Elastic interaction between 90° domain walls and misfit dislocations in epitaxial ferroelectric thin films

Abstract

The elastic interaction between 90° domain walls and misfit dislocations in epitaxial ferroelectric thin films is studied theoretically for the first time. The interaction energy is evaluated via the calculation of a work done by the dislocation stress field during the development of spontaneous strains in a polydomain film. For a single wall separating c and a domains, the energy and force of the interaction with individual misfit dislocations and periodic dislocation arrays are computed as a function of the wall position in a film. The results obtained are used to describe the pinning of 90° walls by misfit dislocations in epitaxial films. The critical electric field E_c , which is necessary for the motion of domain walls through the potential relief created by misfit dislocations, is evaluated and compared with the observed coercive fields of ferroelectric thin films.     

Piezoelectric properties and domain structure of the orthorhombic (K,Na,Li)(Nb,Ta,Sb)O3 single crystal

ABSTRACT

In this work, (K,Na,Li)(Nb,Ta,Sb)O₃ (KNLNTS) crystal is in the orthorhombic phase at room temperature. The orthorhombic-tetragonal phase transition temperature is 50.0°C, and the Curie temperature T_C of the tetragonal-cubic phase transition temperature is 253.8°C. The crystal is poled, the defects were “pinned” in specific position, and has positive effect in domain wall motions and better ferroelectric property (P_r is 6.49 µC/cm², E_c is 6.66 kV/cm). The domain configrations of the crystal were studied by means of a polarizing light microscopy (PLM), with poling along [100]_C direction.     

Conduction mechanism in antiferrolectric PbZrO3 thin films—analysis of charge carrier trapping phenomenon

Abstract

Antiferroelectric compositions, such as PbZrO₃, are attractive candidates in charge storage devices and actuator/transducer applications in MEMs technology. Thin films of PbZrO₃were deposited on Pt coated Si substrates by a pulsed excimer ablation process. The process of field induced ferroelectric phase switching involves the domain wall reorientation in the polycrystalline thin films. The presence of grain boundaries and various defects in the polycrystalline thin films acts as the pinning sources for the various domain walls. These defects capture the charge carriers in the presence of external applied field and hinders further switching of the dipoles in the domains, thereby increases the response times and threshold voltages for the devices operations. Understanding of the trapping phenomenon in these films is very essential. Using Lampert's theory of space charge limited conduction both shallow and deep trap energies were estimated approximately from charge transport analysis.     

Relaxation Motion and Possible Memory of Domain Structures in Barium Titanate Ceramics Studied by Mechanical and Dielectric Losses

The relaxation motion and memory effect of domain structures have been investigated using mechanical and dielectric loss measurements in BaTiO₃ ceramics with grains sizes varied from 1 m to 50 m. The measurements of mechanical loss, elastic modulus, dielectric loss and permittivity show that each phase transition induces a loss peak and an anomaly in the dielectric constants and elastic modulus, furthermore, a number of relaxation loss peaks due to ferroelectric domains in the samples with large grain have been observed. All the relaxation peaks can be analysed by Arrhenius relationship for a wide range of frequency from 10^–2 to 10⁶ Hz. The activation energies of relaxation peaks have been determined as 0.92 eV, 0.68 eV, 0.47 eV, and 0.29 eV for the peaks located in the tetragonal, orthorhombic, and rhombohedral phase, with Arrhenius perfactor in the order of 10^–13 s. Moreover, one relaxation process is insensitive to ferroelectric phase transitions, and it can exist in all the ferroelectric phases. This implies a possible memory effect of ferroelectric domain structures. Such a motion of domain wall is limited in fine-grained materials. Effect of vacuum annealing on the relaxation peak in the tetragonal phase is also studied to clarify the mechanisms of the peak. These relaxation peaks could be explained by the interaction between different domain walls and the diffusion of oxygen vacancy in the domains.     

Piezoelectric and dielectric aging in pb(zr,ti)o3 thin films and bulk ceramics

Abstract

Piezoelectric and dielectric aging was studied in Pb(Zr,Ti)O₃ (PZT) thin films and bulk ceramics. It was found that piezoelectric aging in thin films obeys the logarithmic time dependence with the relative aging rate much higher than that of the dielectric constant, while comparable aging rates of piezoelectric and dielectric constants were found in PZT ceramics. The origin of piezoelectric aging in PZT films was related to depolarization of the films rather than to suppression of the domain wall motion as was generally accepted for PZT ceramics.     

