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A nonvolatile highly scalable multilevel memory based on ferroelectric domain walls is demonstrated., Ferroelectric domain walls are atomically sharp topological defects that separate regions of uniform polarization. The discovery of electrical conductivity in specific types of walls gave rise to “domain wall nanoelectronics,” a technology in which the wall (rather than the domain) stores information. This paradigm shift critically hinges on precise nanoengineering of reconfigurable domain walls. Using specially designed nanofabricated electrodes and scanning probe techniques, we demonstrate a prototype nonvolatile ferroelectric domain wall memory, scalable to below 100 nm, whose binary state is defined by the existence or absence of conductive walls. The device can be read out nondestructively at moderate voltages (

We report on the crystallographic orientational dependence of the electrochemical behaviour in nanostructured manganese oxide. Manganese Oxide (Mn 3 O 4 ) nanocrystals have been deposited on Nb-doped Strontium Titanate (Nb:SrTiO 3 ) substrate via pulsed laser deposition. (001), (101) and (112) orientated nanocrystals were successfully grown on (100), (110) and (111) Nb:SrTiO 3 substrates respectively. Analysis of the lattice arrangements suggests that nanostructure growth may be driven more by polarity rather than the epitaxial strain such that electrostatic repulsion can be minimised. Cyclic voltammetry (CV) in 1 M Na 2 SO 4 electrolyte was performed to understand how specific capacitance values vary with changing lattice orientations. The maximum specific capacitance calculated for the (100) orientation was 34 F/g, obtained after the 3000 th cycle. Beyond this the CV loop plateaus rapidly and structural analysis of this sample revealed a morphological transformation from the (001) orientation to the (101) platelet structures. The maximum specific capacitance obtained was for the (112) sample (120F/g) suggesting that such non-primary planes in spinel oxides may be most attractive for electrochemical applications.

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.

Epitaxial self-assembled ferro(i)magnetic spinel (CoFe2O4 (CFO)) and ferroelectric bismuth layered perovskite (Bi5Ti3FeO15 (BTFO)) pillar-matrix nanostructures are demonstrated on (001) single-crystalline strontium titanate substrates. The CFO remains embedded in the BTFO matrix as vertical pillars (∼50 nm in diameter) up to a volume fraction of 50%. Piezoresponse force microscopy experiments evidence a weak out-of-plane and a strong in-plane ferroelectricity in the BTFO phase, despite previously reported paraelectricity along the c-axis in a pure BTFO film. Phenomenological Landau-Ginzburg-Devonshire-based thermodynamic computations show that the radial stress induced by the CFO nanopillars can influence these ferroelectric phases, thus signifying the importance of the nanopillars. The CFO pillars demonstrate robust ferromagnetic hysteresis loops with little degradation in the saturation magnetization (ca. 4 μB/f.u.). Thus BTFO-CFO nanocomposites show significant promise as a lead-free magnetoelectric materials system.

In specific solid-state materials, under the right conditions, collections of magnetic dipoles are known to spontaneously form into a variety of rather complex geometrical patterns, exemplified by vortex and skyrmion structures. While theoretically, similar patterns should be expected to form from electrical dipoles, they have not been clearly observed to date: the need for continued experimental exploration is therefore clear. In this Letter we report the discovery of a rather complex domain arrangement that has spontaneously formed along the edges of a thin single crystal ferroelectric sheet, due to surface-related depolarizing fields. Polarization patterns are such that nanoscale "flux-closure" loops are nested within a larger mesoscale flux closure object. Despite the orders of magnitude differences in size, the geometric forms of the dual-scale flux closure entities are rather similar.

Piezoelectric membranes were produced by “poling” prefabricated polyvinylidene fluoride (PVDF) membranes in an electric field. Scanning electron microscopy (SEM) studies revealed that electrical poling changed the microstructure of membranes significantly. Infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) measurements confirmed the formation of β-phase crystals in PVDF membranes as a result of poling in an intense electric field. Dynamic mechanical analysis (DMA) and measurement the surface displacement when the membranes were energized by AC signals, confirmed that the poled membranes were piezoelectric. Piezo-response force microscopy (PFM) analysis showed that the piezoelectric domains were not uniformly distributed over the sample.

