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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.

BiInO3 is a potentially polar oxide with distinct optical properties whose origin could result from a surface that has not been well characterized. The surface properties of BiInO3 thin films have been characterized here by temperature dependent angle-resolved x-ray photoemission spectroscopy. A large surface to bulk core-level binding energy shift for the In 3d5/2 core-level is identified, indicating a surface very different from the bulk. BiInO3 terminates in indium oxide and loses bismuth from the surface of the film at T = 573 K. The Debye–Waller plots suggest effective Debye temperatures of 263 ± 10 and 556 ± 27 K for the surface and bulk components of In 3d core-level, respectively.

Bismuth ferrite (BiFeO3 – BFO) is a prototypical lead-free single phase multiferroic which shows strong ferroelectric and antiferromagnetic properties simultaneously, together with high ferroelectric Curie temperature (TFE ∼ 1103 K) and Neel temperature (TN ∼ 643 K). BFO thin films and nanomaterials (collectively termed here ‘nanostructured BFO’) show many fascinating functional properties are not observed in the parent bulk compound. In this review we focus on chemical route-derived nanostructured BFO: thin films, nanowires and nanoparticles. The review covers governing factors in a detailed and systematic manner so as to give readers a clear picture of the current state of the art in the development of nanostructured BFO via chemical routes. We discuss the process pathways for each of the chemical (or soft) based synthesis techniques, highlighting both the advantages and challenges faced for each method. Specific emphasis is placed on understanding the role of each processing ingredient and development of optimized precursors. Finally we identify the opportunities posed by further development of chemical routes for this fascinating materials system. The review thus sheds significant insight into successfully achieving high-quality nanostructured BFO via chemical process techniques.

International audience; In ferroelectric thin films, the domain structure defines ferroelectric switching pathways and thus influences device performance. In epitaxial bismuth ferrite (BiFeO3) films, fractal-like domains have been observed, but direct evidence of their origins has remained unclear. Here, we show that the nature of the ferroelectric domain structure-i.e. striped vs. fractal-like-in epitaxial BiFeO3 is defined by the strain profile across the film-substrate interface. In samples with fractal-like domains, X-ray diffraction analysis reveals strong strain gradients, while geometric phase analysis using atomic resolution scanning transmission electron microscopy reveals that within a few nanometers of the film-substrate interface, the out of plane strain shows an anomalous dip while the in-plane strain is constant. Electron energy-loss near edge structure at the oxygen K edge shows that in the vicinity of the interface, the oxygen coordination is locally modified; this combined with the anomalous strain behavior thus drives the formation of fractal-like domains. Conversely, if uniform strain is maintained across the interface, characteristic striped domains are formed. Interestingly, conversion from the fractal-like arrangement to striped domains is found possible by an ex-situ thermal treatment step. Critically, the antiferromagnetic state of the BiFeO3 is influenced by the domain structure, whereby the fractal-like domains disrupt the long-range spin cycloid. Finally, as a demonstration of the applicability of this concept, we show that a carefully engineered lower electrode with large strain gradient can be used to induce fractal domains.

Strain engineering of epitaxial ferroelectrics has emerged as a powerful method to tailor the electromechanical response of these materials, although the effect of strain at the atomic scale and the interplay between lattice displacements and electronic structure changes are not yet fully understood. Here, using a combination of scanning transmission electron microscopy (STEM) and density functional theory (DFT), we systematically probe the role of epitaxial strain in mixed phase bismuth ferrite thin films. Electron energy loss O K and Fe L2,3 edge spectra acquired across the rhombohedral (R)–tetragonal (T) phase boundary reveal progressive, and systematic, changes in electronic structure going from one phase to the other. The comparison of the acquired spectra with theoretical simulations using DFT suggests a breakage in the structural symmetry across the boundary due to the simultaneous presence of increasing epitaxial strain and off-axial symmetry in the T phase. This implies that the imposed epitaxial strain plays a significant role in not only changing the crystal-field geometry, but also the bonding environment surrounding the central iron cation at the interface thus providing new insights and a possible link to understand how the imposed strain could perturb magnetic ordering in the T phase BFO.

We present an in situ high-temperature confocal laser microscopy study on the thermal stability of 40 nm thick gold thin films grown on 40 nm Ti, TiO x and ZnO adhesion layers on (0 0 1) Si. In situ observation of the dewetting process was performed over a wide range of set temperatures (400–800 °C) and ramp rates (10–50 °C min −1 ) for each gold/adhesion layer combination. We found that significant dewetting and subsequent formation of gold islands occurs only at and above 700 °C for all adhesion layers. The dewetting is driven to equilibrium for gold/ZnO compared to gold/Ti and gold/TiO x as confirmed by ex situ X-ray diffraction and scanning electron microscopy characterization. Quantification of the in situ data through stretched exponential kinetic models reveals an underlying apparent activation energy of the dewetting process. This energy barrier for dewetting is higher for gold/Ti and gold/TiO x compared to gold/ZnO, thus confirming the ex situ observations. We rationalize that these apparent activation energies correspond to the underlying thermal stability of each gold/adhesion layer system.

