Search

Ferroelectric domain walls, across which there is a divergence in polarization, usually have enhanced electrical conductivity relative to bulk. However, in lead germanate, head-to-head and tail-to-tail walls are electrically insulating. Recent studies have shown that this is because, when oppositely oriented domains meet, polar divergence is obviated by a combination of domain bifurcation and suspected local dipolar rotation. To explore the uniqueness, or otherwise, of this microstructure, we have used tomographic piezoresponse force microscopy to map three-dimensional domain morphologies in another uniaxial ferroelectric system: triglycine sulfate. This mapping reveals an abundance of domain wall saddle points, which are characteristic of interlocking bifurcated domains. Conducting atomic force microscopy, performed close to the saddle points, showed no evidence for highly localized conducting domain wall sections, across which a divergence in polarization might be implied; this supports the notion that localized dipolar rotation occurs to minimize any potential polar discontinuity. Overall, our study, therefore, confirms that mutual domain bifurcation and suspected local dipolar rotation are not unique to lead germanate and instead may be widely present in other uniaxial ferroelectrics. [ABSTRACT FROM AUTHOR]

The electrical conductivity of lithium niobate thin film capacitor structures depends on the density of conducting 180 ° domain walls, which traverse the interelectrode gap, and on their inclination angle with respect to the polarization axis. Both microstructural characteristics can be altered by applying electric fields, but changes are time-dependent and relax, upon field removal, into a diverse range of remanent states. As a result, the measured conductance is a complex history-dependent function of electric field and time. Here, we show that complexity in the kinetics of microstructural change, in this ferroelectric system, can generate transport behavior that is strongly reminiscent of that seen in key neurological building blocks, such as synapses. Successive voltage pulses, of positive and negative polarity, progressively enhance or suppress domain wall related conductance (analogous to synaptic potentiation and depression), in a way that depends on both the pulse voltage magnitude and frequency. Synaptic spike-rate-dependent plasticity and even Ebbinghaus forgetting behavior, characteristic of learning and memory in the brain, can be emulated as a result. Conductance can also be changed according to the time difference between designed identical voltage pulse waveforms, applied to top and bottom contact electrodes, in a way that can mimic both Hebbian and anti-Hebbian spike-timing-dependent plasticity in synapses. While such features have been seen in, and developed for, other kinds of memristors, few have previously been realized through the manipulation of conducting ferroelectric domain walls. [ABSTRACT FROM AUTHOR]

This Perspective briefly summarizes what is known about electrically conducting domain walls. It highlights insights into the underlying causes of enhanced current transport, developed despite the frustrations and limitations of the standard two-probe source and drain measurements that have dominated the field to date (because of the pervasive use of conventional conducting atomic force microscopy). This perspective gives a feel for the unique possibilities offered by conducting domain walls, in future forms of agile electronics. Indeed, it is imagined that domain walls and domain wall junctions might eventually allow for entire nanoscale circuits (devices and their interconnects) to be created in one instant, for one purpose, only to be wiped clean and rewritten in a different form, for a different purpose, in the next instant. Malleable domain wall network architecture that can continually metamorphose could represent a kind of technological genie, granting wishes on demand for radical moment-to-moment changes in electronic function. [ABSTRACT FROM AUTHOR]

Charged domain walls form spontaneously in Cu-Cl boracite on cooling through the phase transition. These walls exhibit changed conductivity compared to the bulk and motion consistent with the existence of negative capacitance. Here, we present the dielectric permittivity and DC resistivity of bulk Cu-Cl boracite as a function of temperature (−140 to 150 °C) and frequency (1 mHz to 10 MHz). The thermal behavior of the two observed dielectric relaxations and the DC resistivity is discussed. We propose that the relaxations can be explained by the existence of point defects, most likely local complexes created by a change of valence of Cu and accompanying oxygen vacancies. In addition, the sudden change in resistivity seen at the phase transition suggests that conductive domain walls contribute significantly to the conductivity in the ferroelectric phase. [ABSTRACT FROM AUTHOR]

