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Describing climate change in terms of spatial velocity is essential to assess the ability for ecosystems or individual species to migrate at a sufficient pace to keep environmental conditions allowing their survival. While climate models provide a temporal evolution of a number of variables at each point of their computational grid, Loarie et al. introduced a velocity of climate change, defined as the ratio of the temporal derivative to the spatial gradient of temperature, or any other variable such as precipitations [1]. This amounts to assume that isotherms shift along the temperature gradient. Although intuitive, this idea is mathematically correct only for straight isotherms parallel to each other [2]. Whenever this condition is not met, e.g., due to complex topography or coastlines, the gradient-based velocity field will display artefacts in the form of local convergence or divergence that are likely to bias the analysis.We show that these artefacts can be fixed by defining a much more regular velocity field. This alternative approach to the velocity of climate change determines the direction of the velocity vector by minimising the local vorticity rather than by the gradient. From a fundamental point of view, the resulting smoother velocity field allow an analysis at finer temporal and spatial scales. It also allows to define the climate trajectory of a given origin point. Our approach also provides tools to estimate the stability of climate trajectories depending on the behaviour of their "return" trajectory obtained by reversing time [3]. From an ecological point of view, we discuss preliminary results on the relevance of each definition of the velocity of climate change, based on comparisons of the obtained climate trajectories with ecological trajectories from observational data relative to species distribution areas.References 1. S. R. Loarie et al., Nature 462, 1052 (2009) 2. J. Rey, G. Rohat, M. Perroud, S. Goyette, J. Kasparian, Env. Res. Lett. 15, 034027 (2020) 3. I. Gaponenko, G. Rohat, S. Goyette, P. Paruch, J. Kasparian, Sci. Rep., 12, 2997, (2022)

International audience; Second-harmonic generation (SHG) is a nonlinear optical method allowing the study of the local structure, symmetry, and ferroic order in noncentrosymmetric materials such as ferroelectrics. The combination of SHG microscopy with local polarization analysis is particularly efficient for deriving the local polarization orientation. This, however, entails the use of tedious and time-consuming modeling methods of nonlinear optical emission. Moreover, extracting the complex domain structures often observed in thin films requires a pixel-by-pixel analysis and the fitting of numerous polar plots to ascribe a polarization angle to each pixel. Here, the domain structure of GeTe films is studied using SHG polarimetry assisted by machine learning. The method is applied to two film thicknesses: A thick film containing large domains visible in SHG images, and a thin film in which the domains' size is below the SHG resolution limit. Machine learning-assisted methods show that both samples exhibit four domain variants of the same type. This result is confirmed in the case of the thick film, both by the manual pixel-by-pixel analysis and by using piezoresponse force microscopy. The proposed approach foreshows new prospects for optical studies by enabling enhanced sensitivity and high throughput analysis.

Ferroelectrics are technologically important, with wide application in micromechanical systems, nonlinear optics, and information storage. Recent discoveries of exotic polarisation textures in these materials, which can strongly influence their properties, have brought to the forefront questions about the nature of their domain walls -- long believed to be primarily Ising, with locally null polarisation. Here, combining three complementary techniques -- second harmonic generation microscopy, piezoresponse force microscopy, and transmission electron microscopy - to cover all the relevant lengthscales, we reveal the N\'eel character (non-Ising polarisation oriented perpendicular to the wall) of 180{\deg} domain walls in c-phase tetragonal ferroelectric lead titanate epitaxial thin films, for both artificial and intrinsic domains at room temperature. Furthermore, we show that variations in the domain density -- detected both optically and via local piezoresponse, then quantified by radial autocorrelation analysis -- can give us insight into the underlying defect potential present in these materials., Comment: Main text: 17 pages, 5 figures. Supporting information: 23 pages, 10 figures

