Gotowa bibliografia na temat „Giant Ferroelectric Polarization”

Thickness effect and mechanical tuning behavior such as strain engineering in thin-film ferroelectrics have been extensively studied and widely used to tailor the ferroelectric properties. However, this is never the case in freestanding single crystals, and conclusions from thin films cannot be duplicated because of the differences in the nature and boundary conditions of the thin-film and freestanding single-crystal ferroelectrics. Here, using in situ biasing transmission electron microscopy, we studied the thickness-dependent domain switching behavior and predicted the trend of ferroelectricity in nanoscale materials induced by surface strain. We discovered that sample thickness plays a critical role in tailoring the domain switching behavior and ferroelectric properties of single-crystal ferroelectrics, arising from the huge surface strain and the resulting surface reconstruction. Our results provide important insights in tuning polarization/domain of single-crystal ferroelectric via sample thickness engineering.

Among several different types of magnetoelectric multiferroics, "magnetically-induced ferroelectrics" in which ferroelectricity is induced by complex spin orders, such as spiral orders, exhibit giant direct magnetoelectric effects, i.e., remarkable changes in electric polarization in response to a magnetic field. Not a few spin-driven ferroelectrics showing the magnetoelectric effects have been found in the past decade.[1] However, their induced ferroelectric polarization is much smaller than that in conventional ferroelectrics and mostly develops only at temperatures much lower than room temperature. Thus, the quest for spin-driven ferroelectrics with room temperature operation and/or robust ferroelectric polarization is still a major challenge in magnetoelectric multiferroics research. In this presentation, I will begin with introducing the background of research on magnetically-induced ferroelectrics, and present the following current progress. Recently, some hexaferrites have been found to show direct magnetoelectric effects at room temperature and relatively low magnetic fields.[2] Furthermore these hexferrites show inverse magnetoelectric effects, that is, induction of magnetization by applying electric fields, at room temperature. The results represented an important step toward practical applications using the magnetoelectric effect in spin-driven ferroelectrics. This presentation introduces magnetism and magnetoelectricity of several types of hexaferrites which show magnetoelectric effect at temperatures above room temperature. In addition, I will also introduce our recent work on magnetoelectric perovskite manganites with large magnetically-induced ferroelectric polarization which is comparable to that in conventional ferroelectrics. This work has been done in collaboration with T. Aoyam, K. Haruki, K. Okumura, A. Miyake, K. Shimizu, and S. Hirose.

Ferroelectric pulsed-power sources with rapid response time and high output energy are widely applied in the defense industry and mining areas. As the core materials, ferroelectric materials with large remnant polarization and high electrical breakdown field should generate high power under compression. Currently, lead zirconate titanate 95/5 ferroelectric ceramics dominated in this area. Due to environmental damage and limited output power of lead-based materials, lead-free ferroelectrics are highly desirable. Here, the electrical response of 0.9BiFeO3-0.1BaTiO3 (BFO-BT) ferroelectric ceramics under shock-wave compression was reported, and a record-high power density of 4.21 × 108 W/kg was obtained, which was much higher than any existing lead-based ceramics and other available energy storage materials. By in situ high-pressure neutron diffraction, the mechanism of shock-induced depolarization of the BFO-BT ceramics was attributed to pressure-induced structural transformation, and the excellent performance was further elaborated by analyzing magnetic structure parameters under high pressures. This work provides a high-performance alternative to lead-based ferroelectrics and guidance for the further development of new materials.

