Ferroelectric Grains Research Articles

Second-harmonic generation in nano-structured [formula omitted]- and [formula omitted]-xerogels with randomly oriented ferroelectric grains

Second-harmonic generation (SHG) in polycrystalline and porous LiTaO 3 - and LiNbO 3 -xerogels is reported. The ferroelectric xerogels are synthesized via the sol–gel method and sintered at different temperatures of 700 °C, 900 °C, 1100 °C and 1150 °C, respectively for 48 h. The nano-structured materials (grain size of 250–1760 nm) are characterized by scanning electron microscopy, thermogravimetry, X-ray diffraction and piezoresponse force microscopy. The ferroelectricity of the grains is determined with piezoresponse force microscopy measurements. While the SHG efficiency grows with increasing grain size, the SHG output energy increases with a higher grade of stoichiometry and increases quadratically as a function of the input energy.

EFFECT OF DEPOLARIZING FIELD AND CHARGED DOPANTS ON POLARIZATION IN POLYCRYSTALLINE Pb(ZrTi)O3 FILM

ABSTRACT A two-dimensional model taking into account the reciprocal effect of electric field on polarization as well as variation of the polarization from saturated to minor hysteresis loops is developed for polycrystalline Pb(ZrTi)O3(PZT) film, where ferroelectric PZT grains are separated by ultrathin semiconductor PbO interlayers. Using the model, it is shown that the depolarizing field, produced by uncompensated polarization charge, strongly depresses the polarization not only on grain boundaries but at appreciable distance inside the grain. The charged dopants in the film cannot suppress the depolarizing field. Instead, the dopants strengthen the coordinate dependency of the polarization along the grain.

Synthesis and magnetoelectric properties of y (Ni0.3Cu0.4Zn0.3Fe2O4)+(1−y) [50% BaTiO3+50% PZT] ME composites

A ME composites system with the general formula y (Ni0.3Cu0.4Zn0.3Fe2O4) + (1 − y) [50% BaTiO3 + 50% PZT] (y = 0.15, 0.30 and 0.45) was prepared by ceramic method. The cubic spinel structure of the ferrite phase and tetragonal pervoskites structure of the ferroelectric phase was confirmed by XRD. SEM micrographs reveal compatibility of ferrite grains with ferroelectric grains. The variation of dielectric constant (ɛ′) and loss tangent (tan δ) with frequency in the range of 20 Hz to 1 MHz was studied. The dielectric constant shows dispersion in the lower frequency region, with almost a constant value at higher frequencies. The variation of dielectric constant with temperature is studied at four different frequencies, viz. 1 kHz, 10 kHz, 100 kHz and 1 MHz. The diffuse phase transition is observed for the composites. There is decrease in transition temperature (Tc) as the ferrites content increases. The magnetoelectric effect (dE/dH)H was measured as a function of intensity of dc magnetic field. The maximum value of ME coefficient was observed to be 692.3 μV Oe−1 cm−1 for the composition 15% (Ni0.3Cu0.4Zn0.3Fe2O4) + 85% [50% BaTiO3 + 50% PZT].

Development of Novel Multiferroic Composites Based on BaTiO3 and Hexagonal Ferrites

AbstractA new multiferroic composite ceramics with the general formula (x)Ba(Sr)Fe12O19-(1-x)BaTiO3 (x=0.1, 0.5) was synthesized via a simple solid-state reaction technique. Crystal structure analysis performed for both materials revealed the presence of two crystalline phases pertinent to the initial composite components. X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to testify the crystallinity, microstructure, and local magnetoelectric interactions between ferroelectric and ferromagnetic grains. Magnetic measurements revealed that the saturation magnetization is proportional to the volume fraction of ferrite phase. Dielectric studies demonstrated strong frequency relaxation due to space charge polarization and high conductivity loss making macroscopic magnetoelectric measurements difficult. Novel nanoscale magnetoelectric effect observed by AFM is discussed.