Electrochemical phenomena in ferroelectrics are of particular interest for catalysis and sensing applications, with recent studies highlighting the combined role of the ferroelectric polarisation, applied surface voltage and overall switching history. Here, we present a systematic Kelvin probe microscopy study of the effect of relative humidity and polarisation switching history on the surface charge dissipation in ferroelectric Pb(Zr0.2Ti0.8)O3 thin films. We analyse the interaction of surface charges with ferroelectric domains through the framework of physically constrained unsupervised machine learning matrix factorisation, Dictionary Learning, and reveal a complex interplay of voltage-mediated physical processes underlying the observed signal decays. Additional insight into the observed behaviours is given by a Fitzhugh–Nagumo reaction–diffusion model, highlighting the lateral spread and charge passivation process contributors within the Dictionary Learning analysis., The authors acknowledge Dr Sergei V. Kalinin of Oak Ridge National Laboratory, for helpful discussions about machine learning and the initial suggestion to explore reaction–diffusion modelling. This work was supported by Division II of the Swiss National Science Foundation under project 200021_178782. A.V. acknowledges support by the Spanish Government under the project PID2019-110907GB-I00 and the “Severo Ochoa” Program for Centres of Excellence in R&D (CEX2019-000917-S). N.D. acknowledges support by the Spanish Government under the project PID2019- 109931GB-I00. N.B.G. acknowledges support by the National Science Foundation under the project DMR-20269676. The authors would like to thank S. Muller for technical support., With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).

Understanding and controlling the motion, stability, and equilibrium configuration of ferroelectric domain walls is key for their integration into potential nanoelectronics applications, such as ferroelectric racetrack memories. Using piezoresponse force microscopy we analyse the growth and roughness of ferroelectric domains in epitaxial thin film Pb(Zr$_{0.2}$Ti$_{0.8}$)O$_3$, driven by the electric fields at straight edges of planar electrodes at two different temperatures. This device relevant geometry allows us to confirm that the domain walls are well described as 1-dimensional monoaffine elastic interfaces driven in random-bond disorder. However, we observe a progressive increase of roughness as initially flat domain walls move through the disorder landscape, which could prove a significant limiting factor for racetrack-type memories using ferroelectrics., To appear in Applied Physics Letters (accepted)

Ferroelectric materials provide a useful model system to explore the jerky, highly nonlinear dynamics of elastic interfaces in disordered media. The distribution of nanoscale switching event sizes is studied in two Pb(Zr_{0.2}Ti_{0.8})O_{3} thin films with different disorder landscapes using piezoresponse force microscopy. While the switching event statistics show the expected power-law scaling, significant variations in the value of the scaling exponent τ are seen, possibly as a consequence of the different intrinsic disorder landscapes in the samples and of further alterations under high tip bias applied during domain writing. Importantly, higher exponent values (1.98-2.87) are observed when crackling statistics are acquired only for events occurring in the creep regime. The exponents are systematically lowered when all events across both creep and depinning regimes are considered-the first time such a distinction is made in studies of ferroelectric materials. These results show that distinguishing the two regimes is of crucial importance, significantly affecting the exponent value and potentially leading to incorrect assignment of universality class.

Self-affine rough interfaces are ubiquitous in experimental systems, and display characteristic scaling properties as a signature of the nature of disorder in their supporting medium, i.e. of the statistical features of its heterogeneities. Different methods have been used to extract roughness information from such self-affine structures, and in particular their scaling exponents and associated prefactors. Notably, for an experimental characterization of roughness features, it is of paramount importance to properly assess sample-to-sample fluctuations of roughness parameters. Here, by performing scaling analysis based on displacement correlation functions in real and reciprocal space, we compute statistical properties of the roughness parameters. As an ideal, artifact-free reference case study and particularly targeting finite-size systems, we consider three cases of numerically simulated one-dimensional interfaces: (i) elastic lines under thermal fluctuations and free of disorder, (ii) directed polymers in equilibrium with a disordered energy landscape, and (iii) elastic lines in the critical depinning state when the external applied driving force equals the depinning force set by disorder. Our results shows that sample-to-sample fluctuations are rather large when measuring the roughness exponent. These fluctuations are also relevant for roughness amplitudes. Therefore a minimum of independent interface realizations (at least a few tens in our numerical simulations) should be used to guarantee sufficient statistical averaging, an issue often overlooked in experimental reports., Significant overlap with arXiv:1904.11726