Nanometer-scale electronic transport in engineered interfaces in ferroelectrics, such as domains and topological defects, has emerged as a topic of broad interest due to potential applications in information storage, sensors and photovoltaic devices. Scanning probe microscopy (SPM) methods led to rapid growth in the field by enabling correlation of the unique functional properties with microstructural features in the aforementioned highly localized phenomena. In addition to conduction localized at interfaces, polarization-mediated control of conduction through domains in nanoscale ferroelectrics suggests significant potential for use in memristor technologies. In parallel with experiment, theory based on thermodynamic Landau-Ginzburg-Devonshire (LGD) framework has seen rapid development, both rationalizing the observations, and hinting at possibilities for local, deterministic control of order parameters. These theories can successfully account for static interface conductivity at charged, nominally uncharged and topologically protected domain walls. Here, recent experimental and theoretical progress in SPM-motivated studies on domain wall conduction in both standard and improper ferroelectrics are reviewed. SPM studies on transport through ferroelectrics reveal that both domains and topological defects in oxides can be exploited as individual elements for use in functional nanoscale devices. Future prospects of the field are discussed.

Electromechanical properties such as d33 and strain are significantly enhanced at morphotropic phase boundaries (MPBs) between two or more different crystal structures. Many actuators, sensors and MEMS devices are therefore systems with MPBs, usually between polar phases in lead (Pb)-based ferroelectric ceramics. In the search for Pb-free alternatives, systems with MPBs between polar and non-polar phases have recently been theorized as having great promise. While such an MPB was identified in rare-earth (RE) modified bismuth ferrite (BFO) thin films, synthesis challenges have prevented its realization in ceramics. Overcoming these, we demonstrate a comparable electromechanical response to Pb-based materials at the polar-to-non-polar MPB in Sm modified BFO. This arises from ‘dual’ strain mechanisms: ferroelectric/ferroelastic switching and a previously unreported electric-field induced transition of an anti-polar intermediate phase. We show that intermediate phases play an important role in the macroscopic strain response and may have potential to enhance electromechanical properties at polar-to-non-polar MPBs.

Using a combination of piezoresponse force microscopy (PFM) and phase-field modeling, we demonstrate ubiquitous formation of center-type and possible ferroelectric closure domain arrangements during polarization switching near the ferroelastic domain walls in (100) oriented rhombohedral BiFeO(3). The formation of these topological defects is determined from the vertical and lateral PFM data and confirmed from the reversible changes in surface topography. These observations provide insight into the mechanisms of tip-induced ferroelastic domain control and suggest that formation of topological defect states under the action of local defect- and tip-induced fields is much more common than previously believed.

This paper proposes a method to engineer the effects of mesa aspect ratio on polarization switching for single-crystal and polycrystalline PZT nanostructures. The out-of-plane polarization switching of single-crystal and polycrystalline structures as a function of crystallographic orientation, epitaxial strain and mesa aspect ratio are explored. The results are summarized in terms of the mesa geometrical parameters, crystallographic orientation and expitaxial strain. The results demonstrate a strong correlation of single-crystal properties to the polarization hysteresis behavior of a central representative grain in a polycrystalline film. The average remnant polarization and its reliability are controlled through the aspect ratio of the mesa. Calculations demonstrate that the stresses at the edges are relaxed for film height, h f , to mesa width, w , ratios h f / w ⩽ 1 × 10 −4 . For h f / w ⩾ 1 × 10 −2 , the effective in-plane stress is relaxed throughout the deposited film. Moreover, the effective stresses at the center of the mesa are 15% of the stresses of an infinite film.

Ferroelectrics are non-centrosymmetric crystalline materials that possess a spontaneous polarization that can be switched by an electric field. The electric-field-dependent optical response of these materials makes them important for optical devices, such as modulators or beam deflectors. In the inexorable drive to miniaturization, the concept of integrated thin film optical devices has led to the incorporation of ferroelectric thin films on single-crystal substrates. These structures have appealing electro-optic modulation characteristics, interesting strain-dependent bandgaps and refractive index, as well as promising possibilities for solar harvesting. Here, we review the work on epitaxial ferroelectric (FE) films for optical applications. We first show that FE thin film materials are attractive for integrated electro-optic modulators and then show that epitaxial strain can be used to enhance the FE and optical functionality of films. Next, we describe some of the photovoltaic functionality of FE thin film materials' systems and conclude the review by highlighting some thin-film devices that exploit the aforementioned optical effects.

We investigate the interfacial chemistry of secondary Fe2O3 phases formed in a BiFeO3 (BFO) layer in BFO/ La0.67Sr0.33MnO3 (LSMO)/SrTiO3 (STO) heterostructures. A combination of high-resolution spherical aberration corrected scanning TEM and spectroscopy results, reveals that specific chemical and crystallographic similarities between Fe2O3 and BFO, enable the BFO layer to form a facile host for Fe2O3.

High-performance piezoelectric devices based on ferroelectric materials rely heavily on ferroelastic domain wall switching. Here we present visual evidence for the local mechanisms that underpin domain wall dynamics in ferroelastic nanodomains. State-of-the-art band excitation switching spectroscopy piezoforce microscopy (PFM) reveals distinct origins for the reversible and irreversible components of ferroelastic domain motion. Extrapolating the PFM images to case for uniform fields, we posit that, while reversible switching is essentially a linear motion of the ferroelastic domains, irreversible switching takes place via domain wall twists. Critically, real-time images of in situ domain dynamics under an external bias reveal that the reversible component leads to reduced coercive voltages. Finally, we show that junctions representing three-domain architecture represent facile interfaces for ferroelastic domain switching, and are likely responsible for irreversible processes in the uniform fields. The results presented here thus provide (hitherto missing) fundamental insight into the correlations between the physical mechanisms that govern ferroelastic domain behavior and the observed functional response in domain-engineered thin film ferroelectric devices.

