Field-induced Polarization Switching Research Articles

Direct observation of intrinsic room-temperature ferroelectricity in 2D layered CuCrP2S6

Multiferroic materials have ignited enormous interest owing to their co-existence of ferroelectricity and ferromagnetism, which hold substantial promise for advanced device applications. However, the size effect, dangling bonds, and interface effect in traditional multiferroics severely hinder their potential in nanoscale device applications. Recent theoretical and experimental studies have evidenced the possibility of realizing two-dimensional (2D) multiferroicity in van der Waals (vdW) layered CuCrP2S6. However, the incorporation of magnetic Cr ions in the ferroelectric framework leads to antiferroelectric and antiferromagnetic orderings, while macroscopic spontaneous polarization is always absent. Herein, we report the direct observation of robust out-of-plane ferroelectricity in 2D vdW CuCrP2S6 at room temperature with a comprehensive investigation. Modification of the ferroelectric polarization states in 2D CuCrP2S6 nanoflakes is experimentally demonstrated. Moreover, external electric field-induced polarization switching and hysteresis loops are obtained in CuCrP2S6 down to ~2.6 nm (4 layers). By using atomically resolved scanning transmission electron microscopy, we unveil the origin of the emerged room-temperature ferroelectricity in 2D CuCrP2S6. Our work can facilitate the development of multifunctional nanodevices and provide important insights into the nature of ferroelectric ordering of this 2D vdW material.

Role of polarization switching and domain patterns in the enhanced piezoelectric characteristics of a Pb-free ferroelectric system

The Pb-free piezoelectric BaTi1−x Sn x O3 system is a promising environmentally friendly ceramic due to its superior piezoresponse properties. The enhanced piezoresponse of this system has a strong correlation with its microscopic and macroscopic characteristics. In this work, the effects of structural and microstructural evolution on the piezoelectric properties of a BaTi0.97Sn0.03O3 (BST) compound are explored. Structural and piezoelectric analysis elucidates that the samples experiencing a field induced tetragonal to orthorhombic phase transformation exhibit better piezoresponse. In addition, the domain configurations have a strong correlation with the synthesis conditions, where the distribution of lamellar and/or stripe types of domain patterns show enhancement in comparison to those of labyrinth type with increase in sintering temperature. Rayleigh analysis confirms that the increase in the irreversible nature of non-180° domain walls can improve the d 33 values of the BST samples. Overall, the results illustrate that the stripe domain configurations and field induced polarization switching from tetragonal to orthorhombic phase favour superior piezoresponse in the samples.

Ferroelectric polarization reversal in multiferroic MnWO4 via a rotating magnetic field up to 52 T

A long-standing issue in multiferroic ${\mathrm{MnWO}}_{4}$ is the electric polarization reversal when magnetic field is applied along the magnetic easy axis $(H||u)$. In this work, we performed comprehensive ferroelectric polarization measurements on ${\mathrm{MnWO}}_{4}$ in fields up to 52 T and with rotating field direction in the $ac$ plane. When the field is rotated from $H||c$ to $H||u$, the induced high-field phase $(\ensuremath{-}P||b)$ favors a sign reversal relative to the low-field $+P||b$ phase, which is attributed to the vector spin chirality changing from one side of the spin connecting vector to the other. Particularly, further slight deviation of the field from the magnetic easy axis towards $H||a$ causes an abrupt reversal of the high-field phase from $\ensuremath{-}P||b$ to $+P||b$. We assume that this unusual angular dependence of polarization behavior can be understood by magnetic control of the helicity of the chiral plane with opposite conical spin structures. In addition, we also propose electric-field control of the chiral plane close to the ferroelectric phase transition at high fields. This study results in an overall understanding of this important multiferroic material and sheds light on field-induced polarization switching in related compounds.

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.

