Polarization Reversal Dynamics Research Articles

Kinetics of N- to M-Polar Switching in Ferroelectric Al1-xScxN Capacitors.

Ferroelectric wurtzite-type aluminum scandium nitride (Al1-xScxN) presents unique properties that can enhance the performance of non-volatile memory technologies. The realization of the full potential of Al1-xScxN requires a comprehensive understanding of the mechanism of polarization reversal and domain structure dynamics involved in the ferroelectric switching process. In this work, transient current integration measurements performed by a pulse switching method are combined with domain imaging by piezoresponse force microscopy (PFM) to investigate the kinetics of domain nucleation and wall motion during polarization reversal in Al0.85Sc0.15N capacitors. In the studied electric field range (from 4.4 to 5.6MVcm-1), ferroelectric switching proceeds via domain nucleation and wall movement. The currently available phenomenological models are shown to not fully capture all the details of the complex dynamics of polarization reversal in Al0.85Sc0.15N. PFM reveals a non-linear increase of both domain nucleation rate and lateral wall velocity during the switching process, as well as the dependency of the domain pattern on the polarization reversal direction. A continuously faster N- to M-polar switching upon cycling is reported and ascribed to an increasing number of M-polar nucleation sites and density of domain walls.

Dynamics of polarization reversal in neodymium-doped PMN-PT ceramic

In this investigation, we present a comparative dynamic scaling analysis of switching and non-switching loops for PMN-PT ceramic and its neodymium-doped counterpart. The results demonstrate that both the samples exhibit similar scaling relations at high fields, suggesting that their distinct piezoelectric behaviour primarily stems from their polarization dynamics at low fields. For E<4kV/cm, the switching polarization for PMN-PT and Nd:PMN-PT follows the scaling relations: <ASW>=1.65*E5.89f−0.27 and <ASW>=34.5*E3.4f−0.2, respectively. On the other hand, non-switching polarization for the two samples follows the relations: <ASW>=2.26*E4.28f−0.15 and <ASW>=65.9*E1.95f−0.05, respectively. These scaling relations suggest that switching as well as non-switching polarization for Nd:PMN-PT respond faster to changing applied field as compared to their PMN-PT counterparts. We argue that the proportionality constants in two scaling relations have significant effect on the electromechanical behaviour of piezoceramics and should be evaluated in forthcoming dynamic scaling investigations. The kinetics of domain reversal, investigated using the Kolmogorov-Avrami-Ishibashi (KAI) model, suggest that the crystallographic domains in both the samples exhibit nucleation limited-switching (NLS) behaviour. We believe that our work will stimulate investigations on other piezoelectrics to corelate their functional response with the underlying dynamics and kinetics of polarization rotation.

Low temperature relaxor, polarization dynamics and energy storage properties of Ca0.28Ba0.72Nb2O6 tungsten bronze ceramics

The development of modern electronic devices requires dielectrics with high energy storage density, high power density and excellent working stability as the dielectric layer of multilayer ceramic capacitors. Herein, Ca0.28Ba0.72Nb2O6-based tungsten bronze ceramics doped by ions with various radii were prepared, and the energy storage performance was improved by adjusting the relaxor behavior. Rare double relaxor behavior was confirmed based on macroscopic phenomenological model and characterization of domain structure, and the low-temperature relaxor behavior was found to be sensitive to the in-plane polarization displacement disorder of Nb2 sites. Meanwhile, a significantly improved energy storage performance was obtained: Wre ∼ 2.59 J/cm3, η ∼ 85.3 %, PD ∼ 130.78 MW/cm3. The excellent temperature stability of the energy storage performance is explained by the polarization reversal dynamics via Vopsaroiu model. This work highlights the potential of tungsten bronze as energy storage devices, and provides an additional feasible paradigm of pulse power capacitors with excellent thermal stability.

