Initial Polarization Direction Research Articles

Novel magnetic ordering in LiYbO2 probed by muon spin relaxation

The stretched diamond lattice material LiYbO2 has recently been reported to exhibit two magnetic transitions ($T_{N1} = 1.1 K$, $T_{N2} = 0.45 K$) via specific heat, magnetization, and neutron scattering measurements [Bordelon et al., Phys. Rev. B 103, 014420 (2021)]. Here we report complementary magnetic measurements down to T = 0.28 K via the local probe technique of muon spin relaxation. While we observe a rapid increase in the zero-field muon depolarization rate at $T_{N1}$, we do not observe any spontaneous muon precession for $T < T_{N1}$, which is typically associated with long-range magnetic ordering. The depolarization rate in the ordered state shows a surprising sensitivity to magnetic fields applied along the initial spin polarization direction. Using a simple one-dimensional model, we show that these results are consistent with the unusual random-phase bipartite incommensurate magnetic structure proposed by Bordelon et al. for the intermediate temperature range $T_{N2} < T < T_{N1}$. We also find evidence for temperature-independent magnetic fluctuations persisting to our lowest temperatures, but no obvious signature of the transition or spontaneous muon precession at and below TN2, respectively. This result is suggestive of quantum dynamics within a highly degenerate ground state.

Numerical investigation of nanoscale electromechanical response in a ferroelectric perovskite through an atomistic field theory

An atomistic field theory was introduced to study the electromechanical response in a ferroelectric perovskite at nanoscale. Based on a shell model potential, the polarization switching process of a ferroelectric perovskite PbTiO3 under electric field and mechanical loading was investigated. The simulation results showed that external electric field applied opposite to the initial polarization direction could result in 180° polarization switching in PbTiO3 nanostructures as the external electric field increased. On the other hand, mechanical loading applied parallel to initial polarization could cause 90° nanoscale polarization rotation in PbTiO3. Ultimately, the simulation results indicated that the coupled loads of electric field and mechanical loading would influence the polarization switching in PbTiO3 nanostructures.

Magnetoelectric memory effect of paramagnetic nonpolar phase in Co4Nb2O9

A polarization memory effect of the paramagnetic nonpolar phase is observed in magnetoelectric antiferromagnet Co4Nb2O9, which has a magnetic field induced polarization in the antiferromagnetic phase. Following magnetoelectric poling in the polar phase, the nonpolar paramagnetic state retains a strong memory of polarization. When reentering the polar phase without applying the electric field, a polarization along the initial poling direction is recovered. If the applied magnetic field while staying in the paramagnetic phase is weakened, the memory effect is enhanced. With reversing this magnetic field, the polarization is also reversed when reentering the polar phase. The memory effect can be attributed to the ferroelectric seeds forming in the nonpolar phase due to short-range magnetic ordering as evidenced by magnetic entropy data.

Polarization switching in the PbMg1/3Nb2/3O3 relaxor ferroelectric: An atomistic effective Hamiltonian study

An atomistic effective Hamiltonian technique is employed to investigate the polarization switching mechanism of the (metastable but long-living) ferroelectric state of ${\mathrm{PbMg}}_{1/3}{\mathrm{Nb}}_{2/3}{\mathrm{O}}_{3}$ relaxor ferroelectric, resulting from the application of a dc electric field along the $[\overline{1}\overline{1}\overline{1}]$ direction---that is, opposite to the initial polarization. It is predicted that such switching is of inhomogeneous type. It involves the creation of intermediate short-range ordered, relaxor-like phases in between long-range ordered states inside which an infinite cluster exists and has dipoles near either the initial polarization direction (for shorter times) or the field's direction (for longer times). In contrast, dipoles belonging to finite clusters or being isolated can deviate away from [111] and $[\overline{1}\overline{1}\overline{1}]$, and, in fact, rotate in average from [111] to $[\overline{1}\overline{1}\overline{1}]$ when time increases. Such rotations govern the reversal of the polarization from [111] to $[\overline{1}\overline{1}\overline{1}]$ occurring within the intermediate relaxor-like states' region, while always resulting in the overall cancellation of any Cartesian component of the polarization that is perpendicular to [111]. These rotations occurring at the atomic scale also naturally imply that some fundamental assumptions of the original nucleation-limited-switching (NLS) model are not valid, despite the fact that we numerically further find that the whole temporal behavior of the macroscopic polarization can be well fitted by the general formula associated with NLS. In other words, we have stumbled into a novel switching or at least a switching that should be denoted as a generalized NLS model. Finally, three different electric-field regimes are predicted, with each of them having its own dependency of the switching time on the magnitude of the applied electric field and only one of them obeying the Merz's law. The existence of these three regimes is explained in terms of specific microscopic features.