Simple meso-scale capacitor structures have been made by incorporating thin (~300 nm) single crystal lamellae of KTiOPO4 (KTP) between two coplanar Pt electrodes. The influence that either patterned protrusions in the electrodes or focused ion beam milled holes in the KTP have on the nucleation of reverse domains during switching was mapped using piezoresponse force microscopy imaging. The objective was to assess whether or not variations in the magnitude of field enhancement at localised "hot-spots," caused by such patterning, could be used to both control the exact locations and bias voltages at which nucleation events occurred. It was found that both the patterning of electrodes and the milling of various hole geometries into the KTP could allow controlled sequential injection of domain wall pairs at different bias voltages; this capability could have implications for the design and operation of domain wall electronic devices, such as memristors, in the future. [ABSTRACT FROM AUTHOR]

Domain microstructures in single crystal lamellae of 88%Pb(Zn1/3Nb2/3)O3-12%PbTiO3 (cut from bulk using focused ion beam milling) have been mapped using both piezoresponse force microscopy and transmission electron microscopy. Dramatic changes from mottled microstructures typical of relaxors to larger scale domains typical of ferroelectrics have been noted. Stresses associated with substrate clamping are suspected as the cause for the transition from short- to long-range polar order, akin to effects induced by cation ordering achieved by thermal quenching. [ABSTRACT FROM AUTHOR]

Ba0.5Sr0.5TiO3 (BST) thin-film capacitor structures with various thicknesses, (50–1200 nm) and different strain conditions (on lanthanum strontium cobalt oxide La0.5Sr0.5CoO3 and strontium ruthenate SrRuO3 buffer layers) were made using pulsed laser deposition, and characterized by x-ray diffraction. The out-of-plane lattice parameter was followed as a function of temperature within the 100–300 K temperature interval. The phase sequence (cubic-tetragonal-orthorhombic-rhombohedral) known to exist in the bulk analog is shown to be strongly affected by both the stress conditions imposed by the buffer layer and the thickness of the BST film itself. Thus, no phase transition was found for the in-plane compressed BST films. On the stress-free BST films, on the contrary, more phase transitions were observed. It appeared that the complexity of structural phase transitions increased as the film thickness in this system was reduced. [ABSTRACT FROM AUTHOR]

Thin film Au/Ba[sub 0.5]Sr[sub 0.5]TiO[sub 3]/SrRuO[sub 3] capacitor structures, with a thickness of dielectric varying between ∼70 and ∼950 nm, were deposited on {001} MgO single-crystal substrates using pulsed laser deposition. Low-field dielectric measurements were performed as a function of temperature and frequency. At all temperatures and frequencies, the dielectric response as a function of thickness was found to adhere reasonably well to the so-called “series capacitor model,” from which nominal “bulk” and “interfacial” capacitance components could be extracted. The bulk component showed weak frequency dependence but strong temperature dependence, with a peak in permittivity and dielectric loss around 250 K and 150 K, respectively. Well above 250 K, reasonable Curie–Weiss behavior was evident. Overall, the extracted bulk component behaved much as would be expected in real bulk ceramics or single crystals lending confidence as to the general applicability of the series capacitor model. The functional behavior of the extracted interfacial capacitance was rationalized as being due to a combination of a thermally independent background, and thermally activated space charge. The activation energy of the space charge (∼0.6 eV) is commensurate with the detrapping of electrons from shallow level traps associated with oxygen vacancies. Importantly, since this component acts in series with the bulk component, the functional analysis performed here implies that the oxygen vacancies lie in a plane parallel to the electrodes. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Pulsed laser deposition was used to grow thin films of several compositions from the Pb(Mg[SUB⅓,Nb[SUB⅔)O[SUB3] - PbTiO[SUB3] (PMN-PT) solid solution as dielectric layers in thin film capacitor structures. They were found to display functional behavior characteristic of relaxors, with the only significant difference between thin film and bulk being a severely reduced dielectric permittivity. Room temperature polarization loops showed a general increase in both the remanent polarization and absolute magnitude of the polarization for a given field with PT content. The electric field-induced strain was primarily examined by in situ x-ray diffraction and although fields greater than 20 MV m[SUP-1] were applied to the films, the maximum observed strain was only ∼0.3%, considerably less than the 1.4% seen in single crystal samples. Piezo-response atomic force microscopy studies were consistent in that the macroscopic effective piezoelectric coefficient d[SUB33], was found to differ from single crystal measurements by an order of magnitude. However, electrostrictive coefficients (Q[SUB33]), were found to be comparable to published values for both PMN- PT single crystals and ceramics, ranging between 1 and 9 ×10[SUP-2]C[SUP-4]cm[SUP2]. Consequently, it seems that the relatively poor strain response in these thin films is intimately linked to the reduced permittivity and not to a fundamental difference in the electrostrictive coefficients between bulk and thin film. [ABSTRACT FROM AUTHOR]