Polarization reversal via a transient relaxor state in nonergodic relaxors near freezing temperature

Among the unresolved issues in the study of relaxor ferroelectrics is the role of freezing temperature, across which the dynamics of polarization reversal in relaxor ferroelectrics changes. The presence of this freezing temperature is best manifested by the appearance of a double polarization hysteresis loop just above the freezing temperature. Given that the polarization pinching evolving into a double hysteresis starts well below the freezing temperature, there exists a transient temperature regime between the nonergodic and the ergodic relaxor states. To clarify the role of the freezing temperature on the pinching, the polarization reversal near the freezing temperature of relaxor (Pb1-xLax)(Zr1-yTy)1-x/4O3 (PLZT) was monitored using three in situ electric field methods: electrocaloric effect, neutron diffraction, and transmission electron microscopy. We demonstrate that the pinching results from a two-step process, 1) domain detexturization in the ferroelectric state and 2) miniaturization of domains. This observation explains the recently reported gap between the depolarization temperature Td and the ferroelectric-to-relaxor transition temperature TF-R in lead-free relaxors. We further show that Td and TF-R, which have long been considered identical in lead-based relaxors, are not the same. The current study suggests that the mismatch between Td and TF-R is an inherent feature in both lead-based and lead-free relaxor ferroelectrics.

Real-time switching dynamics of ferroelectric tunnel junctions under single-shot voltage pulses

In ferroelectric memory devices, information is stored within the polarization direction whose reversal usually occurs by the nucleation and propagation of domains. In ultrathin ferroelectrics, ultrafast dynamics may be achieved by nucleation-limited switching, avoiding the inherently speed-limited propagation of domain walls. Here, we investigate polarization reversal dynamics in ultrathin ferroelectric films by transient current measurements. Thanks to the tunnel electroresistance, the start of polarization reversal induces sharp variations of the transmitted current under voltage pulses. These single-shot measurements show extremely fast switching with durations down to 3 ns that is only limited by the current device geometry. While the OFF-to-ON switching shows finite nucleation times that scale with the pulse amplitude, the ON-to-OFF switching speed cannot be detected under such rectangular pulses. Resorting to triangular pulse excitations allows us to detect the dynamics of this switching direction. Both cases can be interpreted by nucleation switching models following Merz's law.

Polarization Reversal Via a Transient Relaxor State in Nonergodic Relaxors Near Freezing Temperature

Among the unresolved issues in the study of relaxor ferroelectrics is the role of freezing temperature, across which the dynamics of polarization reversal in relaxor ferroelectrics changes. The presence of this freezing temperature is best manifested by the appearance of a double polarization hysteresis loop just above the freezing temperature. Given that the pinching evolving into the double hysteresis starts well below the freezing temperature, there exists a transient temperature regime between the nonergodic and ergodic relaxor states. To clarify the role of the freezing temperature in developing the constriction, the polarization reversal near the freezing temperature of relaxor (Pb1-xLax)(Zr1-yTy)1-x/4O3 (PLZT) was monitored using three in situ electric field methods: electrocaloric effect, neutron diffraction, and transmission electron microscopy. We demonstrate that the appearance of constriction results from a two-step process, 1) reduction in domain texture in the ferroelectric state and 2) transformation to a relaxor state with nanodomains. This observation explains the recently reported gap between the depolarization temperature Td and the ferroelectric-to-relaxor transition temperature TF-R in lead-free relaxors. We further show that Td and TF-R, which have long been considered identical in lead-based relaxors, are not the same. The current study suggests that the mismatch between Td and TF-R is an inherent feature in both lead-based and lead-free relaxor ferroelectrics.

Ferroelectric properties and polarization dynamics in Ba4Sm2Ti4Ta6O30 tungsten bronze ceramics

Ferroelectricity and polarization reversal dynamics in Ba4Sm2Ti4Ta6O30 tungsten bronze ceramics were investigated by measuring dielectric spectra and the evolution of hysteresis loops over a wide temperature range. With decreasing temperature, the dielectric properties and differential scanning calorimetry results showed diffuse peaks at ∼280 K with large thermal hysteresis, suggesting a first order ferroelectric transition. A dielectric relaxation was observed at low temperature that followed the Vogel–Fulcher relationship. The saturation and remanent polarizations of the Ba4Sm2Ti4Ta6O30 ceramics showed remarkable dependence on the applied field and temperature. The temperature dependence of the coercive field was divided into three linear regions and fitted to the Vopsaroiu model. Activation energies for polarization reversal of 0.73, 0.79, and 0.65 eV were determined for the following three regions: (I) the diffuse ferroelectric transition region (323.15–293.15 K), (II) the region just below the ferroelectric transition temperature (293.15–233.15 K), and (III) the low temperature relaxation region (233.15–173.15 K), respectively. The decrease of the activation energy in region III was attributed to the low temperature relaxation of Ba4Sm2Ti4Ta6O30.

