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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.

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.

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

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

Direct evidence of controlling the population of magnetic domains in BiFeO${}_{3}$ thin films through electric field is reported using neutron diffraction. By fabricating BiFeO${}_{3}$ thin films on vicinal SrTiO${}_{3}$ substrates, we have achieved ferroelectric monodomains as confirmed by piezoresponse force microscopy. The application of an electric field between the bottom SrRuO${}_{3}$ and the top electrode switches the ferroelectric domain state with concomitant changes in magnetic reflections observed with neutron diffraction, indicating changes in the antiferromagnetic domain populations. The observed magnetoelectric switching behavior by neutron diffraction is compared with the electric-field effect on the magneto-optical Kerr effect measurement on patterned pads of exchange coupled Co film deposited on top of the BiFeO${}_{3}$ films. The present result shows possible new directions for the realization of magnetoelectric devices.

The dynamic switching of ferroelastic nanodomains in ferroelectric PbZr0.3Ti0.7O3/PbZr0.7Ti0.3O3 bilayers was investigated. Synchrotron microdiffraction using a high-speed pulse generator reveals that electric field pulses as short as 200 ns can switch the ferroelastic domain. Multiples of random distribution analysis of the field-induced changes in diffraction peak intensities finds a dynamic strain change from 0.2 to 1% with increasing the pulse width. Raman spectroscopy shows considerable decreases in A1(1TO) soft mode intensity after applications of short pulses, confirming the ferroelastic switching. The results demonstrate that ferroelastic domains can indeed move at time scales of the order of hundreds of nanoseconds.

Ferroelectric oxide thin films are currently used in ultra high density non-volatile memories (FeRAM) and Nano/Micro-Electro-Mechanical Systems (NEMS/MEMS). In all these application, the functional property is determined by the ferroelectric and ferroelastic domain wall movement. Moreover, commercially developed systems are based on polycrystalline (or textured) ferroelectric thin films and therefore their performance rely heavily on the microstructural features such as orientation, grain size, grain boundary contribution etc. Furthermore, as the thin films are patterned onto the substrate for any device fabrication, the film-substrate interface defects such as lattice misfit, dislocations etc., highly influence the domain wall behavior. This dissertation, investigates the nanoscale domain switching behavior in polycrystalline perovskite lead zirconate titanate, Pb(ZrxTi(l-x)03 (PZT) ferroelectric thin films. Systematic Piezoresponse Force Microscopy (PFM) studies provide direct visual evidence of the complex interplay between electrical and mechanical fields in a polycrystalline system, which causes effects such as correlated switching between grains, ferroelastic domain switching, inhomogeneous piezostrain profiles and domain pinning on very minute length scales. Furthermore, the grain to grain long range interaction and ensuing collective dynamics in the domain switching behavior have been investigated using the time resolved PFM and Switching Spectroscopy PFM (SSPFM). Finite element method (FEM) has been employed to quantify the local ferroelectric interaction and assess the 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. Having studied in detail the nanoscale domain wall behavior, we are now able to control and engineer the domain behavior in the PZT ferroelectric thin film materials. In the special case of thin film ferroelectrics that have a large ferroelastic self-strain associated with their phase transformation, a key aspect is the interaction of this self-strain with the boundary conditions of the film. In this thesis, by depositing a strongly tetragonal ferroelectric thin film on a soft rhombohedral bottom layer, we show that these elastic interactions result in a ferroelastic domain structure in the tetragonal film that is susceptible to external perturbation. High-resolution piezoresponse force microscopy images demonstrate gross movement (nm scale) of the ferroelastic domains under local bias and shows enhanced electromechanical response through this movement. Band excitation piezoforce spectroscopy investigation presents visual evidence for the local mechanism that underpins the ferroelastic domain wall movement and reveals distinct origins for the reversible and irreversible components of ferroelastic domain motion. We find 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. 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 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

We report on the evolution of the magnetic structure of BiFeO3 thin films grown on SrTiO3 substrates as a function of Sm doping. We determined the magnetic structure using neutron diffraction. We found that as Sm increases, the magnetic structure evolves from a cycloid to a G-type antiferromagnet at the morphotropic phase boundary, where there is a large piezoelectric response due to an electric-field induced structural transition. The occurrence of the magnetic structural transition at the morphotropic phase boundary offers another route towards room temperature multiferroic devices.

Structure-electromechanical property correlations in rare-earth (RE)-substituted (001) BiFeO3 (BFO) epitaxial thin films have been investigated. Quantitative piezoelectric coefficient (d33) and dielectric constant (e33) measurements, in conjunction with selected area electron diffraction, reveal that the enhancement in d33 and e33 at the morphotropic phase boundary (MPB) of the RE-substituted films (RE=Dy3+, Gd3+, and Sm3+) is correlated with the presence of a competing intermediate antipolar phase with the rhombohedral ferroelectric and nonpolar orthorhombic phase. This leads to a complex nanoscale phase coexistence at the MPB. Extending the studies to RE=La3+ case, we find the nanoscale phase coexistence to be less pronounced. This explains the lack of increase in d33 for the La3+-substituted BFO films, in contrast to the Dy3+, Gd3+, and Sm3+-substituted films.

We investigate theoretically ferroelastic domain fractions in a heteroepitaxial bilayer consisting of (001) tetragonal PbZrxTi1-xO3and (001) rhombohedral PbZrxTi1-xO3on a thick (001) passive substrate as a function of the lattice misfit strain between layers and the substrate. By considering the self-strain in each layer and the indirect elastic interaction between the layers, we provide a numerical analysis of the relative domain fractions in the tetragonal layer of a (001)PbZr0.2Ti0.8O3/ (001) PbZr0.8Ti0.2O3and (001)PbZr0.4Ti0.6O3/ (001) PbZr0.6Ti0.4O3bilayer structure as a function of the tetragonal layer thickness on (001) LaAlO3, (001) SrTiO3, and (001) MgO. It is found that the elastic coupling between the tetragonal and rhombohedral layers leads to an excess elastic energy in the tetragonal layer, resulting in a two to three times increase in the ferroelastic domain volume fraction of the tetragonal layer compared to single-layer films of similar thickness. These results show alternate ways of engineering ferroelastic domain structures in ferroelectric thin films. Refereed/Peer-reviewed

Multiferroic BiFeO3 (BFO) thin films have been fabricated via flux mediated epitaxy with varying oxygen partial pressure and flux composition (Bi2O3:CuO) conditions. Transmission electron microscopy coupled with energy dispersive x-ray spectroscopy as well as piezoresponse force microscopy confirm, that with the correct flux and seed layer conditions, even at very low partial pressures (3mTorr) no secondary phases are formed. The study reveals the crucial role of the bottom seed layer and flux chemistry in epitaxy of BFO thin films and provides alternate routes to BFO epitaxy in oxygen-deficient environments.

High-resolution piezoresponse force microscopy is used to visualize imprint in polycrystalline PbZr0.25Ti0.75O3 thin films. Three-dimensional domain images show the formation of a thin bright band (∼8nm in width) running along the grain boundary after local application of a negative bias. Such bands extend completely over the region under local bias thereby forming networks. Cross-section profile analysis reveals that these are not pinned regions, rather they are formed during the switching process. This demonstrates an active role of grain boundaries in pinning a preferential polarization state. Piezoresponse hysteresis loops confirm that these regions are imprinted.