Domain stability and structures in Pb(Zr0.3Ti0.7)O3/Pb(Zr0.7Ti0.3)O3 bilayer films under different substrate strains are studied using the phase field method. It is demonstrated that the domain structure of the bilayer film is very different from those of the corresponding single layer films grown on the same silicon substrate with an incoherent interface. Moreover, the predicted rhombohedral domains in the Pb(Zr0.7Ti0.3)O3 layer of the bilayer film have smaller sizes than those in the single layer case. These results are compared with experimental observations and previous thermodynamic analyses. The polarization distributions of the ferroelectric–paraelectric bilayer are analyzed as a function of the thickness of the bilayer film, where there is a “ferroelectric proximity effect” due to dipole–dipole interactions. The phase diagrams for both the bilayer and single layer films as a function of temperature and effective in-plane substrate strain are constructed.

We present the study of the synthesis of (001) nickel oxide (NiO) epitaxial nanocrystals grown on (001) strontium titanate (SrTiO3) single crystal substrates. Pulsed laser deposition of the bismuth nickel oxide (BiNiO3, BNO) perovskite precursor followed by post-deposition processing is carried out to form the NiO nanocrystals. A detailed analysis of the dimensions of nanocrystals reveals that the morphology attained differs from the thermodynamically expected equilibrium shape. The deviations from the equilibrium shape are found to follow a systematic trend where the in-plane basal dimensions, that is, the length and width of the nanocrystals grown differ in discretized dimensions. This discretization suggests that for a given interfacial area of nanocrystals there are multiple stable basal rectangular geometries attainable.

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.

The rare-earth (RE)-modified bismuth ferrite (BiFeO3 or BFO) family of ferroelectrics have uncomplicated lead-free chemistries and simple perovskite structures. Due to the high Curie transition temperature of the parent BiFeO3 perovskite (similar to 830 °C), they are promising piezoelectric materials for use at elevated temperatures. However, the influence of the specific RE species on the electromechanical behavior at high temperatures and above the coercive electric-field is not widely reported. Here, structural analysis over multiple length scales using X-ray diffraction, transmission electron microscopy and piezoresponse force microscopy is coupled with a high electric-field cycling study and in situ converse d33 measurements up to 325 °C for three RE-BFO ceramic compositions, Bi0.86Sm0.14FeO3, Bi0.88Gd0.12FeO3 and Bi0.91Dy0.09FeO3. The ceramics exhibit different phase assemblages with varying amounts of polar rhombohedral R3c and intermediate antipolar orthorhombic Pbam phases as a function of the RE species. During electric-field cycling at electric-fields with amplitudes of 160 kV cm-1, peak-to-peak strains of 0.23-0.27% are reached for all three compositions. However, there are qualitative differences in the field-induced strain and electric current behavior as a function of electric-field cycling and the materials exhibit an electrical-history dependent behavior. Bi0.91Dy0.09FeO3 possesses an improved d33 stability as a function of temperature relative to the parent BFO perovskite and the highest depolarization temperature among the three RE-BFO compositions, with a stable d33 of similar to 22 pC N-1 up to 325 °C.

A new paradigm of domain wall nanoelectronics has emerged recently, in which the domain wall in a ferroic is itself an active device element. The ability to spatially modulate the ferroic order parameter within a single domain wall allows the physical properties to be tailored at will and hence opens vastly unexplored device possibilities. Here, we demonstrate via ambient and ultrahigh-vacuum (UHV) scanning probe microscopy (SPM) measurements in bismuth ferrite that the conductivity of the domain walls can be modulated by up to 500% in the spatial dimension as a function of domain wall curvature. Landau-Ginzburg-Devonshire calculations reveal the conduction is a result of carriers or vacancies migrating to neutralize the charge at the formed interface. Phase-field modeling indicates that anisotropic potential distributions can occur even for initially uncharged walls, from polarization dynamics mediated by elastic effects. These results are the first proof of concept for modulation of charge as a function of domain wall geometry by a proximal probe, thereby expanding potential applications for oxide ferroics in future nanoscale electronics.