Two-Step Polarization Switching in Ferroelectric Polymers.

The correlation between hierarchical structures and polarization switching in ferroelectric poly(vinylidene fluoride-ran-trifluoroethylene) has been probed by combining transmission electron microscopy studies with piezoresponse force microscopy observations. Differences are demonstrated between homogeneous and anisotropic thin films with well-defined lamellar orientation, with the later exhibiting quadrangular domain shape and double hysteresis. We propose that the polarization switching within lamella is dominated by domain wall flow motion, while the amorphous components between lamellae impede full polarization switching. The coupling between lamellae is controlled by a creep process. These results and interpretations explain well the seemingly contradicting polarization reversal dynamics reported and offer opportunities to change domain reversal speed by making ferroelectric polymer nanostructures.

Highly Textured BaTiO3 via Templated Grain Growth and Resulting Polarization Reversal Dynamics

Samples of bulk textured polycrystalline BaTiO3 ceramics were fabricated using a templated grain growth (TGG) approach in order to investigate effects of polycrystallinity and texture related to ferroelectric domain reversal under high‐power drive conditions. Barium titanate platelets were formed via two‐step topochemical conversion of bismuth titanate platelets grown via molten salt synthesis, then aligned via tape casting within a matrix of fine BaTiO3 powder. The coarse‐grained parts showed a high degree of crystallographic texture after sintering. Combined with ceramics of similar density and polycrystallinity, but random orientation and commercial single‐crystal specimens, this sample set enabled direct isolation of crystallographic texture and polycrystallinity as the primary variables for high‐power polarization reversal studies. These studies have also demonstrated a link between grain size and polarization reversal time that strongly suggests that grain boundaries serve effectively as nucleation sites during the ferroelectric switching process.

Electromechanics of Ferroelectric‐Like Behavior of LaAlO3 Thin Films

Electromechanical coupling in complex oxide heterostructures opens new possibilities for the development of a broad range of novel electronic devices with enhanced functionality. In this article, the switchable hysteretic electro­mechanical behavior of crystalline epitaxial LaAlO3 (LAO) thin films associated with polarization induced by electrical and mechanical stimuli is investigated. The field–time‐dependent testing of the induced polarization states along with transport measurements and theoretical modeling suggests that the ferroelectric‐like response of the LAO thin films is mediated by the field‐induced ion migration in the bulk of the film. Comparative analysis of the dynamics of polarization reversal under the electrical field and mechanical stress applied via a tip of a scanning probe microscope demonstrates that both electrical and mechanical stimulus can be used to effectively control polarization at least at the submillisecond timescale. However, the mechanical writing is more localized than the electrical one. A combined electrical/mechanical approach for tuning the physical properties of oxide hetero­structures may potentially facilitate novel memory and logic devices, in which the data bits are written mechanically and read electrically.

Finite-size effects of hysteretic dynamics in multilayer graphene on a ferroelectric

The origin and influence of finite-size effects on the nonlinear dynamics of space charge stored by multilayer graphene on a ferroelectric and resistivity of graphene channel were analyzed. Here, we develop a self-consistent approach combining the solution of electrostatic problems with the nonlinear Landau-Khalatnikov equations for a ferroelectric. The size-dependent behaviors are governed by the relations between the thicknesses of multilayer graphene, ferroelectric film, and the dielectric layer. The appearance of charge and electroresistance hysteresis loops and their versatility stem from the interplay of polarization reversal dynamics and its incomplete screening in an alternating electric field. These features are mostly determined by the dielectric layer thickness. The derived analytical expressions for electric fields and space-charge-density distribution in a multilayer system enable knowledge-driven design of graphene-on-ferroelectric heterostructures with advanced performance. We further investigate the effects of spatially nonuniform ferroelectric domain structures on the graphene layers' conductivity and predict its dramatic increase under the transition from multi- to single-domain state in a ferroelectric. This intriguing effect can open possibilities for the graphene-based sensors and explore the underlying physical mechanisms in the operation of graphene field-effect transistor with ferroelectric gating.