We present a comprehensive study of the physical properties of epitaxial cobalt-doped BiFeO3 films ∼50 nm thick grown on (001) LaAlO3 substrates. X-ray diffraction and magnetic characterization demonstrate high quality purely tetragonal-like (T′) phase films with no parasitic impurities. Remarkably, the step-and-terrace film surface morphology can be fully recovered following a local electric-field-induced rhombohedral-like to T′ phase transformation. Local switching spectroscopy experiments confirm the ferroelectric switching to follow previously reported transition pathways. Critically, we show unequivocal evidence for conduction at domain walls between polarization variants in T′-like BFO, making this material system an attractive candidate for domain wall-based nanoelectronics.

The phase stability of ultra-thin (0 0 1) oriented ferroelectric PbZr1-xTixO3(PZT) epitaxial thin films as a function of the film composition, film thickness, and the misfit strain is analyzed using a non-linear Landau-Ginzburg-Devonshire thermodynamic model taking into account the electrical and mechanical boundary conditions. The theoretical formalism incorporates the role of the depolarization field as well as the possibility of the relaxation of in-plane strains via the formation of microstructural features such as misfit dislocations at the growth temperature and ferroelastic polydomain patterns below the paraelectric-ferroelectric phase transformation temperature. Film thickness-misfit strain phase diagrams are developed for PZT films with four different compositions (x = 1, 0.9, 0.8 and 0.7) as a function of the film thickness. The results show that the so-called rotational r-phase appears in a very narrow range of misfit strain and thickness of the film. Furthermore, the in-plane and out-of-plane dielectric permittivities ε11and ε33, as well as the out-of-plane piezoelectric coefficients d33for the PZT thin films, are computed as a function of misfit strain, taking into account substrate-induced clamping. The model reveals that previously predicted ultrahigh piezoelectric coefficients due to misfit-strain-induced phase transitions are practically achievable only in an extremely narrow range of film thickness, composition and misfit strain parameter space. We also show that the dielectric and piezoelectric properties of epitaxial ferroelectric films can be tailored through strain engineering and microstructural optimization. Refereed/Peer-reviewed

Grain-to-grain long-range interactions and the ensuing collective dynamics in the domain behavior of nanostructured polycrystalline Pb(Zr,Ti)O 3 ferroelectric thin films have been investigated. To identify the key factors and interactions controlling local polarization dynamics we utilize a synergistic approach based on focused ion beam (FIB) milled damage-free nanostructures to isolate single grains and grain clusters, time-resolved piezoresponse force microscopy and switching spectroscopy PFM (SSPFM) (PFM) to address polarization dynamics within individual grains, and finite-element simulations to quantify the local ferroelectric interactions and hence assess the weight of several possible switching mechanisms. The experiments find that of the three possible switching mechanisms, namely direct electromechanical coupling, local built-in electric field and strain, and grain boundary electrostatic charges, the last one is the dominant mechanism. Although finite-element simulations find that direct electromechanical coupling and local built-in field-induced switching are possible, calculations confirm that for the utilized material properties, the aforementioned mechanisms are energetically unfavored.

We present a theoretical investigation of the elastic interactions in a heteroepitaxial bilayer consisting of a (001) tetragonal PbZrxTi1-xO3and (001) rhombohedral PbZr1-xTixO3on a thick (001) passive substrate. Analytical expressions for the elastic interaction energies between the layers and the resultant ferroelastic twin formation have been derived as a function of the lattice misfit strain (between layers and the substrate), composition of the ferroelectric and thickness. It is found that the elastic coupling between the tetragonal and rhombohedral layers leads to the equilibrium domain fraction in the tetragonal layer several time larger than that in single-layer films of similar thickness. Most critically, the model finds a significant change in the ferroelastic domain volume fraction in the presence of an applied electric field and hence enhanced piezoelectric properties compared to single-layered epitaxial PZT thin films. Refereed/Peer-reviewed

We present a detailed investigation of the chemistry at the growth interface between the bottom electrode and ferroelectric film in (001)-oriented epitaxial ferroelectric thin-film heterostructures. Three different ferroelectric systems, namely PbZr0.2Ti0.8O3, PbZr0.52Ti0.48O3, and BaTiO3 deposited on SrRuO3/SrTiO3, were investigated to compare and contrast the role of lattice mismatch versus the volatility of the deposited cation species. A combination of transmission electron microscopy-based imaging and spectroscopy reveals distinct correlations among the ferroelectric thin-film composition, the deposition process, and chemical gradients observed across the ferroelectric–electrode interface. Sr diffusion from the electrode into the ferroelectric film was found to be dominant in PbZr0.2Ti0.8O3/SrRuO3/SrTiO3 thin films. Conversely, Pb diffusion was found to be prevalent in PbZr0.52Ti0.48O3/SrRuO3/SrTiO3 thin films. The BaTiO3/SrRuO3/SrTiO3 heterostructure was found to have atomically sharp interfaces with no signature of any interdiffusion. We show that controlling the volatility of the cation species is as crucial as lattice mismatch in the fabrication of defect-free ferroelectric thin-film devices.

