Article

Shape and Surface Charge Modulation of Topological Domains in Oxide Multiferroics

Authors:
  • Institute of Metal Research, Chinese Academy of Sciences, shenyang
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Abstract

Topological defects showing exotic properties and diverse functionalities provide us a potential utilization in nanoscale electronic devices. However, the formation mechanism and density manipulation of topological defects such as center-type domains which are crucial for applications remain elusive. Usually, these center-type domains are generated by applying external electric fields in ferroelectrics. In contrast, here we have prepared high density center-divergent domains in BiFeO3 as self-assembled nano-islands deposited on both Nb and Fe doped SrTiO3 substrates. The size and density of these domains can be easily manipulated by varying doping level in substrates. The panorama polar configurations of the center-divergent domains are revealed by piezoresponse force microscopy (PFM) and Cs-corrected scanning transmission electron microscopy. Phase-field simulations prove that both the surface charge accumulation and the shape of the nano-islands take great effect in the formation of center-type domains. The controllable growth of the nano-islands offers us a promising way to acquire high density nanoscale non-volatile memories.

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... Recent years, several exotic polar states have been gradually revealed in different ferroelectric systems, thanks to the advancements in theoretical work and high resolution domain probing techniques. A series of exciting discoveries of exotic ferroelectric topological objects, such as flux-closure domain, [16][17][18][19][20] quadrant vortex, [21][22][23][24] circular vortices lattice, 25 skyrmion, 26 bubble, 27 meron, 28 and center domain state (monopole-like structure) [29][30][31][32][33] in nanometer-scale thin films/superlattices/nanostructures have been witnessed, as well as the multifold vortex network in improper ferroelectric single crystals (e.g., YMnO 3 ) 34,35 and metal-organic single crystal films. 36,37 Some typical examples of experimentally observed exotic polar topologies are summarized in Fig. 1 for reference. ...
... 29 Such a center domain can be reversibly switched between the convergent and divergent center states triggered by the electric field, thus offering a possibility for storing "0" and "1" data bits. A similar domain structure was also found by Han et al. 30 using the HAADF-STEM technique. More recently, quadrant center domains were reported by Ma et al. in rhombohedral BFO nanoislands embedded in the tetragonal phase matrix, 31 where large domain wall conduction was identified, which enables current readout of different types of center states and is promising for developing nonvolatile memory with current readout. ...
... Similar to the magnetic analogs, polar topological domains are largely determined by the competition between exchange, anisotropy, elastic, and electrostatic interactions, which are sensitive to boundary conditions, strain, charge screening, and so on. In oxide ferroelectrics, point defects and charge carriers also play critical roles, for instance, leading to the formation of center topological states 29,30 that are rarely seen in magnetic systems. In a recent work, the DM interaction that is critical for the formation of the spiral configurations or complicated topologies such as skyrmion was also theoretically predicted in perovskite ferroelectrics, which implies the existence of more complicated mechanisms and possible new topologies. ...
Article
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In the past decade, a series of breakthrough discoveries in new exotic polar topological states have been witnessed, e.g., vortex, skyrmion, and meron. These tantalizing findings open a new avenue toward a plethora of emerging physical phenomena and offer opportunities for a wide range of future configurable electronic devices, which might eventually lead to an exciting area, the so-called “topotronics.” Although this field has seen a rapid progress, especially in revealing various novel topological states, the associated emerging phenomena and functionalities as well as application potentials yet remain largely unexplored, which might become fruitful areas in the upcoming years and thus deserve more attention. In this perspective, we give a brief overview on the recent advances in the field of exotic polar topological states, highlighting the emerging phenomena and efforts to control these functional topological objects. Finally, we present a concluding summary with some suggestions for future directions.
... These topological states not only potentially host a wealth of emergent physical phenomena and exotic functionalities, for example, conductivity, magnetism, and photovoltaics, [1][2][3][4][5][6][7][8][9][10][11][12] but also allow being controlled and manipulated by external fields, making them ideal elemental building blocks for a wide range of future programmable nanoelectronic devices. [13][14][15] In the past few years, substantial efforts have been made in searching for novel topological structures, [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31] leading to a series of exciting discoveries of new exotic topological states, for example, flux-closure, 16,20 vortices, [17][18][19][20][21][22] center domain, 13,24,25 skyrmion, 26 bubble, [27][28][29] and meron. 30 Particularly, the observation of stable center-type topological domains in isolated BiFeO 3 (BFO) nanoislands not only enables the electric control of individual topological states, 13 but also allows the non-destructive readout of different topological states via their corresponding domain wall conductive current levels. ...
... These topological states not only potentially host a wealth of emergent physical phenomena and exotic functionalities, for example, conductivity, magnetism, and photovoltaics, [1][2][3][4][5][6][7][8][9][10][11][12] but also allow being controlled and manipulated by external fields, making them ideal elemental building blocks for a wide range of future programmable nanoelectronic devices. [13][14][15] In the past few years, substantial efforts have been made in searching for novel topological structures, [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31] leading to a series of exciting discoveries of new exotic topological states, for example, flux-closure, 16,20 vortices, [17][18][19][20][21][22] center domain, 13,24,25 skyrmion, 26 bubble, [27][28][29] and meron. 30 Particularly, the observation of stable center-type topological domains in isolated BiFeO 3 (BFO) nanoislands not only enables the electric control of individual topological states, 13 but also allows the non-destructive readout of different topological states via their corresponding domain wall conductive current levels. ...
... 13 It was also suggested that charge defects (oxygen vacancies) at the surface of the BFO nanoisland are an important factor that drives the formation of center domains. 13,25 In our case, the samples with nanoislands were patterned from epitaxial thin film deposited by PLD, which may contain some extent of oxygen vacancies (from the low deposition oxygen pressure of 15 Pa) as well as some misfit dislocations from the deposition process. 39 In addition, the ion beam bombarding during the nanoislands etching process can also introduce some charge or charge defects accumulating in the edge of nanoislands. ...
