Giant elastic tunability in strained BiFeO3 near an electrically induced phase transition

Artificial Domain Patterning in Ultrathin Ferroelectric Films via Modifying the Surface Electrostatic Boundary Conditions

Nanoscale spatially controlled modulation of the properties of ferroelectrics via artificial domain pattering is crucial to their emerging optoelectronics applications. New patterning strategies to achieve high precision and efficiency and to link the resultant domain structures with device functionalities are being sought. Here, we present an epitaxial heterostructure of SrRuO3/PbTiO3/SrRuO3, wherein the domain configuration is delicately determined by the charge screening conditions in the SrRuO3 layer and the substrate strains. Chemical etching of the top SrRuO3 layer leads to a transition from in-plane a domains to out-of-plane c domains, accompanied by a giant (>105) modification in the second harmonic generation response. The modulation effect, coupled with the plasmonic resonance effect from SrRuO3, enables a highly flexible design of nonlinear optical devices, as demonstrated by a simulated split-ring resonator metasurface. This domain patterning strategy may be extended to more thin-film ferroelectric systems with domain stabilities amenable to electrostatic boundary conditions.

Facile Control of Ferroelastic Domain Patterns in Multiferroic Thin Films by a Scanning Tip Bias

BiFeO₃, known as the "holy grail of all multiferroics", provides an appealing platform for exploration of multifield coupling physics and design of functional devices. Many fantastic properties of BiFeO₃ are regulated by its ferroelastic domain structure. However, a facile programable control on the ferroelastic domain structure in BiFeO₃ remains challenging and our understanding on the existing control strategies is also far from complete. This work reports a facile control of ferroelastic domain patterns in BiFeO₃ thin films under area scanning poling by exploiting the tip bias as the control parameter. Combining scanning probe microscopy experiments and simulations, we found that BiFeO₃ thin films with pristine 71° rhombohedral-phase stripe domains exhibit at least four switching pathways solely by controlling the scanning tip bias. As a result, one can readily write mesoscopic topological defects into the films without the necessity to change the tip motion. The correlation between conductance of the scanned region and the switching pathway is further investigated. Our results extend the current understanding on the domain switching kinetics and the coupled electronic transport properties in BiFeO₃ thin films. The facile voltage control of ferroelastic domains should facilitate the development of configurable electronic and spintronic devices.

Oxygen Vacancy Injection as a Pathway to Enhancing Electromechanical Response in Ferroelectrics

Since their discovery in late 1940s, perovskite ferroelectric materials have become one of the central objects of condensed matter physics and materials science due to the broad spectrum of functional behaviors they exhibit, including electro-optical phenomena and strong electromechanical coupling. In such disordered materials, the static properties of defects such as oxygen vacancies are well explored but the dynamic effects are less understood. In this work, the first observation of enhanced electromechanical response in BaTiO₃ thin films is reported driven via dynamic local oxygen vacancy control in piezoresponse force microscopy (PFM). A persistence in peizoelectricity past the bulk Curie temperature and an enhanced electromechanical response due to a created internal electric field that further enhances the intrinsic electrostriction are explicitly demonstrated. The findings are supported by a series of temperature dependent band excitation PFM in ultrahigh vacuum and a combination of modeling techniques including finite element modeling, reactive force field, and density functional theory. This study shows the pivotal role that dynamics of vacancies in complex oxides can play in determining functional properties and thus provides a new route toward- achieving enhanced ferroic response with higher functional temperature windows in ferroelectrics and other ferroic materials.

Anisotropic epitaxial stabilization of a low-symmetry ferroelectric with enhanced electromechanical response

Piezoelectrics interconvert mechanical energy and electric charge and are widely used in actuators and sensors. The best performing materials are ferroelectrics at a morphotropic phase boundary, where several phases coexist. Switching between these phases by electric field produces a large electromechanical response. In ferroelectric BiFeO₃, strain can create a morphotropic-phase-boundary-like phase mixture and thus generate large electric-field-dependent strains. However, this enhanced response occurs at localized, randomly positioned regions of the film. Here, we use epitaxial strain and orientation engineering in tandem-anisotropic epitaxy-to craft a low-symmetry phase of BiFeO₃ that acts as a structural bridge between the rhombohedral-like and tetragonal-like polymorphs. Interferometric displacement sensor measurements reveal that this phase has an enhanced piezoelectric coefficient of ×2.4 compared with typical rhombohedral-like BiFeO₃. Band-excitation frequency response measurements and first-principles calculations provide evidence that this phase undergoes a transition to the tetragonal-like polymorph under electric field, generating an enhanced piezoelectric response throughout the film and associated field-induced reversible strains. These results offer a route to engineer thin-film piezoelectrics with improved functionalities, with broader perspectives for other functional oxides.