Heterostructured thin films differing either in their structure, composition, or in both have shown novel magnetic, superconducting, ferroelectric, or electromechanical responses. In the case of ferroelectrics, multilayers or superlattices have displayed enhanced polarization, high dielectric permittivity and in some instances, entirely new structural phases. These observations have been accounted for on the basis of electric-field-induced coupling, epitaxial strain, and specific polar interactions between the interfacial layers. As most ferroelectrics have a strong non-negligible ferroelastic self-strain associated with their phase transformation, an aspect that should be equally fascinating is that of the elastic interactions between such multilayers. The interactions lead to the formation of ferroelastic domains, arranged in the form of periodalternating lamellae in order to relax the excess elastic energy. When these lamellae are of the same phase but of different crystallographic orientations, they are generally referred to as ‘‘twins’’. The ferroelastic-domain-wall (extrinsic) contribution to the dielectric, piezoelectric, and elastic properties of ferroelectrics is indeed quite significant in the case of bulk ceramics and single-crystals, and this can be several times larger than the intrinsic lattice piezoresponse. In the case of ferroelectric thin films, the issue of movement of ferroelastic domains is not without debate. Earlier experimental studies proposed that the ferroelastic ‘‘herringbone’’ pattern in thin films demonstrated very limited (if any) ability to move under external stress or electric field. This has been contradicted in more recent experiments on polycrystalline films. Nevertheless, gross quantitative enhancement has been reported only under special conditions, such as films patterned into

Multiferroic epitaxial Bi-Fe-O thin films of different thicknesses (15–500 nm) were grown on SrTiO3 (001) substrates by pulsed laser deposition under various oxygen partial pressures to investigate the microstructural evolution in the Bi-Fe-O system and its effect on misfit strain relaxation and on the magnetic properties of the films. Films grown at low oxygen partial pressure show the canted antiferromagnetic phase α-Fe2O3 embedded in a matrix of BiFeO3. The ferromagnetic phase, γ-Fe2O3 is found to precipitate inside the α-Fe2O3 grains. The formation of these phases changes the magnetic properties of the films and the misfit strain relaxation mechanism. The multiphase films exhibit both highly strained and fully relaxed BiFeO3 regions in the same film. The magnetization in the multiphase Bi-Fe-O films is controlled by the presence of the γ-Fe2O3 phase rather than heteroepitaxial strain as it is the case in pure single phase BiFeO3. Also, our results show that this unique accommodation of misfit strain by the formation of α-Fe2O3 gives rise to significant enhancement of the piezo electric properties of BiFeO3.

We investigate photo-induced ferroelectric domain switching in a series of Pb(Zr0.2Ti0.8)O3/La0.7Sr0.3MnO3 (PZT/LSMO) bilayer thin films with varying surface morphologies by piezoresponse force microscopy under light illumination. We demonstrate that reverse poled ferroelectric regions can be almost fully recovered under laser irradiation of the PZT layer and that the recovery process is dependent on the surface morphology on the nanometer scale. The recovery process is well described by the Kolmogorov-Avrami-Ishibashi model, and the evolution speed is controlled by light intensity, sample thickness, and initial write voltage. Our findings shed light on optical control of the domain structure in ferroelectric thin films with different surface morphologies.

We investigate ferroelectric domain structure and piezoelectric response under variable mechanical compressive stress in Pb(Zr0.2TiO0.8)O3 (PZT) thin films using high-resolution piezoresponse force microscopy (PFM) and an in situ sample bending stage. Measurements reveal a drastic change in the ferroelectric domain structure which is presented along with details of the mediating switching process involving domain wall motion, nucleation, and domain wall roughening under an applied external mechanical stimulus. Furthermore, local PFM hysteresis loops reveal significant changes in the observed coercive biases under applied stress. The PFM hysteresis loops become strongly imprinted under increasing applied compressive stress.

Near-field optical second harmonic microscopy has been applied to imaging of the c/a/c/a polydomain structure of epitaxial PbZrxTi1–xO3 thin films in the 0

We report observations of the ferroelectric domain structure in epitaxial lead zirconate titanate (PZT) ferroelectric thin films using piezoresponse microscopy. EpitaxialPZT films with a nominal composition of PbZr0.2Ti0.8O3were deposited onto single crystal SrTiO3 substrates by pulsed laser deposition, with an epitaxial layer of La-Sr-Co-O as the bottom electrode. By manipulating thefilm thickness, a uniform 2-dimensional grid of 90° domains (a-domains, i.e., c-axis in the plane of the film) has been induced. Our studies show that the polarization direction in the film is substantially preferentially oriented and that nucleation of reverse 180° domains occurs preferentially at 90° domain interfaces. Polarization reversal occurs through the nucleation and subsequent growth of “semicircular/elliptical” reverse domains, which subsequently consume the entire region as a function of reversal time. The reversal is seen to fit a stretched exponential.

