Orientation Of Domain Walls Research Articles

Strain phase equilibria and phase‐field method of ferroelectric polydomain: A case study of monoclinic KxNa1 − xNbO3 thin films

AbstractKnowledge of the thermodynamic equilibria and domain structures of ferroelectrics is critical to establishing their structure–property relationships that underpin their applications from piezoelectric devices to nonlinear optics. Here, we establish the strain condition for strain phase separation and polydomain formation and analytically predict the corresponding domain volume fractions and wall orientations of, relatively low symmetry and theoretically more challenging, monoclinic ferroelectric thin films by integrating thermodynamics of ferroelectrics, strain phase equilibria theory, microelasticity, and phase‐field method. Using monoclinic KxNa1 − xNbO3(0.5 < x < 1.0) thin films as a model system, we establish the polydomain strain–strain phase diagrams, from which we identify two types of monoclinic polydomain structures. The analytically predicted strain conditions of formation, domain volume fractions, and domain wall orientations for the two polydomain structures are consistent with phase‐field simulations and in good agreement with experimental results in the literature. The present study demonstrates a general, powerful analytical theoretical framework to predict the strain phase equilibria and domain wall orientations of polydomain structures applicable to both high‐ and low‐symmetry ferroelectrics and provide fundamental insights into the equilibrium domain structures of ferroelectric KxNa1 − xNbO3 thin films that are of technology relevance for lead‐free dielectric and piezoelectric applications.

Non‐Destructive Tomographic Nanoscale Imaging of Ferroelectric Domain Walls

AbstractExtraordinary physical properties arise at polar interfaces in oxide materials, including the emergence of 2D electron gases, sheet‐superconductivity, and multiferroicity. A special type of polar interface is ferroelectric domain walls, where electronic reconstruction phenomena can be driven by bound charges. Great progress has been achieved in the characterization of such domain walls and, over the last decade, their potential for next‐generation nanotechnology has become clear. Established tomography techniques, however, are either destructive or offer insufficient spatial resolution, creating a pressing demand for 3D imaging compatible with future fabrication processes. Here, non‐destructive tomographic imaging of ferroelectric domain walls is demonstrated using secondary electrons. Utilizing conventional scanning electron microscopy (SEM), the position, orientation, and charge state of hidden domain walls are reconstructed at distances up to several hundreds of nanometers away from the surface. A mathematical model is derived that links the SEM intensity variations at the surface to the local domain wall properties, enabling non‐destructive tomography with good noise tolerance on the timescale of seconds. The SEM‐based approach facilitates high‐throughput screening of materials with functional domain walls and domain‐wall‐based devices, which is essential for monitoring during the production of device architectures and quality control in real‐time.

Effects of Annealing and Domain Structure on the Photovoltaic Response in Pb[(Mg1/3Nb2/3)0.70Ti0.30]O3 Single Crystal

Crystalline acentricity leading to the photovoltaic (PV) effect in ferroelectrics (FEs) emerges these materials as important candidates for the PV cells potentially overcoming the Shockley–Queisser limit of semiconductors. However, this research direction still requires more investigations to develop reliable pathways for PV efficiency optimization. Herein, it is reported that due to the sample annealing above the Curie point the PV effect asymmetry with respect to the poling sign can be compensated and intrinsic PV response of the FE crystal can be increased by more than 20% in comparison with the pristine sample. It is also reported how the orientation of domain walls can be used to engineer the PV effect in the subcoercive electric field region. The reported results contribute to better understanding of PV effect tuning in the FEs paving the way for more efficient and sustainable solar energy utilization.

Modelling domain-wall orientation in antiferromagnets driven by magnetoelastic interactions and volume variations

Modelling domain-wall orientation in antiferromagnets driven by magnetoelastic interactions and volume variations

The Third Dimension of Ferroelectric Domain Walls.

Ferroelectric domain walls are quasi-2D systems that show great promise for the development of nonvolatile memory, memristor technology, and electronic components with ultrasmall feature size. Electric fields, for example, can change the domain wall orientation relative to the spontaneous polarization and switch between resistive and conductive states, controlling the electrical current. Being embedded in a 3Dmaterial, however, the domain walls are not perfectly flat and can form networks, which leads to complex physical structures. In this work, the importance of the nanoscale structure for the emergent transport properties is demonstrated, studying electronic conduction in the 3D network of neutral and charged domain walls in ErMnO3 . By combining tomographic microscopy techniques and finite element modeling, the contribution of domain walls within the bulk is clarified and the significance of curvature effects for the local conduction is shown down to the nanoscale. The findings provide insights into the propagation of electrical currents in domain wall networks, reveal additional degrees of freedom for their control, and provide quantitative guidelines for the design of domain-wall-based technology.

Manipulation of charged domain walls in geometric improper ferroelectric thin films: A phase-field study

Using phase-field simulations, we show how interfaces acting on the geometric-improper ferroelectric polarization of hexagonal manganite and ferrite thin films can be used to control the formation of charged domain walls. We modify the Landau expansion of the free energy valid in bulk to emulate interface effects known from previous cross-sectional experiments, and we verify our model by comparing our results with images obtained in these experiments. We then show how the interface affects the orientation of ferroelectric domain walls in the fully three-dimensional case. Furthermore, we demonstrate that interface effects combined with an external electric field enable us to specifically choose the dominant domain-wall type (head-to-head, tail-to-tail, or neutral). We also find that an electric field can stabilize a novel domain-wall type which only emerges in the improper ferroelectric order but not in the primary structural distortion. Since the domain walls have a conductivity that is different from the interior of the domains, the influence of the interfaces of a thin film on the type and distribution of the walls gives us the possibility to control the transport properties of a material by appropriate thin-film engineering.