Ferroelectric materials are defined as the materials, of which spontaneous polarization can be switched by an external electric field. Their crystal structural symmetry makes them exhibit a variety of electric properties, such as piezoelectricity, pyroelectricity, and ferroelectricity. Because of their characteristics, they are expected to be adapted for various applications, including sensors, actuators, and non-volatile memories. Over the past decades, perovskite type ferroelectric materials have occupied the central position in both fundamental studies and applications of ferroelectric materials. On the other hand, there are a few studies on ferroelectrics with other crystal structures. This regime is now changing since HfO2-based new ferroelectric materials have been discovered. The HfO2-based dielectric materials are employed as high-k insulators of the metal-oxide-semiconductor field-effect-transistors instead of the conventional SiOx gate dielectrics, suggesting the high compatibility with semiconductor technologies. Thus, discovering ferroelectricity in HfO2-based materials strongly encourages us to develop highly integrated ferroelectric devices that are difficult to fabricate with traditional perovskite-type ferroelectrics. Amid increasing interest in ferroelectric materials, ferroelectricity is demonstrated on another new (Al, Sc)N, which has a wurtzite structure. Both fluorite structure, the parent structure of HfO2-based ferroelectrics, and wurtzite structure are simple compounds, having only a single anion and cation sites in the crystal structure. This feature contrasts the complex crystal structure of conventional perovskite structure. This presentation will give a brief outline of these new ferroelectric materials and introduce our recent studies from the viewpoint of crystal chemistry. It is well-known that HfO2 undergoes successive phase transitions from monoclinic to tetragonal and tetragonal to cubic phases. However, these phases cannot show ferroelectricity because of their inversion center in the crystal structure. It is widely accepted that the ferroelectricity in HfO2-based materials originates from the metastable orthorhombic structure. This orthorhombic structure was confirmed by the convergence electron diffraction and scanning transmission electron microscopy. Among the HfO2-based materials, HfO2- ZrO2 materials are most extensively studied. However, the thickness that can exhibit ferroelectricity in these materials is limited to less than 50 nm because of their strong preference for the monoclinic structure. In order to investigate structural features of the HfO2-based materials, materials are demanded that have ferroelectricity over the wide thickness range. The Y-doped HfO2 meets the requirement, allowing us to grow the ferroelectric film over 1 μm in thickness. Furthermore, we demonstrated ferroelectricity in epitaxial films using this composition. A recent report on ferroelectricity in bulk single-crystal also employed the Y-doped HfO2 system. The ferroelectricity in the wurtzite structure has been discussed for a long time. Moriwake et al. put forward giant spontaneous polarization in wurtzite materials by calculation based on density functional theory. The proposed mechanism of polarization reversal is accompanied by the change in the outermost surface, namely a cation surface to an anion surface and vice versa. Such large polarization was demonstrated in (Al1-x Scx)N films by Fitchtner et al. They also confirmed the change in the surface by performing chemical etching. In addition to (Al1-x Scx)N films, the ferroelectricity has been confirmed in (Al1-x B x )N, (Ga1-x Sc x )N, and (Zn1-x Mg x )O. For the wurtzite structure, we can consider the virtual paraelectric BN phase, in which both anions and cations are located in the same plane. As the paraelectric phase is deemed an intermediate state during polarization reversal, easy polarization reversal is expected as the u-parameter of the wurtzite structure approaches 0.5. It is considered that the u parameter is closely related to the axial ratio of the c- and a-axes. In fact, the reduction of coercive field and remanent polarization is ascertained experimentally. The “simple compound” ferroelectrics have attracted much attention due to their unique features, e.g., outstanding compatibility to semiconductor technologies in HfO2-based materials and giant remanent polarization in wurtzite materials. However, quite a large coercive field compared to conventional ferroelectrics reduces the reliability of the devices, particularly endurance properties. Further studies and developments to unveil microstructures under and after applying a strong electric field will lead to the next application of these ferroelectrics.

A ferroelectret cellular structure of poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] is fabricated by a 3D printing technique that exhibits a giant piezoelectric coefficient of 1200 pC/N, which is 40 times higher than its commonly known film counterpart. It attributes that the bi-polar charge separation in cellular voids upon the corona discharge behaves as macroscopic dipoles. An increase in the surface potential and dielectric constant (from 10 to 20 at 1 kHz) also attributes to charged voids. Furthermore, the deviation of ferroelectric behavior, for instance, the continuous increasing trend in dielectric constant and remanent polarization as a function of temperature attributes to ferroelectret behavior of a 3D printed P(VDF-TrFE) specimen. The mechanical energy harvester (MEH) made with this ferroelectret structure shows prompt response with [Formula: see text]4 W/m2 of the power density. Furthermore, the benefit of the giant piezoelectric coefficient of the MEH is used to demonstrate self-powered tactile mapping.

By analogy with ferromagnetism and the hysteresis of the magnetic moment with a magnetic field, materials that exhibit a macroscopic spontaneous polarization Ps, which can be reversed under electric field E were defined as ferroelectrics. Ps, the directional order parameter can give rise to different polar structural phase transitions and finally disappear as a function of temperature T and/or hydrostatic pressure P in a transformation from a non-centrosymmetric to a centrosymmetric space group. The physical properties of ferroelectric materials are the basis of many technological applications based on their hysteretic properties (Ps / E in ferroelectric random access memories) or based on their coupled properties (η (mechanical strain)/ E in piezoelectric applications). In order to understand the origin and the mechanisms associated with the ferroelectric properties, "in-situ" structural studies as a function of E, T and P have to be performed. In addition ferroelectric materials exhibit based on their directional properties (Ps) a particular domain configuration which makes the structural understanding of these compounds much more complex. Different scales should be taken into account: from the atomic scale (individual polar displacements) to the macroscopic scale (macroscopic piezoelectric effect) and finally the mesoscopic scale in between, which is governed by the domain wall motion. High piezoelectric/ferroelectric properties in lead perovskite materials (PZT, PMN, PZN) are structurally linked to strong disorder which can be characterized by the presence of diffuse scattering in diffraction experiments and by nanosized domains. Here we will present "in-situ" characterization in lead perovskite materials as a function of the applied electric field based on X-ray and neutron diffraction and EXAFS techniques. A brief overview of the challenges to solve in future studies as a function of pressure and temperature will also be discussed.