Ferroelastic Nanodomain-mediated Mechanical Switching of Ferroelectricity in Thick Epitaxial Films

Mechanical switching of ferroelectric polarization, typically realized via a scanning probe, holds promise in (multi)ferroic device applications. Whereas strain gradient-associated flexoelectricity has been regarded to be accountable for mechanical switching in ultrathin (<10 nm) films, such mechanism can hardly be extended to thicker materials due to intrinsic short operating lengths of flexoelectricity. Here, we demonstrate robust mechanical switching in ∼100 nm thick Pb(Zr_0.2Ti_0.8)O₃ epitaxial films with a characteristic microstructure consisting of nanosized ferroelastic domains. Through a combination of multiscale structural characterizations, piezoresponse force microscopy, and phase-field simulations, we reveal that the ferroelastic nanodomains effectively mediate the 180° switching nucleation in a dynamical manner during tip scanning. Coupled with microstructure engineering, this newly revealed mechanism could boost the utility of mechanical switching through extended material systems. Our results also provide insight into competing polarization switching pathways in complex ferroelectric materials, essential for understanding their electromechanical response.

Local electronic transport across probe/ionic conductor interface in scanning probe microscopy

Charge carrier transport through the probe-sample junction can have substantial consequences for outcomes of electrical and electromechanical atomic-force-microscopy (AFM) measurements. For understanding physical processes under the probe, we carried out conductive-AFM (C-AFM) measurements of local current-voltage (I-V) curves as well as their derivatives on samples of a mixed ionic-electronic conductor Li_1-xMn₂O₄ and developed an analytical framework for the data analysis. The implemented approach discriminates between contributions the highly resistive sample surface layer and the bulk with the account of ion redistribution in the field of the probe. It was found that, with increasing probe voltage, the conductance mechanism in the surface layer transforms from Pool-Frenkel to space-charge-limited current. The surface layer significantly alters the ion dynamics in the sample bulk under the probe, which leads, in particular, to a decrease of the effective electromechanical AFM signal associated with the ionic motion in the sample. The framework can be applied for the analysis of electronic transport mechanisms across the probe/sample interface as well as to uncover the role of the charge transport in the electric field distribution, mechanical, and other responses in AFM measurements of a broad spectrum of conducting materials.

Phase transition enhanced superior elasticity in freestanding single-crystalline multiferroic BiFeO3 membranes

The integration of ferroic oxide thin films into advanced flexible electronics will bring multifunctionality beyond organic and metallic materials. However, it is challenging to achieve high flexibility in single-crystalline ferroic oxides that is considerable to organic or metallic materials. Here, we demonstrate the superior flexibility of freestanding single-crystalline BiFeO₃ membranes, which are typical multiferroic materials with multifunctionality. They can endure cyclic 180° folding and have good recoverability, with the maximum bending strain up to 5.42% during in situ bending under scanning electron microscopy, far beyond their bulk counterparts. Such superior elasticity mainly originates from reversible rhombohedral-tetragonal phase transition, as revealed by phase-field simulations. This study suggests a general fundamental mechanism for a variety of ferroic oxides to achieve high flexibility and to work as smart materials in flexible electronics.

Structure, Performance, and Application of BiFeO3 Nanomaterials

Multiferroic nanomaterials have attracted great interest due to simultaneous two or more properties such as ferroelectricity, ferromagnetism, and ferroelasticity, which can promise a broad application in multifunctional, low-power consumption, environmentally friendly devices. Bismuth ferrite (BiFeO3, BFO) exhibits both (anti)ferromagnetic and ferroelectric properties at room temperature. Thus, it has played an increasingly important role in multiferroic system. In this review, we systematically discussed the developments of BFO nanomaterials including morphology, structures, properties, and potential applications in multiferroic devices with novel functions. Even the opportunities and challenges were all analyzed and summarized. We hope this review can act as an updating and encourage more researchers to push on the development of BFO nanomaterials in the future.