Article
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In recent years, there is a surge of research interest in exotic ferroelectric topological states, motivated by their rich emerging physical properties and potential applications in nanoelectronic devices. Here, we demonstrate the observation of a sort of complex center-type topological domain structures, which exhibit a quadrant center-type (with polarization pointing to the center) topological texture for an in-plane polarization component and a cylinder domain pattern along the vertical direction, in rhombohedral structured Pb(Zr0.7Ti0.3)O3 (R-PZT) nanoislands. Such a center domain state exhibits a rather high stability, which can well maintain its topological texture after heating to above curie temperature and subsequently cooling down to room temperature. Moreover, it allows erasure by a scanning electric field, yet it can also be recovered by a similar heating and cooling process. The observation of these unique topological textures in R-PZT nanoislands might provide a good playground for further exploring their topological phase transition properties, emerging novel functionalities, and application potential.
... On the other hand, various domain textures in well-confined nanoislands, such as closure quadrant domains (square shape flux-closure), vortex structures, and center-type domains (with polarization pointing out (or in) from (or to) a center region) have been identified in BaTiO 3 (BTO) or BFO nanodots [26][27][28][29][30][31]. In particular, identification of electrically controllable center-type domains and the unique domain wall conduction features in small nanoislands [27][28][29] holds promise for implementation in novel topological memory devices utilizing current readout of topological states [28]. ...
... Ferroic domain structure is rather essential, not only for its role as data storage media in memory devices but also considering its intimate relation with a wide range of functionalities such as piezoelectricity, conductivity, magnetic exchange bias, as well as ME coupling [52,53]. Since domain structures depend critically on various interactions including exchange coupling, strain energy, electrostatic energy, and others, which, however, are highly competitive, REVIEW Tian et al. 687 dimension shrinkage into nanostructures becomes an effective tool to manipulate domain structures and thus relevant properties [26][27][28][29][30][31][54][55][56][57][58][59]. In particular, size-confined ferroic nanostructures favor intriguingly exotic topological domains. ...
... This texture only occurs close to insulating interfaces, indicating the critical role of depolarization energy [16]. It is also worth mentioning that these textures are not the classical Enhanced conductivity in vortex core [17]; High-density memory [19]; Negative capacitance, energy-efficient transistors [79] Six-fold vortex/ anti-vortex RMnO3 single crystal (R=Y, Ho, ..., Lu, Sc): bulk [71] Conductive domain walls [13]; Magnetoelectric effects [73] Flux-closure quadrant STO/PTO multilayers: ∼20-40 nm thick PTO [22]; BTO and PZT nanoplates: lateral size ∼1 μm [54,69,70] High-density non-volatile memories [22] Other closure domain PZT films: ∼11 nm [15]; BFO films: ∼20 nm [16] Ultrahigh-density non-volatile memories [15] Center domain BFO nanoislands: ∼60 nm [27], ∼300 nm [28], ∼300 nm [31] in diameter; BFO thin films: ∼800 nm thick [20] High-density non-volatile memories [27]; Conductive domain walls [28] Skyrmion bubble STO/PTO superlattice: 6-8 nm thick PTO [25] Memories and other electronic nano-devices [25] four-fold quadrant domains with four 90 • walls converging onto a central core, but rather considered as half-closure quadrants with walls converging at angles of 90 • Fig. 2b) [23], elu-cidating that size confinement is another ingredient for stabilizing the vortex domain structure. This effect was further supported by an observation of ultra-small polarization curling and closure domains (∼2 nm in diameter) in several unit-cell thick nonpoled PTO ferroelectric tunneling junctions between Co and (La, Sr)MnO 3 electrodes [24]. ...
Article
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Multiferroic nanostructures have been attracting tremendous attention over the past decade, due to their wealthy cross-coupling effects and prospective electronic applications. In particularly, the emergences of some exotic phenomena in size-confined multiferroic systems, including topological domain states such as vortex, center-domains, and skyrmion bubble domains, open a new avenue to a number of intriguing physical properties and functionalities, and thus underpin a wide range of applications in future nanoelectronic devices. It is also highly appreciated that nano-domain engineering provides a pathway to control the magnetoelectric properties, promising for future energy-efficient spintronic devices. In recent years, this field, yet in its infancy state, has witnessed a rapid development and a number of challenges either. In this article, we shall review the recent advances in the emergent domain-related exotic phenomena in multiferroic nanostructures. Specific attention is paid to the topological domain structures and related novel physical behaviors as well as the electric field driven magnetic switching via domain engineering. This review will be ended with a prospect of future challenges and potential directions.
... Exotic topological domain structures in ferroelectric materials, such as vortices, 1-3 flux-closures, 4-6 center domains, 7-12 skyrmions, [13][14][15] bubbles, [16][17][18][19][20][21] and merons, 22 have been extensively studied to comprehend their physical properties as well as potential for high-density memories, [23][24][25][26][27][28] low-power consumption transistors, 29 and ferroelectricbased nanoelectronic devices. 30 These topological domains exhibit intriguing physical properties, including high stability attributable to topological protection, enhanced piezoelectric properties, 31 enhanced conductive channels, [32][33][34] and negative capacitance 35 among others. ...
... In nanoscale confined systems, e.g., nanodots/nanoislands, the surface or edge effects and flexoelectric effect generated by the possible nonuniform strain can drive the polarization away from their original directions to form more complex domain textures. 7,27,42,43 Therefore, it is intriguing to investigate whether these topological center-type states can maintain stability in structural systems apart from rhombohedral phase via the size effect. For this, we explore the domain structure in the PTO nanodots via tailoring of tetragonal films. ...
Article
Full-text available
In this work, we demonstrated that tunable topological domain structures, e.g., center-type domains and skyrmion-like polar bubbles, can be generated at room temperature in high-density epitaxial PbTiO3 nanodots fabricated via the template-assisted tailoring of thin films. These topological domain structures can be manipulated electrically by applying an appropriate bias on the conductive atomic force microscopy tip, allowing for writing, erasing, and rewriting of topological domains into the nanodot. Moreover, ring-shaped conductive channels are observed around the center-type domain states. These findings provide a playground for further exploring their emerging functionalities and application potentials for nanoelectronics.