Research in materials science increasingly depends on the correlation of information from multiple characterisation techniques, acquired in ever larger datasets. Efficient methods of processing and storing these complex datasets are therefore crucial. Reliably keeping track of data processing is also essential to conform with the goals of open science. Here, we introduce Hystorian, a generic materials science data analysis Python package built at its core to improve the traceability, reproducibility, and archival ability of data processing. Proprietary data formats are converted into open hierarchical data format (HDF5) files, with both datasets and subsequent workflows automatically stored into a single location, thus allowing easy management of multiple data types. At present, Hystorian provides a basic scanning probe microscopy and x-ray diffraction analysis toolkit, and is readily extensible to suit user needs. It is also able to wrap over any existing processing functions, making it easy to append in an extant workflow.

Describing the spatial velocity of climate change is essential to assessing the challenge of natural and human systems to follow its pace by adapting or migrating sufficiently fast. We propose a fully-determined approach, “MATCH”, to calculate a realistic and continuous velocity field of any climate parameter, without the need for ad hoc assumptions. We apply this approach to the displacement of isotherms predicted by global and regional climate models between 1950 and 2100 under the IPCC-AR5 RCP 8.5 emission scenario, and show that it provides detailed velocity patterns especially at the regional scale. This method thus favors comparisons between models as well as the analysis of regional or local features. Furthermore, the trajectories obtained using the MATCH approach are less sensitive to inter-annual fluctuations and therefore allow us to introduce a trajectory regularity index, offering a quantitative perspective on the discussion of climate sinks and sources.

P.P. and C.W. acknowledge partial support by Swiss National Science Foundation Division II grant 200021_178782. L.R.D. acknowledges support from the US National Science Foundation under grant DMR‐1708615. L.W.M. acknowledges support from the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division under Contract No. DE‐AC02‐05‐CH11231 (Materials Project program KC23MP) for the growth and study of defect structures in ferroic materials. A.B.N. gratefully acknowledges support from the Engineering and Physics Sciences Research Council (EPSRC) through grants EP/R023751/1 and EP/L017008/1. Deterministic control of the intrinsic polarization state of ferroelectric thin films is essential for device applications. Independently of the well-established role of electrostatic boundary conditions and epitaxial strain, the importance of growth temperature as a tool to stabilize a target polarization state during thin film growth is shown here. Full control of the intrinsic polarization orientation of PbTiO3 thin films is demonstrated-from monodomain up, through polydomain, to monodomain down as imaged by piezoresponse force microscopy-using changes in the film growth temperature. X-ray diffraction and scanning transmission electron microscopy reveal a variation of c-axis related to out-of-plane strain gradients. These measurements, supported by Ginzburg-Landau-Devonshire free energy calculations and Rutherford backscattering spectroscopy, point to a defect mediated polarization gradient initiated by a temperature dependent effective built-in field during growth, allowing polarization control not only under specific growth conditions, but ex-situ, for subsequent processing and device applications. Publisher PDF

