Observation of Electric-Field-Induced Structural Dislocations in a Ferroelectric Oxide

Dislocations are 1D topological defects with emergent electronic properties. Their low dimensionality and unique properties make them excellent candidates for innovative device concepts, ranging from dislocation-based neuromorphic memory to light emission from diodes. To date, dislocations are created in materials during synthesis via strain fields or flash sintering or retrospectively via deformation, for example, (nano)-indentation, limiting the technological possibilities. In this work, we demonstrate the creation of dislocations in the ferroelectric semiconductor Er(Mn,Ti)O3 with nanoscale spatial precision using electric fields. By combining high-resolution imaging techniques and density functional theory calculations, direct images of the dislocations are collected, and their impact on the local electric transport behavior is studied. Our approach enables local property control via dislocations without the need for external macroscopic strain fields, expanding the application opportunities into the realm of electric-field-driven phenomena.publishedVersio

Characterization of BaTiO3/La0.7Sr0.3MnO3 Thin Films on SrTiO3(111) Substrates - A Transmission Electron Microscopy Study

In this work, an oxide thin film system was investigated by using transmission electron microscopy (TEM) techniques. It is of great interest to study such systems due to their current applications as well as potential future applications in electronic devices. One sample consists of a 10 nm BaTiO3 thin film deposited on a 10 nm La0.7Sr0.3MnO3 thin film deposited on a (111)-oriented SrTiO3 substrate, in short BTO(10nm)/LSMO(10nm)/STO(111). The other sample is the same, except for having a BTO film of thickness 3 nm. BTO is a tetragonal perovskite with space group P4mm (99) and STO is cubic perovskite with space group Pm-3m (221). LSMO is a rhombohedral perovskite with space group R-3c (167) in bulk form. TEM specimens with cross-sectional and plan-view geometries were prepared using a mechanical polishing routine combined with precision ion polishing, and by a focused ion-beam (FIB) procedure. High-resolution TEM (HR-TEM), brightfield TEM (BF-TEM), scanning precession electron diffraction (SPED), scanning transmission electron microscopy electron energy loss spectroscopy (STEM-EELS), high angle annular dark-field STEM (HAADF-STEM), geometrical phase analysis (GPA) and selected area diffraction (SAD) were performed on the Jeol-Jem 2100F and the Jeol-Jem ARM 200F. The data obtained in this work suggests that the BTO thin films are in a ferroelectric mono-domain state. Due to a large amount of edge dislocations, it was not possible to map the precise positions of the Ti columns and thus the ferroelectric polarization. Such dislocations occur naturally when there is misfit between a thin film and the substrate, and the thin film is relaxed. Since the crystallographic difference between ferroelectric domains in BTO is very small, it becomes very difficult to deconvolve the strain fields around dislocations from a potential ferroelectric domain structure. The LSMO thin film was found to have undergone a reduction in symmetry from its bulk structure. This conclusion was reached based on the fact that additional super-reflections were observed in the SAD patterns. Based on the literature available on LSMO thin film systems deposited on STO(111), it was concluded that LSMO has undergone a monoclinic distortion. However, a precise determination of its space group is beyond the scope of this work. Furthermore, two domain states were found in LSMO. The EELS analysis indicates that LSMO has a large amount of oxygen vacancies and a Mn oxidation state far below its bulk value. In addition, a tensile strain in the direction normal to the thin film interface was observed in the GPA analysis, which is inconsistent with X-ray diffraction measurements done on the same system. This discrepancy is probably linked to the large amount of oxygen vacancies in LSMO, which may dramatically alter its behavior. The oxygen vacancies may have been created during the TEM specimen preparation in the FIB. Another explanation is that the oxygen vacancies were induced by the electron beam in the TEM, as this has been observed to occur in LSMO in several earlier studies. The oxygen vacancies are probably not ordered, since the observed super-reflections were not consistent with any known ordering, such as the Brownmillerite phases. In conclusion, an important and interesting oxide thin film system has been characterized using TEM techniques. An important observation is the fact that there are many obstacles to extracting the relevant data. Firstly, misfit dislocations are not easily avoided, but make it difficult to map small changes in the positions of the Ti columns. Secondly, the electron beam and the specimen preparation may induce oxygen vacancies that dramatically alter the structure of LSMO. Hopefully, the findings in this work will give an insight into the structure-property relations of these systems, as well as an overview of the challenges associated with studying them. A recommendation for the future is to produce specimens using several different types of preparation techniques, to find out how these affect the specimen quality. It may be necessary to alter the parameters used during the thin film synthesis as well. For example, annealing the sample in an oxygen rich atmosphere after synthesis and after preparation in the FIB to reduce the amount of oxygen vacancies. Finally, using a low voltage and shorter exposure times in the TEM may help reduce the creation of oxygen vacancies in LSMO

