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Topological defects in ferroelectric materials have attracted much attention due to the emergence of conductive, ferroic, and magnetic functionalities. However, many topological configurations dynamically evolve during the switching processes, making them a challenge to characterize via traditional techniques. Here, we implement an automated experimentation approach for the exploration of functional properties in BiFeO3 thin films. Specifically, we visualize the ferroelectric domain structures via single frequency piezoresponse force microscopy and implement a computer vision-based algorithm to discover features of interest at which spectroscopic measurements are taken. Subsequently, we employ dimensionality reduction techniques to reveal characteristic polarization behaviors at these features. This approach can be extended to other spectroscopies and modalities to probe only specific features of interest, ultimately enabling dynamical processes in ferroelectrics to be studied.

The mechanisms of phase transitions have been previously explored at various theoretical and experimental levels. For a wide variety of compounds, the majority of studies are limited by observations at fixed temperature and composition, in which case, relevant information can be determined only from the behaviors at topological and structural defects. All analyses to date utilize macroscopic descriptors derived from structural information such as polarization or octahedral tilts extracted from the atomic positions, ignoring the multiple degrees of freedom observable from atomically resolved images. In this article, we provide a solution, by exploring the mechanisms of a phase transition between the trigonal prismatic and distorted octahedral phases of layered chalcogenides in the 2D MoS2–ReS2 system from the observations of local degrees of freedom, namely atomic positions by scanning transmission electron microscopy. We employ local crystallographic analysis based on statistical learning of atomic environments to build a picture of the transition from the atomic level up and determine local and global variables controlling the local symmetry breaking. We highlight how the dependence of the average symmetry-breaking distortion amplitude on global and local concentration can be used to separate local chemical as well as global electronic effects on the transition. This approach allows for the exploring of atomic mechanisms beyond the traditional macroscopic descriptions, utilizing the imaging of compositional fluctuations in solids to explore phase transitions over a range of observed local stoichiometries and atomic configurations.

The predictability of a certain effect or phenomenon is often equated with the knowledge of relevant physical laws, typically understood as a functional or numerically derived relationship between the observations and known states of the system. Correspondingly, observations inconsistent with prior knowledge can be used to derive new knowledge on the nature of the system or indicate the presence of yet unknown mechanisms. Here we explore the applicability of Gaussian Processes (GP) to establish predictability and uncertainty of local behaviors from multimodal observations, providing an alternative to this classical paradigm. Using atomic-resolution Scanning Transmission Electron Microscopy (STEM) of multiferroic Sm-doped BiFeO3 across a broad composition range, we directly visualize the atomic structure and structural, physical, and chemical order parameter fields for the material. GP regression is used to establish the predictability of the local polarization field from different groups of parameters, including the adjacent polarization values and several combinations of physical and chemical descriptors, including lattice parameters, column intensities, etc. We observe that certain elements of microstructure including charged and uncharged domain walls and interfaces with the substrate are best predicted with specific combinations of descriptors, and this predictability and their associated uncertainties are consistent across the composition series. The associated generative physical mechanisms are discussed. We argue that predictability and uncertainty in observational data offers a new pathway to probe the physics of condensed matter systems from multimodal local observations., Update the accepted version

The emergence of scanning probe and electron beam imaging techniques has allowed quantitative studies of atomic structure and minute details of electronic and vibrational structure on the level of individual atomic units. These microscopic descriptors, in turn, can be associated with local symmetry breaking phenomena, representing the stochastic manifestation of the underpinning generative physical model. Here, we explore the reconstruction of exchange integrals in the Hamiltonian for a lattice model with two competing interactions from observations of microscopic degrees of freedom and establish the uncertainties and reliability of such analysis in a broad parameter-temperature space. In contrast to other approaches, we specifically specify a loss function inherent to thermodynamic systems and utilize it to estimate uncertainty in simulated realizations of different models. As an ancillary task, we develop a machine learning approach based on histogram clustering to predict phase diagrams efficiently using a reduced descriptor space. We further demonstrate that reconstruction is possible well above the phase transition and in the regions of parameter space when the macroscopic ground state of the system is poorly defined due to frustrated interactions. This suggests that this approach can be applied to the traditionally complex problems of condensed matter physics such as ferroelectric relaxors and morphotropic phase boundary systems, spin and cluster glasses, and quantum systems once the local descriptors linked to the relevant physical behaviors are known.