... On the other hand, due to the irregularities in size, shape and strain, some regions maybe easier to switching under the depolarization field than others. 40,41 For instance, large strain energy exists near the center of the topological domain where dipoles turn their directions sharply. 34,41 Therefore, the opposite polarizations are very likely to appear in the center of the nanostructure, as shown in Figs. ...
... 40,41 For instance, large strain energy exists near the center of the topological domain where dipoles turn their directions sharply. 34,41 Therefore, the opposite polarizations are very likely to appear in the center of the nanostructure, as shown in Figs. 1(e) and 1(f). ...
Article
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Ferroelectric nanocapacitors have attracted intensive research interest due to their novel functionalities and potential application in nanodevices. However, due to the lack of knowledge of domain evolution in isolated nanocapacitors, precise manipulation of topological domain switching in the nanocapacitor is still a challenge. Here, we report unique bubble and cylindrical domains in the well-ordered BiFeO3 nanocapacitor array. The transformation of bubble, cylindrical and mono domains in isolated ferroelectric nanocapacitor has been demonstrated via scanning probe microscopy (SPM). The bubble domain can be erased to mono domain or written to cylindrical domain and mono domain by positive and negative voltage, respectively. Additionally, the domain evolution rules, which are mainly affected by the depolarization field, have been observed in the nanocapacitors with different domain structures. This work will be helpful in understanding the domain evolution in ferroelectric nanocapacitors and providing guidance on the manipulation of nanoscale topological domains.
... The emerging topological structures in ferroelectric materials, such as vortexes, 1,2 flux-closure domains, 3 central domains, [4][5][6][7][8] skyrmions, 9 and merons, 10 have attracted intensive attention regarding physical mechanism research and potential device functionalities, [11][12][13][14][15][16][17] which open pathways to facilitate and miniaturize future electronic devices. For instance, central domains and vortexes have been observed on BiFeO 3 (BFO) nanoislands, enabling individual access. ...
... 19 In the past few years, substantial efforts have been made to obtain topological domains in BFO nanoislands. 2,[4][5][6][7][8][18][19][20] For instance, Li et al. demonstrate various types of spontaneous centertype domains in high-density ordered nanoisland arrays on SrTiO 3 (STO) by using a developed top-down ion-etching method using a sacrificed template. The domain textures can be stabilized by the charge accumulation on the top surface of the nanoislands. ...
Article
Exotic topological domains in BiFeO 3 nanoislands have attracted much attention regarding their potential applications in advanced electronic devices. Here, different from the earlier reported disordered distributed BiFeO 3 nanoislands formed by a self-assembly method, we fabricated an ordered BiFeO 3 nanoisland array by mask-assisted pulsed laser deposition on a SrTiO 3 substrate, which exhibits a center-converged in-plane polarization component and a monodomain pattern along the vertical direction. Such center-type quad-domain structures exhibit high stability, maintaining their topological structures after heating to 250 °C and subsequently cooling to room temperature. Moreover, they can be switched by applying a scanning electric field and recovered by applying a heating and cooling process. Observing this topological structure in BiFeO 3 nanoislands might provide a suitable platform for further exploration of its topological phase transition properties, new functions, and potential applications.
... [23][24][25] A series of exotic domains have been discovered in ferroelectric nanodots or nanoislands, e.g., vortices, 26,27 flux-closure domains, 28 bubble domains, 29,30 and center domains. [31][32][33][34][35][36][37] In particular, the electric-field-controlled reversible switching and enhanced conductivity of the center-type domains observed in ferroelectric nanoislands make them ideal candidates for future programmable nanoelectronic devices. 33 For instance, center-type topological domains can be reversibly switched between the divergent state with highly conductive confined walls and convergent state with insulating confined walls, demonstrating its potential for data storage applications with nondestructive readout of the topological center-domain state. ...
Article
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We describe the impact of post-annealing on ferroelectric-domain structures in arrays of BiFeO3 (BFO) epitaxial nanoislands, which exhibit a domain evolution from an initial 71° stripe/vortex domains to center-convergent topological domains. These results suggest that the increase and redistribution of charged defects, e.g., oxygen vacancies, in BFO nanoislands play a crucial role in driving the formation of center-type domain structures. The observation of defect-driven domain evolution in BFO nanoislands provides a path for further exploring their formation mechanism, topological properties, novel functionalities, and potential applications.
... These entities can be scaled down to nanometre sizes, controlled with energy-efficient electric fields and detected via a non-destructive resistive read-out, making them interesting for information technologies and in particular memory applications. Among them, topological centre-type domains in ferroelectric thin films are currently garnering attention for their stability, switchability and electrical conduction properties [6][7][8][9][10][11] . Topological defects here refer to local singularity regions in an ordered medium wherein the order parameter ceases to vary continuously 12 . ...
Article
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Topologically protected spin whirls in ferromagnets are foreseen as the cart-horse of solitonic information technologies. Nevertheless, the future of skyrmionics may rely on antiferromagnets due to their immunity to dipolar fields, straight motion along the driving force and ultrafast dynamics. While complex topological objects were recently discovered in intrinsic antiferromagnets, mastering their nucleation, stabilization and manipulation with energy-efficient means remains an outstanding challenge. Designing topological polar states in magnetoelectric antiferromagnetic multiferroics would allow one to electrically write, detect and erase topological antiferromagnetic entities. Here we stabilize ferroelectric centre states using a radial electric field in multiferroic BiFeO3 thin films. We show that such polar textures contain flux closures of antiferromagnetic spin cycloids, with distinct antiferromagnetic entities at their cores depending on the electric field polarity. By tuning the epitaxial strain, quadrants of canted antiferromagnetic domains can also be electrically designed. These results open the path to reconfigurable topological states in multiferroic antiferromagnets.
... In parallel studies, several groups prepared similar self-assembled BFO nanostructures with different growth conditions and found various exotic properties. Through varying the doping level in Nb-doped SrTiO 3 substrates, self-assembled BFO nanoislands with different sizes and densities (Fig. 30b) have been obtained [374]. STEM reveals the center-divergent domain structure, which may be caused by surface charge accumulation and the shape of the nanoislands. ...