We have studied the effect of misfit strain on the microstructure and properties of ferroelectric lead zirconate titanate thin films. We have changed the misfit strain by varying the film thickness and studied the thickness effect on the domain formation of epitaxial PbZr0.2Ti0.8O3 (PZT) films grown by pulsed laser deposition on (001) LaAlO3 substrates with La0.5Sr0.5CoO3 (LSCO) electrodes. The nominal thickness of the PZT films was varied from 60 to 400 nm with the LSCO electrode thickness kept constant at 50 nm. X-ray diffraction experiments show that the films relax via the formation of a domains, the fraction of which increase with the ferroelectric film thickness. The c-axis lattice constant of PZT films calculated from the 002 reflection decreases with increasing film thickness and approaches the bulk value of ∼0.413 nm in the films thicker than 300 nm. Cross-sectional transmission electron microscopy images reveal that the a-domain fraction and period increase with increasing film thickness. The relaxation of misfit strain in the film is accompanied by systematic changes in the polarization properties, as well as the switching fields, quantified by the coercive field and the activation field.

The origin of giant electromechanical response in a mixed-phase rhombohedral-tetragonal BiFeO${}_{3}$ thin film is probed using subcoercive scanning probe microscopy based multiple-harmonic measurements. Significant contributions to the strain arise from a second-order harmonic response localized at the phase boundaries. Strain and dissipation data, backed by thermodynamic calculations, suggest that the source of the enhanced electromechanical response is the motion of phase boundaries. These findings elucidate the key role of labile phase boundaries, both natural and artificial, in achieving thin films with giant electromechanical properties.

90° elastic domains were observed after local polings in epitaxial ferroelectric Pb(Zr0.2Ti0.8)O3 thin films via piezoresponse force microscopy. An area of internal stress arises under a conductive atomic force microscope tip due to the opposite signs of the converse piezoelectric effects in the switched domain and the unswitched films surrounding. The formation of 90° domains leads to the relaxation of the internal stress and stabilization of 180° domain after turning off the electric field applied by the tip. The criterion that formulates the necessary condition for realization of the relaxation mechanism is presented as well.

We report on the out-of-plane piezoelectric response (d33), measured via piezoresponse scanning force microscopy, of submicron capacitors fabricated from epitaxial PbZrxTi1−xO3 thin films. Investigations on 1 μm2 and smaller capacitors show that the substrate-induced constraint is dramatically reduced by nanostructuring. At zero field, the experimentally measured values of d33 for clamped as well as submicron capacitors are in good agreement with the predictions from thermodynamic theory. The theory also describes very well the field dependence of the piezoresponse of clamped capacitors of key compositions on the tetragonal side of the PbZrxTi1−xO3 phase diagram as well as the behavior of submicron PbZr0.2Ti0.8O3 (hard ferroelectric) capacitors. However, the field-dependent piezoresponse of submicron capacitors in compositions closer to the morphotropic phase boundary (soft ferroelectrics) is different from the behavior predicted by the theoretical calculations.

Use of an epitaxial conducting template has enabled the integration of epitaxial ferroelectric perovskites on silicon. The conducting template layer, LaxSr1−xTiO3 (LSTO), deposited onto (001) silicon wafers by molecular-beam epitaxy is then used to seed {001}-oriented epitaxial perovskite layers. We illustrate the viability of this approach using PbZr0.4Ti0.6O3 (PZT) as the ferroelectric layer contacted with conducting perovskite La0.5Sr0.5CoO3 (LSCO) electrodes. An important innovation that further facilitates this approach is the use of a low-temperature (450 °C) sol–gel process to crystallize the entire ferroelectric stack. Both transmission electron microscopy and x-ray diffraction analysis indicate the LSCO/PZT/LSCO/LSTO/Si heterostructures are epitaxial. The electrical response of ferroelectric capacitors (for pulse widths down to 1 μs) measured via the underlying silicon substrate is identical to measurements made using conventional capacitive coupling method, indicating the viability of this approach.

Direct hysteresis measurements on single submicron structure sizes were performed on epitaxial ferroelectric Pb(Zr,Ti)O3 thin films grown on SrTiO3 with La0.5Sr0.5CoO3 (LSCO) electrodes. The samples were fabricated by focused-ion-beam milling resulting in pad sizes down to 200 nm×200 nm. The influence of parasitic capacitance of the measurement setup was eliminated by applying an enhanced compensation procedure. No size effects were observed in capacitors milled down to 400 nm×400 nm. Thus, a published increase of Pmax1 for decreasing pad size can be explained by the parasitic influence of the setup. Finally, the inaccuracy of increasing coercive voltage due to the coating of the cantilever of the atomic force microscope is discussed.

We have investigated the polydomain formation in 100–200-nm-thick PbZr0.2Ti0.8O3 epitaxial thin films on vicinally cut (100) oriented SrTiO3 substrates. Our results show that there is a preferential location of the nucleation of the a domains along the step edges of the underlying substrate. By piezo-response microscopy, we show that all a domains have their polarization aligned along the same direction. This result is in contrast to flat substrates where fourfold symmetry of a domains is observed. We observe that the critical thickness for a domain formation is much lower than that for PbZr0.2Ti0.8O3 films grown on flat substrates. We have developed a model based on minimization of elastic energy to describe the effect of localized stresses at step edges on the formation of a domains in the ferroelectric layer.

