Polarization Switching Process Research Articles

Direct imaging of the spatial and energy distribution of nucleation centres in ferroelectric materials

Macroscopic ferroelectric polarization switching, similar to other first-order phase transitions, is controlled by nucleation centres. Despite 50 years of extensive theoretical and experimental effort, the microstructural origins of the Landauer paradox, that is, the experimentally observed low values of coercive fields in ferroelectrics corresponding to implausibly large nucleation activation energies, are still a mystery. Here, we develop an approach to visualize the nucleation centres controlling polarization switching processes with nanometre resolution, determine their spatial and energy distribution and correlate them to local microstructure. The random-bond and random-field components of the disorder potential are extracted from positive and negative nucleation biases. Observation of enhanced nucleation activity at the 90 composite function domain wall boundaries and intersections combined with phase-field modelling identifies them as a class of nucleation centres that control switching in structural-defect-free materials.

Parameters of Domain Boundaries in Thin PbTiO3 Films

Features of polarization switching processes in lead titanate thin films were investigated. The films under discussion were synthesized by plasma-enhanced magnetron sputtering of titanium and lead on Al 2 O 3 substrates in an argon medium, and subsequent annealing in an oxygen atmosphere. In terms of theory of lateral motion of domain boundaries the main parameters of domain walls, characterizing kinetics of domain structure in thin polycrystalline lead titanate films, were determined.

Observation of inhomogeneous domain nucleation in epitaxial Pb(Zr,Ti)O3 capacitors

We investigated domain nucleation process in epitaxial Pb(Zr,Ti)O3 capacitors under a modified piezoresponse force microscope. We obtained domain evolution images during polarization switching process and observed that domain nucleation occurs at particular sites. This inhomogeneous nucleation process should play an important role in an early stage of switching and under a high electric field. We found that the number of nuclei is linearly proportional to log(switching time), suggesting a broad distribution of activation energies for nucleation. The nucleation sites for a positive bias differ from those for a negative bias, indicating that most nucleation sites are located at the ferroelectric/electrode interfaces.

Nanoscale Electromechanics of Ferroelectric and Biological Systems: A New Dimension in Scanning Probe Microscopy

Functionality of biological and inorganic systems ranging from nonvolatile computer memories and microelectromechanical systems to electromotor proteins and cellular membranes is ultimately based on the intricate coupling between electrical and mechanical phenomena. In the past decade, piezoresponse force microscopy (PFM) has been established as a powerful tool for nanoscale imaging, spectroscopy, and manipulation of ferroelectric and piezoelectric materials. Here, we give an overview of the fundamental image formation mechanism in PFM and summarize recent theoretical and technological advances. In particular, we show that the signal formation in PFM is complementary to that in the scanning tunneling microscopy (STM) and atomic force microscopy (AFM) techniques, and we discuss the implications. We also consider the prospect of extending PFM beyond ferroelectric characterization for quantitative probing of electromechanical behavior in molecular and biological systems and high-resolution probing of static and dynamic polarization switching processes in low-dimensional ferroelectric materials and heterostructures.

Characterization of ferroelectric/metal interface under the repeated polarization switching

Polarization fatigue mechanism in organic ferroelectrics, structures at interface between ferroelectric vinylidene fluoride oligomer and Al electrode under the repeated polarization switching process were investigated by high-resolution X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR). Al 2O 3 layer at interface was formed with increasing the number of polarization reversal. The formation of oxide layer will be strongly related to polarization reversal, thus the repeated charge and discharge process by polarization reversal may promote the oxidation of Al electrode. Furthermore, the structural and orientation changes in ferroelectric molecular films by applying the electric field were observed. The formation of Al 2O 3 layer, as well as the structural changes in thin films, is affected to polarization fatigue process of ferroelectric organic devices.

All-optical switching of continuous wave, microsecond lasers with a dye-doped nematic liquid crystal

The authors present a theoretical analysis and experimental demonstration of optically self-activated polarization switching process in a 90° twist alignment dye-doped nematic liquid crystal. The underlying mechanisms are laser induced dye-assisted director axis reorientation and order parameter modifications. Experimental observations are in good agreement with the theoretical expectations derived from a modified Jones matrix calculation, and demonstrate the feasibility of designing efficient low threshold polarization and fast switching and optical limiting devices for cw and microsecond lasers.