Imaging mechanisms in contact Kelvin Probe Force Microscopy (cKPFM) are explored via information theory-based methods. Gaussian Processes are used to achieve super-resolution in the cKPFM signal, effectively extrapolating across the spatial and parameter space. Tensor matrix factorization is applied to reduce the multidimensional signal to the tensor convolution of the scalar functions that show clear trending behavior with the imaging parameters. These methods establish a workflow for the analysis of the multidimensional data sets, that can then be related to the relevant physical mechanisms. We also provide an interactive Google Colab notebook (http://bit.ly/39kMtuR) that goes through all the analysis discussed in the paper., Comment: Update with accepted version

The universal tendency in scanning probe microscopy (SPM) over the last two decades is to transition from simple 2D imaging to complex detection and spectroscopic imaging modes. The emergence of complex SPM engines brings forth the challenge of reliable data interpretation, i.e. conversion from detected signal to descriptors specific to tip-surface interactions and subsequently to materials properties. Here, we implemented a Bayesian inference approach for the analysis of the image formation mechanisms in band excitation (BE) SPM. Compared to the point estimates in classical functional fit approaches, Bayesian inference allows for the incorporation of extant knowledge of materials and probe behavior in the form of corresponding prior distribution and return the information on the material functionality in the form of readily interpretable posterior distributions. We note that in application of Bayesian methods, special care should be made for proper setting on the problem as model selection vs. establishing practical parameter equivalence. We further explore the non-linear mechanical behaviors at topological defects in a classical ferroelectric material, PbTiO3. We observe the non-trivial evolution of Duffing resonance frequency and the nonlinearity of the sample surface, suggesting the presence of the hidden elements of domain structure. These observations suggest that the spectrum of anomalous behaviors at the ferroelectric domain walls can be significantly broader than previously believed and can extend to non-conventional mechanical properties in addition to static and microwave conductance., Comment: Supplementary materials is located at the end of the manuscript

Advances in predictive modeling across multiple disciplines have yielded generative models capable of high veracity in predicting macroscopic functional responses of materials. Correspondingly, of interest is the inverse problem of finding the model parameter that will yield desired macroscopic responses, such as stress-strain curves, ferroelectric hysteresis loops, etc. Here we suggest and implement a Gaussian Process based methods that allow to effectively sample the degenerate parameter space of a complex non-local model to output regions of parameter space which yield desired functionalities. We discuss the specific adaptation of the acquisition function and sampling function to make the process efficient and balance the efficient exploration of parameter space for multiple possible minima and exploitation to densely sample the regions of interest where target behaviors are optimized. This approach is illustrated via the hysteresis loop engineering in ferroelectric materials, but can be adapted to other functionalities and generative models. The code is open-sourced and available at [github.com/ramav87/Ferrosim]., Update title to match the journal one

Magnetoelectric (ME) composite thin films are promising candidates for novel applications in future nanoelectronics, spintronics, memory, and other multifunctional devices as they exhibit much higher ME coupling and transition temperatures (Tc) than well-known single phase multiferroics discovered to date. Among the three types of multiferroic composite nanostructures, (2–2) layered and (1–3) vertically aligned composite nanostructures exhibit comparatively smaller ME coupling due to different shortcomings that restrict their use in many applications. Here, we study the morphological, piezoresponse force microscopic (PFM), ferroelectric, magnetic, and magnetodielectric properties of 0–3 [magnetic nanoparticles (0) homogeneously distributed in ferroelectric matrices (3)] multiferroic composite thin films. The Pb(Fe0.5Nb0.5)O3 (PFN)–Ni0.65Zn0.35Fe2O4 (NZFO) particulate composite films were synthesized by pulsed laser deposition. These particulate composite thin films are completely c-axis oriented with very low surface roughness. We observed magnetic and ferroelectric Tc above room temperature (RT) for all composite thin films. The PFN–NZFO 0–3 composites exhibit large polarization, high saturated magnetization with low coercive field, and low dielectric loss along with magnetodielectric coupling at RT. These nanocomposites might be utilized in next generation nano/microelectronics and spintronic devices.