Domain wall mobility and roughening in doped ferroelectric hexagonal manganites

The macroscopic performance of ferroelectric and piezoelectric devices depends strongly on domain wall dynamics. It is clear that structural defects, such as vacancies, interstitials, and dopants codetermine the dynamics, but the microscopic understanding of the wall-defect interactions is still at an early stage. Hexagonal manganites are among of the most intensively studied systems with respect to static domain wall properties and thus are ideal model materials for studying domain wall mobility in the presence of defects. Here we study the mobility of domain walls in the hexagonal manganites and how it is affected by cation dopants using density functional theory calculations. The results are correlated with scanning probe microscopy measurements on single crystals, to confirm an increasing domain wall roughness for the dopants we predict to pin the walls. The pinning originates from elastic strain fields around the walls interacting with the local crystal perturbations surrounding a dopant. The pinning strength is correlated with the local change in order parameter amplitude caused by the dopant. As a computationally friendly alternative to large supercell calculations, we demonstrate that domain wall pinning can be predicted from the dopants’ effect on the free-energy landscape of polarization switching. This approach allows to directly probe the effect of defects on domain wall mobility using a fraction of the computational cost, opening the door to detailed modeling and understanding of the critical pinning process of domain walls

Anisotropic in-plane dielectric and ferroelectric properties of tensile-strained BaTiO3 films with three different crystallographic orientations

Ferroelectric properties of films can be tailored by strain engineering, but a wider space for property engineering can be opened by including crystal anisotropy. Here, we demonstrate a huge anisotropy in the dielectric and ferroelectric properties of BaTiO3 films. Epitaxial BaTiO3 films deposited on (100), (110), and (111) SrTiO3 substrates were fabricated by chemical solution deposition. The films were tensile-strained due to thermal strain confirmed by the enhanced Curie temperature. A massive anisotropy in the dielectric constant, dielectric tunability, and ferroelectric hysteresis loops was observed depending on the in-plane direction probed and the orientation of the films. The anisotropy was low for (111) BaTiO3, while the anisotropy was particularly strong for (110) BaTiO3, reflecting the low in-plane rotational symmetry. The anisotropy also manifested at the level of the ferroelectric domain patterns in the films, providing a microscopic explanation for the macroscopic response. This study demonstrates that the properties of ferroelectric films can be tailored not only by strain but also by crystal orientation. This is particularly interesting for multilayer stacks where the strain state is defined by the boundary conditions. We propose that other materials can be engineered in a similar manner by utilizing crystal anisotropy

Observation of Electric-Field-Induced Structural Dislocations in a Ferroelectric Oxide

Dislocations are 1D topological defects with emergent electronic properties. Their low dimensionality and unique properties make them excellent candidates for innovative device concepts, ranging from dislocation-based neuromorphic memory to light emission from diodes. To date, dislocations are created in materials during synthesis via strain fields or flash sintering or retrospectively via deformation, for example, (nano)-indentation, limiting the technological possibilities. In this work, we demonstrate the creation of dislocations in the ferroelectric semiconductor Er(Mn,Ti)O3 with nanoscale spatial precision using electric fields. By combining high-resolution imaging techniques and density functional theory calculations, direct images of the dislocations are collected, and their impact on the local electric transport behavior is studied. Our approach enables local property control via dislocations without the need for external macroscopic strain fields, expanding the application opportunities into the realm of electric-field-driven phenomena

Observation of Electric-Field-Induced Structural Dislocations in a Ferroelectric Oxide

