Polarization Switching Mechanism Research Articles

Improved polarization switching and piezoresponse in Nd and Mn co-doped BiFeO3 ceramics

Nano-to-micro polarization switching and electromechanical mechanisms in lead-free bismuth ferrite have shown opportunities in nonvolatile logic and optoelectronic applications. This report highlights improved ferroelectric polarization switching and electromechanical responses in (Nd, Mn) co-doped BiFeO3 ceramics via modulation of the B-site Mn substitution. The out-of-plane piezoresponse hysteresis loops reveals enhanced local polarization and electromechanical strain with increasing B-site Mn substitution. The B-site Mn substitution increases lattice distortion (or octahedral tilt) in rhombohedral R3c unit cell, confirmed by the enhanced ½(000)c superlattice electron diffraction. Average size of polar nano-regions (PNRs) and dielectric permittivity were also increased as Mn content increases. The synchrotron X-ray absorption spectra reveal a Mn3+ → Mn4+ valence shift and modulated orbital hybridizations between the O 2p and cation’s orbitals (Fe 3d and Bi 6 s(p)), which are critical factors for the improved polarization and dielectric responses.

Effects of Process-Induced Defects on Polarization Switching in Ferroelectric Tunneling Junction Memory

The scaling effects of ferroelectric tunnel junction (FTJ) memory are investigated by evaluating remanent polarization ( ${P} _{\mathrm {r}}$ ), coercive field ( ${E} _{\mathrm {c}}$ ), and polarization switching speed. Through the analysis on dc tunneling currents and frequency responses of ${E} _{\mathrm {c}}$ and ${P} _{\mathrm {r}}$ for FTJs with various sizes, it is found that polarization switching mechanism (domain nucleation limit) is not changed by the width reduction. However, by the length scaling, domain wall spreading-limited switching becomes dominant by process-induced damaged edge regions, leading to the larger ${P}_{\mathrm {r}}$ and the faster switching speed.

Quantitative Characterization of Ferroelectric/Dielectric Interface Traps by Pulse Measurements

The ferroelectric (FE) polarization switching behavior in the HfZrO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (HZO) FE/dielectric (FE/DE) stack is investigated systematically by charge responses from pulse measurements. The trapped charge density at the FE/DE interface related with the FE polarization switching is found to be 1.2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> according to the leakage-current-assist polarization switching mechanism. Furthermore, by the time-dependent nonswitching charge responses, the extra FE/DE interface trap density of 1.1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> is confirmed, which is not related but can be detected along with the FE polarization switching. The quantitative characterization reveals the huge amount of FE/DE interface traps and their dominant role in the FE operation of HZO FE/DE stack, which improves the proposed leakagecurrent-assist polarization switching model. This improved model provides a more comprehensive understanding of the polarization switching in the HZO FE/DE stack and new insights on HZO negative-capacitance (NC) and FE fieldeffect transistors (FETs).

Toward Decoding the Relationship between Domain Structure and Functionality in Ferroelectrics via Hidden Latent Variables

Polarization switching mechanisms in ferroelectric materials are fundamentally linked to local domain structure and the presence of the structural defects, which both can act as nucleation and pinning centers and create local electrostatic and mechanical depolarization fields affecting wall dynamics. However, the general correlative mechanisms between domain structure and polarization dynamics are only weakly explored, precluding insight into the associated physical mechanisms. Here, the correlation between local domain structures and switching behavior in ferroelectric materials is explored using convolutional encoder-decoder networks, enabling image to spectral (im2spec) and spectral to image (spec2im) translations via encoding of latent variables. The latter reflect the assumption that the relationship between domain structure and polarization switching is parsimonious, i.e., is based upon a small number of local mechanisms. The analysis of latent variables distributions and their real-space representations provides insight into the predictability of the local switching behavior and hence associated physical mechanisms. We further pose that the regions where these correlative relationships are violated, i.e., predictability of the polarization dynamics from domain structure is reduced, represent the obvious target for detailed studies, e.g., in the context of automated experiments. This approach provides a workflow to establish the presence of correlation between local spectral responses and local structure and can be universally applied to spectral imaging techniques such as piezoresponse force microscopy (PFM), scanning tunneling microscopy (STM) and spectroscopy, and electron energy loss spectroscopy (EELS) in scanning transmission electron microscopy (STEM).