Juxtacellular interactions play an essential but still not fully understood role in both normal tissue development and tumour invasion. Using proliferating cell fronts as a model system, we explore the effects of cell-cell interactions on the geometry and dynamics of these one-dimensional biological interfaces. We observe two distinct scaling regimes of the steady state roughness of in-vitro propagating Rat1 fibroblast cell fronts, suggesting different hierarchies of interactions at sub-cell lengthscales and at a lengthscale of 2–10 cells. Pharmacological modulation significantly affects the proliferation speed of the cell fronts, and those modulators that promote cell mobility or division also lead to the most rapid evolution of cell front roughness. By comparing our experimental observations to numerical simulations of elastic cell fronts with purely short-range interactions, we demonstrate that the interactions at few-cell lengthscales play a key role. Our methodology provides a simple framework to measure and characterise the biological effects of such interactions, and could be useful in tumour phenotyping.Proliferating cell fronts underpin wound healing, tumour invasion, and morphogenesis, and are governed by a complex and still not fully understood interplay of multiple mechanical and chemical factors. From a physics perspective, these cell fronts can be described as elastic interfaces in disordered media. Thus, analysing their geometric properties and dynamics can reveal the characteristic lengthscales of the dominant interactions, and point towards the underlying biological pathways. Here, we show that the roughness of proliferating fronts of Rat1 fibroblasts is governed by two different hierarchies of interactions, with distinct behaviour at sub-cell and few-cell lengthscales. Using in-vitro scratch assays, we find moreover that pharmacological modulators significantly affect the proliferation speed of the cell fronts as well as the evolution of their roughness, increased when cell-cell communication via gap junctions is perturbed, and decreased when cell division is repressed. We determine that our experimental observations cannot be reproduced by numerical simulations of elastic cell fronts with purely short-range interactions, demonstrating the key role of juxtacellular interactions at few-cell lengthscales. Our approach provides a simple framework to measure and characterise the biological effects of such interactions, and could be useful in tumour phenotyping.

Ferroelectric materials provide a useful model system to explore the jerky, highly nonlinear dynamics of elastic interfaces in disordered media. The distribution of nanoscale switching event sizes is studied in two Pb(Zr_{0.2}Ti_{0.8})O_{3} thin films with different disorder landscapes using piezoresponse force microscopy. While the switching event statistics show the expected power-law scaling, significant variations in the value of the scaling exponent τ are seen, possibly as a consequence of the different intrinsic disorder landscapes in the samples and of further alterations under high tip bias applied during domain writing. Importantly, higher exponent values (1.98-2.87) are observed when crackling statistics are acquired only for events occurring in the creep regime. The exponents are systematically lowered when all events across both creep and depinning regimes are considered-the first time such a distinction is made in studies of ferroelectric materials. These results show that distinguishing the two regimes is of crucial importance, significantly affecting the exponent value and potentially leading to incorrect assignment of universality class.

The combination of scanning probe microscopy and ambient pressure X-ray photoelectron spectroscopy opens up new perspectives for the study of combined surface chemical, electrochemical and electromechanical properties at the nanoscale, providing both nanoscale resolution of physical information and the chemical sensitivity required to identify surface species and bulk ionic composition. In this work, we determine the nature and evolution over time of surface chemical species obtained after water-mediated redox reactions on Pb(Zr0.2,Ti0.8)O3 thin films with opposite as-grown polarization states. Starting with intrinsically different surface chemical composition on the oppositely polarized films (as a result of their ferroelectric-dominated interaction with environmental water), we identify the reversible and irreversible electrochemical reactions under an external electric field, distinguishing switching and charging events. We find that while reversible ionic displacements upon polarization switching dominate screening in the bulk of the sample, polarization dependent irreversible redox reactions determine surface chemical composition, which reveals itself as a characteristic fingerprint of the ferroelectric polarization switching history.

Radio-frequency off-axis magnetron sputtering is a well established technique to produce high quality epitaxial thin films of complex oxides. It has been successfully used for over two decades to grow thin films, superlattices and even solid solutions. The main drawback is the common lack of in situ monitoring of the growth, which can significantly slow down the optimisation of the many growth parameters. However, once the optimal parameters are found, they are usually very stable in time, leading to consistently high quality thin films. One of the main growth parameters is the growth temperature, with typical optimal ranges as narrow as 20 °C. Here, using the prototypical ferroelectric PbTiO3 as a model system, we show that by periodically interrupting the deposition process to allow the deposited material to relax, we can significantly increase the temperature range over which we obtain atomically flat surfaces to more than 50 °C. Moreover, the overall crystalline quality is greatly improved, as shown by X-ray diffraction. Finally, we demonstrate the applicability of this method to other materials.