Dislocations are 1D topological defects with emergent electronic properties. Their low dimensionality and unique properties make them excellent candidates for innovative device concepts, ranging from dislocation-based neuromorphic memory to light emission from diodes. To date, dislocations are created in materials during synthesis via strain fields or flash sintering or retrospectively via deformation, for example, (nano)-indentation, limiting the technological possibilities. In this work, we demonstrate the creation of dislocations in the ferroelectric semiconductor Er(Mn,Ti)O3 with nanoscale spatial precision using electric fields. By combining high-resolution imaging techniques and density functional theory calculations, direct images of the dislocations are collected, and their impact on the local electric transport behavior is studied. Our approach enables local property control via dislocations without the need for external macroscopic strain fields, expanding the application opportunities into the realm of electric-field-driven phenomena

Observation of Unconventional Dynamics of Domain Walls in Uniaxial Ferroelectric Lead Germanate

Application of scanning probe microscopy techniques such as piezoresponse force microscopy (PFM) opens the possibility to re‐visit the ferroelectrics previously studied by the macroscopic electrical testing methods and establish a link between their local nanoscale characteristics and integral response. The nanoscale PFM studies and phase field modeling of the static and dynamic behavior of the domain structure in the well‐known ferroelectric material lead germanate, Pb5Ge3O11, are reported. Several unusual phenomena are revealed: 1) domain formation during the paraelectric‐to‐ferroelectric phase transition, which exhibits an atypical cooling rate dependence; 2) unexpected electrically induced formation of the oblate domains due to the preferential domain walls motion in the directions perpendicular to the polar axis, contrary to the typical domain growth behavior observed so far; 3) absence of the bound charges at the 180° head‐to‐head (H–H) and tail‐totail (T–T) domain walls, which typically exhibit a significant charge density in other ferroelectrics due to the polarization discontinuity. This strikingly different behavior is rationalized by the phase field modeling of the dynamics of uncharged H–H and T–T domain walls. The results provide a new insight into the emergent physics of the ferroelectric domain boundaries, revealing unusual properties not exhibited by conventional Ising‐type walls

Application of a long short-term memory for deconvoluting conductance contributions at charged ferroelectric domain walls

Ferroelectric domain walls are promising quasi-2D structures that can be leveraged for miniaturization of electronics components and new mechanisms to control electronic signals at the nanoscale. Despite the significant progress in experiment and theory, however, most investigations on ferroelectric domain walls are still on a fundamental level, and reliable characterization of emergent transport phenomena remains a challenging task. Here, we apply a neural-network-based approach to regularize local I(V)-spectroscopy measurements and improve the information extraction, using data recorded at charged domain walls in hexagonal (Er0.99,Zr0.01)MnO3 as an instructive example. Using a sparse long short-term memory autoencoder, we disentangle competing conductivity signals both spatially and as a function of voltage, facilitating a less biased, unconstrained and more accurate analysis compared to a standard evaluation of conductance maps. The neural-network-based analysis allows us to isolate extrinsic signals that relate to the tip-sample contact and separating them from the intrinsic transport behavior associated with the ferroelectric domain walls in (Er0.99,Zr0.01)MnO3. Our work expands machine-learning-assisted scanning probe microscopy studies into the realm of local conductance measurements, improving the extraction of physical conduction mechanisms and separation of interfering current signals

Conductivity control via minimally invasive anti-Frenkel defects in a functional oxide

Utilizing quantum effects in complex oxides, such as magnetism, multiferroicity and superconductivity, requires atomic-level control of the material’s structure and composition. In contrast, the continuous conductivity changes that enable artificial oxide-based synapses and multiconfigurational devices are driven by redox reactions and domain reconfigurations, which entail long-range ionic migration and changes in stoichiometry or structure. Although both concepts hold great technological potential, combined applications seem difficult due to the mutually exclusive requirements. Here we demonstrate a route to overcome this limitation by controlling the conductivity in the functional oxide hexagonal Er(Mn,Ti)O3 by using conductive atomic force microscopy to generate electric-field induced anti-Frenkel defects, that is, charge-neutral interstitial–vacancy pairs. These defects are generated with nanoscale spatial precision to locally enhance the electronic hopping conductivity by orders of magnitude without disturbing the ferroelectric order. We explain the non-volatile effects using density functional theory and discuss its universality, suggesting an alternative dimension to functional oxides and the development of multifunctional devices for next-generation nanotechnology