A Wideband Non-Uniform Metasurface-Based Circularly Polarized Reconfigurable Antenna

This paper presents a low-profile circularly polarized (CP) antenna with wideband and polarization-reconfigurable characteristics. The proposed antenna consists of a truncated-corner patch working as a CP source and a non-uniform metasurface (MS) acting as parasitic elements for performance enhancement. The non-uniform MS is utilized to produce additional operating bands in the higher frequency range, which are then combined with the lower band produced by the driven patch to significantly broaden the overall operating bandwidth. The polarization switching mechanism is based on the ON/OFF states of two p-i-n diodes. The fabricated prototype shows the measured overlapped bandwidth between -10 dB impedance and 3-dB axial ratio is 25.6% and peak broadside gain is 7.3 dBic.

Polarization switching in thin doped HfO2 ferroelectric layers

The deployment of ferroelectrics in device concepts such as Ferroelectric Random Access Memory and Ferroelectric Field Effect Transistors requires a good understanding of the polarization switching mechanisms. While several reports already exist involving classical perovskite ferroelectrics, only recently has the switching dynamics in HfO2-based layers started to be addressed. In this work, the Kolmogorov–Avrami–Ishibashi (KAI), the Nucleation Limited Switching (NLS), the Landau–Khalatnikov (LK), and the Inhomogeneous Field Mechanism (IFM) models for polarization switching are surveyed and evaluated with the existing body of literature. Data concerning NLS and IFM are compared to experiments undertaken in this study. After excluding the KAI model because of considerations dealing with film morphology and domain wall energy, we conclude that the NLS, the LK, and the IFM models do not necessarily mutually exclude each other, but rather give a diverse perspective on the switching phenomenon based on thermodynamic, kinetic, statistic, microscopic, and/or macroscopic points of view.

Polarization Spinodal at Ferroelectric Morphotropic Phase Boundary.

Here, we report a new phenomenon of uniform and continuous transformation of a single polarization domain into alternating nanodomains of two polarization vectors with the same magnitude but different directions at ferroelectric morphotropic phase Boundary (MPB). The transformation is fully reversible and could enhance the piezoelectric coefficient d_{33}. Further free energy calculations illustrate that such a polarization "decomposition" process occurs within the region on the Landau free energy curve with respect to the polarization direction where the second derivative becomes negative, which is similar to spinodal instability in phase transformations such as spinodal ordering or isostructural phase separation (e.g., spinodal decomposition). This "polarization spinodal" uncovers a new mechanism of polarization switching that may account for the ultrahigh ahysterestic piezoelectric strain at the MPB. This work could shed light on the development of phase transition theory and the design of novel ferroelectric memory materials.

Evolution of domain structure and ferroelectric polarization in praseodymium doped BiFeO3 ceramics

Nanoscale domain structure, electric-field-induced polarization switching and electromagnetic strain in lead-free perovskite bismuth ferrite are substantial parameters for nonvolatile magnetoelectric applications. This work highlights nano-to-micro domain morphology, polarization switching and electromechanical mechanisms in (Bi1-xPrx)FeO3 ceramics in the vicinity of the morphotropic phase boundary (MPB). A coexistence of ferroelectric rhombohedral R3c and antiferroelectric orthorhombic Pbam symmetries was identified in conjunction with antiphase domain boundaries and polar nano-regions in the interiors of grains as the system crosses the MPB. Out-of-plane piezoresponse force microscopy indicates decreased ferroelectric polarization and capability of polarization switching as the system approaches the MPB. Frequency- and temperature-dependent dielectric permittivity indicates a relaxor characteristic in all compositions. The O 2p-Fe 3d and O 2p-Bi 6s(p) orbital hybridizations play key roles for evolution of structural distortion, polarization and electromechanical.

Split-off states in tunnel-coupled semiconductor heterostructures for ultrafast modulation of spin and optical polarization

We present a theoretical analysis of the split-off states emerging due to a tunnel coupling between a remote bound state and a semiconductor quantum well (QW). The on-site Coulomb repulsion and the spin splitting of the bound state have been considered. The split-off states emerge in the band gap of the QW and reveal themselves as two solitary peaks in the photoluminescence (PL) from the QW. The peaks have opposite circular polarization and their spectral position strongly depends on the tunnel coupling strength. We suggest a mechanism of ultrafast PL polarization switching by means of electrical modulation of the tunnel coupling by an external gate. The obtained results open a new possibility for the spin and optical polarization control in nanoscale systems.