Fully spin-polarized quadratic non-Dirac bands realized quantum anomalous Hall effect

The quantum anomalous Hall effect is an intriguing quantum state that exhibits chiral edge states in the absence of a magnetic field. While the search for quantum anomalous Hall insulators is still active, researchers mainly search for the systems containing a magnetic atom. Here, based on first-principles density functional theory, we predict a new family of Chern insulators with fully spin-polarized quadratic px,y non-Dirac bands in the alkaline earth metal BaX (X = Si, Ge, and Sn) system. We show that BaX monolayer has a half-metallic ferromagnetic ground state. The ferromagnetism mainly originates from the p orbitals of Si, Ge and Sn atoms. The 2D BaSn monolayer exhibits a large magnetocrystalline anisotropic energy of 12.20 meV per cell and a nontrivial band gap of 159.10 meV. Interestingly, both the chiral edge current direction and the sign of Chern number can be tuned by doping. Furthermore, the 4% compressive strain in the 2D BaX systems can drive a structural phase transition but the nontrivial topological properties remain reserved. Our findings not only extend the novel topological physics but also provide fascinating opportunities for the realization of the quantum anomalous Hall effect experimentally.

Simultaneous scanning near-field optical and X-ray diffraction microscopy for correlative nanoscale structure-property characterization

A multimodal imaging instrument has been developed that integrates scanning near-field optical microscopy with nanofocused synchrotron X-ray diffraction imaging. The instrument allows for the simultaneous nanoscale characterization of electronic/near-field optical properties of materials together with their crystallographic structure, facilitating the investigation of local structure-property relationships. The design, implementation and operating procedures of this instrument are reported. The scientific capabilities are demonstrated in a proof-of-principle study of the insulator-metal phase transition in samarium sulfide (SmS) single crystals induced by applying mechanical pressure via a scanning tip. The multimodal imaging of an in situ tip-written region shows that the near-field optical reflectivity can be correlated with the heterogeneously transformed structure of the near-surface region of the crystal.

Deterministic optical control of room temperature multiferroicity in BiFeO3 thin films

Controlling ferroic orders (ferroelectricity, ferromagnetism and ferroelasticity) by optical methods is a significant challenge due to the large mismatch in energy scales between the order parameter coupling strengths and the incident photons. Here, we demonstrate an approach to manipulate multiple ferroic orders in an epitaxial mixed-phase BiFeO₃ thin film at ambient temperature via laser illumination. Phase-field simulations indicate that a light-driven flexoelectric effect allows the targeted formation of ordered domains. We also achieved precise sequential laser writing and erasure of different domain patterns, which demonstrates a deterministic optical control of multiferroicity at room temperature. As ferroic orders directly influence susceptibility and conductivity in complex materials, our results not only shed light on the optical control of multiple functionalities, but also suggest possible developments for optoelectronics and related applications.

Evaluation of the Structural Phase Transition in Multiferroic (Bi1−x Prx)(Fe0.95 Mn0.05)O3 Thin Films by A Multi-Technique Approach Including Picosecond Laser Ultrasonics

Picosecond laser ultrasonics is an experimental technique for the generation and detection of ultrashort acoustic pulses using ultrafast lasers. In transparent media, it is often referred to as time-domain Brillouin scattering (TDBS). It provides the opportunity to monitor the propagation of nanometers-length acoustic pulses and to determine acoustical, optical, and acousto-optical parameters of the materials. We report on the application of TDBS for evaluating the effect of Praseodymium (Pr) substitution on the elasticity of multiferroic (Bi1−xPrx)(Fe0.95Mn0.05)O3 (BPFMO) thin films. The films were deposited on Si and LaAlO3 (LAO) substrates by a sol-gel method. X-ray diffraction and Raman spectra revealed earlier that a phase transition from rhombohedral to tetragonal structure occurs at about 15% Pr substitution and is accompanied by the maxima of remnant magnetization and polarization. Combining TDBS with optical spectral reflectometry, scanning electron microscopy, and topographic measurements by atomic force microscopy, we found that the structural transition is also characterized by the maximum optical dielectric constant and the minimum longitudinal sound velocity. Our results, together with earlier ones, suggest that BiFeO3-based films and ceramics with compositions near phase boundaries might be promising materials for multifunctional applications.