Spontaneous ferroelectricity in strained low-temperature monoclinic Fe3O4: A first-principles study

As a single-phase multiferroic material, Fe3O4 exhibits spontaneous ferroelectric polarization below 38 K. However, the nature of the ferroelectricity in Fe3O4 and effect of external disturbances such as strain on it remains ambiguous. Here, the spontaneous ferroelectric polarization of low-temperature monoclinic Fe3O4 was investigated by first-principles calculations. The pseudo-centrosymmetric FeB42–FeB43 pair has a different valence state. The noncentrosymmetric charge distribution results in ferroelectric polarization. The initial ferroelectric polarization direction is in the -x and -z directions. The ferroelectricity along the y axis is limited owing to the symmetry of the Cc space group. Both the ionic displacement and charge separation at the FeB42–FeB43 pair are affected by strain, which further influences the spontaneous ferroelectric polarization of monoclinic Fe3O4. The ferroelectric polarization along the z axis exhibits an increase of 45.3% as the strain changes from 6% to -6%.

Minicharged particles search by strong laser pulse-induced vacuum polarization effects

Laser-based searches of the yet unobserved vacuum birefringence might be sensitive for very light hypothetical particles carrying a tiny fraction of the electron charge. We show that, with the help of contemporary techniques, polarimetric investigations driven by an optical laser pulse of moderate intensity might allow for excluding regions of the parameter space of these particle candidates which have not been discarded so far by laboratory measurement data. Particular attention is paid to the role of a Gaussian wave profile. It is argued that, at energy regimes in which the vacuum becomes dichroic due to these minicharges, the transmission probability of a probe beam through an analyzer set crossed to the initial polarization direction will depend on both the induced ellipticity as well as the rotation of the initial polarization plane. The weak and strong field regimes, relative to the attributes of these minicharged particles, and the relevance of the polarization of the strong field are investigated.

Polarization reversal by tip of scanning probe microscope in SBN

&lt;p&gt;We present the results of experimental study of the influence of initial domain state on the shape and size of isolated domains created by the conductive tip of scanning probe microscope during local polarization reversal in relaxor ferroelectric strontium barium niobate doped with nickel and cerium. The domain radius was found to increase with increasing voltage and time and depend on the initial polarization direction. Circular domains of the opposite sign were found to appear due to polarization backswitching. The obtained results can be used for practical applications of domain and domain wall engineering in ferroelectrics.&lt;/p&gt;

Compact spherical neutron polarimeter using high-T(c) YBCO films.

We describe a simple, compact device for spherical neutron polarimetry measurements at small neutron scattering angles. The device consists of a sample chamber with very low (<0.01 G) magnetic field flanked by regions within which the neutron polarization can be manipulated in a controlled manner. This allows any selected initial and final polarization direction of the neutrons to be obtained. We have constructed a prototype device using high-T(c) superconducting films and mu-metal to isolate regions with different magnetic fields and tested device performance in transmission geometry. Finite-element methods were used to simulate the device's field profile and these have been verified by experiment using a small solenoid as a test sample. Measurements are reported using both monochromatic and polychromatic neutron sources. The results show that the device is capable of extracting sample information and distinguishing small angular variations of the sample magnetic field. As a more realistic test, we present results on the characterization of a 10 μm thick Permalloy film in zero magnetic field, as well as its response to an external magnetic field.