The nonlinear response of a ferroic to an applied fi eld has been studied through the phenomenological Rayleigh Law for over a hundred years. Yet, despite this, the fundamental physical mechanisms at the nanoscale that lead to macroscopic Rayleigh behavior have remained largely elusive, and experimental evidence at small length scales is limited. Here, it is shown using a combination of scanning probe techniques and phase fi eld modeling, that nanoscale piezoelectric response in prototypical Pb(Zr,Ti)O 3 fi lms appears to follow a distinctly non-Rayleigh regime. Through statistical analysis, it is found that an averaging of local responses can lead directly to Rayleigh-like behavior of the strain on a macroscale. Phase-fi eld modeling confi rms the twist of the ferroelastic interface is key in enhancing piezoelectric response. The studies shed light on the nanoscale origins of nonlinear behavior in disordered ferroics.

Recent advances in thin-fi lm engineering leads to a new type of mixed-phase system in BiFeO 3 (BFO) epitaxial fi lms driven by substrate strain. [ 1 ] Single-phase multiferroic BFO had attracted great interests due to its robust ferroelectric and antiferromagnetic orderings at room temperatures. [ 2–11 ] Under a strong compressive strain ( > 4%), the stable crystal structure of BFO transformed from the rhombohedral-like monoclinics (R) to tetragonal-like monoclinics (T), [ 12 , 13 ] and with suitable strain relaxation through thickness, the coexistence of R-BFO and T-BFO phases can be obtained in the same fi lm. [ 1 , 14–18 ]

Development of magnetoelectric, electromechanical, and photovoltaic devices based on mixed-phase rhombohedral-tetragonal (R-T) BiFeO(3) (BFO) systems is possible only if the control of the engineered R phase variants is realized. Accordingly, we explore the mechanism of a bias induced phase transformation in this system. Single point spectroscopy demonstrates that the T → R transition is activated at lower voltages compared to T → -T polarization switching. With phase field modeling, the transition is shown to be electrically driven. We further demonstrate that symmetry of formed R-phase rosettes can be broken by a proximal probe motion, allowing controlled creation of R variants with defined orientation. This approach opens a pathway to designing next-generation magnetoelectronic and data storage devices in the nanoscale.

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.

Flux-mediated epitaxy of self-assembled ferroelectric Bi4Ti3O12 (BIT) nanoplates on SrTiO3 (001) is investigated with vanadium oxide (VOx) as the flux medium. It is shown that precoverage of a SrTiO3 (001) substrate with VOx helps to nucleate thin rutile TiO2 (001) crystallites that act as templates for the subsequent growth of (110)-oriented BIT nanoplates. Piezoresponse force microscopy reveals the presence of 90° domain walls even within a single BIT nanoplate, similar to BIT bulk single crystals.

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.

Experimental results on entirely complex oxide ferromagnetic/ferroelectric/ ferromagnetic tunnel junctions are presented in which the tunneling magnetoresistance is modified by applying low electric field pulses to the junctions. The experiments indicate that ionic displacements associated with the polarization reversal in the ferroelectric barrier affect the complex band structure at ferromagnetic-ferroelectric interfaces. The results are discussed in the framework ofthe theoretically predicted magnetoelectric interface effect and may lead to novel multistate memory devices.

The discovery of a universal behavior in rare-earth (RE)-substituted perovskite BiFe0 3 is reported. The structural transition from the ferroelectric rhombohedral phase to an orthorhombic phase exhibiting a double-polarization hysteresis loop and substantially enhanced electromechanical properties is found to occur independent of the RE dopant species. The structural transition can be universally achieved by controlling the average ionic radius of the A-site cation. Using calculations based on first principles, the energy landscape of BiFe0 3 is explored, and it is proposed that the origin of the double hysteresis loop and the concomitant enhancement in the piezoelectric coefficient is an electric-field-induced transformation from a paraelectric orthorhombic phase to the polar rhombohedral phase.

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 local atomic structure and nanoscale chemistry of an antipolar phase in Bi0.9Sm0.1FeO3 epitaxial thin films are examined by an array of transmission electron microscopy (TEM) coupled with electron diffraction and electron energy-loss spectroscopy methods. The observations are tied to macroscopic properties of the films, namely, polarization-electric field hysteresis loops, dielectric constant-electric field hysteresis loops, and the dielectric loss. At room temperature, the local Sm deficiency was determined to destabilize the long-range ferroelectric state, resulting in the formation of local antipolar clusters with the appearance of PbZrO3-like antiparallel cation displacements, which give rise to 1/4{011} and 1/4{211} reflections as well as 1/2{321}, because of in-phase oxygen octahedral tilts. Aberration-corrected TEM analysis reveals that the antipolar structure is actually a lamellar of highly dense ferroelectric domains with alternating polarizations. With increasing temperature, a phase transi...