Machine learning-enabled identification of material phase transitions based on experimental data: Exploring collective dynamics in ferroelectric relaxors

Exploration of phase transitions and construction of associated phase diagrams are of fundamental importance for condensed matter physics and materials science alike, and remain the focus of extensive research for both theoretical and experimental studies. For the latter, comprehensive studies involving scattering, thermodynamics, and modeling are typically required. We present a new approach to data mining multiple realizations of collective dynamics, measured through piezoelectric relaxation studies, to identify the onset of a structural phase transition in nanometer-scale volumes, that is, the probed volume of an atomic force microscope tip. Machine learning is used to analyze the multidimensional data sets describing relaxation to voltage and thermal stimuli, producing the temperature-bias phase diagram for a relaxor crystal without the need to measure (or know) the order parameter. The suitability of the approach to determine the phase diagram is shown with simulations based on a two-dimensional Ising model. These results indicate that machine learning approaches can be used to determine phase transitions in ferroelectrics, providing a general, statistically significant, and robust approach toward determining the presence of critical regimes and phase boundaries.

Ultrafast electron-phonon coupling and photo-induced strain in the morphotropic phase boundary of BixDy1-xFeO3 films

The interplay among ferroelectric, magnetic and elastic degrees of freedom in multiferroics is the key issues in condensed matters, which has been widely investigated by various methods. Here, using ultrafast two-color pump-probe spectroscopy, the picosecond electron-phonon and spin-lattice coupling process in Dysprosium doped-BiFeO₃ (BDFO) films on SrTiO₃ (STO) substrate have been investigated systematically. The Dy-doping induced structural transition and magnetic enhancement in BDFO is observed by ultrafast electron-phonon and spin-lattice interaction, respectively. The elastic anomalies in BDFO films are revealed by the photo-induced coherent acoustic phonon. With increasing the Dy doping ratio, the frequencies of the acoustic phonon in the films are modulated, and the phonon transmission coefficient between films and substrate is found to approach unity gradually. The ultrafast observation of the tunability of the ferroelectric, magnetic and the elastic properties in the morphotropic phase boundary of rare-earth doped BFO films provides new insights into the integration of BFO in next-generation high frequency electro-magnetic and electroacoustic devices.

Enhanced photocatalytic efficiency of C3N4/BiFeO3heterojunctions: the synergistic effects of band alignment and ferroelectricity

C₃N₄/BiFeO₃heterojunction shows enhanced visible light photocatalytic efficiency.

Three-State Ferroelastic Switching and Large Electromechanical Responses in PbTiO3 Thin Films

Leveraging competition between energetically degenerate states to achieve large field-driven responses is a hallmark of functional materials, but routes to such competition are limited. Here, a new route to such effects involving domain-structure competition is demonstrated, which arises from strain-induced spontaneous partitioning of PbTiO₃ thin films into nearly energetically degenerate, hierarchical domain architectures of coexisting c/a and a₁ /a₂ domain structures. Using band-excitation piezoresponse force microscopy, this study manipulates and acoustically detects a facile interconversion of different ferroelastic variants via a two-step, three-state ferroelastic switching process (out-of-plane polarized c⁺ → in-plane polarized a → out-of-plane polarized c^- state), which is concomitant with large nonvolatile electromechanical strains (≈1.25%) and tunability of the local piezoresponse and elastic modulus (>23%). It is further demonstrated that deterministic, nonvolatile writing/erasure of large-area patterns of this electromechanical response is possible, thus showing a new pathway to improved function and properties.

Highly (001)-Textured Tetragonal BiFeO3 Film and Its Photoelectrochemical Behaviors Tuned by Magnetic Field

Highly (001)-textured BiFeO₃ film in tetragonal phase (T-BFO) with a giant c/a ratio was first obtained on quartz/polycrystalline ITO substrate. Our results indicate that the polycrystalline ITO layer with small surface roughness is a critical point to control the growth of T-BFO structure. It should be ascribed to the fact that a Bi-rich phase interlayer (∼5 nm) could be formed on ITO, which acted as a crystal seed layer and thus induced the growth of (001)-textured T-BFO structure. The observed weak room temperature ferromagnetism should be attributed to Fe valence change. Open circuit potential measurements under 360 μW/cm² full spectra irradiation show that the open circuit potential and the lifetime of photo-induced carriers increased under applied magnetic field, which reveals that the applied magnetic field can manipulate the photo electrochemical behaviors of BFO film. Our findings offer a simple way to fabricate highly (001)-textured T-BFO film, which make it desirable to obtain extensive applications for these oriented BFO films.