Article
Bismuth ferrite (BiFeO3, BFO) as one of the few single-phase room-temperature multiferroics, has aroused ever-increasing enthusiasm in research communities during the past two decades. The robust ferroelectricity, promising magnetoelectric coupling and remarkable optical behaviors of BFO all enrich its physical phenomena and functional properties. The microscopic ferroic domain structures in BFO determine both the static configurations and dynamic behaviors of order parameters, which is the fundamental basis for understanding and controlling of macroscopic properties. Here, we provide a comprehensive and up-to-date review of the intensive research advances of BFO, in the framework of domain engineering. We begin with an introduction to the rich domain structures of BFO and typical domain engineering strategies, such as chemical modification, electrostatic boundary control, strain engineering, substrate engineering, etc. Then, electrical properties (ferroelectricity, piezoelectricity and conduction), magnetoelectric couplings and optical effects (photovoltaic, photocatalytic, mechanical-optical, etc.) modulated by domain engineering in BFO are discussed in sequence. Remarkable electrical, magnetic and optical phenomena at the domain walls of BFO, which have been discovered and intensively explored recently, are also summarized. Finally, remaining challenges and perspectives are proposed for further domain engineering in BFO-based functional materials, devices and applications.
... In experiment, center-type quadrant topological domains can be obtained from the self-assembled methods [31] (square samples) and template methods [32] (circular samples) in BFO nanostructures, which show great differences in domain patterns. Han et al verified the shape effect and the accumulations of surface charge on center-type quadrant domains using a cylindrical and Gauss-distribution sample model [33], which demonstrated the surface charge-induced domain formation mechanisms. Recently, Yang et al discovered in experiment that a center-type quadrant topological domain and quadrant vortex domain can be created at the same time [34]. ...
Article
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Ferroelectric vortices are characterized by their small size and exotic physical properties, which have great potential for application in high-density information storage. However, the influence of the shape of the external field on such topological vortex domain morphology and domain percentage is still lacking research. Here, the ferroelectric domain evolution induced by a circular-, square-, and triangular-shaped surface charge are investigated in BiFeO3 thin films by phase-field simulations. The shape symmetry of the surface charge distribution determines the 4R domain percentage in one single vortex. Triangle-shaped surface charge distribution with odd number symmetry axes has the greatest influence on the percentage of 4R domain, while that of circular- and square-shaped with even number symmetry axes have almost no influence on the 4R domain ratio. Compared to the triangle, an antivortex can be observed around the vortex domain in the circular- and square-shaped surface charge. The rotating triangle affects the domain percentage of the vortex and the emergence of an antivortex. Here, the difference of domain percentage is determined by the choice of nucleation position and the domain switching mechanism. It was found that the percentage of domain along the edge is greater than the corner of the triangle. Among them, 71° domain switching is more likely to happen than 109° and 180° domain switching. These findings provide a fundamental understanding for ferroelectric domain manipulation in a single vortex from an external field with different shapes.
... The density of the self-assembled BiFeO 3 nano-islands grown on NSTO (0.7% wt) substrate with 6 nm in thickness achieves 50 Gbit/inch 2 , which is comparable to the nanocapacitor arrays fabricated using anodic alumina template [28,29]. Besides, based on our previous analyses [30], the density of the nano-islands would drastically increase with the amount of doping in the substrate and it could be much higher than the nanocapacitor arrays. Thus, it is possible to achieve high density nanoislands in a more convenient and economical way. ...
Article
Ferroelectrics exhibit polarization tunable resistance switching behaviors, which are promising for next-generation non-volatile memory devices. For technological applications, thinner nanoscale arrays are expected, which feature with higher density and larger ON/OFF (RON/OFF) ratios in the metal/ferroelectrics/semiconductor heterojunction. Here, we acquire high density BiFeO3 (BFO) nano-islands around 10 nm in thickness displaying a high RON/OFF ratio of 10³, comparable to the tunnel junctions. Moreover, both the macroscopic and microscopic resistive switching behaviors of the present BFO films reveal an unexpected filamentary-type resistive switching which is modified by the charged domain walls in nano-islands dominated by the center-type domains. Particularly, the charged domain walls spontaneously formed within the BFO nano-islands are proposed as the conductive paths based on the redistribution of carriers under the applied voltages. Potential applications for memories with large RON/OFF ratios of such kind of configurable charged domain walls are demonstrated.
... Instead of the flexoelectric effect, it is also reported that the surface charge accumulation in ferroelectric BiFeO 3 nanoislands can modulate the polarization distribution of the nanoislands. 32,33 When PTO is deposited on the SRO-buffered STO substrates in high vacuum, positively charged oxygen vacancies are inevitably formed on the surface of the PTO layer. 34 Thus, we consider that the formation of exotic polarization rotation in our [101]PTO nanostructures may be ascribed to the surface oxygen vacancy accumulation. ...
Article
The topological domains in ferroelectrics have garnered significant attention for their potential applications in nanoelectronics. However, current research is predominantly limited to rhombohedral BiFeO3 materials. To validate the universality of topological domains in non-rhombohedral ferroelectrics, it is crucial to explore the existence and characteristics of topological states in alternative material systems. In this work we successfully construct topological polar structures in PbTiO3 nano-islands with a tetragonal structure. Furthermore, the topological structures can be well manipulated by electric field and mechanical stress, making them switchable between center-divergent structure and center-converging types. Phase-field simulations revealed that the aggregation and redistribution of free charges play a decisive role in the formation and manipulation of topological states. These findings not only verify the feasibility of constructing topological domains in universal ferroelectrics, but also validate the multiple manipulability of these topological domains, displaying their significant potential in high-density nonvolatile memory devices.
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The electrical boundary condition plays an important role in the manipulation of domain structures in low-dimensional ferroelectric materials, especially ferroelectric nanowires. Here, using phase-field simulations, we systematically investigate the influence of electrical boundary conditions on the domain structure in porous PbTiO3 ferroelectric nanowires. Our results demonstrate the formation of four types of domain structures via varying electrical boundary conditions, which possess distinguished local polarization and energy configurations. We further show that the domain structures are also dependent on the nanowire radius, including the breakdown of a metastable concentric toroidal domain structure upon reducing the radius to 14 nm. The present work provides guidance for further experimental studies on the control of polar domain structures through manipulating the electrical boundary condition and the ferroelectric size, which paves the way for developing multi-functions of low-dimensional ferroelectric systems.