Long-range strain fields associated with dislocation cores at an oxide interface are shown to be sufficient enough to create significant variations in the chemical composition around the core (Cottrell atmospheres). Such stress-assisted diffusion of cations towards the cores is proposed to significantly impact the properties of nanoscale functional devices. The figure shows a Z-contrast image of a single dislocation core at an oxide interface. (Figure Presented) Refereed/Peer-reviewed

We have investigated heteroepitaxial films of Sm-doped BiFeO3 with a Sm-concentration near a morphotropic phase boundary. Our high-resolution synchrotron X-ray diffraction, carried out in a temperature range of 25C to 700C, reveals substantial phase coexistence as one changes temperature to crossover from a low-temperature PbZrO3-like phase to a high-temperature orthorhombic phase. We also examine changes due to strain for films greater or less than the critical thickness for misfit dislocation formation. Particularly, we note that thicker films exhibit a substantial volume collapse associated with the structural transition that is suppressed in strained thin films.

The effect of various substrates on the electrical and electromechanical properties of 100-nm-thick epitaxial 0.9[Pb(Mg1/3Nb2/3)O3]–0.1[PbTiO3](0.9PMN–0.1PT) thin films is investigated. (001) 0.9PMN–0.1PT films are grown on (001)LaAlO3(LAO), (La, Sr)(Al, Ta)O3(LSAT), SrTiO3(STO), and MgO substrates with 40-nm-thick top and bottom La0.5Sr0.5CoO3 electrodes by pulsed laser deposition. X-ray diffraction results indicate that the films on LAO, LSAT, and STO are stressed biaxially in compression in the film-substrate interface whereas the films on MgO are stressed in tension. A decrease in the temperature of dielectric maximum (Tm) together with an increase in the dielectric constant and the longitudinal piezomodulus is observed with decreasing in-plane epitaxial stresses for LAO, LSAT, and STO substrates. The films on MgO substrates have the highest dielectric constant and piezomodulus with Tm below room temperature. The variation in Tm may be attributed to the shift in the transformation temperature from the...

We report observations of the ferroelectric domain structure in epitaxial lead zirconate titanate (PbZr0.2Ti0.8O3) ferroelectric thin films using piezoresponse microscopy. By manipulating the film thickness, a uniform two-dimensional grid of 90° domains (a domains, i.e., c axis in the plane of the film) has been induced. Our studies show that the out of plane polarization direction in the film is preferentially oriented. 90° domains have been observed as regions of low piezoresponse, as compared to the fully c axis oriented regions. We have studied the influence of these 90° domains and the domain walls on the nucleation of polarization reversal. We observe that the nucleation occurs preferentially at 90° domain interfaces. Polarization reversal is seen to occur through the nucleation and subsequent growth of “semicircular/elliptical” reverse domains, which eventually consume the entire region as a function of reversal time.

The effect of heteroepitaxy-induced constraint on the structure and piezoelectric properties of the relaxor ferroelectric lead magnesium niobate–lead titanate (PMN–PT) were investigated. Relaxor PMN–PT epitaxial thin films with oxide electrodes were grown by pulsed-laser deposition on (100) LaAlO3 substrates. We observe a systematic decrease in the phase transition temperature (temperature at which a maximum in dielectric response occurs), from around 250 to around 60 °C as the relaxor thickness is increased from 100 to 400 nm. This is accompanied by an increase in the relative dielectric constant (er), measured at room temperature and 10 kHz, from 300 to 2000. The piezoelectric coefficient d33 measured using a scanned probe microscope, increase by almost an order of magnitude with increasing film thickness.

We report on the discovery of a lead-free morphotropic phase boundary in Sm doped BiFeO3 with a simple perovskite structure using the combinatorial thin film strategy. The boundary is a rhombohedral to pseudo-orthorhombic structural transition which exhibits a ferroelectric (FE) to antiferroelectric (AFE) transition at approximately Bi0.86Sm0.14FeO3 with dielectric constant and out-of-plane piezoelectric coefficient comparable to those of epitaxial (001) oriented Pb(Zr,Ti)O3 (PZT) thin films at the MPB. The discovered composition may be a strong candidate of a Pb-free piezoelectric replacement of PZT., 16 pages, 5 figures, submitted to Applied Physics Letters

Scaling of the structural order parameter, polarization, and electrical properties was investigated in model ultrathin epitaxial SrRuO3/PbZr0.2Ti0.8O3/SrRuO3/SrTiO3 heterostructures. High-resolution transmission electron microscopy images revealed the interfaces to be sharp and fully coherent. Synchrotron x-ray studies show that a high tetragonality (c/a similar to 1.058) is maintained down to 50 angstrom thick films, suggesting indirectly that ferroelectricity is fully preserved at such small thicknesses. However, measurement of the switchable polarization (Delta P) using a pulsed probe setup and the out-of-plane piezoelectric response (d(33)) revealed a systematic drop from similar to 140 mu C/cm(2) and 60 pm/V for a 150 angstrom thick film to 11 mu C/cm(2) and 7 pm/V for a 50 angstrom thick film. This apparent contradiction between the structural measurements and the measured switchable polarization is explained by an increasing presence of a strong depolarization field, which creates a pinned 180 degrees polydomain state for the thinnest films. Existence of a polydomain state is demonstrated by piezoresponse force microscopy images of the ultrathin films. These results suggest that the limit for a ferroelectric memory device may be much larger than the fundamental limit for ferroelectricity. (c) 2006 American Institute of Physics.

