Effect of dimensions and anisotropy on the formation of domain structures of uniaxial single crystals

Distribution of magnetization in infinitely long single-crystal prisms of Co, Nd₂Fe₁₄B, and Ni₈₀Fe₂₀ with different cross sections has been calculated using the method of minimization of the free-energy functional and by solving the Landau-Lifshitz equation. It has been shown that in cobalt single crystals, domain structures of the Landau type arise. In Nd₂Fe₁₄B single crystals the domain structures have a stripe type; in Ni₈₀Fe₂₀ single crystals, the domain structure is in a single-domain state.     

Evolution of domain structure and frequency effect on ferroelectric properties in BIT ferroelectrics

Based on the time-dependent Ginzburg-Landau （TDGL） equation, the temporal evolution of the domain structure and hysteresis loops of polarization versus electric field were simulated by a phase-field model for Bi4Ti3O12 （BIT） ferroelectric single crystal under an applied electric field. In the static electric energy induced by an applied alternating electric field, the effects of field frequency on the ferroelectric properties of BIT ferroelectrics were investigated. The results show that the evolution of ferroelectric domain structure is a gradual process including domain nucleation, domain wall motion, domain growth and domain combination. In the boundary regions of ferroelectric domain, the new domain nucleations occur and the old domains disappear. The coercive field increases with the field frequency, and it is in good agreement with the previous experiment.     

Rearrangement of dislocation structures in the aging of ice single crystals

Aging of ice single crystals subjected to creep exhibits peculiar behavior. If the sample is unloaded after sufficient strain, a forward jump in creep rate is observed at reloading. Sequences of loading periods alternated with either increasing or decreasing unloading intervals were performed to document this phenomenon. During the tests, acoustic emission was recorded in order to characterize dislocation activity and spatial distribution. Predictions obtained from a field dislocation theory coupling the evolution of statistical and polar dislocation densities compare fairly well with experimental results. Polar dislocation density reflects lattice incompatibility and long-range internal stresses. The associated back-stress and its relaxation during aging are seen as the origin of the acceleration effect. The interplay between dislocation velocity enhancement and polar dislocation annihilation during aging controls the phenomenon, whereas statistical dislocations only play a minor role. The reverse relaxation deformation observed during unloading periods is reproduced well by the model.     

Ferroelastic domain wall dynamics in ferroelectric bilayers

High-performance piezoelectric devices based on ferroelectric materials rely heavily on ferroelastic domain wall switching. Here we present visual evidence for the local mechanisms that underpin domain wall dynamics in ferroelastic nanodomains. State-of-the-art band excitation switching spectroscopy piezoforce microscopy (PFM) reveals distinct origins for the reversible and irreversible components of ferroelastic domain motion. Extrapolating the PFM images to case for uniform fields, we posit that, while reversible switching is essentially a linear motion of the ferroelastic domains, irreversible switching takes place via domain wall twists. Critically, real-time images of in situ domain dynamics under an external bias reveal that the reversible component leads to reduced coercive voltages. Finally, we show that junctions representing three-domain architecture represent facile interfaces for ferroelastic domain switching, and are likely responsible for irreversible processes in the uniform fields. The results presented here thus provide (hitherto missing) fundamental insight into the correlations between the physical mechanisms that govern ferroelastic domain behavior and the observed functional response in domain-engineered thin film ferroelectric devices.     

Magnetocrystalline anisotropy,magnetization curves,and domain structure of FeB single crystals