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Ferroelectric and multiferroic materials have gained significant attention due to their potential for investigating emergent cross-coupling phenomena among spin, charge, orbit, and lattice in correlated electron systems, as well as promising candidates for prospective applications in advanced industries,e.g. data memory/processing, sensors, actuators, and energy-relevant devices. The structure and dynamic characteristics of ferroelectric domains can significantly affect the physical properties and device functions of materials, such as electrical conductivity, photovoltaics, and magnetoelectric coupling, particularly, novel topological domains can bring many new physical properties. These make it possible to design materials and devices through domain engineering methods. Therefore, exploring the microdomain structures and related physical property is expected to bring new material and device solutions for post-Moore's era information technology. Accurately comprehending domain structures and their corresponding functionalities pose challenges to characterization techniques. In particular, it remains challenging to investigate the dynamics and cross-coupling behaviors at the nanoscale in situ. Nowadays, it is worthwhile to dedicate more attention to the multifunctional scanning probe microscopy technique, as it serves as a versatile and powerful nanoscale probe capable of exploring multifunctionalities. Multi-field stimuli such as electric field, magnetic field, light illumination, strain field, and thermal field, are able to be integrated with the advance scanning probe microscopy technique, made it an ideal platform to in situ manipulate domain structure and its related functional response at the nanoscale. In this article, we give a brief overview on the recent advances of our research group in detection and manipulation of ferroelectric domains and microscopic physical properties through multifunctional scanning probe microscopy technique. Special attention is given to those topological domain structures such as vortex, center domain state and bubble domain in size-confined systems (ultrathin films/multilayers and nanodots/nanoislands) and their associated novel physical phenomena. In addition, the controllability of electric field driven magnetic switching in multiferroic heterostructures is also studied through size effect, interfacial coupling and domain engineering. Finally, we present a concluding summary with some suggestions for future directions. Most of these studies are conducted using the tip probe, so it is nicknamed the “Laboratory experiments based on the tip probe”.
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Topological quad-domain textures in ferroelectric nanoislands have been considered as enablers for nanoelectric devices. However, the fabrication of ordered arrays of ferroelectric islands exhibiting this domain structure is a challenge. By using substrate patterning to create nucleation sites, highly ordered quad-domain ferroelectric polarization configurations were achieved in BiFeO3 nanoisland arrays. Reversible switching of the quad-domain between the center divergent state with highly conductive domain walls and the center convergent state with insulating domain walls can be realized, resulting in a resistance change with a large on/off ratio. This templated growth strategy enables the controllable fabrication of exotic topological domains and sheds light on their applications for configurable electronic devices.
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Ferroelectric crystals feature asymmetric or polar structures that are switchable under an external electric field, holding promise for information storage. Nanoscale ferroelectrics might exhibit various exotic domain configurations and polar topologies, such as full flux-closure, vortex, skyrmion, and meron. These topological domains were theoretically switchable and may give rise to an unusually high density of memory bits. They would also undergo unusual phase transitions and form hidden, collective polar topological states under external stimulations. Similar domains and spin topologies are well known in ferromagnetic materials, and their topological properties and dynamics are under intensive investigation. However, in ferroelectric materials, the coupling of polarizations to spontaneous strains would be so pronounced that the formations of polar topologies were believed to be impossible. How to stabilize the polar topologies in ferroelectrics, especially in nanoscale ferroelectrics, was known as a big challenge. In this overview, we summarize the recent progress in polar topologies in ferroelectric oxides. We start from a review the discovery of polar topologies, including flux-closure quadrant, vortex, bubble, skyrmion, meron lattice, polar waves, and center-type domains. We also focus on the effects of mechanical and electrical boundary conditions and sample size on the formation of topological structures. In the meanwhile, we emphasize the use of aberration-corrected transmission electron microscope which enables to visualize the ion displacement at a sub-Ångström resolution in real space. And at the end, we envision several aspects to be considered in the future, such as imaging three dimensional (3D) atomic morphology of the topological polar structures, exploring novel polar topologies in other possible systems, and addressing the coupling of polar topologies with flexoelectricity by a combination of quantitative transmission electron microscopy and relevant theoretical approaches.
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Self-assembled BiFeO3-CoFe2O4 (BFO-CFO) vertically aligned nanocomposites are promising for logic, memory, and multiferroic applications, primarily due to the tunability enabled by strain engineering at the prodigious epitaxial vertical interfaces. However, local investigations directly revealing functional properties in the vicinity of such critical interfaces are often hampered by the size, geometry, microstructure, and concomitant experimental artifacts. Ferroelectric switching in the presence of lateral distributions of vertical strain thus remains relatively unexplored, with broader implications for all strain-engineered functional devices. By implementing tomographic atomic force microscopy, 3D domain orientation mapping, and spatially-resolved ferroelectric switching movies, local tensile strain significantly impacts the ferroelectric switching, principally by retarding domain nucleation in the BFO nearest to the vertically epitaxial tensile-strained interfaces. The relaxed centers of the BFO pillars become preferred domain nucleation and growth sites for low biases, with up to an order of magnitude change in the edge:center switching ratio for high biases. The new, multi-dimensional imaging approach—and its corresponding insights especially for directly strained interface effects on local properties—thereby advances the fundamental understanding of polarization switching and provides design principles for optimizing functional response in confined nanoferroic systems.
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Exotic ferroelectric topological states (such as vortex state) have received intensive attention in the past decade, creating a new area for exploring the emerging physical phenomena and functionalities, as well as new applications (such as memory). In recent years, a series of discoveries in novel topological states, such as vortex, central domain, skyrmion and meron states, has inspired an upsurge of research interests. Moreover, the effort to manipulate such a topological domain structure hints the possibilities for the local, deterministic control of order parameters so that the static interface conductivity can be successfully controlled at topologically protected domain walls. These encouraging discoveries create a new avenue to the fertile emerging physic phenomena, and offer new possibilities for developing potential high-performance materials and new nano-electronic devices based on these exotic states. In the past decade, this field has developed rapidly and become a hot research topic in ferroelectrics. In this paper, we review the recent progress in the field of exotic topological state in nanoferroelectrics, and discuss some existing problems and potential directions.