Enhancing Speed and Stability of Polarization Reversal in Predominantly a/b‐Axes‐Oriented Bi4Ti3O12 Thin Films Deposited on Pt/Ti/SiO2/Si

Bi4Ti3O12‐based thin films of a/b‐axes orientation grown on standard‐type Pt/Ti/SiO2/Si substrates are desirable for designing reliable ferroelectric devices. Herein, a/b‐axes‐oriented and randomly oriented Bi4Ti3O12 thin films are deposited on Pt/Ti/SiO2/Si through a sol–gel process. As expected, the a/b‐axes‐oriented films show a higher remnant polarization than that of randomly oriented films. The characteristic time τ of domain reversal in 74% a/b‐axes‐oriented Bi4Ti3O12 films is 1.1 ms, notably shorter than that (τ &gt; 2.5 ms) for randomly oriented films. The dynamic hysteresis scaling relations of the randomly oriented 56% and 74% a/b‐axes‐oriented Bi4Ti3O12 films take the forms of ⟨A⟩ ∝ f−0.106E01.667, ⟨A⟩ ∝ f−0.035E00.952, and ⟨A⟩ ∝ f−0.026E00.600, respectively. The difference indicates that domain reversal in Bi4Ti3O12 films of predominant a/b‐axes orientation exhibits higher stability compared with randomly oriented Bi4Ti3O12 films under varying electric fields E(E0, f). The enhancing speed and stability of polarization reversal in predominantly a/b‐axes‐oriented Bi4Ti3O12 films can be understood in terms of their layered‐perovskite structure, large columnar grains, oxygen vacancies mobility, domain states, and polarization switching mechanism.

Enhancement of local piezoresponse in samarium and manganese co-doped bismuth ferrite ceramics

Field-induced polarization switching and electromechanical mechanisms in lead-free multiferroic BiFeO3 materials are essential for applications in nanoscale logic and magnetoelectric-controlled devices. This work highlights enhanced ferroelectric polarization switching and electromechanical strain behaviors in (Sm, Mn) co-doped (Bi0.95Sm0.05)(Fe1-xMnx)O3 ceramics with increasing Mn substitution. The Mn L-edge and oxygen K-edge synchrotron X-ray absorption spectra confirm a Mn3+→Mn4+ valence shift and reduced O 2p–Fe 3d and O 2p–Bi 6sp orbital hybridizations in the Mn-doped specimens. High-resolution transmission electron microscopy (TEM) confirms larger polar nano-regions (PNRs) and oxygen octahedral tilt in the Mn-doped specimens. Out-of-plane piezoresponse force microscopy (OP-PFM) shows improved local ferroelectric polarization and electromechanical response within grains by the Mn substitution. The enhanced ferroelectricity and piezoelectricity may originate from collective effects, including fewer oxygen vacancies, reduced covalent bonding, larger oxygen octahedral tilt and PNRs, caused by the Sm and Mn co-substitution.

Local piezoresponse in BiFeO3–HoFeO3 ceramics across morphotropic phase boundary

This study highlights structural evolution, nanoscale polarization switching and electromechanical mechanisms in lead-free perovskite (1-x)BiFeO3–xHoFeO3 ceramics in the vicinity of the morphotropic phase boundary (MPB). A phase crossover from ferroelectric rhombohedral R3c phase to nonpolar orthorhombic Pnma phase is identified as the system crosses the MPB. Local electric-field switched piezoresponse microscopies indicate a decreasing switchable-polarization percentage near the MPB. Ferroelectric-relaxor phase crossover takes place within the grains, accompanied by a V-shape hysteresis loop of electromechanical strain vs. bias voltage in the vicinity of the MPB. The oxygen K-edge synchrotron X-ray absorption suggests a decreased Bi 6s2 lone pair as the system approaches the MPB, which plays an important role in modification of local ferroelectric polarization. The enhanced ferromagnetic behavior may originate from collective effects, including structural distortion, exchange interactions, suppression of antiferromagnetic spin structure, and fewer oxygen vacancies, caused by the A-site Ho substitution.

One-Dimensional Polarization Dynamics in Ferroelectric Polymers.

Despite the realization of ferroelectricity in the δ-phase of poly(vinyleden difluoride) (PVDF) nearly four decades ago, the dynamics of polarization switching has not been studied yet. Here, we unravel the polarization switching mechanism as a one-dimensional process that is nucleated by a 90° rotation of a CH2-CF2 repeat unit, forming a kink with reversed dipole along the polymer chain. The kink subsequently propagates in time, yielding full polarization reversal along the chain while preserving TGTG' chain conformation. We show that the domain wall mobility in δ-phase PVDF is faster than both conventional ferroelectric β-phase PVDF and its copolymers with trifluoroethylene, P(VDF-TrFE). The switching time at infinite electric field for δ-phase PVDF is ten times faster and amounts to 500 ps. Fast switching dynamics combined with the low voltage operation and high thermal stability of polarization make δ-PVDF a suitable candidate for microelectronic applications.