Ferroelectric and dielectric properties of sol-gel and excimer-laser-deposited lead zirconate titanate and barium titanate films

Abstract

Switched remanent polarization was measured as a function of accumulated switching cycles for a variety of ferroelectric films using sinusoidally driven hysteresis loops. Switched remanent polarization and dielectric constant and loss were also obtained as a function of the cycling frequency. PZT films with niobium additives appeared to lose switched remanent polarization with accumulated cycles at a lesser rate than films without niobium. The switched remanent polarization was found to decrease with increasing frequency, which we attribute to the effect of grain size. Also, a decrease of dielectric constant with increasing frequency and an increase of dielectric constant with increased applied voltage are attributed to the effects of domain wall motion contributions to dielectric constant.     

Fatigue and dielectric properties of undoped and Ce doped PZT thin films

Abstract

Ce doping improves the ferroelectric properties of sol-gel derived PZT thin films by facilitating easier domain wall movement. It also decreases the leakage current densities by reducing the concentration of free carriers through a decrease in concentration of Pb and O vacancies. Ce-PZT films retain good dielectric dispersion characteristics since the concentration of defects and defect dipoles are reduced. Ce doping dramatically improves the fatigue resistance of PZT thin films. We have studied the frequency dependence of fatigue behavior and shown that the loss of polarization due to fatigue follows a universal scaling behavior with N/f², where N is the number of the switching cycles and f is the frequency. The origin of the scaling is attributed to the drift of oxygen vacancies, which is the rate limiting process in the growth of the interface layer responsible for fatigue. Empirical fits for both undoped and cerium doped samples show that switchable polarization follows a stretched exponential decay with time or N/f. Cerium doping is believed to improve fatigue resistance by impeding the motion of oxygen vacancies.     

Investigation of ferroelectric domains and structural twins in barium titanate crystal

Abstract

The Ferroelectric domain patterns and the {111} twin boundaries are frequently observed in the BaTiO₃ ceramics. The ferroelectric domain structures in BaTiO₃ single crystal were investigated by a conventional and a synchrotron white beam X-ray topographies. The domain structure of {111} twinned crystal sample was also observed under a polarizing microscope and under a TEM. The relation between the {111} twin and the ferroelectric domain structure was verified by insitu observation of the behaviors of 90° domains during the phase transitions. It is observed that the 90° ferroelectric domain boundaries around the {111} twin boundary which was produced during the crystal growth obey the symmetry relation of the {111} twinning and that they meet each other coherently at the same position of the {111} twin boundary.     

Efficient Switching and Domain Interlocking Observed in Polyaxial Ferroelectrics

We present transmission electron microscopy observations of domain wall motion in thin freestanding KNbO 3 crystals under applied electric fields. Since there is no substrate, there is no elastic clamping of 90 domains. We observe that curved and tilted 90 domain walls are the most mobile, whereas untilted 90 domain walls are resistant to field-induced motion. We explain this result in terms of two factors. First, the switching pressure on a domain wall ( P 2 m P 1 ) E is determined by the relative electrostatic energies of the neighboring polarizations P 1 and P 2 . Consequently, some 90 domain walls are immobile under certain field directions, leading to domain interlocking. Second, domain walls experiencing a high switching pressure move by a ripple mechanism, and do not move as rigid sheets. The tilted wall region in such a ripple has a polarization charge, and an associated depolarization field, which reduces the local switching barrier. An accumulation of polarization charge can result in a tilted or curved wall, as occurs at the mobile tips of 90 domain needles. Any increase in density of immobile wall configurations with cycle time represents an inherent contribution to fatigue. Uniaxial ferroelectrics, with polarizations parallel to the field, should not experience such domain interlocking.     

Accelerated multi-contrast high isotropic resolution 3D intracranial vessel wall MRI using a tailored k-space undersampling and partially parallel reconstruction strategy

Objective

To develop a 3D multi-contrast IVW protocol with 0.5-mm isotropic resolution and a scan time of 5 min per sequence.

Materials and methods

Pre-contrast T1w VISTA, DANTE prepared PDw VISTA, SNAP, and post-contrast T1w VISTA were accelerated using cartesian undersampling with target ordering method (CUSTOM) and self-supporting tailored k-space estimation for parallel imaging reconstruction (STEP). CUSTOM + STEP IVW was compared to full-sample IVW, SENSE-accelerated IVW, and CUSTOM + zero-filled Fourier reconstruction in normal volunteers and subjects with intracranial atherosclerotic disease (ICAD). Image quality, vessel delineation, CSF suppression, and blood suppression were compared.