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

The property of ferroelectricity was first reported in 1921 in Rochelle salt (sodium potassium tartrate). Once considered a rare phenomenon limited to a comparatively small class of crystals, the discovery of ferroelectricity in the binary oxide compound barium titanate (BaTiO3) in the mid-1940s paved the way for rapid advances in the search of new materials. Since that time, a great many other ferroelectric compounds and solid solutions have been found and there has been broad interest in ferroelectric materials that has continued essentially uninterrupted until the present time. Ferroelectric materials remain subjects of intensive investigation today for three principal reasons. First, the unique dielectric, pyroelectric, piezoelectric, and electro-optic properties exhibited by ferroelectric crystals, ceramics, composites, and thin films can be exploited in great many devices of commercial importance. Second, apart from their many technological applications, ferroelectric materials as a class exhibit a great diversity of phase transitions that make them ideal objects for scientific investigations into the origins and mechanisms of a wide range of structural transformation phenomena. Finally, advances in thin-film deposition and nanoscale fabrication techniques made over the past two decades have created new possibilities for the integration of these materials into the ever expanding array of microelectronic devices. Ferroelectrics form a sub-group of functional (or smart) materials whose physical properties are sensitive to changes in external conditions such as temperature, pressure, and electric fields. Below some critical temperature TC, the dielectric displacement (electric polarization) spontaneously assumes non-zero values in the absence of any externally applied force. The transition between non-ferroelectric and ferroelectric phases is accompanied by a loss of symmetry, characterized by double-well minima below the transition temperature, resulting in a switchable polarization that is accompanied by a hysteresis in the electric field–dielectric displacement response. As a class of materials, ferroelectrics also exhibit unusually large and nonlinear generalized susceptibilities; their dielectric, piezoelectric, elastic, and other properties display critical behavior near the ferroelectric phase transformation temperature. Ferroelectrics may be regarded as high energydensity materials as well that can be configured to store, release or interconvert electrical and mechanical energy in a well-controlled manner. Their exceptionally large piezoelectric compliances, pyroelectric coefficients, and dielectric susceptibilities can be exploited in a variety of microelectronic devices. Important examples of these include piezoelectric sensors and actuators, pyroelectric thermal imaging devices, high-dielectric constant capacitors, electro-optic light valves, and thin-film memories. Because of their strongly non-linear dielectric response ferroelectrics can be utilized in applications such as frequency-agile phase shifters and filters in wireless telecommunications systems. Because the properties of ferroelectric materials that are exploited in technological applications are intimately S. P. Alpay (&) G. A. Rossetti Jr. Materials Science and Engineering Program, Departments of Chemical, Materials, and Biomolecular Engineering, University of Connecticut, Storrs, CT, USA e-mail: p.alpay@ims.uconn.edu

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.

It has been well established that both in bulk at ambient pressure and for films under modest strains, cubic SrCoO$_{3-\delta}$ ($\delta < 0.2$) is a ferromagnetic metal. Recent theoretical work, however, indicates that a magnetic phase transition to an antiferromagnetic structure could occur under large strain accompanied by a metal-insulator transition. We have observed a strain-induced ferromagnetic to antiferromagnetic phase transition in SrCoO$_{3-\delta}$ films grown on DyScO$_3$ substrates, which provide a large tensile epitaxial strain, as compared to ferromagnetic films under lower tensile strain on SrTiO$_3$ substrates. Magnetometry results demonstrate the existence of antiferromagnetic spin correlations and neutron diffraction experiments provide a direct evidence for a G-type antiferromagnetic structure with Ne\'el temperatures between $T_N \sim 135\,\pm\,10\,K$ and $\sim 325\,\pm\,10\,K$ depending on the oxygen content of the samples. Therefore, our data experimentally confirm the predicted strain-induced magnetic phase transition to an antiferromagnetic state for SrCoO$_{3-\delta}$ thin films under large epitaxial strain., Comment: 6 pages, 4 figures