Single crystals of an orthorhombic compound FeB have been synthesized. Based on the data of magnetic measurements and X-ray diffraction analysis, it has been shown that two mutually perpendicular preferred magnetization axes exist in the orthorhombic single crystals of FeB: the easy axis (EA) corresponds to the crystallographic axis [010], and the hard axis (HA), to the [100] axis. The function of the energy of magnetocrystalline anisotropy (MCA) of the orthorhombic magnets has been analyzed for extrema. Orientations of the EAs and HAs have been determined. It has been established that in the orthorhombic magnets there is always realized a single type of MCA: two preferred mutually perpendicular axes always exist in the crystals, one of which is an EA, and the other is an HA. Expressions for the anisotropy fields of orthorhombic magnets have been obtained. The constants of MCA of the orthorhombic compound FeB have been calculated to be K ₁^* ≈ 4 × 10⁵ J/m³ and K ₂^* 7×10⁴ J/m³. The domain structure on the planes (100), (010), and (001) of the FeB single crystals has been studied. It has been shown that the main volume of the sample is occupied by stripe domains ∼40 μm wide with the walls oriented along (100) crystallographic planes. It has been found that the structure of closure domains on the plane (010) of the FeB single crystals has some specific features, namely, there exists a preferred direction in the structure for the orientation of the boundaries of the main domains, and the closure domains have the shape of regular rhombs. A model of the domain structure of FeB single crystals is suggested.     

湿度对恒电场下BaTiO3单晶中畴变的影响

通过偏振光显微镜观察了在恒电场作用下湿空气对BaTiO3单晶畴变的影响.结果表明,在恒电场下,湿度对不同的畴结构有不同的影响.较低的湿度对a-b和b-c畴结构没有影响.而在较高的湿度下,随着放置时间的增加,b-c畴结构发生了变化,b畴逐渐向c畴转变;但是较高的湿度对a-b畴结构无影响.可用水分子在a(b)畴和c畴上的吸附差别解释上述现象.     

Role of grain orientation distribution in the ferroelectric and ferroelastic domain switching of ferroelectric polycrystals

The non-linear electromechanical behavior of ferroelectric polycrystals stems from polarization/domain switching, which are affected by the grain boundaries and grain orientations. The effects of grain orientation distribution on the domain switching and non-linear behavior of a two-dimensional ferroelectric polycrystal subjected to an electric or/and mechanical load are investigated by computer simulations with a real-space phase-field model. Phase-field simulations indicate that the macroscopic coercive field, remanent polarization and remanent strain in the polycrystal with a random distribution of grain orientation are correspondingly smaller than those in the polycrystal with a uniform distribution of grain orientation. However, the polycrystal with randomly distributed grain has a larger strain variation with the electric field than the polycrystal with uniformly distributed grains, which suggests that the random polycrystal has a better piezoelectric property than the uniform one. The different macroscopic non-linear behaviors of the ferroelectric polycrystals are attributed to different microscopic domain switching processes. For the polycrystal with randomly distributed grains, the domain switching takes place from the regions near the large angle grain boundaries, while new domains nucleate from the cross sections between the grain boundaries and the material surface in the polycrystal with uniform grain orientation.     

A distributed step-like switching model of the continuous field-driven phase transformations observed in PMN–xPT relaxor ferroelectric single crystals

The field-driven phase transformation behavior of relaxor ferroelectric single crystal PZN–xPT is discontinuous and displays well-defined forward and reverse coercive fields, whereas the same transformation in PMN–xPT is nearly continuous and occurs over a range of field levels. In analogy to the broad Curie range in relaxor ferroelectrics arising from property fluctuations at the nanometer length scale, the continuous field-driven phase transformations in PMN–xPT are modeled as a step-like series of discontinuous transformations associated with similar spatial property fluctuations. An increase in the applied field gradually increases the volume fraction of the new phase at the expense of the old phase, resulting in a continuous transition between phases. The model simulation produces excellent agreement with the measured material response of 〈0 1 1〉 cut PMN–0.32PT single crystals under conditions of cooperative stress and electric field loading.     

Phase transitions and domain structures of ferroelectric nanoparticles: Phase field model incorporating strong elastic and dielectric inhomogeneity

An efficient numerical algorithm based on a Fourier spectral iterative perturbation method is proposed to accurately compute the electrostatic fields in three-dimensional (3D) microstructures with arbitrary dielectric inhomogeneity and anisotropy. The method can be conveniently implemented in phase field modeling of microstructure evolution in systems with inhomogeneous dielectric constants as well as inhomogeneous polarization and charge distributions. It is employed to determine the temperature–shape (aspect ratio) phase diagram, domain structures, and domain switching of PbTiO₃ nanoparticles using phase field simulations. It is shown that the Curie temperature is enhanced for nanowires and nanorods and reduced for nanodots. The critical sizes below which the ferroelectricity disappears for the nanowire and thin film are estimated to be around 1.4 nm. Vortex domain structures are found in nanorods, nanodots, and nanodisks. Results are in general agreement with existing experimental observations and first principle calculations.     