Competing polarization reversal mechanisms in ferroelectric nanowires

Polarization reversal in ferroelectrics has been a subject of intense interest for many years owing to both its scientific appeal and practical utility. In recent years the interest has increased even further thanks to the expectations of achieving ultrafast polarization reversal at the nanoscale. While most of the studies up to now are focused on the polarization reversal in ferroelectric thin films, we report the intrinsic dynamics of ultrafast polarization reversal in ferroelectric nanowires. Using atomistic first-principles-based simulations, we trace the time evolution of polarization under applied electric field to reveal the existence of two competing polarization reversal mechanisms: (i) domain-driven and (ii) homogeneous. The analysis of their microscopic origin allows us to postulate the associated laws and leads to a deeper understanding of polarization reversal dynamics in general. In addition, we find that in defect-free nanowires the polarization reversal can occur within picoseconds, which potentially is very promising for ultrafast memory and other applications.

Dynamics of polarization reversal in virgin and fatigued ferroelectric ceramics by inhomogeneous field mechanism

Temporal behavior of ferroelectric ceramics during the polarization switching cannot be satisfactorily explained by simple Debye or even stretched exponential laws. These materials exhibit rather a wide spectrum of characteristic times interpreted by different authors as switching or nucleation waiting times, the physical reasons for a wide time distribution still remaining unclear. A new model of polarization switching presented here suggests that the characteristic time variance in the ferroelectrics originates from the random distribution of the local electric fields due to intrinsic randomness of the material. The presented theory allows a direct extraction of the distribution of field values from the experiment. Systematic studies of polarization switching in fatigued lead zirconate titanate demonstrate the evolution of the field distribution with increasing level of fatigue. Plausible cause of the formation of regions subject to different field strengths is the generation of defects such as microcracks, pores, or voids in the course of fatigue. Suitability of the proposed model is demonstrated by an excellent correlation between experimental and calculated data for virgin and differently fatigued samples in a broad time-field region covering the electric field values of 0.5--2.5 kV/mm and nine orders of the magnitude of poling time.

Numerical Investigation of Polarization Reversal Characteristics in a Ferroelectric Thin Film

A small-sized symmetric thin ferroelectric (FE) film, assumed second order and in the FE phase, of thickness l is considered. The Landau-Devonshire free energy of the film is expressed using the Tilley-Zeks model. A special case, in which the polarization values at both film surfaces are zero, p± = 0, is assumed. The initial spontaneous polarization profile of the film at equilibrium is approximated using a cosine function. The Landau-Khalatnikov equation of motion is then used to obtain numerically the initial stable polarization profile; and to describe the dynamics of polarization reversal in the film under the action of a step driving electric fields. A finite-difference time domain method is utilized to simulate the switching process. Changes in the average polarization and the average polarization current during the switching are calculated and studied. The influence of temperature and thickness on switching is investigated.

Dynamics of polarization reversal in thin PZT films

New experimental data on the dynamics of polarization reversal in thin PZT films prepared by rf sputtering are reported. The frequency and amplitude dependences of the hysteresis loop area and coercive field are investigated. It is shown that, at infralow frequencies, there exists a critical frequency at which the shape of the hysteresis loop changes stepwise. Theoretical analysis of the related problems is given.

Dynamics of Polarization Reversal in Ferroelectric Thin Films-Phenomenological Analysis of Polarization Reversal Mechanism Using Lattice Model-

Phenomenological analysis of polarization reversal of ferroelectric thin films has been done on the basis of Landau theory using a lattice model. Polarization hystereses have been obtained in the lattice models in which dead-ferroelectric and built-in field regions are distributed at random. Dependences of saturation polarization and coercive field on remanent polarization are used for comparison between the calculated and experimental data. The polarization fatigue analyzed in the model including bipolar built-in fields agrees best with the experimental data of sol-gel PZT films. So, it is certified that these analyses using polarization lattice model are powerful to clarify the mechanism of polarization reversal dynamics in ferroelectric thin films. These analyses are also expected to be applied to the electrical characterization of ferroelectric capacitors in FeRAM.