The evolution of polarization inside ultrathin PbTiO3 films: a theoretical study

How to control the material properties by manipulating the unitcell thickness is crucial for applications of ferroelectric ultrathin films. To understand the polarization behaviour of ultrathin PbTiO3 (PTO) films grown on SrTiO3 (STO) substrate, we have systematically explored the strength and direction of polarization in each unitcell layer, using density functional theory combined with Born effective charge method. Strikingly, we find that the polar state of ultrathin PTO films is a composite result depending not only on thickness but also on boundary condition, initial polarization direction, etc. Besides, we also studied the surface effect on the polarization in the thicker PTO films for comparison with the ultrathin ones, which suggests that the surface effect is basically confined in a small range (3–5 unitcells thick at surface region) no matter what kinds of surface terminations and polarization directions.

Two-step polarization reversal in biased ferroelectrics

Polarization reversal in polycrystalline ferroelectrics is shown to occur via two distinct and sequential domain reorientation steps. This reorientation sequence, which cannot be readily discriminated in the overall sample polarization, is made apparent using time-resolved high-energy x-ray diffraction. Upon application of electric fields opposite to the initial poling direction, two unique and significantly different time constants are observed. The first (faster time constant) is shown to be derived by the release of a residual stress due to initial electrical biasing and the second (slower time constant) due to the redevelopment of residual stress during further domain wall motion. A modified domain reorientation model is given that accurately describes the domain volume fraction evolution during the reversal process.

GAMMA-RAY POLARIZATION INDUCED BY COLD ELECTRONS VIA COMPTON PROCESSES

The polarization measurement is an important tool to probe the prompt emission mechanism in gamma-ray bursts (GRBs). The synchrotron photons can be scattered by cold electrons in the outflow via Compton scattering processes. The observed polarization depends on both the photon energy and the viewing angle. With the typical bulk Lorentz factor $\Gamma \sim 200$, photons with energy $E>10$ MeV tend to have smaller polarization than photons with energy $E<1$ MeV. At the right viewing angle, i.e. $\theta \sim \Gamma^{-1}$, the polarization achieves its maximal value, and the polarization angle changes $90^{\circ}$ relative to the initial polarization direction. Thus, the synchrotron radiation plus Compton scattering model can naturally explain the $90^{\circ}$ change of the polarization angle in GRB 100826A.

3D Larmor clock and barrier interaction times for tunneling

We study a three-dimensional spin dynamics of tunneling particles with spin 1/2 through a barrier in the presence of a static and a small time-dependent magnetic fields, perpendicular to each other. The spin dynamics is adopted as a 3D Larmor clock to derive the traversal and reflection times for tunneling (Büttiker, 1983) [14]. We show that the two times can be determined by a real three-dimensional rotation of spin, without requiring the effective rotation due to probability imbalance, and are directly related to the change of spin along its initial polarization direction. We also discuss the experimental feasibility of the present model.

Nonlinear dielectric properties and polarization in thin ferroelectric P(VDF-TrFE) copolymer films

Thin VDF-TrFE copolymer films of molar composition 70/30 and thickness from 25 nm to 175 nm have been prepared by spin coating on a glass substrate covered with an aluminum electrode. The linear, second and third order permittivities ϵ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> , ϵ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and ϵ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> of poled and unpoled samples have been studied versus temperature in heating and cooling cycles up to 120 °C, i.e., above the Curie temperature. The polarization and its temperature dependence are derived from ϵ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> and ϵ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> . This allows a non-destructive readout of the polarization state. It is found that above the Curie temperature and after cooling to room temperature a non-switchable polarization remains. This nonswitchable polarization points from the bottom to the top electrode and is not influenced by the initial polarization direction.

