromechanical properties, and this issue is essential when designing devices that exploit the highest material responses by correctly orienting single crystals [8, 33-35]. The crystal axes, along which the highest material response occurs, may not coincide with the polar directions of spontaneous polarization for a given ferroelectric phase. This is the case... 

Ferroelectrics may And application in magnetoelectric devices, either in combination with magnetic materials or intrinsically as a multiferroic material. The latter represents a rare class of materials that are both ferroelectric and ferromagnetic, and often also ferroelastic, in the same phase [113]. This means that, simultaneously, they can undergo spontaneous polarization, magnetization, and deformation that can be reoriented by the respective application of an electric or magnetic fleld, or by... 

C-V Characteristics of a Ferroelectric Thin Film Sample. In the semiconductor industry, C-V curves are frequently utilized for studying the characteristics of devices. In general, a C- V curve can show ferroelectric memory effect of a ferroelectric thin film sample. A voltage applied on the sample is swept from 0 to a positive value (for example, -1-5 V), then from -h5 to -5 V and returned from to -i-5 V. It is demonstrated that the C-V curve has a counterclockwise hysteresis loop, probably due to the ferroeletric nature of the sample. This C- V hysteresis loop can indirectly reflect that a sample may have ferroelectric characteristics (Tokumitsu, 2002). However, the hysteresis characteristics of a C-V curve is not only caused by spontaneous polarization, but also included other effects, like space charge and interface charge. 

This surface bistability is at the basis of chiral smectic C surface stabilized ferroelectric liquid crystal (SSFLC) devices [92]. As their name indicates, these devices are made of thin cells in which the walls, imposing the orientation of the molecules at the surfaces, unwind the spontaneous smectic C helix and stabilize two uniform configurations of the director in the cell. Switching between these two states can be done by applying an electric field. 

On a macroscopic scale, the spontaneous polarization vector in the optically active phase spirals about an axis perpendicular to the smectic layers (Fig. 20), and sums to zero. This macroscopic cancellation of the polarization vectors can be avoided if the helical structure is unwound by surface forces, by an applied field, or by pitch compensation with an oppositely handed dopant. The surface stabilized ferroelectric liquid crystal display utilizes this structure and uses coupling between the electric field and the spontaneous polarization of the smectic C phase. The device uses a smectic C liquid crystal material in the so-called bookshelf structure shown in Fig. 21a. This device structure was fabricated by shearing thin (about 2 i,m) layers of liquid crystal in the... 

Approximately 20 different smectic phases have been identified up to now [3]. Eight among them consist of so-called tilted phases, Le.. the long axes of the moleciiles are tilted with respect to the layer normal (, Sb Sr. So. Sh, Si, Sk. and Sm). If these latter mesophases consist of chiral molecules, they in principle match the requiremeni for intrinsic ferroelectric polarization. In six of these tilled chiral phases (denoted herein by an asterisk), spontaneous polarization has been measured (Sr. Sr. , So St. Sy ). For technological application in electrooptical devices, the chiral smectic C phase Sc is prominent due to its lowest ordering a hence highest fluidity, making reorientation processes caused by electric fields very find. 

All ferroelectric materials are also piezo- and pyroelectric, although there are no known examples of ferroelectric LB films. However, bulk phase phthalocyanine (2) is known to undergo a ferroelectric transition at low temperature. By modifying the structure to achieve reversible spontaneous polarization upon application of an external electric field, it may be possible to produce ferroelectric LB films of such materials at more accessible temperatures. Applications for materials of this type include bistable switching devices and memory storage systems. 

The switching memory effect is a reflection of the fact that the electric displacement, being the function of both the applied field and the material s properties, needs some finite time to adjust to the value of the electric field. The widely accepted model of the instantaneous relationship between the electric displacement and the electric field in the NLC is invalid when the characteristic times of the director dynamics are close to the relaxation times for molecular permanent dipoles. This time scale is typically in the submillisecond range which is of great interest for modem fast-switching devices. The electric displacement (as well as the dielectric torque density) becomes a function of the static dielectric properties of the NLC, the present and past electric field, and the present and past director. We discussed the recently proposed theory and experimental verification of the phenomenon [11]. The model in Ref [11] should be applicable to dynamic reorientation of other LC phases in the appropriate range of times/frequencies. In the case of ferroelectric LCs, the theory should be supplemented by the consideration of spontaneous electric polarization. A similar approach should be also... 

A number of moleoiles that exhibit a ferroelectric (F) phase or an anti-ferroelectric (AF) phase (Figure 2) such as methyl-substituted MHPOBC (18) and the trifluoromethyl analogue, TFMHPOBC (19) with greater spontaneous polarization (Ps) (Figure 3), have been prepared and their physical properties have been investigated because of potential application in electro-optic devices for liquid-CTystalline displays (LCDs) (20). The AFLC molecules show quite useful characteristics such as a tri-stable switching, sharp DC threshold, and double-loop-hysteresis. 

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