Recent advances in scanning transmission electron microscopy (STEM) have enabled direct visualization of the atomic structure of ferroic materials, enabling the determination of atomic column positions with ~pm precision. This, in turn, enabled direct mapping of ferroelectric and ferroelastic order parameter fields via the top-down approach, where the atomic coordinates are directly mapped on the mesoscopic order parameters. Here, we explore the alternative bottom-up approach, where the atomic coordinates derived from the STEM image are used to explore the extant atomic displacement patterns in the material and build the collection of the building blocks for the distorted lattice. This approach is illustrated for the La-doped BiFeO3 system.The full analysis procedure is available as an interactive paper in a form of a Google Colab (Jupyter) notebook where a classical paper organization is augmented with code cells that appear hidden by default (when viewed in Google Colab). This should allow a reader to retrace the analysis and, more importantly, it enables the readers to use the same codes for their data. The same paper is also available in a standard pdf format (without code).

We study the local conductivity properties of KTiOPO4 using conductive atomic force microscopy in ultrahigh vacuum (UHV). We show that domains with opposite orientations have different conductivity values. We investigate the temperature dependence of the local conductivity and report a difference in the activation energy of 25% between different domains. Furthermore, we show that the local conductivity increases with the number of biased-scans. Finally, it is found that the domain wall conductivity previously observed at ambient conditions vanishes in UHV. Our results are discussed in terms of the screening effects and surface conditions.

With the rapid development of new device miniaturization technology, there is invigorated interest in magnetic nanostructures for potential application in novel multifunctional devices. In continuation to our search for a suitable magnetic material having Curie temperature (Tc) well above room temperature for multifunctional applications, we have studied the dielectric, optical, magnetic, and resistance switching characteristics of Ni0.65Zn0.35Fe2O4 (NZFO) thin films. The observation of only (004) reflection in the X-ray diffraction patterns confirms the c-axis orientation and high quality growth of NZFO thin films. The presence of mixed valences of Fe2+/Fe3+ cations is probed by X-ray photon spectroscopy, which supports the cationic ordering-mediated large dielectric response. Our investigations reveal NZFO to be an indirect band gap material (∼1.8 eV) with a direct gap at ∼2.55 eV. These nanostructures exhibit high saturation magnetization and a low coercive field with a ferrimagnetic–paramagnetic phase transition of ∼713 K. Magnetic force microscopy studies revealed the stripe-like domain structure of the investigated thin films. In addition, these thin films exhibit reliable and repeatable unipolar resistive switching characteristics. The observed high dielectric permittivity with low loss tangent, large magnetization with soft magnetic behavior, striped magnetic domain structure and reliable resistance switching in NZFO thin films above room temperature suggest potential application in memory, spintronics, and multifunctional devices.

Ferroelectric materials have remained one of the foci of condensed matter physics and materials science for over 50 years. In the last 20 years, the development of voltage-modulated scanning probe microscopy techniques, exemplified by Piezoresponse force microscopy (PFM) and associated time and voltage spectroscopies, opened a pathway to explore these materials on a single-digit nanometer level. Consequently, domain structures, walls and polarization dynamics can now be imaged in real space. More generally, PFM has allowed studying electromechanical coupling in a broad variety of materials ranging from ionics to biological systems. It can also be anticipated that the recent Nobel prize in molecular electromechanical machines will result in rapid growth in interest in PFM as a method to probe their behavior on single device and device assembly levels. However, the broad introduction of PFM also resulted in a growing number of reports on nearly ubiquitous presence of ferroelectric-like phenomena including remnant polar states and electromechanical hysteresis loops in materials which are non-ferroelectric in the bulk, or in cases where size effects are expected to suppress ferroelectricity. While in certain cases plausible physical mechanisms can be suggested, there is remarkable similarity in observed behaviors, irrespective of the materials system. In this review, we summarize the basic principles of PFM, briefly discuss the features of ferroelectric surfaces salient to PFM imaging and spectroscopy, and summarize existing reports on ferroelectric like responses in non-classical ferroelectric materials. We further discuss possible mechanisms behind observed behaviors, and possible experimental strategies for their identification., Comment: 74 pages, 16 figures

The influence of surface morphology on the local piezoelectric response of highly (100)-textured 0.70PbMg2/3Nb1/3O3-0.30PbTiO3 thin films is studied using piezoresponse force microscopy in band-excitation mode. The local electromechanical response is mostly suppressed in direct proximity of the grain boundaries. However, within 100–200 nm of the grain boundary, the piezoresponse is substantially enhanced, before decaying again within a region at the center of the grain itself. Nested piezoresponse hysteresis curves confirm the influence of topography descriptors on parameters affecting the hysteresis loop shape. The enhancement of the electromechanical response is rationalized through reduced lateral clamping in the grains with deep trenched boundaries, as well as an expected lower energy for complex domain wall structures, due to curved ferroelectric surfaces. The lower piezoresponse at the center of the grain is assigned to the lateral clamping by the surrounding piezoelectric material.