Numerical analysis of the effect of poling‐loading misalignment and mechanical boundary conditions on piezoelectric actuation

PurposeThe paper aims to highlight the computational implementation of a nonlinear piezoelectric constitutive model and its application in determining the impact of misalignment between initial poling direction and applied electrical field, and mechanical boundary conditions on actuator performance.Design/methodology/approachThe numerical analysis is based on an existing three‐dimensional model, where the original rate‐independent evolution equations are replaced by their rate‐dependent counterparts to facilitate implementation, which is performed in a partial differential equation solver. The execution of the model is verified through several benchmark constitutive responses.FindingsThe analysis shows that small angles of poling and loading axes misalignment such as may occur in fabrication (less than 5○) have minor impact on piezoelectric performance regardless of the type of imposed mechanical boundary conditions. On the other hand, larger angles of misalignment can have a significant impact, the feasibility of which in actuator design remains to be seen. Furthermore, it is shown that the linear response range of these actuators can be expanded by increased levels of mechanical constraint at the cost of maximum actuation stroke regardless of the degree of misalignment.Originality/valueThe misalignment, which occurs accidentally, but can also be introduced purposefully during the fabrication process when poled material is cut into specimen form, may exhibit desirable performance features for actuator design when combined with appropriate mechanical constraints.

Investigation of orientation effects on the electro-mechanical coupling behavior of 1–3piezoelectric composites under compression

The electro-mechanical responses of 1–3 piezoelectric composites with PZT5A1fibers embedded in an epoxy matrix are investigated under compressive loading.When the applied loading direction is not parallel to the polarization and fiberdirections of the PZT fibers, the corresponding electrical and mechanical responsescan be dramatically affected. Here both experimental testing and theoreticalmodeling are carried out to study such orientation effects on 1–3 piezoelectriccomposites. The testing results indicate that, as the loading direction changes from0°,to 30°, 45°,60° andfinally 90° to the initial poling direction (which is also the PZT fiber direction),the magnitude of the electric displacement and mechanical strain of thesystem all decrease and the electrical response will diminish to zero at90°. On the other hand, a two-level micromechanics theory based on irreversiblethermodynamics and the physics of domain switching is applied to predict suchorientation dependency of the electro-mechanical coupling behavior of the compositesystem. The first level is based on PZT fibers only which consist of parent andswitched domains due to external loading, and the second level is based on a largerscale consisting of PZT fibers and an electrically inactive polymer matrix. Thetheoretical results are found to be in reasonable agreement with our experimental data.

Generation and Modulation of Phase Difference of Output Intensities in a Feedback Nd:YAG Laser with an Extracavity Waveplate Rotated

External anisotropic feedback effects on the phase difference behaviour of output intensities in a microchip Nd:YAG laser are presented. By rotating a quarter wave plate placed in the external cavity, the angle between laser initial polarization direction and o-axis of the wave plate is tuned from -45° to 45°, which results in variable extra-cavity birefringence along two orthogonal detection directions. With only one optical path and one wave plate, laser intensities of the two orthogonal directions, both modulated by the external cavity length, are output with a tunable phase difference, which can be continuously changed from zero to twice as large as that of the waveplate. Experimental results as well as a theoretical analysis based on Fabry–Perot cavity equivalent model and the refractive index ellipsoid, are presented. The potential applications of this phenomenon are also discussed.

Orientation dependence on electromechanical coupling behavior of ferroelectrics under compression

When a compression is applied to a fully poled ferroelectric polycrystal at an angle θ to the original poling direction, its nonlinear electromechanical response is dependent on such an angle. Here we report our investigation on such an orientation dependence on the electromechanical behavior of lead zirconium titanate (PZT) under compression. The measured stress-electric displacement and stress-strain curves exhibit strong angle dependence and nonlinear behavior under compression. The experimental results indicate that as the loading direction changes from 0°, to 30°, 45°, 60°, and finally 90° to the initial poling direction, the magnitude of the electric displacement and mechanical strain all decrease. Then a micromechanics theory based on irreversible thermodynamics and physics of domain switch was applied to study this orientation effect on the electromechanical coupling behavior under compression. The comparisons between our experimental data and theoretical prediction are found to be in reasonable agreement.