Thin film parallel-plate capacitor structures with Ba[sub 0.5]Sr[sub 0.5]TiO[sub 3] as the dielectric layer were fabricated using pulsed-laser deposition. The thickness of the dielectric was varied between 7.5 nm and 1.4 µm. Low-field measurements of the dielectric constant showed the system to possess a nonzero interfacial capacitance. Since there is considerable debate about the origin of interfacial capacitance, further functional analysis was performed: zero-field dielectric susceptibility information was extracted using Landau-Ginzburg-Devonshire fitting of saturated polarization loops. This analysis revealed that the zero-field dielectric susceptibility extracted from P-E loops does not possess a finite interfacial component. The origin of interfacial capacitance therefore disappears (albeit temporarily) after a significant field has been applied and then removed during the P-E loop measurement cycle. [ABSTRACT FROM AUTHOR]

Strongly oriented thin film capacitors of Pb(Mg[sub 1/3]Nb[sub 2/3])O[sub 3] were made by pulsed laser deposition and found to show a decreased temperature of the dielectric maximum (T[sub m]) relative to bulk. The in-plane strain was measured and found to be compressive, a feature that is normally associated with increased Curie temperatures in conventional ferroelectrics. Other features, such as a more pronounced frequency dependence of T[sub m] in thin films than in bulk, were also noted. An attempt to rationalize these differences by consideration of thermal expansion mismatch between substrate and film is presented. The shifts in T[sub m] are modeled using a semiempirical Landau–Ginzburg–Devonshire model, in which the first coefficient of the thermodynamic potential has been modified to have a quadratic, rather than linear, temperature dependence. The predictions of the model are compared with experimental results from this work and other reports on relaxor thin films extracted from the literature. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Thin film capacitors of Au/Pb(Mg[sub 1/3]Nb[sub 2/3])O[sub 3]–7%PbTiO[sub 3] [PMN–(0.07)PT]/(La[sub 1/2]Sr[sub 1/2])CoO[sub 3] were made by pulsed laser deposition on single crystal {001} MgO substrates. The PMN–(0.07)PT dielectric was perovskite dominated, and demonstrated functional behavior typical of relaxors. Electrostrictive behavior was observed at fields up to 200 kV cm[sup -1], however, the maximum strain was disappointingly low at only ∼0.14%. The macroscopic electromechanical d[sub 33] and Q[sub 33] coefficients were determined using piezo-response atomic force microscopy. At 100 kV cm[sup -1] the macroscopic Q[sub 33] was found to be (2.6±0.2)×10[sup -2] C[sup -2] m[sup 4]. The crystallographic electrostrictive coefficient was determined using in-situ x-ray diffraction and at the same field found to be significantly higher: (4.9±0.2)×10[sup -2] C[sup -2] m[sup 4]. Since these electrostrictive coefficients are of the same order of magnitude as found in single crystal experiments (2.5–3.8×10[sup -2] C[sup -2] m[sup 4]), it appears that the low out-of-plane strain is simply the result of poor polarizability in the thin films. An effective Q[sub 13] component of the electrostrictive tensor was also determined, and found to be ∼-0.32 ×10[sup -2] C[sup -2] m[sup 4]. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Recent experimental work shows that ferroelectric switching can occur in large jumps in which ferroelastic superdomains switch together, rather than having the numerous smaller ferroelectric domains switch within them. In this sense, the superdomains play a role analogous to that of Abrikosov vortices in thin superconducting films under the Kosterlitz-Thouless framework, which control the dynamics more than individual Cooper pairs within them do. Here, we examine the dynamics of ferroelastic superdomains in ferroelastic ferroelectrics and their role in switching devices such as memories. Jamming of ferroelectric domains in thin films has revealed an unexpected time dependence of t-1/4 at long times (hours), but it is difficult to discriminate between power-law and exponential relaxation. Other aspects of this work, including spatial period doubling of domains, led to a description of ferroelastic domains as nonlinear processes in a viscoelastic medium, which produce folding and metastable kinetically limited states. This 1/4 exponent is a surprising agreement with the well-known value of 1/4 for coarsening dynamics in viscoelastic media. We try to establish a link between these two processes, hitherto considered unrelated, and with superdomains and domain bundles. We note also that high-Tc superconductors share many of the ferroelastic domain properties discussed here and that several new solar cell materials and metalinsulator transition systems are ferroelastic. [ABSTRACT FROM AUTHOR]