This paper reviews nanoscale phenomena in ferroelectric thin films. Specific focus is polarization relaxation dynamics and piezoelectric characterization in model thin films and nanostructures using voltage modulated scanning force microscopy coupled with nanofabrication techniques. We present results of studies on the time dependent relaxation of the remanent polarization in model epitaxial PZT ferroelectric thin films, containing a uniform 2-dimensional grid of 90° domains (c-axis in the plane of the film). Clear evidence of the role of 90° domain walls in the nucleation of the 180° reverse domains and on a very local scale, pinning and bowing of domain walls is shown. The kinetics of relaxation is modeled through a stretched exponential approach. The second part of this paper illustrates how a conventional AFM can be used as a “nano” probe for ferroelectric properties. The characterization of the longitudinal piezoelectric constant (d33) in nanoscale capacitors (or islands) of various PZT compositions is elucidated. We show that by altering the electromechanical interplay between the substrate and the ferroelectric thin film, in compositions closer to the morphotropic phase boundary, an unusual field dependence of the d33 exists. Due to this effect, the change in strain at saturation field is twice the theoretical prediction, opening new applications such as highly strain tunable devices. Secondly we discuss in highly tetragonal Pb(Zr0.2Ti0.8)O3, movement of elastic 90° domains with applied DC field, a phenomenon hitherto observed only in bulk single crystals or ceramics. This results in a d33 of ∼ 250 pm/V at remanence, which is approximately 3–4 times the predicted value of 87 pm/V for a single domain single crystal. We summarize this review with some possible directions for future work.

This paper reviews nanoscale phenomena such as polarization relaxation dynamics and piezoelectric characterization in model ferroelectric thin films and nanostructures using voltage-modulated scanning force microscopy. Using this technique we show the three-dimensional reconstruction of the polarization vector in lead zirconate titanate (PZT) thin films. Second, the time-dependent relaxation of remanent polarization in epitaxial PZT ferroelectric thin films, containing a uniform two-dimensional grid of 90° domains (c-axis in the plane of the film), has been investigated extensively. The 90° domain walls preferentially nucleate the 180° reverse domains during relaxation. Relaxation occurs through the nucleation and growth of reverse 180° domains, which subsequently coalesce and consume the entire region as a function of relaxation time. In addition we also present results on investigation of the relaxation phenomenon on a very local scale, where pinning and bowing of domain walls has been observed. We also show how this technique is used for obtaining quantitative information on piezoelectric constants and by engineering special structures, and how we realize ultrahigh values of piezoconstants. Last, we also show direct hysteresis measurements on nanoscale capacitors, where there is no observable loss of polarization in capacitors as small as 0.16 μm2 in area.

Enhancement of polarization and related properties in heteroepitaxially constrained thin films of the ferroelectromagnet, BiFeO 3 , is reported. Structure analysis indicates that the crystal structure of film is monoclinic in contrast to bulk, which is rhombohedral. The films display a room-temperature spontaneous polarization (50 to 60 microcoulombs per square centimeter) almost an order of magnitude higher than that of the bulk (6.1 microcoulombs per square centimeter). The observed enhancement is corroborated by first-principles calculations and found to originate from a high sensitivity of the polarization to small changes in lattice parameters. The films also exhibit enhanced thickness-dependent magnetism compared with the bulk. These enhanced and combined functional responses in thin film form present an opportunity to create and implement thin film devices that actively couple the magnetic and ferroelectric order parameters.

Dynamics of domain interfaces in a broad range of functional thin-film materials is an area of great current interest. In ferroelectric thin films, a significantly enhanced piezoelectric response should be observed if non-180 degrees domain walls were to switch under electric field excitation. However, in continuous thin films they are clamped by the substrate, and therefore their contribution to the piezoelectric response is limited. In this paper we show that when the ferroelectric layer is patterned into discrete islands using a focused ion beam, the clamping effect is significantly reduced, thereby facilitating the movement of ferroelastic walls. Piezo-response scanning force microscopy images of such islands in PbZr0.2Ti0.8O3 thin films clearly point out that the 90 degrees domain walls can move. Capacitors 1 microm2 show a doubling of the remanent polarization at voltages higher than approximately 15 V, associated with 90 degrees domain switching, coupled with a d33 piezoelectric coefficient of approximately 250 pm V-1 at remanence, which is approximately three times the predicted value of 87 pm V-1 for a single domain single crystal.

Voltage-modulated scanning force microscopy (Piezoresponse microscopy) is applied to investigate the domain structure in epitaxial PbZr0.2Ti0.8O3 ferroelectric thin films grown on (001) SrTiO3. By monitoring the vertical and lateral differential signals from the photodetector of the atomic force microscope it is possible to separate out and observe the out-of-plane and in-plane polarization vectors in the thin film individually. The relative orientation of the polarization vectors across a 90degrees domain wall is observed. Nucleation of new reversed 180degrees domains at the 90degrees domain wall is studied and its impact on the rotation of polarization within the a domain is analyzed as a function of reversal time. (C) 2002 American Institute of Physics.