Anisotropy of domain switching in prepoled lead titanate zirconate ceramics under multiaxial electrical loading

The authors report an observation of anisotropic domain switching process in prepoled lead titanate zirconate (PZT) ceramics under multiaxial electrical loading. Prepoled PZT blocks were obliquely cut to apply an electric field at discrete angles θ (0°–180°) to the initial poling direction. Both the coercive field and switchable polarization are found to decrease significantly when sinθ increases from zero to unity. The measured strain curves show that most domains that accomplished 180° domain switching actually experienced two successive 90° switching. The oriented domain texture after poling plus the induced nonuniform stress are used to explain the observed domain switching anisotropy.

Effects of in-plane electric fields on the toughening behavior of ferroelectric ceramics

Mode-I steady state crack growth in poled ferroelectric ceramics subjected to simultaneous electrical and mechanical loading is analyzed to investigate the effect of in-plane electric fields and polarization on the toughening behavior. A multiaxial, incremental constitutive law for domain switching is implemented within the finite element method to obtain the electromechanical crack tip fields. Simulation results are presented for the cases of initial remanent polarization states and applied electric fields perpendicular to the crack plane and parallel to the crack growth direction. Specific results from the calculations include the shapes and sizes of switching zones, and the toughening effects due to domain switching near the crack tip. Ferroelectric ceramics have been widely used in smart structure applications due to their large electromechanical coupling effects. Since ferroelectricdevices oftenoperate understrong mechanicaland electrical loading conditions, the brittle ferroelectric ceramic material is susceptible to fracture. Therefore, an understanding of ferroelectric fracture is a key issue for the efficient and reliable design of these devices. This paper is concerned with the study of the Mode-I fracture behavior of ferroelectric ceramics under combined in-plane electrical and mechanical loading. The scenario investigated here is illustrated in Figure 1. A ferroelectric material is initially poled by an electric field either perpendicular or parallel to the direction of crack growth. After the initial poling, an electric field is applied along (positive electric field) or opposite (negative electric field) to the initial poling direction. Finally, mechanical loading is applied and crack growth occurs. Experimental observations of the fracture properties of ferroelectrics under such conditions have been obtained on several materials from indentation tests and compact tension specimens. Using indentation tests on a PZT-8 material composition, Tobin and Pak [1993] showed that for cracks perpendicular to the poling direction, the apparent fracture toughness decreases with positive electric field and increases with a negative field. For cracks parallel to the poling direction, their results indicated that both positive and negative electric fields have little influence on the toughening. Tobin and Pak [1993] also observed that with no applied electric field the fracture toughness was greater for cracks parallel to the poling direction than for cracks perpendicular to the poling direction. Park and Sun [1995] investigated electric field effects on crack growth in a PZT-4 ceramic by using conventional compact tension fracture tests. Their results agree with those of Tobin and Pak [1993] for the case of electric field applied perpendicular to the cracksurface. However, theVickersindentationtestsofWangandSingh[1997]showedthatiftheapplied electricfields are perpendicular to the crack in a PZT EC-65 ceramic, then a positive electricfield impedes

Size dependence of initial polarization direction in nanosized epitaxial PbTiO3 islands fabricated by hydrothermal epitaxy below Curie temperature

Nanosized epitaxial PbTiO3 (PTO) islands with a mono c domain were fabricated via self-assembly on a Nb-SrTiO3 (NSTO) substrate at 160 °C by hydrothermal epitaxy without undergoing the paraelectric to ferroelectric phase transition. The direction of the polarization at the as-fabricated state was observed with an island size up to a lateral area of 104nm2 and thickness of 12 nm (determined by atomic force microscope) using a piezoresponse force microscope. We found a critical size below which the negative polarization is stable at the as-fabricated state and above which the positive polarization is stable. This phenomenon was discussed under two assumptions: that the PTO islands are grown initially with positive polarization during fabrication, and the energy barrier for the transition from the positive polarization state to the negative polarization state is dependent on the ferroelectric island size.