Superdomain structure and high conductivity at the vertices in the (111)-oriented epitaxial tetragonal Pb(Zr,Ti)O3 thin film

Recently, in ferroelectric materials, there have been many experimental efforts to find out more intriguing topological objects and their functionalities, such as conduction property. Here we investigated ferroelectric domain structures and related topological defects in the (111)-oriented epitaxial tetragonal PbZr0.35Ti0.65O3 thin film. Systematic piezoresponse force microscopy measurements revealed that the field-induced polarization switching can form thermodynamically stable superdomain structures composed of nano-sized stripe subdomains. Within such superdomain structures, we observed the exotic equilateral triangular in-plane flux-closure domains composed of three stripe domain bundles with 120/120/120 degrees of separation. The conductive-atomic force microscopy measurements under vacuum showed that some vertices have significantly higher conductivity compared to other surrounding regions. This work highlights electric field-driven polarization switching and unique crystallographic symmetry (here, three-fold rotational symmetry) can generate exotic ferroelectric domain structures and functional topological defects, such as conductive vertices.

Hydrogen-Bonded Architectures and Field-Induced Polarization Switching in Bridged Bis(benzimidazole) Crystals

The imidazole ring is chemically stable and forms an excellent building block for compounds that show externally switchable polarization in their crystalline states. We examined the structural prop...

Simultaneous dynamic characterization of charge and structural motion during ferroelectric switching

Monitoring structural changes in ferroelectric thin films during electric field-induced polarization switching is important for a full microscopic understanding of the coupled motion of charges, atoms and domain walls. We combine standard ferroelectric test-cycles with time-resolved x-ray diffraction to investigate the response of a nanoscale ferroelectric oxide capacitor upon charging, discharging and switching. Piezoelectric strain develops during the electronic RC time constant and additionally during structural domain-wall creep. The complex atomic motion during ferroelectric polarization reversal starts with a negative piezoelectric response to the charge flow triggered by voltage pulses. Incomplete screening limits the compressive strain. The piezoelectric modulation of the unit cell tweaks the energy barrier between the two polarization states. Domain wall motion is evidenced by a broadening of the in-plane components of Bragg reflections. Such simultaneous measurements on a working device elucidate and visualize the complex interplay of charge flow and structural motion and challenges theoretical modelling.

A Fluid Liquid-Crystal Material with Highly Polar Order.

An anomalously large dielectric permittivity of ≈104 is found in the mesophase temperature range (MP phase) wherein high fluidity is observed for a liquid-crystal compound having a 1,3-dioxane unit in the mesogenic core (DIO). In this temperature range, no sharp X-ray diffraction peak is observed at both small and wide Bragg angles, similar to that for a nematic phase; however, an inhomogeneous sandy texture or broken Schlieren one is observed via polarizing optical microscopy, unlike that for a conventional nematic phase. DIO exhibits polarization switching with a large polarization value, i.e., P = 4.4 µC cm-2 , and a parallelogram-shaped polarization-electric field hysteresis loop in the MP phase. The inhomogeneously aligned DIO in the absence of an electric field adopts a uniform orientation along an applied electric field when field-induced polarization switching occurs. Furthermore, sufficiently larger second-harmonic generation is observed for DIO in the MP phase. Second-harmonic-generation interferometry clearly shows that the sense of polarization is inverted when the +/- sign of the applied electric field in MP is reversed. These results suggest that a unidirectional, ferroelectric-like parallel polar arrangement of the molecules is generated along the director in the MP phase.

Critical slowing down of polar nano regions ensemble in Gd3+-substituted PbMg1/3Nb2/3O3 ceramics