The nonlinear response of a ferroic to external fields has been studied for decades, garnering interest for both understanding fundamental physics, as well as technological applications such as memory devices. Yet, the behavior of ferroelectrics at mesoscopic regimes remains poorly understood, and the scale limits of theories developed for macroscopic regimes are not well tested experimentally. Here, we test the link between piezo-nonlinearity and local piezoelectric strain hysteresis, via AC-field dependent measurements in conjunction with hysteresis measurements with varying voltage windows on (K,Na)NbO3 crystals with band-excitation piezoelectric force microscopy. The correlation coefficient between nonlinearity amplitude and the amplitude during hysteresis loop acquisition shows a clear decrease with increasing AC bias. Further, correlation of polynomial fitting terms from the nonlinear measurements with the hysteresis loop area reveals that the largest correlations are reserved for the quadratic term...

Compositional and charge disorder in ferroelectric relaxors lies at the heart of the unusual properties of these systems, such as aging and non-ergodicity, polarization rotations, and a host of temperature and field-driven phase transitions. However, much information about the field-dynamics of the polarization in the prototypical ferroelectric relaxor (1−x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-xPT) remains unprobed at the mesoscopic level. Here, we use a piezoresponse force microscopy-based dynamic multimodal relaxation spectroscopy technique, enabling the study of ferroelectric switching and polarization relaxation at mesoscopic length scales, and carry out measurements on a PMN-0.28PT sample with minimal polishing. Results indicate that beyond a threshold DC bias the average relaxation increases as the system attempts to relax to the previous state. Phenomenological fitting reveals the presence of mesoscale heterogeneity in relaxation amplitudes and clearly suggests the presence of two distinct amplitudes. Independent component analysis reveals the presence of a disorder component of the relaxation, which is found to be strongly anti-correlated with the maximum piezoresponse at that location, suggesting smaller disorder effects where the polarization reversal is large and vice versa. The disorder in the relaxation amplitudes is postulated to arise from rhombohedral and field-induced tetragonal phase in the crystal, with each phase associated with its own relaxation amplitude. These studies highlight the crucial importance of the mixture of ferroelectric phases in the compositions in proximity of the morphotropic phase boundary in governing the local response and further highlight the ability of PFM voltage and time spectroscopies, in conjunction with big-data multivariate analyses, to locally map disorder and correlate it with parameters governing the dynamic behavior.

The doped manganese oxides (manganites) have been widely studied for their colossal magnetoresistive effects, for potential applications in oxide spintronics, electroforming in resistive switching devices, and are materials of choice as cathodes in modern solid oxide fuel cells. However, little experimental knowledge of the dynamics of the surfaces of perovskite manganites at the atomic scale exists. Here, through in-situ scanning tunneling microscopy (STM), we demonstrate atomic resolution on samples of La0.625Ca0.375MnO3 grown on (001) SrTiO3 by pulsed laser deposition. Furthermore, by applying triangular DC waveforms of increasing amplitude to the STM tip, and measuring the tunneling current, we demonstrate the ability to both perform and monitor surface electrochemical processes at the atomic level, including formation of oxygen vacancies and removal and deposition of individual atomic units or clusters. Our work paves the way for better understanding of surface oxygen reactions in these systems.

Significant advances in atomically resolved imaging of crystals and surfaces have occurred in the last decade allowing unprecedented insight into local crystal structures and periodicity. Yet, the analysis of the long-range periodicity from the local imaging data, critical to correlation of functional properties and chemistry to the local crystallography, remains a challenge. Here, we introduce a Sliding Fast Fourier Transform (FFT) filter to analyze atomically resolved images of in-situ grown La5/8Ca3/8MnO3 (LCMO) films. We demonstrate the ability of sliding FFT algorithm to differentiate two sub-lattices, resulting from a mixed-terminated surface. Principal Component Analysis and Independent Component Analysis of the Sliding FFT dataset reveal the distinct changes in crystallography, step edges, and boundaries between the multiple sub-lattices. The implications for the LCMO system are discussed. The method is universal for images with any periodicity, and is especially amenable to atomically resolved probe and electron-microscopy data for rapid identification of the sub-lattices present.