Results

CUSTOM + STEP vessel wall delineation was comparable to full-sample IVW and better than SENSE IVW for vessel wall delineation on T1w VISTA and luminal contrast on SNAP. Average image quality and wall depiction were significantly improved using STEP reconstruction compared with zero-filled Fourier reconstruction, with no significant difference in CSF or blood suppression.

Conclusions

CUSTOM + STEP allowed multi-contrast 3D 0.5-mm isotropic IVW within 30 min. Although some quantitative and qualitative scores for CUSTOM − STEP were lower than fully sampled IVW, CUSTOM + STEP provided comparable vessel wall delineation as full-sample IVW and was superior to SENSE. CUSTOM + STEP IVW was well tolerated by patients and showed good delineation of ICAD plaque.

     

3D IMAGING OF INVERTED DOMAIN STRUCTURES BY CONFOCAL SHG INTERFERENCE MICROSCOPE

ABSTRACT

A novel nonlinear optical interference microscope which enables us to take 3D images of ferroelectric domain structures is developed. This microscope is of a transmission type equipped with the confocal optical system, and exploits, as a probe, second harmonic waves generated in a sample by an incident laser beam. We apply the microscope to the observation of periodically poled domain structure in a LiTaO₃ quasi-phase matching device.     

Domain Wall Dynamics Near the Phase Transition in PZN-9%PT

To investigate the paraelectric–ferroelectric phase transition in relaxor Pb(Zn_1/3Nb_2/3)O₃-9% PbTiO₃, correlations and scaling relations were measured from the domain structure of single crystals near the Curie temperature using a multifractal analysis. The dynamics of the domain walls were observed in (100) platelets using polarized light microscopy. The complexity of the domain wall distribution was evaluated by means of its fractal dimension and singularity spectrum. The changes in the energy state of the samples are assessed by measuring changes of the spatial distribution of the walls as a function of temperature; this can be used to estimate the specific heat.     

Modification of PZT nucleation and growth using oxide layer in multi-layered electrodes

Abstract

The ferroelectric properties of PZT on RuO₂ electrodes were compared to those on RuO₂/Pt electrodes. The better hysteretic properties were obtained from Pt/RuO₂/PZT/RuO₂/Pt ferroelectric capacitors. The enhancement of ferroelectric properties is likely attributed to the modification in the microstructure of PZT film. The interfacial modification would be affected by the factors such as surface roughness, stress, and porosity of RuO₂ film. As the result of the interfacial modification, better quality PZT films are produced, thereby resulting in better ferroelectric properties. We made an effort to understand the relationship between the grain size and the coercive voltage in terms of the domain formation and the domain pinning in connection with defects like grain boundaries.     

Terahertz absorption characteristics of multilayer graphene/polymer

Abstract

In this article, we fabricated two different graphene structure samples, composed of single-layer graphene and multilayer graphene/PMMA structures. When w samples are pumped by the 980?nm laser with different powers, we measure the time-domain and frequency domain spectra of the terahertz transmission using the terahertz time domain spectroscopy system. The measured results show that the average terahertz transmission can be actively controlled by changing the pump power or the layer number of graphene, which exhibits potential applications in the terahertz absorption devices.     

Fast and Superfast Motion of Ferroelectric Domain Boundaries

We present the review of our systematic investigation of the propagation of domain walls/boundaries in wide velocity range in uniaxial ferroelectrics with optically distinguished domains, such as lead germanate Pb₅Ge₃O₁₁, congruent and stoichiometric lithium tantalate LiTaO₃ and lithium niobate LiNbO₃. In situ optical observation of domain kinetics was widely used. Three different regimes of boundary propagation have been revealed and investigated in details. It was shown that except of conventional “slow” sideways domain wall motion “fast” and “superfast” domain growth regimes through formation of complicated self-maintained domain boundaries could be obtained. The crucial role of the retardation of screening/compensation of depolarization field has been pointed out. It was demonstrated that realization of proper regime is determined by the ratio of switching rate to bulk screening one. Computer simulation has been applied for verification of proposed approach.     