Since its inception, scanning probe microscopy (SPM) has established itself as the tool of choice for probing surfaces and functionalities at the nanoscale. Although recent developments in the instrumentation have greatly improved the metrological aspects of SPM, it is still plagued by the drifts and nonlinearities of the piezoelectric actuators underlying the precise nanoscale motion. In this work, we present an innovative computer-vision-based distortion correction algorithm for offline processing of functional SPM measurements, allowing two images to be directly overlaid with minimal error – thus correlating position with time evolution and local functionality. To demonstrate its versatility, the algorithm is applied to two very different systems. First, we show the tracking of polarisation switching in an epitaxial Pb(Zr0.2Ti0.8)O3 thin film during high-speed continuous scanning under applied tip bias. Thanks to the precise time-location-polarisation correlation we can extract the regions of domain nucleation and track the motion of domain walls until the merging of the latter in avalanche-like events. Secondly, the morphology of surface folds and wrinkles in graphene deposited on a PET substrate is probed as a function of applied strain, allowing the relaxation of individual wrinkles to be tracked.

From the survival of biological systems to the surface electrochemistry of materials, ambient and surface water plays a key role, and understanding its contribution is essential to describe the observed behaviours fully. Thus, precise measurement and control of relative humidity are essential in laboratory applications. In this paper, we present an open source hardware and software design of a low-noise and high precision standalone relative humidity controller, whose versatility and potential applications we demonstrate through basic calibration and performance tests. The hardware and software implementation are discussed in detail, with all plans and codes provided.

© 2015 AIP Publishing LLC. Control over the localised conductance recently observed at ferroelectric domain walls is key for their integration into potential nanoelectronics devices. Using a combination of piezoresponse force microscopy and local conductance mapping, we demonstrate switching between conducting and insulating behavior at 180° domain walls in epitaxial Pb(Zr0.2Ti0.8)O3thin films subjected to ultrahigh vacuum thermal annealing or exposed to ambient conditions, respectively. The reversibility of this behavior is demonstrated in repeated annealing-exposure cycles. To explain these observations, we propose a mechanism based on changes in electrostatic and chemical boundary conditions through oxygen vacancy redistribution and the removal of surface adsorbates.

A multiscale model has been implemented that provides accurate predictions of the behaviour of ferroelectric materials in electric fields, and might aid efforts to design devices such as sensors and digital memory. See Letter p.360

In ferroelectrics, domain walls separate regions with different polarization. At domain walls, modified symmetry and electronic structure, or chemical segregation can lead to unusual, extremely localized functional properties, quite different from those of the parent phase. Particularly exciting has been the discovery of domain-wall-specific electrical conductivity in multiferroic BiFeO3 [1]. Here, I will present our generalization of these observations to Pb(Zr,Ti)O3 [2], highlighting the key role of oxygen vacancies, whose distribution can be modulated to reversibly control domain wall transport. For these studies, we have also developed carbon-nanotube-based probes with exceptional performance [3]. [1] Seidel, Nat. Mat. 8, 229 (2009) [2] Guyonnet, Adv. Mat. 23, 5377 (2011) [3] Lisunova, Nano Lett. 13, 4527 (2013)

Using conductive and piezoforce microscopy, we reveal a complex picture of electronic transport at weakly conductive 109 degrees domain walls in bismuth ferrite films. Even once initial ferroelectric stripe domains are changed/erased, persistent conductive paths signal the original domain wall position. The conduction at such domain wall "footprints" is activated by domain movement and decays rapidly with time, but can be re-activated by opposite polarity voltage. The observed phenomena represent true leakage conduction rather than merely displacement currents. We propose a scenario of hopping transport in combination with thermionic injection over interfacial barriers controlled by the ferroelectric polarization. (C) 2014 AIP Publishing LLC.