Observation of photoassisted polarization switching in BiFeO3 thin films probed by terahertz radiation

The authors have examined the effect of photoexcitation in a multiferroic BiFeO3 thin film using terahertz radiation as the probe. The illumination of femtosecond laser pulses with a center wavelength of 400nm gave rise to a significant improvement in the polarization switching process by the application of bias electric field. This effect is attributed to the depinning of domain walls induced by carrier excitation as reported in ordinary ferroelectrics. The authors revealed that a significantly weak laser power in the order of submilliwatt is sufficient to invoke this effect.

Polarization Switching and Domain Structure in LaBGeO5 Crystals

Some new features of the polarization switching processes in the high temperature (T c = 533°C) ferroelectric LaBGeO 5 crystal regarding the stability of the domain structure are discussed. The shape of the switching current for different values of the electric field in rectangular pulses is described in the framework of the model taking into account the possible heterogeneity of the crystal—the different probability of the domain nucleation in different parts of the crystal. Stability of the mono-domain state was studied by means of pyroelectric measurements. The “natural” domain structure and artificial one are visualized.

Polarization switching of triglycine sulfate and deuterated triglycine sulfate single crystals irradiated with a high-current pulsed electron beam

The influence of high-current pulsed electron beams on the processes of polarization switching in triglycine sulfate and deuterated triglycine sulfate single crystals is investigated using the Barkhausen effect.

Switching Processes and Formation of the Stable Artificial Domain Structure in Ferroelectric LaBGeO5

The processes of polarization switching in LaBGeO 5 (LBGO) crystals are studied in details for the triangle and pulse electric field in the wide temperature range from room temperature (RT) to Tc = 530°C. It is revealed that the temperature dependence of a coercive field (f = 0.1 Hz) has an exponential form and this field reaches a value 150 kV/cm at the RT. The temperature evolution of the switching time and the form of the switching current are obtained in the mode of the successive solitary pulses and in the steady conditions. The features of “non-classical” behavior are noted (the dependence of switching parameters on the prehistory of samples, frequency dependence of the switching time). It is shown that the stable artificial domain structure at RT with the periodicity of 200 μm can be obtained by applying about 300–350 V/mm near 300°C. The stable domain structure was contrasted by means of SHG microscope probing.

Nanoelectromechanics of piezoresponse force microscopy

To achieve quantitative interpretation of Piezoresponse Force Microscopy (PFM), including resolution limits, tip bias- and strain-induced phenomena and spectroscopy, analytical representations for tip-induced electroelastic fields inside the material are derived for the cases of weak and strong indentation. In the weak indentation case, electrostatic field distribution is calculated using image charge model. In the strong indentation case, the solution of the coupled electroelastic problem for piezoelectric indentation is used to obtain the electric field and strain distribution in the ferroelectric material. This establishes a complete continuum mechanics description of the PFM contact mechanics and imaging mechanism. The electroelastic field distribution allows signal generation volume in PFM to be determined. These rigorous solutions are compared with the electrostatic point charge and sphere-plane models, and the applicability limits for asymptotic point charge and point force models are established. The implications of these results for ferroelectric polarization switching processes are analyzed.

Theoretical and experimental investigations of polarization switching in ferroelectric materials

The polarization switching process is certainly the most important feature of ferroelectric materials from fundamental as well as practical points of view. In this paper, a one-dimensional lattice model is presented in order to describe the polarization switching process in ferroelectric materials, incorporating the contribution of both dipolar defects and depolarizing fields to the domain reorientation. The influence of the interaction strength between switchable dipoles and dipolar defects, the medium viscosity, the depolarizing fields, and the frequency of the external electric field on the polarization switching were simulated. It was found that the degree of interaction between domains and dipolar defects has a strong influence on the coercive field, polarization, and backswitching behavior. Through an adequate analysis of the variables in the model it was also possible to describe the evolution of the polarization switching with the number of electric field cycles, which is commonly observed in the fatigue or depinning process. Comparison between simulated and experimental results revealed a remarkable concordance.

90° domain wall relaxation and frequency dependence of the coercive field in the ferroelectric switching process

The mechanisms involved in the polarization switching process in soft and hard Pb(Zr53,Ti47)O3 (PZT) bulk ceramics were investigated through the dependency of the hysteresis loop on the frequency. In order to determine the influence of the defects on the domain switching dynamics, the samples were characterized in the virgin state and after a fatigue or a depinning process. The frequency dependence of the polarization revealed a strong relaxation of the 90° domain walls at ∼100 Hz. The results also revealed a strong influence of the kind of defect and their distribution in the ferroelectric matrix on the domain switching dynamics, which were reflected in the frequency dependence of the coercive field and the percentage of the backswitching. Initially, it was observed that the frequency dependence of the coercive field for the soft and the hard PZT in the virgin state had just one rate of change per decade in the entire frequency range investigated, which is the standard behavior found in the literature. However, after the fatigue or the depinning process, two rates of changes were noticed. Consequently, evidence of an upper-frequency limit for the coercive field changes was found. The percentage of the backswitching and its behavior for the soft PZT was almost independent of the fatigue state in the entire frequency range investigated. Nevertheless, for the hard PZT, an opposite behavior was verified. The reorientation of the domains was modeled as occurring in a viscous medium where several forces, such as viscous and restoring forces, act on them.

