Jacob R. Jokisaari

Domain Dynamics During Ferroelectric Switching

The role of defects and interfaces on switching in ferroelectric materials is observed with high-resolution microscopy. The utility of ferroelectric materials stems from the ability to nucleate and move polarized domains using an electric field. To understand the mechanisms of polarization switching, structural characterization at the nanoscale is required. We used aberration-corrected transmission electron microscopy to follow the kinetics and dynamics of ferroelectric switching at millisecond temporal and subangstrom spatial resolution in an epitaxial bilayer of an antiferromagnetic ferroelectric (BiFeO3) on a ferromagnetic electrode (La0.7Sr0.3MnO3). We observed localized nucleation events at the electrode interface, domain wall pinning on point defects, and the formation of ferroelectric domains localized to the ferroelectric and ferromagnetic interface. These results show how defects and interfaces impede full ferroelectric switching of a thin film.

Revealing the role of defects in ferroelectric switching with atomic resolution.

Ferroelectric materials are characterized by a spontaneous polarization, which can be reoriented with an applied electric field. The switching between polarized domains is mediated by nanoscale defects. Understanding the role of defects in ferroelectric switching is critical for practical applications such as non-volatile memories. This is especially the case for ferroelectric nanostructures and thin films in which the entire switching volume is proximate to a defective surface. Here we report the nanoscale ferroelectric switching of a tetragonal PbZr(0.2)Ti(0.8)O(3) thin film under an applied electric field using in situ transmission electron microscopy. We found that the intrinsic electric fields formed at ferroelectric/electrode interfaces determine the nucleation sites and growth rates of ferroelectric domains and the orientation and mobility of domain walls, whereas dislocations exert a weak pinning force on domain wall motion.

Ferroelastic domain switching dynamics under electrical and mechanical excitations.

In thin film ferroelectric devices, switching of ferroelastic domains can significantly enhance electromechanical response. Previous studies have shown disagreement regarding the mobility or immobility of ferroelastic domain walls, indicating that switching behaviour strongly depends on specific microstructures in ferroelectric systems. Here we study the switching dynamics of individual ferroelastic domains in thin Pb(Zr0.2,Ti0.8)O3 films under electrical and mechanical excitations by using in situ transmission electron microscopy and phase-field modelling. We find that ferroelastic domains can be effectively and permanently stabilized by dislocations at the substrate interface while similar domains at free surfaces without pinning dislocations can be removed by either electric or stress fields. For both electrical and mechanical switching, ferroelastic switching is found to occur most readily at the highly active needle points in ferroelastic domains. Our results provide new insights into the understanding of polarization switching dynamics as well as the engineering of ferroelectric devices.

Direct Observations of Retention Failure in Ferroelectric Memories

Nonvolatile ferroelectric random-access memory uses ferroelectric thin films to save a polar state written by an electric field that is retained when the field is removed. After switching, the high energy of the domain walls separating regions of unlike polarization can drive backswitching resulting in a loss of switched domain volume, or in the case of very small domains, complete retention loss.

Atomic Scale Structure Changes Induced by Charged Domain Walls in Ferroelectric Materials

Charged domain walls (CDWs) are of significant scientific and technological importance as they have been shown to play a critical role in controlling the switching mechanism and electric, photoelectric, and piezoelectric properties of ferroelectric materials. The atomic scale structure and properties of CDWs, which are critical for understanding the emergent properties, have, however, been rarely explored. In this work, using a spherical-aberration-corrected transmission electron microscope with subangstrom resolution, we have found that the polarization bound charge of the CDW in rhombohedral-like BiFeO3 thin films not only induces the formation of a tetragonal-like crystal structure at the CDW but also stabilizes unexpected nanosized domains with new polarization states and unconventional domain walls. These findings provide new insights on the effects of bound charge on ferroelectric domain structures and are critical for understanding the electrical switching in ferroelectric thin films as well as in memory devices.