Phase transitions in ferroelectric liquid crystals

Molecular chirality plays an important role in the science of liquid crystals, leading to cholesteric liquid crystal, blue phases, ferroelectric and antiferroelectric smectic phases and twist grain boundary phases. In all of these mesogens, chirality is an intrinsic property built into the chemical structure of mesogenic molecules. The study of ferroelectric liquid crystals has seen substantial experimental strides. In theoretical aspects, there has been relatively little basic work on this fascinating class of material. This review will try to present a comprehensive overview of the current status of the phase transitions in ferroelectric smectic liquid crystals. The article begins with a brief introduction about the symmetry and structure of ferroelectric mesophases. An attempt is made to identify a range of problems and related questions associated with the study of phase transitions. In the remaining parts of the article the important experimental and theoretical developments are summarized. Finally, some of the future directions have been identified.

Effective permittivity of a nanostructured film consisting of elliptic ferroelectric grains

The effective dielectric permittivity of a nanostructured ferroelectric film consisting of elliptic ferroelectric grains has been theoretically studied as a function of the grain size and the thickness of a dead (nonferroelectric) surface layer in inhomogeneous grains.

Progress in Research on Critical Size of Phase Transition in Ferroelectrics

Theoretical approaches(such as macroscopic thermodynamics phenomenological approach, microscopic transverse field Ising model and first-principles calculation)in dealing with size effect and predicts on critical size of ferroelectric phase transition are reviewed from two aspects,ferroelectric film and ferroelectric grain,respectively.Critical sizes of some ferroelectrics observed in experiments are summarized from 1950s.Some size effects of ferroelectric ceramics are introduced simply.Ferroelectric phase transitions of ferroelectric nanowires reported recently are narrated too.Finally,some problems in researching progress are pointed out.

Enhanced pyroelectric effect in self-supported films of BaTiO3 with polycrystalline macrodomains

Self-supported films of nanocrystalline BaTiO3 exhibit a two orders of magnitude enhancement of the pyroelectric coefficient (≈1μC∕cm2K) with respect to the value measured for a single BaTiO3 crystal. The enhancement strongly depends on film geometry and appears only in buckled films where ferroelectric grains undergo self-organization into polycrystalline macrodomains. The authors posit that the enhancement of the pyroelectric effect is related to 90° polarization switching and is, therefore, similar in nature to an “extrinsic” piezoelectric effect.

Effect of electromagnetic environment on the dielectric resonance in the ferroelectric-ferromagnetic composite

The influence of electromagnetic environment on resonance character of dielectrics was studied in the ferroelectric-ferromagnetic composite. The magnetic phase induces inhomogeneous electromagnetic environment, which will baffle the motion of charged particles in neighboring ferroelectric grains. The influence is not strong enough to change the dielectric properties of composite except for its dielectric resonance character. With the rise of permeability or proportion of magnetic phase, the dielectric resonance peak turns wider and smoother. The dielectric response tends to transit from resonance to relaxation. The electromagnetic interaction between components will prevent the resonance character for the equilibrium of the system.

Micro-structure and ferroelectric properties of BiFeO3 thin films formed on Pt-coated r-plane sapphire substrates

Magnetoelectric BiFeO3 (BFO) materials exhibit ferroelectric and ferromagnetic properties simultaneously, therefore they have a potential to be applied in magnetic as well as ferroelectric devices. BFO thin films were formed by depositing sol-gel solutions on Pt-coated r-plane sapphire dielectric substrates. We did not observe any secondary phase such as Bi2Fe4O9 on the r-plane sapphire substrates, which is generally observed on Si substrates. We observed small ferroelectric grains of about 0.1 μm on Pt/sapphire structures. The leakage current density in BFO films was found to be decreased dramatically after optimizing process conditions of stoichiometric BFO chemical solution. The leakage current densities were in the range of 10− 7 A/cm2 at room temperature and 10− 9 A/cm2 at 80 K under 0.4 MV/cm applied electric field. The main reason for low leakage current is considered to be reduction of oxygen vacancies due to the presence of exclusive Fe3 + valance state in the films. An applied electric field higher than 0.5 MV/cm was required to pole the BFO films, which made it difficult to obtain the saturated polarization at room temperature. We could measure the saturated remanent polarization in the BFO films at 80 K and the obtained remanent polarization was 100 μC/cm2.