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Domain walls in ferroelectric materials attract great interest since they can possess fascinating functionalities. Therefore, it is very important to modulate domain structures. Our recent experiments showed that oxygen vacancy plates could induce charged domain walls with different types. However, the detailed transition behavior between different charged domain walls was not explored. In this work, systematical phase field simulations were performed to reveal the evolution of domain structures with the size and charge density of the oxygen vacancy plate. These results could provide a route to build complex patterns of charged domain walls.
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Nanoscale ferroelectrics are expected to exhibit various exotic domain configurations, such as the full flux-closure pattern that is well known in ferromagnetic materials. Here we observe not only the atomic morphology of the flux-closure quadrant but also a periodic array of flux closures in ferroelectric PbTiO3 films, mediated by tensile strain on a GdScO3 substrate. Using aberration-corrected scanning transmission electron microscopy, we directly visualize an alternating array of clockwise and counterclockwise flux closures, whose periodicity depends on the PbTiO3 film thickness. In the vicinity of the core, the strain is sufficient to rupture the lattice, with strain gradients up to 10(9) per meter. Engineering strain at the nanoscale may facilitate the development of nanoscale ferroelectric devices. Copyright © 2015, American Association for the Advancement of Science.
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We show experimental switching data on microscale capacitors of lead–zirconate–titanate (PZT), which reveal time-resolved domain behavior during switching on a 100 ns scale. For small circular capacitors, an unswitched domain remains in the center while complete switching is observed in square capacitors. The observed effect is attributed to the formation of a vortex domain during polarization switching in small circular capacitors. This dynamical behavior is modeled using the Landau–Lifshitz–Gilbert equations and found to be in agreement with experiment. This simulation implies rotational motion of polarization in the xy plane, a Heisenberg-like result supported by the recent model of Naumov and Fu (2007 Phys. Rev. Lett. 98 077603), although not directly measurable by the present quasi-static measurements.
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Topological defects in ferroic materials are attracting much attention both as a playground of unique physical phenomena and for potential applications in reconfigurable electronic devices. Here, we explore electronic transport at artificially created ferroelectric vortices in BiFeO{sub 3} thin films. The creation of one-dimensional conductive channels activated at voltages as low as 1 V is demonstrated. We study the electronic as well as the static and dynamic polarization structure of several topological defects using a combination of first-principles and phase-field modelling. The modelling predicts that the core structure can undergo a reversible transformation into a metastable twist structure, extending charged domain walls segments through the film thickness. The vortex core is therefore a dynamic conductor controlled by the coupled response of polarization and electron-mobile-vacancy subsystems with external bias. This controlled creation of conductive one-dimensional channels suggests a pathway for the design and implementation of integrated oxide electronic devices based on domain patterning.
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Typically, polarization and strain in ferroelectric materials are coupled, leading to the generally accepted direct relation between polarization and unit-cell tetragonality. Here, by means of high-resolution transmission electron microscopy we map, on the unit-cell scale, the degree of tetragonality and the displacements of cations away from the centrosymmetry positions in an ultrathin epitaxial PbZr0.2Ti0.8O3 film on a SrRuO3 electrode layer deposited on a SrTiO3 substrate. The lattice is highly tetragonal at the centre of the film, whereas it shows reduced tetragonality close to the interfaces. Most strikingly, we find that the maximum off-centre displacements for the central area of the film do not scale with the tetragonality. This challenges the fundamental belief in a strong polarization–tetragonality coupling in PbTiO3-based ferroelectrics, at such thicknesses. Furthermore, a systematic reduction of the atomic displacements is measured at the interfaces, suggesting that interface-induced suppression of the ferroelectric polarization plays a critical role in the size effect of nanoscale ferroelectrics.
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Multiferroic materials showing coupled electric, magnetic and elastic orderings provide a platform to explore complexity and new paradigms for memory and logic devices. Until now, the deterministic control of non-ferroelectric order parameters in multiferroics has been elusive. Here, we demonstrate deterministic ferroelastic switching in rhombohedral BiFeO(3) by domain nucleation with a scanning probe. We are able to select among final states that have the same electrostatic energy, but differ dramatically in elastic or magnetic order, by applying voltage to the probe while it is in lateral motion. We also demonstrate the controlled creation of a ferrotoroidal order parameter. The ability to control local elastic, magnetic and torroidal order parameters with an electric field will make it possible to probe local strain and magnetic ordering, and engineer various magnetoelectric, domain-wall-based and strain-coupled devices.
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Voltage-modulated scanning force microscopy (Piezoresponse microscopy) is applied to investigate the domain structure in epitaxial PbZr0.2Ti0.8O3 ferroelectric thin films grown on (001) SrTiO3. By monitoring the vertical and lateral differential signals from the photodetector of the atomic force microscope it is possible to separate out and observe the out-of-plane and in-plane polarization vectors in the thin film individually. The relative orientation of the polarization vectors across a 90degrees domain wall is observed. Nucleation of new reversed 180degrees domains at the 90degrees domain wall is studied and its impact on the rotation of polarization within the a domain is analyzed as a function of reversal time. (C) 2002 American Institute of Physics.
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Oxide layers grown on the surface provide an effective way of protecting metallic materials against corrosion for sustainable use in a broad range of applications. However, the growth of cavities at the metal/oxide interface weakens the adherence of the protective layer and can promote its spallation under service conditions, as observed for alumina layers formed by selective oxidation of aluminide intermetallic alloys used in high-temperature applications. Here we show that direct atomic-scale observations of the interface between an ultrathin protective oxide layer (alumina) grown on an intermetallic titanium aluminide substrate (TiAl) can be performed with techniques sensitive to the topmost atomic layers at the surface. Nanocavities resulting from the self-assembling of atomic vacancies injected at the interface by the growth mechanism of the protective oxide are observed for the first time, bringing new insight into the understanding of the fate of injected cavities in oxidation processes.