Thin film manganese oxides (manganites) display remarkable properties, such as colossal magnetoresistance and charge ordered phases, and became a focal point of research in the past two decades owing to potential applications ranging from oxide spintronics to resistive switching-based memories. LaxCa1−xMnO3 (LCMO), a widely studied manganite, is known to substantially improve its transport properties when doped with Ag. However, despite the abundance of studies on LCMO, the effect of silver on the surface structure is unknown. Here, through in-situ methods, scanning tunneling microscopy (STM) is performed on La5/8Ca3/8MnO3 films grown by pulsed laser deposition. Films doped by silver, as confirmed by in-situ X-ray photoelectron spectroscopy, display large-scale reconstructions, interpreted as being of type (√10 × √10)R18.4°, while films lacking silver display a (√2 × √2)R45° reconstruction that may be associated with a surface charge-ordered state. It is posited that the possible cause of the varied reconstructions is due to a vacancy ordering on top of the existing (√2 × √2)R45° reconstruction. These studies highlight the influence of Ag on the surface structure, and therefore a route towards modifying the surface properties of manganites.

Laser shock peening (LSP) is an advanced surface enhancement technique used to enhance the fatigue strength of metal parts by imparting deep compressive residual stresses. In the present study, LSP was performed on IN718 SPF alloy, a fine grained nickel-based superalloy, with three different power densities and depth resolved residual strain and stress characterization was conducted using high energy synchrotron x-ray diffraction in beam line 1-ID-C at the Advanced Photon Source at the Argonne National laboratory. A fine probe size and conical slits were used to non-destructively obtain data from specific gauge volumes in the samples, allowing for high-resolution strain measurements. The results show that LSP introduces deep compressive residual stresses and the magnitude and depth of these stresses depend on the energy density of the laser. The LSP induced residual stresses were also simulated using three-dimensional nonlinear finite element analysis, with employment of the Johnson-Cook model for describing the nonlinear materials constitutive behavior. Good agreement between the experimental and simulated data was obtained. These various results are presented and discussed.

Ferroelectric BiFeO3 (BFO) nanoparticles deposited on epitaxial substrates of SrRuO3 (SRO) and La1−xSrxMnO3 (LSMO) were studied using band excitation piezoresponse spectroscopy (BEPS), piezoresponse force microscopy (PFM), and ferromagnetic resonance (FMR). BEPS confirms that the nanoparticles are ferroelectric in nature. Switching behavior of nanoparticle clusters were studied and showed evidence for inhomogeneous switching. The dimensionality of domains within nanoparticles was found to be fractal in nature, with a dimensionality constant of ∼1.4, on par with ferroelectric BFO thin-films under 100 nm in thickness. Ferromagnetic resonance studies indicate BFO nanoparticles only weakly affect the magnetic response of LSMO.

Self‐consistent field (SCF) and large‐scale configuration interaction (CI) studies with double zeta (DZ) and extended basis sets were performed on F2O for its geometry, vertical electronic excitation energies, and vertical ionization potentials (VIP). A DZ plus polarization basis gives the SCF bond distance too short by 0.05 A, and CI methods are required for the experimental distance (1.41 A). A large number of low‐lying excited electronic states are predicted, the lowest ones being 3A2 at 4.1 (2b1→3py), 3B1 at 4.2 (2b1→7a1) and 1B1 at 5.6 eV (2b1→7a1), where 7a1 is essentially a valence orbital. Comparison with the H2O spectrum is made. For the VIP’s, CI results obtained for various states of F20+ suggest some reassignments of experimental values, although unresolved questions remain.

Large scale ab initio self‐consistent field and configuration–interaction calculations were carried out to evaluate the electron affinity of the HC2 radical. The adiabatic electron affinity is calculated to be 2.14, the vertical 2.09, and the vertical detachment energy 2.17 eV. Corrections for the zero‐point vibrational energy amount to less than 0.01 eV. From these values, the heat of formation of HC2, and the C–H bond dissociation energy of C2H2 were estimated to be 121±3 and 119±3 kcal mole−1, respectively.

A very simple circuit for driving two indicators showing the direction of rotation of reversible ac motors is described.