Investigations on Gd-substituted lead magnesium niobate (Pb1-xGdxMg(1+x)/3Nb(2-x)/3O3; varying x = 0.01–0.1) ceramics have revealed critical slowing down of the polar nano regions (PNRs) ensemble into a “super-dipolar glass state” for higher Gd-substitution x ≥ 0.05. Low temperature electric field induced polarization switching study (P-E) has revealed a sharp decrease in the remanent polarization up to x = 0.03, which strengthen the critical slowing down of polar nano-domains dynamics, suggesting a reduction in the correlation between or within polar nano regions (PNRs) leading to a reduction in its size. Bright field imaging by using transmission electron microscope has also confirmed the reduction of the size of polar nano regions with increasing “x.” Selected area electron diffraction pattern along ⟨110⟩ unit axis has revealed enhancement in intensity of the superlattice reflections spot at ½ ½ ½ along ⟨111⟩ unit axis with increasing “x,” which is associated with the enhancement of chemical ordered regions and correlate well to enhancement in the degree of diffuseness parameters “δA” determined from fitting of the temperature dependent dielectric constant ε(T) plot above the dielectric maximum peak (εmax). The enhanced “δA” for x ≥ 0.05 is due to additional disorder created by the Gd-ions substitution at the Mg-site, which is consistent with the phase and microstructural analysis. Fitting of frequency dependent Tm (temperature of εmax) to the power law of critical dynamic has revealed realistic pre-factor fitting parameters for x ≥ 0.05 suggesting critical slowing down of the polar nano-domains dynamics ensemble resulting in super-dipolar glass state.

Modulated spin structure responsible for the magnetic-field-induced polarization switching in multiferroicTbMn2O5

Orthorhombic ${\mathrm{TbMn}}_{2}{\mathrm{O}}_{5}$ ($o\text{-TMO}$) is a well-known multiferroic manganite with the remarkable property of polarization switching at 3 K under a bias magnetic (H) field along the $a$ axis of $Pb{2}_{1}m$. To theoretically account for this outstanding observation, we have proposed a modulated spin structure under the saturated bias $H$ field by considering the relative strength of the three relevant exchange parameters in $o\text{-TMO}$. The proposed modulated structure based on density-functional theory (DFT) calculations is described in terms of the spin angle $\ensuremath{\phi}$ between the neighboring $\mathrm{M}{\mathrm{n}}^{4+}\text{-M}{\mathrm{n}}^{3+}$ spin moments on the a-b plane. We have shown that the computed DFT polarization plotted as a function of $\ensuremath{\phi}$ satisfactorily accounts for the observed $H$-field-induced polarization switching. We have further theoretically shown that the square of the critical field strength (${H}_{c}$) needed for the polarization switching is inversely proportional to the degree of the extrinsic magnetoelectric coupling. The computed partial charge density demonstrates that the $H$-field-induced polarization switching also accompanies with the switching in the sign of the excess valence-electron density.

Ionic field effect and memristive phenomena in single-point ferroelectric domain switching

Electric field-induced polarization switching underpins most functional applications of ferroelectric materials in information technology, materials science and optoelectronics. Recently, much attention has been focused on the switching of individual domains using scanning probe microscopy. The classical picture of tip-induced switching, including formation of cylindrical domains with size, is largely determined by the field distribution and domain wall motion kinetics. The polarization screening is recognized as a necessary precondition to the stability of ferroelectric phase; however, screening processes are generally considered to be uniformly efficient and not leading to changes in switching behaviour. Here we demonstrate that single-point tip-induced polarization switching can give rise to a surprisingly broad range of domain morphologies, including radial and angular instabilities. These behaviours are traced to the surface screening charge dynamics, which in some cases can even give rise to anomalous switching against the electric field (ionic field effect).

Longitudinal and Transversal Polarization Switching in Strained Ferroelectrics

Using the multicomponent Landau-Devonshire functional we study the field-induced polarization switching in c-oriented ferroelectric phase of epitaxial perovskite film and found that two different regimes are possible: (i) longitudinal switching with change of the polarization modulus and (ii) transversal switching with virtual admixing of the a-component of polarization. The results are specified for the case of barium titanate oxide BaTiO3where the crossover line between two types of switching is found at strain-temperature phase diagram.