Solid state dewetting and the subsequent morphological changes for platinum thin films grown on zinc oxide (ZnO) buffered (001) silicon substrates (Pt/ZnO/SiO2/(001)Si system) is investigated under vacuum conditions via a custom-designed confocal laser microscope coupled with a laser heating system. Live imaging of thin film dewetting under a range of heating and quenching vacuum ambients reveals events including hillock formation, hole formation, and hole growth that lead to formation of a network of Pt ligaments, break up of Pt ligaments to individual islands and subsequent Pt islands shape reformation, in chronological fashion. These findings are corroborated by ex-situ materials characterization and quantitative electron microscopy analysis. A secondary hole formation via blistering before film rupture is revealed to be the critical stage, after which a rapid dewetting catastrophe occurs. This process is instantaneous and cannot be captured by ex-situ methods. Finally, an intermetallic phase forms at 900 °C and alters the morphology of Pt islands, suggesting a practical limit to the thermal environments that may be used for these platinized silicon wafers in vacuum conditions.

Cation intermixing at functional oxide interfaces remains a highly controversial area directly relevant to interface-driven nanoelectronic device properties. Here, we systematically explore the cation intermixing in epitaxial (001) oriented multiferroic bismuth ferrite (BFO) grown on a (001) lanthanum aluminate (LAO) substrate. Aberration corrected dedicated scanning transmission electron microscopy and electron energy loss spectroscopy reveal that the interface is not chemically sharp, but with an intermixing of ∼2 nm. The driving force for this process is identified as misfit-driven elastic strain. Landau-Ginzburg-Devonshire-based phenomenological theory was combined with the Sheldon and Shenoy formula in order to understand the influence of boundary conditions and depolarizing fields arising from misfit strain between the LAO substrate and BFO film. The theory predicts the presence of a strong potential gradient at the interface, which decays on moving into the bulk of the film. This potential gradient...

Focused ion beam milling was used to fabricate ferroelectric islands in Pb-Zr-Ti-O thin films. The islands ranged in size from 200μm×200μm to 0.3μm×0.3μm. The inverse piezoelectric effect was studied in these islands as a function of their size by tracking the surface displacement of the top electrode of the island (under an applied electric field) using an atomic force microscope (AFM). It was found that there was a substantial increase in the piezoresponse as the size of the island decreased below 100μm×100μm. This increase was attributed to the elastic deformation of the substrate.

Near field optical second harmonic microscopy has been applied to imaging of the c/a/c/a polydomain structure of epitaxial PbZrxTi1−xO3 thin films in the 0

The relations between electrical and mechanical properties of constrained ferroelectric films are analyzed. It is shown that the internal stresses and the elastic constants can be determined through the measurement of the electrical response. The change in the polarization is proportional to internal stresses due to film-substrate misfit, whereas the linear electrical and electromechanical responses to external field do not depend on the misfit and are determined by the film constraint. The theoretical results are successfully applied to PbZr0.2Ti0.8O3 films on (001) LaAlO3 substrate which exhibit a considerable increase in the saturation polarization due to epitaxial stresses. Significant recovery in the piezoelectric constant and susceptibility is theoretically predicted and experimentally verified for specific film configurations which reduce the degree of constraint. The concept presented in this Letter can be expanded to constrained ferromagnetic and superconductor films. PACS numbers: 77.22.Ch, 77.22.Ej, 77.55. + f, 77.65. – j Our recent experimental work has demonstrated that there is a considerable increase in the spontaneous polarization in thin films of epitaxial PbZr 0.2Ti0.8O3 (PZT) grown by pulsed laser deposition on (001) LaAlO3 (LAO) substrates with La0.5Sr0.5CoO3 (LSCO) electrodes in comparison with relatively thicker films [1]. This effect is accompanied with an increase in the out-of-plane (c-axis) lattice parameter. Both observations can be interpreted as an effect of internal stresses due to lattice mismatch which for the first time allow us to determine internal stresses using the polarization. In this Letter we present an analysis of these facts. Our goal is to formulate an approach to determine internal stresses in the constrained ferroelectric (FE) films using its electrical and electromechanical characteristics. Furthermore, this analysis leads to the engineering of the constraint to govern and enhance the electrical and electromechanical properties of FE films. To start with, we clarify the theoretical background for electromechanical properties of constrained FE films. Afterwards, we analyze our experimental work from this point of view and discuss perspectives. The dependence of the polarization on internal stresses follows directly from the Helmholtz free energy of a constrained film with top and bottom electrodes

The effect of substrate induced constraint and misfit strain on the structure and piezoelectric properties of epitaxial relaxor ferroelectric lead magnesium niobate-lead titanate (PMN-PT) films were investigated. Relaxor PMN-PT epitaxial 100–400 nm thick films with top and bottom oxide electrodes were grown by pulsed laser deposition on (100) LaAlO3 substrates. A systematic decrease in the temperature of the dielectric maximum with increasing film thickness was observed. This is accompanied by an increase in the room temperature relative dielectric constant. The longitudinal piezoelectric coefficient measured using a scanned probe microscope, increases by almost an order of magnitude with increasing film thickness.