Using the model system of ferroelectric domain walls, we explore the effects of long-range dipolar interactions and periodic ordering on the behavior of pinned elastic interfaces. In piezoresponse force microscopy studies of the characteristic roughening of intrinsic 71{\deg} stripe domains in BiFeO$_3$ thin films, we find unexpectedly high values of the roughness exponent {\zeta} = 0.74 $\pm$ 0.10, significantly different from those obtained for artificially written domain walls in this and other ferroelectric materials. The large value of the exponent suggests that a random field-dominated pinning, combined with stronger disorder and strain effects due to the step-bunching morphology of the samples, could be the dominant source of pinning in the system., Comment: 5 pages, 4 figures

Carbon nanotubes used as conductive atomic force microscopy probes are expected to withstand extremely high currents. However, in existing prototypes, significant self-heating results in rapid degradation of the nanotube probe. Here, we investigate an alternative probe design, fabricated by dielectric encapsulation of multiwalled carbon nanotubes, which can support unexpectedly high currents with extreme stability. We show that the dielectric coating acts as a reservoir for Joule heat removal, and as a chemical barrier against thermal oxidation, greatly enhancing transport properties. In contact with Au surfaces, these probes can carry currents of 0.12 mA at a power of 1.5 mW and show no measurable change in resistance at current densities of 10(12) A/m(2) over a time scale of 10(3) s. Our observations are in good agreement with theoretical modeling and exact numerical calculations, demonstrating that the enhanced transport characteristics of such probes are governed by their more effective heat removal mechanisms.

The competition between elasticity and pinning of an interface in a fluctuating potential energy landscape gives rise to characteristic self-affine roughening and a complex dynamic response to applied forces. This statistical physics approach provides a general framework in which the behaviour of systems as diverse as propagating fractures, wetting lines, burning fronts or surface growth can be described. Domain walls separating regions with different polarisation orientation in ferroelectric materials are another example of pinned elastic interfaces, and can serve as a particularly useful model system. Reciprocally, a better understanding of this fundamental physics allows key parameters controlling domain switching, growth, and stability to be determined, and used to improve the performance of ferroelectric materials in applications such as memories, sensors, and actuators. In this review, we focus on piezoresponse force microscopy measurements of individual ferroelectric domain walls, allowing their static configuration and dynamic response to be accessed with nanoscale resolution over multiple orders of length scale and velocity. Combined with precise control over the applied electric field, temperature, and strain, and the ability to influence the type and density of defects present in the sample, this experimental system has allowed not only a direct demonstration of creep motion and roughening, but provides an opportunity to test less-well-understood aspects of out-of-equilibrium behaviour, and the effects of greater complexity in the structure of both the interface and the disorder landscape pinning it.

To minimize parasitic doping effects caused by uncontrolled material adsorption, graphene is often investigated under vacuum. Here we report an entirely unexpected phenomenon occurring in vacuum systems, namely strong n-doping of graphene due to chemical species generated by common ion high-vacuum gauges. The effect --reversible upon exposing graphene to air-- is significant, as doping rates can largely exceed 10^{12} cm^{-2}/hour, depending on pressure and the relative position of the gauge and the graphene device. It is important to be aware of the phenomenon, as its basic manifestation can be mistakenly interpreted as vacuum-induced desorption of p-dopants., 10 pages, 4 figures

Using high-quality epitaxial c-axis Pb(Zr0.2Ti0.8)O3 films grown by off-axis magnetron sputtering onto metallic (001) Nb-doped SrTiO3 substrates, a nonconventional thin-film surface acoustic wave device based on periodic piezoelectric transducers was realized. The piezoelectric transducers consist of a series of ferroelectric domains with alternating polarization states. The artificial modification of the ferroelectric domain structure is performed by using an atomic force microscope tip as a source of electric field, allowing local switching of the polarization. Devices with 1.2 and 0.8μm wavelength, defined by the modulation period of the polarization, and corresponding to central frequencies in the range 1.50–3.50GHz have been realized and tested.