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Distinct and novel features of nanometric electric topological defects, including dipole waves and dipole disclinations, are presently revealed in the PbTiO3 layers of PbTiO3/SrTiO3 multilayer films by means of quantitative high-resolution scanning transmission electron microscopy. These original dipole configurations are confirmed and explained by atomistic simulations and have the potential to act as functional elements in future electronics.
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Continuous developments in nanotechnology require new approaches to materials synthesis that can produce novel functional structures. Here, we show that nanoscale defects, such as nonstoichiometric nanoregions (NSNRs), can act as nano-building blocks for creating complex electrical polarization structures in the prototypical multiferroic BiFeO3. An array of charged NSNRs are produced in BiFeO3 thin films by tuning the substrate temperature during film growth. Atomic-scale scanning transmission electron microscopy imaging reveals exotic polarization rotation patterns around these NSNRs. These polarization patterns resemble hedgehog or vortex topologies and can cause local changes in lattice symmetries leading to mixed-phase structures resembling the morphotropic phase boundary with high piezoelectricity. Phase-field simulations indicate that the observed polarization configurations are mainly induced by charged states at the NSNRs. Engineering defects thus may provide a new route for developing ferroelectric- or multiferroic-based nanodevices.
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Systems that exhibit phase competition, order parameter coexistence, and emergent order parameter topologies constitute a major part of modern condensed-matter physics. Here, by applying a range of characterization techniques, and simulations, we observe that in PbTiO3/SrTiO3 superlattices all of these effects can be found. By exploring superlattice period-, temperature- and field-dependent evolution of these structures, we observe several new features. First, it is possible to engineer phase coexistence mediated by a first-order phase transition between an emergent, low-temperature vortex phase with electric toroidal order and a high-temperature ferroelectric a1/a2 phase. At room temperature, the coexisting vortex and ferroelectric phases form a mesoscale, fibre-textured hierarchical superstructure. The vortex phase possesses an axial polarization, set by the net polarization of the surrounding ferroelectric domains, such that it possesses a multi-order-parameter state and belongs to a class of gyrotr
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A novel mesoscale state comprising of ordered polar vortex lattice has been demonstrated in ferroelectric superlattices of PbTiO3/SrTiO3. Here, we employ phase-field simulations, analytical theory, and experimental observations to evaluate thermodynamic conditions and geometric length scales that are critical for the formation of such exotic vortex states. We show that the stability of these vortex lattices involve an intimate competition between long-range electrostatic, long-range elastic, and short-range polarization gradient-related interactions leading to both an upper- and a lower- bound to the length scale at which these states can be observed. We found that the critical length is related to Pi times the intrinsic domain wall width, which could serve as simple intuitive design rule for the discovery of novel ultrafine topological structures in other ferroic systems.
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Topological structures in multiferroic materials have recently received considerable attention because of their potential use as nanoscale functional elements. Their reduced size in conjunction with exotic arrangement of the ferroic order parameter and potential order parameter coupling allows for emergent and unexplored phenomena in condensed matter and functional materials systems. This will lead to exciting new fundamental discoveries as well as application concepts that exploit their response to external stimuli such as mechanical strain, electric and magnetic fields. In this review we capture the current development of this rapidly moving field with specific emphasis on key achievements that have cast light on how such topological structures in multiferroic materials systems can be exploited for use in complex oxide nanoelectronics and spintronics.
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Here we describe the Wulff shape of strontium titanate nanocuboids prepared by a hydrothermal method and annealed at high temperature. Transmission electron microscopy was used to measure the faceting ratios d(110):d(100) which are compared with surface energy ratios γ(110):γ(100) from first-principles calculations. Internal voids attributed to the Kirkendall effect were also measured and show agreement with the external faceting. Experiment and theory are shown to agree strongly within statistical and density functional theory error.
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This work reports a resistive switching effect observed at rectifying Pt/Bi1–δFeO3 interfaces and the impact of Bi deficiencies on its characteristics. Since Bi deficiencies provide hole carriers in BiFeO3, Bi-deficient Bi1–δFeO3 films act as a p-type semiconductor. As the Bi deficiency increased, a leakage current at Pt/Bi1–δFeO3 interfaces tended to increase, and finally, rectifying and hysteretic current–voltage (I–V) characteristics were observed. In I–V characteristics measured at a voltage-sweep frequency of 1 kHz, positive and negative current peaks originating from ferroelectric displacement current were observed under forward and reverse bias prior to set and reset switching processes, respectively, suggesting that polarization reversal is involved in the resistive switching effect. The resistive switching measurements in a pulse-voltage mode revealed that the switching speed and switching ratio can be improved by controlling the Bi deficiency. The resistive switching devices showed endurance of >105 cycles and data retention of >105 s at room temperature. Moreover, unlike conventional resistive switching devices made of metal oxides, no forming process is needed to obtain a stable resistive switching effect in the ferroelectric resistive switching devices. These results demonstrate promising prospects for application of the ferroelectric resistive switching effect at Pt/Bi1–δFeO3 interfaces to nonvolatile memory.
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We have investigated the effect of the deposition rate, as a direct function of the laser pulse repetition rate, on the surface morphology of CeO2 films deposited by the pulsed laser deposition (PLD) technique on r-cut Al2O3 (11¯02) substrates. The critical thickness is defined as the thickness before the onset of increased growth of large islands and abrupt increase in surface roughening. Two regimes of growth were found within the investigated range of deposition rate. It is found out that in the high deposition rate-regime (within 2-4 nm/min), the critical thickness is ~90 nm, but in the low deposition rate-regime (less than 1 nm/min), the critical thickness is shifted to ~40 nm. Films belonging to these two regimes of crystalline growth were found to have characteristically different formations and surface morphologies. As observed through atomic force microscopy (AFM), the surface morphology is composed of longitudinal islands forming a maze-like pattern in the high deposition rate-regime, while the characteristic morphology was composed of rounded islands in the low deposition rate-regime. Significant reduction in the areal density of large islands and characteristically smoother films was achieved using a low deposition rate.