Crystallographic pitting in magnesium single crystals

Abstract

The corrosion behaviour of three low index planes of an hcp magnesium (Mg) single crystal was investigated in 10^?2M NaCl/10^?4M dichromate solution. The Mg (0001) surface exhibited pitting corrosion susceptibility at the open circuit potential. However, the (1010), and (1120) surfaces were passive at open circuit and pitting only initiated at potentials slightly anodic to their open circuit potentials. The onset of stable dissolution in the three surfaces was marked by the formation and propagation of corrosion filaments. For the three surfaces investigated, pit shape was non-geometric, however, pit propagation was generally found to occur along preferential crystallographic directions. An analysis of the preferred crystallographic directions is presented.     

Direct observation of asymmetric domain wall motion in a ferroelectric capacitor

We report in situ transmission electron microscopy observations of the 180° polarization switching process of a PbZr_0.2Ti_0.8O₃ (PZT) capacitor. The preferential, but asymmetric, nucleation and forward growth of switched c-domains were observed at the PZT/electrode interfaces, arising due to the built-in electric field induced at each interface. The subsequent sideways growth of the switched domains was inhibited by the depolarization field due to the imperfect charge compensation at the counter-electrode and also at the boundaries with preexisting a-domains, which contributed further to the asymmetric switching behavior. It was found that the preexisting a-domains split into fine a- and c-domains constituting a 90° stripe domain pattern during the 180° polarization switching process, revealing that these domains also actively participated in the out-of-plane polarization switching. The real-time observations uncovered the origin of the switching asymmetry and further clarified the importance of charged domain walls and the interfaces with electrodes in the ferroelectric switching processes.     

Phase-field simulation of polarization switching and domain evolution in ferroelectric polycrystals

A phase-field model is developed for predicting the polarization switching and domain structure evolution under an applied electric field in ferroelectric polycrystals. The model takes into account realistic grain structures as well as various energetic contributions, including elastic energy, electrostatic energy, and domain wall energy. A hysteresis loop – average polarization as a function of applied electric field – is computed, and the detailed domain evolution process during switching is analyzed. In particular, the role of grain boundaries in the nucleation and growth of new domains is studied. It is shown that switching takes place through the nucleation of 90° domains at grain boundaries and subsequent growth into the grain interiors instead of direct 180° domain switching. A correlation between the domain structures in neighboring grains was observed, and polarization switching in one grain was found to affect the switching in neighboring grains.     

Dynamics of the domain structure and magnetic losses of Fe-3% Si single crystals in rotating magnetic fields

Dynamic domain-structure behavior during rotational magnetization reversal of a single-crystal Fe-3% Si disk with the [001] crystallographic axis deviated slightly (at an angle β = 1.5°) from the specimen plane have been investigated on the surface of the specimen. New regularities of the changes in the closure domain structure have been revealed in an induction interval of 0.25–1.7 T and their effect on the magnetic losses due to rotational magnetization reversal of the sample have been analyzed. The level of magnetic losses at low induction amplitudes has been estimated from the velocities of displacement of 180° domain walls of the stripe domain structure.     

Evolution of microstructure and thermomechanical properties during superelastic compression cycling in Cu–Al–Ni single crystals

The aim of this work is to relate the macroscopic evolution of the compression superelastic effect in Cu–Al–Ni shape memory alloy single crystals with the evolution of the microstructure during cycling. The analysis has been carried out as a function of the number of cycles, the maximum reached deformation and the kind of induced martensite. Moreover, the new microstructure after mechanical cycling and the evolution of the thermal transformation have been also studied. The presence of two new families of dislocations created by different mechanisms has been observed and the influence of each one on the stress-induced and thermal transformations has been analyzed. In the samples where both kinds of dislocations are present at the same time, the observed behaviour is a combination of their effects in proportion to their density.