Water-Free Titania-Bronze Thin Films With Superfast Lithium Ion Transport

Using pulsed laser deposition, TiO2 (-) B and its recently discovered variant Ca:TiO2 (-) B (CaTi5O11) are synthesized as highly crystalline thin films for the first time by a completely water-free process. Significant enhancement in the Li-ion battery performance is achieved by manipulating the crystal orientation of the films, used as anodes, with a demonstration of extraordinary structural stability under extreme conditions.

Giant Resistive Switching via Control of Ferroelectric Charged Domain Walls

Controlled switching of resistivity in ferroelectric thin films is demonstrated by writing and erasing stable, nanoscale, strongly charged domain walls using an in situ transmission electron microscopy technique. The resistance can be read nondestructively and presents the largest off/on ratio (≈10(5) ) ever reported in room-temperature ferroelectric devices, opening new avenues for engineering ferroelectric thin-film devices.

Defect-assisted Reorganization of Ferroelectric Domain Walls Revealed by Aberration-corrected Electron Microscopy

Domain walls (DWs) in ferroelectrics are quasi-2D functional units possessing peculiar properties that are absent in the bulk materials. The ability to produce ordered patterns of DWs is a prerequisite both for the fundamental study of DW properties and the design of novel nanodevices based on DW functionalites. In recent years, extensive efforts have been made towards fabricating periodic domain and DW structures in ferroelectric thin films, mainly through modifying elastic and electrostatic boundary conditions at the film interfaces. One of the major limitations of such a method, however, has been that once the choice of substrate is set further modifications to control or alter domain patterns during material synthesis becomes difficult, reducing the parameter space for creating more complex structures with ordered DW patterns and thus imposing resitrictions on the functionalities of the system. Here, with a combination of aberration-corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS), we show that atomically-thin charged defects that are deliberately introduced during the film growth can be used as nano-building-blocks for tailoring and reorganizing DW patterns. Such defect engineering can produce novel periodic mixed-type DW structures that are inaccessible by conventional boundary-condition-tuning methods

Control of Domain Structures in Multiferroic Thin Films through Defect Engineering

Domain walls (DWs) have become an essential component in nanodevices based on ferroic thin films. The domain configuration and DW stability, however, are strongly dependent on the boundary conditions of thin films, which make it difficult to create complex ordered patterns of DWs. Here, it is shown that novel domain structures, that are otherwise unfavorable under the natural boundary conditions, can be realized by utilizing engineered nanosized structural defects as building blocks for reconfiguring DW patterns. It is directly observed that an array of charged defects, which are located within a monolayer thickness, can be intentionally introduced by slightly changing substrate temperature during the growth of multiferroic BiFeO3 thin films. These defects are strongly coupled to the domain structures in the pretemperature-change portion of the BiFeO3 film and can effectively change the configuration of newly grown domains due to the interaction between the polarization and the defects. Thus, two types of domain patterns are integrated into a single film without breaking the DW periodicity. The potential use of these defects for building complex patterns of conductive DWs is also demonstrated.

Atomic-scale Mechanisms of Defect-Induced Retention Failure in Ferroelectric Materials

The ease of reversibly switching the spontaneous polarization of a ferroelectric with an applied electric field has made this material attractive for application of high-density nonvolatile memories. One of the major challenges impeding this application, however, has been the socalled “retention failure” phenomenon – a self back-switching process of the written polarization that can lead to data loss. An understanding of the atomic-scale mechanism of the retention failure process is thus necessary to engineer reliable ferroelectric devices. Here, using in situ transmission electron microscopy (TEM), we report direct observation of polarization backswitching induced by non-stoichiometric defects that commonly exist in ferroelectrics. Our results of atomic-resolution scanning transmission electron microscopy (STEM) show a novel mechanism of the retention failure process, revealing that the process is induced by the strong atomic interaction between the defects and the surrounding domains.