We demonstrate the construction of a novel low-noise continuous flow humidity controller and its integration with a commercial variable-temperature atomic force microscope fluid cell, allowing precise control of humidity and temperature at the sample during nanoscale measurements. Based on wet and dry gas mixing, the design allows a high mechanical stability to be achieved by means of an ultrasonic atomiser for the generation of water-saturated gas, improving upon previous bubbler-based architectures. Water content in the flow is measured both at the inflow and outflow of the fluid cell, enabling the monitoring of water condensation and icing, and allowing controlled variation of the sample temperature independently of the humidity. To benchmark the performance of the controller, the results of detailed noise studies and time-based imaging of the formation of ice layers on highly oriented pyrolytic graphite are shown.

The combination of complementary measurement techniques has become a frequent approach to improve scientific knowledge. Pairing of the high lateral resolution scanning force microscopy (SFM) with the spectroscopic information accessible through scanning transmission soft x-ray microscopy (STXM) permits assessing physical and chemical material properties with high spatial resolution. We present progress from the NanoXAS instrument towards using an SFM probe as an x-ray detector for STXM measurements. Just by the variation of one parameter, the SFM probe can be utilised to detect either sample photo-emitted electrons or transmitted photons. This allows the use of a single probe to detect electrons, photons and physical forces of interest. We also show recent progress and demonstrate the current limitations of using a high aspect ratio coaxial SFM probe to detect photo-emitted electrons with very high lateral resolution. Novel probe designs are proposed to further progress in using an SFM probe as a STXM detector.

We have studied the built-in voltage in heterostructures composed of ferroelectric PbTiO3 and dielectric spacers of SrTiO3, grown on SrTiO3 substrates with SrRuO3 bottom electrodes. We used piezoresponse force microscopy to determine the domain structure of the samples and perform local switching experiments. By increasing the thickness of the SrTiO3 spacer layers, we were able to tune the polarization configuration in the PbTiO3 films by gradually going from a monodomain configuration to a polydomain one. The built-in voltage was found to be linked to the choice of electrodes and to the thickness of the spacer layer. The stability of artificially poled regions was also affected, with a faster back-switching for the samples with the thicker SrTiO3 spacers. We have additionally compared samples with and without a top SrRuO3 electrode, showing that a top electrode layer results in a stronger built-in voltage, responsible for a preferential monodomain configuration and a very fast back-switching of artificially poled regions.

Using piezoresponse force microscopy on epitaxial ferroelectric thin films, we have measured the evolution of domain wall roughening as a result of heat-quench cycles up to 735C, with the effective roughness exponent \zeta\ changing from 0.25 to 0.5. We discuss two possible mechanisms for the observed \zeta\ increase: a quench from a thermal 1-dimensional configuration, and from a locally-equilibrated pinned configuration with a crossover from a 2- to 1-dimensional regime. We find that the post-quench spatial structure of the metastable states, qualitatively consistent with the existence of a growing dynamical length scale whose ultra slow evolution is primarily controlled by the defect configuration and heating process parameters, makes the second scenario more plausible. This interpretation suggests that pinning is relevant in a wide range of temperatures, and in particular, that purely thermal domain wall configurations might not be observable in this glassy system. We also demonstrate the crucial effects of oxygen vacancies in stabilizing domain structures., Comment: 17 pages (preprint), 4 figures