Bilateral switching of the modulated electrooptic contrast in PLZTN ceramics

Spatial distribution of ferroelectric polarization in Pb(Zr,Ti)O 3 ceramics doped with lanthanum and niobium has been investigated using scanning electrooptic confocal microscopy. Measurement of the modulated electrooptic signal in presence of a sinusoidal electric field reveals ferroelectric domains at microscopic scale not accessible to conventional polarized optical imaging. Both electrooptic images and local electrooptic loops have been observed after applying a bias electric field to the ceramic samples, being initially non-ferroelectric macroscopically. Bilateral reversal of the electrooptic contrast during the poling cycle has been detected and explained taking into account non-180° switching processes in ferroelectric grains of arbitrary orientation.

Polycrystalline Macro‐Domains Formed by Self‐Organization of Ferroelectric Grains

Nanometer-sized grains may self-organize into macro-domains in a polycrystal undergoing a structural phase transition. Formation of such macro-domains is a very efficient strain-relieving mechanism, as demonstrated here. The Figure shows a tethered self-supported BaTiO3 film viewed with cross-polarized light, illustrating a large in-plane anisotropy due to the self-organization of ferroelectric grains in curved regions. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2089/2005/c2083_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Registered Deposition of Nanoscale Ferroelectric Grains by Template-Controlled Growth.

Regular patterns of ferroelectric PbTiO3 nanostructures with lateral dimensions down to 30-60 nm have been grown onto platinized Si substrates by chemical solution deposition. The high registration of the grains is achieved by "top-down"-generated templates of TiO2 dots as seeds for their deposition. Ferroelectricity was verified by piezoresponse force microscopy.

Piezoresponse force microscopy studies of nanoscale domain structures in ferroelectric thin film

Piezoresponse force microscopy was used to perform studies of nanoscale domain imaging, limit of ferroelectric nano-sized grains and electric field-induced displacement behavior of domain structures in ferroelectric PZT thin film. Nanoscale 180° and 90° domain configurations as small as 30 nm in size were clearly visualized in the individual grains. It was demonstrated that domain configuration was strongly dependent on the size of the grains. The limit of ferroelectric nano-sized grains was found to be smaller than 25 nm. Nanoscale displacement versus field hysteresis loops were obtained in ferroelectric domains of PZT thin film, and discussed in terms of phenomenological theory.

Self-organization of ferroelectric grains in self-supported nanocrystalline films

The dielectric properties of self-supported tethered ferroelectric films were investigated. It was found that the films possess a dielectric constant of more than 2500 and a pyroelectric coefficient comparable with that of a single BaTiO 3 crystal. The temperature and frequency dependence of the dielectric and pyroelectric properties suggest that some self-organization of the ferroelectric domains occurs. In this view one may expect that electromechanical (piezoelectric coupling) is responsible for the observed behavior.

The effective pyroelectric and thermal expansion coefficients of ferroelectric ceramics

We present a micromechanical analysis on the effective pyroelectric and thermal expansion coefficients of ferroelectric ceramics in terms of their microstructural information. The overall behaviors of ferroelectric ceramics are profoundly influenced by the microstructural phenomena, where the macroscopic pyroelectric effect can be induced in an originally isotropic, thus non-pyroelectric ceramic composed of randomly oriented ferroelectric grains through poling, during which the polar axes of grains are switched by the applied electric field or mechanical stress. To analyze these complicated phenomena, we will first establish an exact connection between the effective thermal moduli and the effective electroelastic moduli of ferroelectric ceramics, and then combine the exact connection with the effective medium approximation to provide an estimate on the effective pyroelectric and thermal expansion coefficients of ferroelectric ceramics in terms of the orientation distribution of grains and poling conditions, where the texture evolution as a result of domain switching during poling has been taken into account. Numerical results are presented and good agreements with known theoretical results and some experimental data are observed.

