The study of local dual‐peak vibratory properties in ferroelectric P(VDF‐TrFE) films

In this article, we reported the studies on the local dual‐peak vibrations in ferroelectric P(VDF‐TrFE) films, induced by an alternating voltage with its amplitude high enough to cause polarization reversal. A further study showed that this dual‐peak vibration was caused by ferroelectric switching process. The buildup of these dual‐peak vibrations had been detected, and the asymmetrical evolution of local butterfly loops was observed. It was believed that the observed time delay during the developments of dual‐peak vibrations and butterfly loops should be attributed to the electrode effect. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3282–3287, 2006     

Ferroelectric Properties of Modified Lead Titanate Thin Films Obtained by a Sol-Gel Method

Calcium substituted lead titanate thin films have been prepared by a sol-gel method, deposited by spin-coating, and thermally treated so as to obtain a perovskite structure. Two types of thermal treatments were given, differing in the heating rate, which in conventional treatments was of some 10°C/min, and in rapid treatments >500°C/min. With rapid heating, materials are obtained in which greater polarizations are commuted and bias fields are smaller. This, together with the fact that additional (undesired) phases are not observed by X-ray diffractometry when rapid treatments are used, shows that these are to be preferred over conventional treatments.     

Phase transition in tungsten–bronze Li2Pb2Nd2W2Ti4Nb4O30 ferroelectric

The polycrystalline sample of Li₂Pb₂Nd₂W₂Ti₄Nb₄O₃₀ was prepared by a solid-state reaction technique. Room temperature X-ray structural analysis confirms the formation of a single-phase compound. The morphology of the sintered sample recorded by scanning electron microscope exhibits a uniform grain distribution. Detailed studies of the nature of variation of dielectric constant, tangent loss, and polarization with temperature and frequency confirmed the existence of ferroelectricity in the material at room temperature. The temperature and frequency dependence of impedance parameters (impedance, modulus, etc.) of the material exhibits a strong correlation of its microstructure (i.e., bulk, grain boundary, etc.). Furthermore, the temperature dependence of DC conductivity shows a typical Arrhenius behavior of the material. The nature of variation of pyroelectric coefficient and current with temperature suggests that material has good pyroelectric properties useful for pyroelectric detector.     

Multiferroicity: the coupling between magnetic and polarization orders

Multiferroics, defined for those multifunctional materials in which two or more kinds of fundamental ferroicities coexist, have become one of the hottest topics of condensed matter physics and materials science in recent years. The coexistence of several order parameters in multiferroics brings out novel physical phenomena and offers possibilities for new device functions. The revival of research activities on multiferroics is evidenced by some novel discoveries and concepts, both experimentally and theoretically. In this review, we outline some of the progressive milestones in this stimulating field, especially for those single-phase multiferroics where magnetism and ferroelectricity coexist. First, we highlight the physical concepts of multiferroicity and the current challenges to integrate the magnetism and ferroelectricity into a single-phase system. Subsequently, we summarize various strategies used to combine the two types of order. Special attention is paid to three novel mechanisms for multiferroicity generation: (1) the ferroelectricity induced by the spin orders such as spiral and E-phase antiferromagnetic spin orders, which break the spatial inversion symmetry; (2) the ferroelectricity originating from the charge-ordered states; and (3) the ferrotoroidic system. Then, we address the elementary excitations such as electromagnons, and the application potentials of multiferroics. Finally, open questions and future research opportunities are proposed.     

First-principles study of the mechanisms of the pressure-induced dielectric anomalies in ferroelectric perovskites

The pressure-induced giant dielectric anomaly at 0 K of ABO₃ perovskites is investigated at the Hartree-Fock, density-functional theory and hybrid levels. Its mechanism is analyzed in terms of thermodynamic phase stability, structural and phonon contributions and Born effective charges. It is shown that the IR-active soft phonon is responsible for the anomaly. This mode always involves a displacement and a deformation of the oxygen octahedra, while the roles of A and B ions vary among the materials and between high- and low-pressure phase transitions. A sharp increase in the phonon amplitude near the phase transition gives rise to the dielectric anomaly. The use of hybrid functionals is required for agreement with experimental data. The calculations show that the dielectric anomaly in the pressure-induced phase transitions of these perovskites is a property of the bulk material.     

Limiting long-range-ordered solids to finite sizes in condensed-matter physics

We have examined some of the consequences of finite size effects in solids which exhibit cooperative behaviour. These are materials with long-range interactions based on ferromagnetism, ferroelectricity, and superconductivity. Limiting the range of interaction is achieved by creating nanocrystals, nanodomains, disorder, high-concentration solid solutions and submicron microstructure in sintered compacts. The attempt is to elucidate underlying principles essential at a microscopic level for each of the three phenomena. Not much work has been reported in this area earlier and most of the results are from our own studies. Some of the work is reported for the first time. The experimental techniques used are Mössbauer, Raman and infrared spectroscopies, high-temperature x-ray diffraction, electron energy loss spectroscopy, dielectric and piezoelectric response, ferromagnetic resonance and spinwave dispersion, 4-probe resistivity, DC & AC magnetic susceptibilities and differential scanning calorimetry.     

Strain-induced insulator–metal transition in ferroelectric BaTiO_3(001) surface:First-principles study

The electronic properties of TiO_2-terminated BaTiO_3(001) surface subjected to biaxial strain have been studied using first-principles calculations based on density functional theory. The Ti ions are always inward shifted either at compressive or tension strains, while the inward shift of the Ba ions occurs only for high compressive strain, implying an enhanced electric dipole moment in the case of high compressive strain. In particular, an insulator–metal transition is predicted at a compressive biaxial strain of 0.0475. These changes present a very interesting possibility for engineering the electronic properties of ferroelectric BaTiO_3(001) surface.     

Temperature‐dependent Raman scattering of KDP:Mn (0.9% weight of Mn) crystal

Low temperature polarized Raman scattering measurements of KDP:Mn (0.9% weight of Mn) were performed at temperatures ranging from 14 to 300 K, over the spectral range 50–1250 cm⁻¹. In the present results we can see that the spectra of undoped and doped samples at room temperature are very different. Doped samples maintain the KDP structure as tetragonal, with the same factor group D_2d but with a different class of the space group, different from the original 12. The results show that the crystal undergoes a phase transition at temperature between 115 and 97 K, which is much lower than the phase transition temperature of undoped KDP that occurs at 122 K, where the crystal changes from the para‐electric to the ferroelectric phase. Further, at very low temperature (14 K) we can see that the spectra of KDP:Mn (0.9% weight of Mn) present a behavior very different from the behavior presented by the spectra of KDP doped with low Mn concentration. Copyright © 2010 John Wiley & Sons, Ltd.