Ferroelectric domain switching in c-axis-oriented epitaxial Pb(Zr0.2Ti0.8)O3 thin films was studied using different field geometries and compared to numerical simulations and theoretical predictions. With carbon nanotubes as electrodes, continuous nanodomains as small as 9 nm in radius in a 270 nm thick film could be switched, remaining stable for over 20 months. Defect pinning of domain walls appears to play a key role in stabilizing such domains, below the predicted thermodynamic size limit., Comment: 5 pages, 4 figures

Among the recent discoveries of domain wall functionalities, the observation of electrical conduction at ferroelectric domain walls in the multiferroic insulator BiFeO3 has opened exciting new possibilities. Here, we report evidence of electrical conduction also at 180{\deg} ferroelectric domain walls in the simpler tetragonal ferroelectric PZT thin films. The observed conduction shows nonlinear, asymmetric current-voltage characteristics, thermal activation at high temperatures and high stability. We relate this behavior to the microscopic structure of the domain walls, allowing local defects segregation, and the highly asymmetric nature of the electrodes in our local probe measurements.

Ferroelectric switching in BiFeO3 multiferroic thin films was studied by piezoresponse force microscopy, as a function of the tip voltage and sweep direction, for samples with two different intrinsic domain structures. In all films, the switched polarization direction follows the in-plane and out-of-plane components of the highly inhomogeneous electric field applied by the microscope tip. In films with 'bubble-like' intrinsic domains, we observed in-plane switching assisted by out-of-plane switching for lower voltage values, and independent in-plane and out-of-plane switching for higher voltages, in both cases allowing full control of the ferroelectric polarization depending on the tip voltage polarity and sweep direction. In films with 'stripe-like' intrinsic domains, independent in-plane and out-of-plane switching was observed, but unswitched stripe domains prevented full control of the ferroelectric polarization over large areas. We correlate the observed switching behavior with the field-driven onset of a highly distorted tetragonal phase predicted by ab initio calculations, which leads to a very high in-plane susceptibility during the return to the non-distorted monoclinic phase when the field is decreased. Depending on the specific strain and disorder present in the sample, the transition towards the highly distorted phase may be asymmetrized, and easier to reach when an electric field opposite to the out-of-plane polarization direction is applied.

In purely c-axis oriented PbZr$_{0.2}$Ti$_{0.8}$O$_3$ ferroelectric thin films, a lateral piezoresponse force microscopy signal is observed at the position of 180{\deg}domain walls, where the out-of-plane oriented polarization is reversed. Using electric force microscopy measurements we exclude electrostatic effects as the origin of this signal. Moreover, our mechanical simulations of the tip/cantilever system show that the small tilt of the surface at the domain wall below the tip does not satisfactorily explain the observed signal either. We thus attribute this lateral piezoresponse at domain walls to their sideways motion (shear) under the applied electric field. From simple elastic considerations and the conservation of volume of the unit cell, we would expect a similar lateral signal more generally in other ferroelectric materials, and for all types of domain walls in which the out-of-plane component of the polarization is reversed through the domain wall. We show that in BiFeO$_3$ thin films, with 180, 109 and 71{\deg}domain walls, this is indeed the case., Comment: 31 pages, 10 figures. to appear in J. Appl. Phys. Special topic: invited papers from the international symposium on piezoresponse force microscopy and nanoscale phenomena in polar materials. Aveiro - portugal 2009

In magnetoelectrically coupled CoFe2O4–BiFeO3 nanostructures vertical and lateral lattice parameters of both phases are determined. We find that the in-plane lattice parameter of CoFe2O4 is fully relaxed whereas it presents compressive strain along the out-of-plane direction. Although the CoFe2O4–BiFeO3 interface is semicoherent, CoFe2O4 out-of-plane lattice strain is not relaxed after selective removal of the matrix and thus it is of nonelastic origin. In spite of the absence of elastic residual strain caused by CoFe2O4–BiFeO3 interfaces, the two phases are mechanically coupled as demonstrated by the electrical switching of the magnetization.