PERFORMANCE ANALYSIS OF A D.C. DRIVE ELECTROMECHANICAL SYSTEM WITH ELASTIC COUPLING AND PULSATING LOAD TORQUE

ABSTRACT

This paper presents the analysis of a d.c. motor drive with a pulsating load torque and elastic mechanical link between the motor and the load. A mathematical model of the system using State Space technique is given and the equations are solved to obtain closed-form solutions for motor speed and current under transient and steady state conditions. The analysis reveals that the system performance is significantly affected by elasticity of the shaft and the nature of pulsation of load torque     

TEM observations of magnetic domains and grain boundaries on spin‐sprayed ferrite films exhibiting high permeability usable for gigahertz noise suppressors

Lorentz TEM observations of magnetic domain wall motion, as well as TEM observations of grain boundaries, were performed on spin‐sprayed ferrite films #1 (Ni_0.17Zn_0.22Fe_2.61O₄) and #2 (Ni_0.19Zn_0.20Co_0.03Fe_2.58O₄), both 0.5 µm in thickness. They exhibit much higher natural resonance frequencies than the bulk ferrite and thus have been applied to gigahertz noise suppressors. Films #1 and #2 exhibit prominent and weak in‐plane uniaxial magnetic anisotropy, respectively, which is induced along the liquid flow direction during spin‐spraying. Both films have columnar crystallites with 100‐200 nm widths aligned perpendicular to the film plane, and the boundaries of the crystallites have no pores or impurity phases. Therefore, the crystallites are magnetically exchange‐coupled, which is responsible for the unusually high permeability and high natural resonance frequencies of the films. Under zero bias magnetic field, film #1 exhibits mosaic‐shaped magnetic domains, whereas film #2 exhibits magnetic domains elongated along the easy magnetization axis, both several hundred nanometers in width. For both films the domain structure remains unchanged when an in‐plane bias DC magnetic field,H_dc, of up to 10 Oe is applied along the hard axis. Under a stronger H_dc, the domain structure prominently changes, and the domain walls disappear when H_dc exceeds ∼100 Oe. This confirms our previous finding that the initial permeability is ascribed only to magnetization rotation, with no contribution from domain wall motion [J. Magn. Magn. Mater., 278 , 256 (2004)]. © 2007 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

New explicit formulas for the effective piezoelectric coefficients of binary 0-3 composites

The explicit formula for the effective dielectric constant of binary 0-3 composites (Poon and Shin, J. Mat. Sc. 39 (2004) 1277–1281) is extended into two explicit formulas for the prediction of the elastic properties of macroscopically isotropic 0-3 composites. By combining them with the explicit effective dielectric formula into a calculation scheme (Wong et al., J. Appl. Phys. 90 (2001) 4690), we obtained two new explicit formulas for the prediction of the d ₃₁ and d ₃₃ values for binary 0-3 piezoelectric composites. These two explicit formulas are applicable even when the inclusion volume fraction is high. Comparing with existing experimental data, they are found to fit more favorably than those predicted by Wong et al. and others. Also, being explicit makes these formulas much easier to be embedded into other effective property calculations for binary 0-3 composite materials.     

LOCAL ELASTICITY RESPONSE IN FERROELECTRIC CRYSTALS STUDIED BY AFM-BASED NOVEL PROBE TECHNIQUES

ABSTRACT

A novel probe technique, low frequency scanning probe acoustic microscope, was firstly developed based on the commercial atomic force microscope. The excitation frequency is in the range of several kHz up to teen-kHz, which is much lower than that in the conventional acoustic microscopes. The acoustic images of nanoscale domain structures in Pb(Mg_1/3Nb_2/3)O₃-33%PbTiO₃ single crystals were successfully visualized. Different electric polarizations show the variation of the elasticity in ferroelectrics, so the contrast of the acoustic images reflects the domain structure or the subsurface elasticity. In addition, the elasticity of the sample in response to the modulation of the excitation source is also discussed.     

SIMULATION FOR SMALL DISTURBANCE STABILITY STUDIES USING FREQUENCY DOMAIN SEPARATION TECHNIQUE

ABSTRACT

This paper describes an application of 2-dimensional frequency domain separation technique for predicting the small disturbance stability for a hydro power system. The stability results were obtained in terms of the two unknown AVR gain control parameters. The effects of two kinds of hydro speed governor–turbine controls on the system's stability limits have been carried out in detail. The system's stability is found greatly affected by different governor–turbine models.