Nanoscale Probing of Elastic-Electronic Response to Vacancy Motion in NiO Nanocrystals

Measuring the diffusion of ions and vacancies at nanometer length scales is crucial to understanding fundamental mechanisms driving technologies as diverse as batteries, fuel cells, and memristors; yet such measurements remain extremely challenging. Here, we employ a multimodal scanning probe microscopy (SPM) technique to explore the interplay between electronic, elastic, and ionic processes via first-order reversal curve I-V measurements in conjunction with electrochemical strain microscopy (ESM). The technique is employed to investigate the diffusion of oxygen vacancies in model epitaxial nickel oxide (NiO) nanocrystals with resistive switching characteristics. Results indicate that opening of the ESM hysteresis loop is strongly correlated with changes to the resonant frequency, hinting that elastic changes stem from the motion of oxygen (or cation) vacancies in the probed volume of the SPM tip. These changes are further correlated to the current measured on each nanostructure, which shows a hysteresis loop opening at larger (∼2.5 V) voltage windows, suggesting the threshold field for vacancy migration. This study highlights the utility of local multimodal SPM in determining functional and chemical changes in nanoscale volumes in nanostructured NiO, with potential use to explore a wide variety of materials including phase-change memories and memristive devices in combination with site-correlated chemical imaging tools.

Nanoscale Bandgap Tuning across an Inhomogeneous Ferroelectric Interface

We report nanoscale bandgap engineering via a local strain across the inhomogeneous ferroelectric interface, which is controlled by the visible-light-excited probe voltage. Switchable photovoltaic effects and the spectral response of the photocurrent were explored to illustrate the reversible bandgap variation (∼0.3 eV). This local-strain-engineered bandgap has been further revealed by in situ probe-voltage-assisted valence electron energy-loss spectroscopy (EELS). Phase-field simulations and first-principle calculations were also employed for illustration of the large local strain and the bandgap variation in ferroelectric perovskite oxides. This reversible bandgap tuning in complex oxides demonstrates a framework for the understanding of the optically related behaviors (photovoltaic, photoemission, and photocatalyst effects) affected by order parameters such as charge, orbital, and lattice parameters.

Depth resolved lattice-charge coupling in epitaxial BiFeO3 thin film

For epitaxial films, a critical thickness (t_c) can create a phenomenological interface between a strained bottom layer and a relaxed top layer. Here, we present an experimental report of how the t_c in BiFeO₃ thin films acts as a boundary to determine the crystalline phase, ferroelectricity, and piezoelectricity in 60 nm thick BiFeO₃/SrRuO₃/SrTiO₃ substrate. We found larger Fe cation displacement of the relaxed layer than that of strained layer. In the time-resolved X-ray microdiffraction analyses, the piezoelectric response of the BiFeO₃ film was resolved into a strained layer with an extremely low piezoelectric coefficient of 2.4 pm/V and a relaxed layer with a piezoelectric coefficient of 32 pm/V. The difference in the Fe displacements between the strained and relaxed layers is in good agreement with the differences in the piezoelectric coefficient due to the electromechanical coupling.

Super-crystals in composite ferroelectrics

As atoms and molecules condense to form solids, a crystalline state can emerge with its highly ordered geometry and subnanometric lattice constant. In some physical systems, such as ferroelectric perovskites, a perfect crystalline structure forms even when the condensing substances are non-stoichiometric. The resulting solids have compositional disorder and complex macroscopic properties, such as giant susceptibilities and non-ergodicity. Here, we observe the spontaneous formation of a cubic structure in composite ferroelectric potassium-lithium-tantalate-niobate with micrometric lattice constant, 10(4) times larger than that of the underlying perovskite lattice. The 3D effect is observed in specifically designed samples in which the substitutional mixture varies periodically along one specific crystal axis. Laser propagation indicates a coherent polarization super-crystal that produces an optical X-ray diffractometry, an ordered mesoscopic state of matter with important implications for critical phenomena and applications in miniaturized 3D optical technologies.