Defect‐Mediated Polarization Switching in Ferroelectrics and Related Materials: From Mesoscopic Mechanisms to Atomistic Control

The plethora of lattice and electronic behaviors in ferroelectric and multiferroic materials and heterostructures opens vistas into novel physical phenomena including magnetoelectric coupling and ferroelectric tunneling. The development of new classes of electronic, energy-storage, and information-technology devices depends critically on understanding and controlling field-induced polarization switching. Polarization reversal is controlled by defects that determine activation energy, critical switching bias, and the selection between thermodynamically equivalent polarization states in multiaxial ferroelectrics. Understanding and controlling defect functionality in ferroelectric materials is as critical to the future of oxide electronics and solid-state electrochemistry as defects in semiconductors are for semiconductor electronics. Here, recent advances in understanding the defect-mediated switching mechanisms, enabled by recent advances in electron and scanning probe microscopy, are discussed. The synergy between local probes and structural methods offers a pathway to decipher deterministic polarization switching mechanisms on the level of a single atomically defined defect.

Peculiar effect of mechanical stress on polarization stability in micrometer-scale ferroelectric capacitors

Piezoresponse force microscopy (PFM) has been used to study the polarization stability in micrometer size Pb(Zr,Ti)O3 capacitors. It is shown that the top electrode thickness has a profound effect on the equilibrium polarization state of poled capacitors triggering spontaneous polarization backswitching in the absence of an applied electric field and leading to the formation of an abnormal domain pattern. PFM examination of poled capacitors with thick (250 nm) top electrodes reveals domain patterns with the central regions always oriented in the direction opposite to the applied field. It is suggested that the driving force behind the observed effect is a transient response to the residual shear stress created by the top electrode in the poled capacitors during field-induced polarization switching. The proposed mechanism is quantified using finite element ferroelectric phase field modeling. The observed effect provides valuable insight into the polarization retention behavior in micrometer size ferroelectric capacitors.

Domain Engineered Relaxor Ferroelectric Single Crystals

Single crystal relaxor ferroelectric materials exhibit extraordinary electromechanical properties. They are being applied in high performance sensors, actuators, and transducers. Field induced polarization switching and phase transitions of these crystals lead to complex nonlinear behavior. In recent years experimental investigations have been conducted to characterize the polarization switching and phase transition behavior as a function of crystallographic orientation, temperature, electric field, and stress. The results give insight into the mechanism underlying the observed large field hysteretic behavior. This review article describes the observed behavior and presents results of multiscale modeling that predicts the macroscopic behavior from the single domain single crystal behavior and evolution of crystal variants at the microscale.

Stress effects in sol-gel derived ferroelectric thin films

Residual stress development during processing of sol-gel derived ferroelectric thin films influences electromechanical properties and performance. The present work investigates the effects of stress on field-induced polarization switching in ferroelectric Pb(Zr0.52Ti0.48)O3 (PZT) (52/48) thin films. Film response is measured as a function of externally applied mechanical stress using a double-beam laser-Doppler heterodyne interferometer. This apparatus successfully eliminates any displacement contribution from flexural vibration of the substrate and enables measurement of the strain-electric field hysteresis loops as a function of applied stress. The field-induced strain in the PZT film increases with increasing compressive stress, while the opposite trend is observed for applied tensile stress. The dependence of electromechanical response on the external stress is attributed to the initial tensile residual stress state in the film. Tensile stress creates an in-plane clamping effect on the domains in the film, hindering polarization switching. The application of a compressive stress reduces tensile residual stress in the film and the constraint on the domains, leading to higher field-induced strains. Applied tensile stress amplifies the clamping effect, leading to lower field-induced strains.

Fine structure of the repolarization current in the B 2 phase of a banana compound

A detailed study of the field induced polarization switching has been carried out over the whole temperature range of the antiferroelectric B2 phase of a banana-shaped compound, the tetradecyl homologue (n=14) of the series 4-chloro-1,3-phenylene bis[4-(4-n -alkylphenylimino)benzoate]s. The fine structure of the current response oscillograms was attributed to the field switching of two antiferroelectric subsystems, most probably the racemic and homogeneously chiral domains which, according to the Boulder group, appear as a result of the spontaneous breaking of mirror symmetry. The temperature and field dependences of polarization as well as switching times and the corresponding viscosity of the substance have been measured.