In ferroelectric tunnel junctions, the tunnel resistance depends on the polarization orientation of the ferroelectric tunnel barrier, giving rise to tunnel electroresistance. These devices are promising to be used as memristors in neuromorphic architectures and as non-volatile memory elements. For both applications, device scalability is essential, which requires a clear understanding of the relationship between polarization reversal and resistance change as the junction size shrinks. Here we show a robust tunnel electroresistance in BiFeO3-based junctions with diameters ranging from 1200 to 180 nm. We demonstrate that the tunnel electroresistance and the corresponding fraction of reversed ferroelectric domains change drastically with the junction diameter: while the micron-size junctions display a reversal in less than 10% of the area, the smallest junctions show an almost complete polarization reversal. Modeling the electric-field distribution, we highlight the critical role of the bottom electrode resistance which significantly diminishes the actual electric field applied to the ferroelectric barrier in the mixed polarization state. A polarization-dependent critical electric field below which further reversal is prohibited is found to explain the large differences between the ferroelectric switchability of nano- and micron-size junctions. Our results indicate that ferroelectric junctions are downscalable and suggest that specific junction shapes facilitate complete polarization reversal. [ABSTRACT FROM AUTHOR]

The standard "Kittel Law" for the thickness and shape of ferroelectric, ferroelastic, or ferromagnet domains assumes mechanical equilibrium. The present paper shows that such domains may be highly nonequilibrium, with unusual thicknesses and shapes. In lead germanate and multiferroic lead zirconate titanate iron tantalate domain wall instabilities resemble hydrodynamics (Richtmyer-Meshkov and Helfrich-Hurault, respectively). [ABSTRACT FROM AUTHOR]

Conducting atomic force microscopy images of bulk semiconducting BaTiO3 surfaces show clear stripe domain contrast. High local conductance correlates with strong out-of-plane polarization (mapped independently using piezoresponse force microscopy), and current-voltage characteristics are consistent with dipole-induced alterations in Schottky barriers at the metallic tip-ferroelectric interface. Indeed, analyzing current-voltage data in terms of established Schottky barrier models allows relative variations in the surface polarization, and hence the local domain structure, to be determined. Fitting also reveals the signature of surface-related depolarizing fields concentrated near domain walls. Domain information obtained from mapping local conductance appears to be more surface-sensitive than that from piezoresponse force microscopy. In the right materials systems, local current mapping could therefore represent a useful complementary technique for evaluating polarization and local electric fields with nanoscale resolution. [ABSTRACT FROM AUTHOR]

Recently, lead iron tantalate/lead zirconium titanate (PZTFT) was demonstrated to possess large, but unreliable, magnetoelectric coupling at room temperature. Such large coupling would be desirable for device applications but reproducibility would also be critical. To better understand the coupling, the properties of all 3 ferroic order parameters, elastic, electric, and magnetic, believed to be present in the material across a range of temperatures, are investigated. In high temperature elastic data, an anomaly is observed at the orthorhombic mm2 to tetragonal 4mm transition, Tot = 475 K, and a softening trend is observed as the temperature is increased toward 1300 K, where the material is known to become cubic. Thermal degradation makes it impossible to measure elastic behavior up to this temperature, however. In the low temperature region, there are elastic anomalies near ≈40 K and in the range 160-245 K. The former is interpreted as being due to a magnetic ordering transition and the latter is interpreted as a hysteretic regime of mixed rhombohedral and orthorhombic structures. Electrical and magnetic data collected below room temperature show anomalies at remarkably similar temperature ranges to the elastic data. These observations are used to suggest that the three order parameters in PZTFT are strongly coupled. [ABSTRACT FROM AUTHOR]