Temperature dependence of the domain switching of (CH3NH3)5Bi2Br11 crystals

The dynamics of polarization reversal in dc field was studied in various temperatures in the course of heating process from room temperature to the Curie point T c. The time dependence of domain pattern appearing during the polarization reversal was analysed using the Avrami theory. Observation of domain switching in the vicinity of T c after quenching the crystal from paraelectric phase was also carried out.

Nanoscale Scanning Force Imaging of Polarization Phenomena in Ferroelectric Thin Films

The science and technology of ferroelectric thin films is currently attracting worldwide attention because of its application to a new generation of novel devices. Prime, among these applications are nonvolatile ferroelectric random-access memories (NVFRAMs), which have high speed and extended endurance. The core of an NVFRAM is a capacitor with a ferroelectric film sandwiched between two electrode layers. The polarization of the ferroelectric layer in two possible opposite directions, upon application of an electric field between the two electrodes, provides the logic “1” and “0” states needed for binary-code memory. In spite of the advances in the science and technology of ferroelectric thin films and their integration into ferroelectric capacitors, some materials-related integration strategies as well as manufacturability issues have delayed commercialization of NVFRAMs. High-density memories require storage elements that approach submicron lateral dimensions. Thus, improved understanding of the materials properties and polarization phenomena is needed in conjunction with the development of new characterization tools that can enable such an understanding. For example, a fundamental issue in ferroelectric thin-film capacitors is the exact nature of the complex domain structure in the polarizable ferroelectric layer and its dynamics under high-speed switching conditions. The miniaturization of NVFRAMs requires understanding of granularity in polarization reversal dynamics, fatigue, and retention characteristics. In this respect, theoretical models and electrical measurements (e.g., polarization hysteresis loops and transient currents) have provided substantial insights into the nature of the switching processes. However, the models (phenomenological in nature) and the electrical measurements provide only a global or macroscopic view of the switching process.

Dynamics of polarization reversal in purified Rb2ZnCl4

The dynamics of polarization reversal in sinusoidal fields (0. 1Hz≤f≤50kHz) was studied in the ferroelectric lock-in phase of purified Rb2ZnCl4 crystals. The frequency dependence of remanent polarization and coercive field Ec is qualitatively the same above and below T*≈160K, where for all measuring frequencies Ec starts to increase rapidly with decreasing temperature. A model which treats sideways shifts of planar, nearly regularly arranged domain walls as the only mechanism of polarization reversal in Rb2ZnCl4 is applicable for the quantitative description of ferroelectric hysteresis loops in the whole temperature interval under investigation. The anomalous increase of Ec at T* seems to be connected to a corresponding increase of the viscosity coefficient.

Computer simulation of domain growth in ferroelectric liquid crystals.

The electro-optic response of a surface-stabilized ferroelectric liquid crystal (SSFLC) cell is determined largely by the dynamics of polarization reversal at the chevron interface. This process involves the nucleation, growth, and ultimate coalescence of polarization reversal domains. These domains are generally faceted, being either polygonal or characteristically boat shaped. We have performed computer simulations in two dimensions (2D) of field-driven domain growth at the chevron interface in SSFLCs. By including elastic anisotropy and the orientational binding of the chevron in the equation of motion, we get anisotropic domain growth and, for a range of applied field strengths E, partially faceted domain shapes. The measured growth rates have the same field dependence as is seen experimentally, the domain area increasing as A\ensuremath{\propto}(Et${)}^{2}$. We have also developed an analytical algorithm similar to the Wulff construction for calculating the spatial evolution of 2D domains as a function of time for arbitrary starting shapes. In the present case of ferroelectric domain growth, we can derive full 2D domain shapes from the directional variation of the velocity of planar walls as measured by computer simulations in only 1D.