Phase-field simulations of surface charge-induced polarization switching

Understanding and controlling surface charge-induced polarization switching have attracted interest of researchers extensively, and the rich physical interactions between ionic and atomic displacements play a significant role in polarization reversal. In this work, we investigated the mechanism of surface charge-induced polarization switching in PbZr0.2Ti0.8O3 and BiFeO3 thin films by phase-field simulations. We observed ferroelectric polarization reversal by applying positive/negative charge on the top of a thin film, because the double well of free energy becomes asymmetric by applying surface charge. It is found that the nucleation of switching always starts from the surface and gradually expands into the whole film. In BiFeO3 thin films, the formation of an anti-vortex domain provides topological protection for hindering surface charge-induced polarization switching. The present study, therefore, contributes to a better understanding of charge-induced polarization switching and provides guidance for the experimental design of reversible electronic devices by selecting the appropriate polarity region.

Phase coexistence near the polymorphic phase boundary

A novel multiphase field theory for ferroelectric systems in the vicinity of a polymorphic phase boundary (PPB) is developed by coupling the Landau-Devonshire thermodynamic potentials of the individual phases. The model naturally predicts metastable coexistence of the rhombohedral (R) and tetragonal (T) phases near the PPB temperature, TPPB=43C∘, for the BZT-40BCT system, and provides a maximum temperature of coexistence, TC,0=49.9C∘, in agreement with experiments. For T>TPPB, results show that metastable coexistence of two ferroelectric phases is a result of a phase transformation-induced polarization rotation plus switching mechanism. Metastable domains of the low-temperature R phase coexist with the high-temperature, thermodynamically stable T phase for long periods of time, from minutes to hours. For T<TPPB, the coexistence time is on the order of tens of seconds due to a decreased thermal energy that suppresses the polarization rotation plus switching mechanism. Further, the kinetics of macroscopic T→R phase transformation is accelerated by a large thermodynamic driving force and high mobility.

1/E2 Law: Solution of the Unsolved Problem in the Physics of Ferroelectrics

An unusual frequency dependence of a coercive field observed in polycrystalline ferroelectric films of Pb(Zr1 − xTix)O3 solid solutions in a wide frequency range has been explained within the model of surface sources emitting thin ferroelectric domains with reversed polarization. This model makes it possible to interpret experimental data as the manifestation of the coercivity paradox in polycrystalline films and predicts the existence of the limiting frequency for switching of domains in agreement with the experiment. The proposed mechanism of polarization switching explains the observed temperature dependence of the activation field Ea ∝ T−1/2. Furthermore, it predicts an increase in the activation energy for the nucleation of domains with an increase in the size of the source, indicating that the coercive field increases with the degradation of small

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.

Theoretical and experimental investigation of polarization coexistence and switching in an optical bistability Tm,Ho:LLF laser.

We first investigate the polarization coexistence and switching between the orthogonal π- and σ-polarization states in a free running optical bistability Tm,Ho:LLF laser. The output performances of the optical bistability Tm,Ho:LLF laser is numerically simulated, based on a new rate equation theory model taking into account the influences of polarization-dependent gain and losses. The simulation results show the polarization coexistence and switching of output laser in the process of decreasing pump power. The physical mechanism of polarization coexistence and switching is clarified by analyzing the polarization-dependent net-gain coefficients. Furthermore, the polarization coexistence and switching are experimentally realized in the optical bistability Tm,Ho:LLF laser, which validates the theoretical analysis. The theoretical model can be applied to analyze the polarization coexistence and switching in other kinds of (quasi-) three-level optical bistable lasers.

Polarization switching in single crystals and films of 2-methylbenzimidazole

The polarization switching was studied in single crystals and thin films of 2- methylbenzimidazole (MBI) obtained by evaporation method from an MBI ethanol solution. Dielectric hysteresis loops were measured in the temperature interval 290-390 K and frequency range 20-1000 Hz for different amplitudes of the electric field. The Kolmogorov β-model with account of Mertz law is used for simulation of hysteresis loops. The temperature dependence of domain wall motion activation field Ea has been obtained. Mechanisms of polarization switching in single crystals and films of MBI are discussed. In MBI films, a comparison of polarization switching for in-plane and out-of-plane electric field orientation is presented.

Low-Cost Ka-band Switchable RHCP/LHCP Antenna Array for Mobile SATCOM Terminal

Achieving a functional antenna for mobile satellite communications terminals in Ka-band is probably one of the most challenging tasks in current antenna engineering, particularly bearing in mind they need to be low profile and “affordable.” This quest is involving many companies in the field. Our contribution represents one of such efforts. The antenna is based on a slotted waveguide array technology to maximize efficiency and it features a number of novel solutions, going from its robust polarization switching mechanism, to the use of a thin wideband polarizer and the utilization of groove gap waveguides. This communication reports the measured data of a fully functional prototype to validate its novel contributions.