Dipole screening regime for pyroelectric and ferroelectric films and grains in semiconductor matrix

We discuss physics of nonlinear screening of thin pyroelectric (or ferroelectric) films or grains in a semiconductor matrix. Such screening is different from a conventional screening of thick pyroelectric films. For thin films or small grains, the polarization dipole is screened by charges that are much smaller than the polarization charge, and the depletion region providing the screening charge might be much larger than the characteristic size of the pyroelectric. These results are important for the analysis and design of numerous devices involving pyroelectric–semiconductor systems.

Towards the limit of ferroelectric nanosized grains

Ferroelectric random access memories are non-volatile, low voltage, highread/write speed devices which have been introduced into the market in recentyears and which show the clear potential of future gigabit scale universalnon-volatile memories. The ultimate limit of this concept will depend onthe ferroelectric limit (synonymous superparaelectric limit), i.e. the sizelimit below which the ferroelectricity is quenched. While there are clearindications that 2D ferroelectric oxide films may sustain their ferroelectricpolarization below 4 nm in thickness (Tybell T, Ahn C H and Triscone JM 1999 Appl. Phys. Lett. 75 856), the limit will be quite different forisolated 3D nanostructures (nanograins, nanoclusters). To investigatescaling effects of ferroelectric nanograins on Si wafers, we studied PbTiO3(PTO) and Pb(ZrxTi1−x)O3grown by a self-assembly chemical solution deposition method. Preparing highlydiluted precursor solutions we achieved single separated ferroelectric grains withgrain sizes ranging from 200 nm down to less than 20 nm. For grains smallerthan 20 nm, no piezoresponse was observed and we suppose this couldbe due to the transition from the ferroelectric to the paraelectric phasewhich has no spontaneous polarization. Recent calculations (Zhong WL, Wang Y G, Zhang P L and Qu B D 1994 Phys. Rev. B 50 698) andexperiments (Jiang B, Peng J L, Zhong W L and Bursill L A 2000 J.Appl. Phys. 87 3462) showed that the ferroelectricity of fine ferroelectricparticles decrease with decreasing particle size. From these experiments theextrapolated critical size of PTO particles was found to be around 4.2–20 nm.

Ferroelectric Properties of Tungsten-Substituted Bi4Ti3O12 Thin Film Prepared by Sol–Gel Method

Bi4Ti3O12 (BIT) and tungsten-substituted Bi4Ti3O12 (BTW) thin films were prepared on Pt/Ti/SiO2/Si substrates by a sol–gel method. There are differences in the ferroelectric properties and grain strucure between BIT and BTW thin films. The crystal structure and the surface grain morphology were characterized by scanning electron microscopy and X-ray diffraction. Grains of BIT were grown with c-axis preferred orientation, while these of the BTW were randomly distributed. The ferroelectric properties and polarization fatigue characteristics were confirmed by the P-E hysteresis loops. The BTW thin film was measured to have remanent polarization (2Pr) of 27 µC/cm2 and a coercive field (2Ec) of 130 kV/cm. The dielectric constant and loss tangent at 1 MHz were measured as 210 and 0.05, respectively. On adding a small amount of tungsten in Bi4Ti3O12, the remanent polarization increased and fatigue resistance improved.

Nanometer-scale characterization and manipulation of ferroelectric films using scanning probe microscopy

Ferroelectric domain structures in Pb(Ni 0.04Zr 0.28Ti 0.68)O 3 and SrBi 2Ta 2O 9 ferroelectric thin films have been non-destructively observed using various scanning probe microscopy (SPM) modes. Using the SPM, it has been shown that it is possible to not only image but also manipulate the local domain structure on a nanometer scale by biasing the SPM tip. In addition, the SPM was applied to measure d C/d V– V spectra on individual ferroelectric grains. The examples illustrate the possibility of using an SPM-based datastorage read/write system.