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The stability balance among the ferroelectric, ferrielectric, and antiferroelectric phases in dielectric materials has been studied by concretely examining the crystallographic features of Pb(Zr1−xTix)O3 samples with 0.07⩽x⩽0.20 by transmission electron microscopy. It was, as a result, found that in this oxide system the increase in the degree of antiferroelectricity into the ferroelectric state resulted in the unique sequence of the phase changes including the appearance of the two-phase state and two ferrielectric phases.
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The application of ferroelectric materials for nonvolatile memory and ferroelectric data storage necessitates quantitative studies of local switching characteristics and their relationship to material microstructure and defects. Switching spectroscopy piezoresponse force microscopy (SS-PFM) is developed as a quantitative tool for real-space imaging of imprint, coercive bias, remanent and saturation responses, and domain nucleation voltage on the nanoscale. Examples of SS-PFM implementation, data analysis, and data visualization are presented for epitaxial lead zirconate titanate (PZT) thin films and polycrystalline PZT ceramics. Several common artifacts related to the measurement method, environmental factors, and instrument settings are analyzed.
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We have presented a systematical study of the domain nucleation and growth behaviors in multiferroic BiFeO3 (BFO) films. Both the ferroelectric and the ferroelastic switching dynamics were investigated. Several environmental parameters, including the polarization orientations, the monodomain-like matrix, and the ordered domain walls as local boundaries, were well controlled by thin-film strain engineering through changing the vicinal angles of the substrates. The tip-based domain dynamics was studied by subsequent piezoresponse force microscope (PFM) imaging of the domain evolution under external voltage pulses. For the nanodomains written in the monodomain-like environment, the domain wall performed the thermal activated motion. The as-grown 71° domain walls can act as pinning centers for the ferroelectric domain growth driven by low fields; moreover, ferroelastic nucleation near a 71° domain wall will cause the deformation of the domain wall. The ferroelastic domain growth possessed relatively small activation fields, and therefore usually performed non-activated motion. This study revealed the effects of local environments on the dynamics forming nanoscale domains, and opened a pathway for applications in novel non-volatile functional devices.
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One problem with the growth of high quality c‐axis oriented YBa 2 Cu 3 O 7-x films is the tendency of the film surface to become rough. We studied the film growth mechanism as a function of deposition rate using pulsed laser deposition. These films form by the classic nucleation and growth process; the thickness at which the nucleated islands coalesce increased with decreasing deposition rate. The film has pinholes prior to coalescence and nucleates outgrowths during coalescence. The outgrowths enlarge rapidly because they contain materials and crystallographic directions with growth rates faster than that of the c‐axis film. A smooth surface is obtained if the substrate temperature and deposition rate are chosen such that coalescence is just completed at the final film thickness. We observed the outgrowths nucleating at coalescence and propose that certain defects, related to the c‐axis growth habit, may be the fundamental cause of outgrowth formation. Outgrowths have not been observed in a‐axis films. Outgrowths are easily confused with the particulate deposition problem associated with laser deposition. In these experiments, the particulate problem was essentially eliminated by using freshly polished targets for each run.
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Aspects of the theory of homotopy groups are described in a mathematical style closer to that of condensed matter physics than that of topology. The aim is to make more readily accessible to physicists the recent applications of homotopy theory to the study of defects in ordered media. Although many physical examples are woven into the development of the subject, the focus is on mathematical pedagogy rather than on a systematic review of applications.
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Using a combination of piezoresponse force microscopy (PFM) and phase-field modeling, we demonstrate ubiquitous formation of center-type and possible ferroelectric closure domain arrangements during polarization switching near the ferroelastic domain walls in (100) oriented rhombohedral BiFeO(3). The formation of these topological defects is determined from the vertical and lateral PFM data and confirmed from the reversible changes in surface topography. These observations provide insight into the mechanisms of tip-induced ferroelastic domain control and suggest that formation of topological defect states under the action of local defect- and tip-induced fields is much more common than previously believed.
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A computationally rapid image analysis method, weighted overdetermined regression, is presented for two-dimensional (2D) Gaussian fitting of particle location with subpixel resolution from a pixelized image of light intensity. Compared to least-squares Gaussian iterative fitting, which is most exact but prohibitively slow for large data sets, the precision of this new method is equivalent when the signal-to-noise ratio is high and approaches it when the signal-to-noise ratio is low, while enjoying a more than 100-fold improvement in computational time. Compared to another widely used approximation method, nine-point regression, we show that precision and speed are both improved. Additionally, weighted regression runs nearly as fast and with greatly improved precision compared to the simplest method, the moment method, which, despite its limited precision, is frequently employed because of its speed. Quantitative comparisons are presented for both circular and elliptical Gaussian intensity distributions. This new image analysis method may be useful when dealing with large data sets such as those frequently met in astronomy or in single-particle and single-molecule tracking using microscopy and may facilitate advances such as real-time quantification of microscopy images.
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Two-dimensional arrays of ferroelectric lead zirconate titanate (PZT) nanodots were fabricated using pulsed laser deposition through ultrathin anodic aluminum oxide membrane stencil masks. The static distribution of polarization configurations was investigated using in- and out-of-plane piezoresponse force microscopy (PFM). The observed presence of an in-plane polarization component in nominally (001) oriented PZT suggests the existence of a significant deviation from the regular tetragonal structure that allows the formation of complex core-polarization states. Core-polarization states may indicate the presence of quasi-toroidal polarization ordering. The experimental results are compared with a theoretical model to determine the fingerprint of a vortex polarization state in PFM.
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Properties of BaTiO3{\mathrm{B}\mathrm{a}\mathrm{T}\mathrm{i}\mathrm{O}}_{3} colloidal quantum dots and wires are simulated using a first-principles-based approach. Large atomic off-center displacements (that are robust against capping matrix materials) are found to exist in very small (<5 nm<5\text{ }\mathrm{n}\mathrm{m}) dots. We further determine the size dependences of electrical and electromechanical responses in the studied nanostructures, as well as provide microscopic understanding of these responses.