Polarization Switching Relaxation in Pb(Zr, Ti)O3 Ceramics*

The polarization switching process was investigated in soft and hard PZT ceramics. The frequency dependence of the polarization revealed a relaxation process, which was related to 90°domain walls. The results revealed that the domain reorientation is strongly dependent of the kind of defects and their distribution into the lattice. An evidence of an upper frequency limit for the coercive field was observed.∗Paper originally presented at 10th European Meeting on Ferroelectricity, Cambridge, U.K., August 3–8, 2003.

Simulation of the Polarization Switching in Ferroelectrics with Dipolar Defects Through the Landau-Theory-Based Lattice Model*

A one dimensional lattice model is presented in order to describe the polarization switching process in ferroelectric materials, incorporating the contribution of both dipolar defects and depolarizing fields to the domain reorientation. Through an adequate analysis of the variables of the model it was possible to describe the evolution of the polarization switching with the number of electric field cycles, which is commonly observed in the depinning process. *Paper originally presented at 10th European Meeting on Ferroelectricity, Cambridge, U.K., August 3–8, 2003. †E-mail: djpr@df.ufscar.br ‡picinin@df.ufscar.br

Polarization Switching Relaxation in Pb(Zr,Ti)O3 Ceramics

The polarization switching process was investigated in soft and hard PZT ceramics. The frequency dependence of the polarization revealed a relaxation process, which was related to 90° domain walls. The results reveled that the domain reorientation is strongly dependent of the kind of defects and their distribution into the lattice. An evidence of an upper frequency limit for the coercive field was observed.

Application of an ultrafast photonic technique to study polarization switching dynamics of thin-film ferroelectric capacitors

We demonstrate the use of jitter-free ultrafast rise-time electrical pulses generated by a semiconductor photoconductive switch with femtosecond laser illumination to study the fast polarization switching process in fully integrated, ferroelectric PNZT thin-film capacitors. The necessary conditions for high-speed polarization switching detection are presented. The switching behavior for various sizes of capacitors and various rise-times of applied voltage pulses are investigated. Circuit influence on the activation field, /spl alpha/, as well as application of the Inshibashi-Merz model, are discussed.

SFM investigation of nanoscale domain structure in ferroelectric PZT thin films

Nanoscale domain structures and polarization reversal behaviour in (111)-oriented PZT60/40 thin film were investigated in-situ with scanning force microscopy p iezoresponse mode. Complex domain contrast is related to the arrangement of doma ins in grains and to the orientation of the grains in the film. The step structu re of ～ 30 nm in width was directly observed, which was formed during the polar ization reversal process. The presence of the step structures reveals that the f orward domain growth mechanism prevails in the polarization switching process of PZT60/40 thin films.

Nanoelectromechanics of Piezoresponse Force Microscopy: Contact Properties, Fields Below the Surface and Polarization Switching

ABSTRACTTo achieve quantitative interpretation of Piezoresponse Force Microscopy (PFM), including resolution limits, tip bias- and strain-induced phenomena and spectroscopy, knowledge of elastic and electrostatic field distributions below the tip is required. The exact closed form solution of the coupled electroelastic problem for piezoelectric indentation is derived and used to obtain the tip-induced electric field and strain distribution in the ferroelectric material. This establishes a complete continuum mechanics description of the PFM imaging mechanism. These solutions are reduced to the point charge/force behavior for large separations from contact, and the applicability limits and charge/force magnitude for these models are established. The implications of these results for ferroelectric polarization switching processes are analyzed.

Investigation of Switching Behaviour in PbZr 0.55 Ti 0.45 O 3 Thin Films by Means of Scanning Probe Microscopy

Polarization switching processes in polycrystalline PbZr 0.55 Ti 0.45 O 3 thin films were investigated using piezoresponse mode of Scanning Probe Microscopy. Analysis of a piezoresponse signal reveals self-polarization of as-grown film caused by internal bias field. The written domains with polarization oriented opposite to self-polarization direction are unstable and switched back with time. The dynamics of backswitching process strongly depends on the films microstructure and is slower in the regions with higher density of grain boundaries.