Strain phase diagram and physical properties of (110)-oriented PbTiO3 thin films by phase-field simulations

Extensive studies have shown that (110)-oriented ferroelectric films exhibit unique structures and excellent physical properties compared with (001)-oriented ferroelectric films. Showing how the domain structure evolve as the function of epitaxial strain, or establishing a strain phase diagram, should provide valuable information for the controlling of domain structures. In this work, three-dimensional phase-field simulations were performed to investigate the phase (domain) structures and physical properties in (110)-oriented PbTiO3 thin films. We constructed the temperature-misfit strain phase diagram of (110)-oriented PbTiO3 thin films. Typical phase (domain) structures were analyzed in detail and various types of topological flux-closure domains were found to exist in mixed phases. The experimental results on a special substrate KTaO3, in particular the orientation of domain walls, corroborate the simulation results. We found that the misfit strain could be used to engineer the phase (domain) structures and tune the dielectric, piezoelectric and thermal conductivity properties in (110)-oriented PbTiO3 thin films. These results deepen the understanding and provide a theoretical guidance for the strain engineering of the structures and properties of high-index (110)-oriented PbTiO3 thin films.

Origin of Ferroelectric Domain Wall Alignment with Surface Trenches in Ultrathin Films.

Engraving trenches on the surfaces of ultrathin ferroelectric (FE) films and superlattices promises control over the orientation and direction of FE domain walls (DWs). Through exploiting the phenomenon of DW-surface trench (ST) parallel alignment, systems where DWs are known for becoming electrical conductors could now become useful nanocircuits using only standard lithographical techniques. Despite this clear application, the microscopic mechanism responsible for the alignment phenomenon has remained elusive. Using ultrathin PbTiO_{3} films as a model system, we explore this mechanism with large scale density functional theory simulations on as many as 5,136 atoms. Although we expect multiple contributing factors, we show that parallel DW-ST alignment can be well explained by this configuration giving rise to an arrangement of electric dipole moments which best restore polar continuity to the film. These moments preserve the polar texture of the pristine film, thus minimizing ST-induced depolarizing fields. Given the generality of this mechanism, we suggest that STs could be used to engineer other exotic polar textures in a variety of FE nanostructures as supported by the appearance of ST-induced polar cycloidal modulations in this Letter. Our simulations also support experimental observations of ST-induced negative strains which have been suggested to play a role in the alignment mechanism.

Dynamic Tilting of Ferroelectric Domain Walls Caused by Optically Induced Electronic Screening.

Optical excitation perturbs the balance of phenomena selecting the tilt orientation of domain walls within ferroelectric thin films. The high carrier density induced in a low-strain BaTiO_{3} thin film by an above-band-gap ultrafast optical pulse changes the tilt angle that 90° a/c domain walls form with respect to the substrate-film interface. The dynamics of the changes are apparent in time-resolved synchrotron x-ray scattering studies of the domain diffuse scattering. Tilting occurs at 298K, a temperature at which the a/b and a/c domain phases coexist but is absent at 343K in the better ordered single-phase a/c regime. Phase coexistence at 298K leads to increased domain-wall charge density, and thus a larger screening effect than in the single-phase regime. The screening mechanism points to new directions for the manipulation of nanoscale ferroelectricity.

Prototype Design of a Domain-Wall-Based Magnetic Memory Using a Single Layer La0.67Sr0.33MnO3 Thin Film.

Magnetic field-free, nonvolatile magnetic memory with low power consumption is highly desired in information technology. In this work, we report a current-controllable alignment of magnetic domain walls in a single layer La0.67Sr0.33MnO3 thin film with the threshold current density of 2 × 105 A/cm2 at room temperature. The vector relationship between current directions and domain-wall orientations indicates the dominant role of spin-orbit torque without an assistance of external magnetic field. Meanwhile, significant planar Hall resistances can be readout in a nonvolatile way before and after the domain-wall reorientation. A domain-wall-based magnetic random-access memory (DW-MRAM) prototype device has been proposed.

Domain walls in ferroelectrics

AbstractSpontaneous unit‐cell deformation accompanies spontaneous polarization in perovskite ferroelectrics, thereby making it energetically favorable for domain walls to form on particular planes that satisfy mechanical compatibility. Historically, domain walls are found analytically, solving for walls with compatible strains within the wall plane. However, analytical solutions do not give any information about relative energetics of nonideal domain walls. Here, the orientation of the most favorable domain walls and the relative energetics of nonideal domain walls are predicted by calculating strain mismatch and charge discontinuity over all possible domain wall orientations. This is done for common ferroelectric phase symmetries of tetragonal, rhombohedral, orthorhombic, and monoclinic type. In tetragonal and rhombohedral symmetry, the domain walls are independent of any external stimulus as long as the symmetry is maintained. In orthorhombic and monoclinic symmetry, the orientation of certain mechanically permissible domain walls changes with temperature and/or electric field as the unit cell distorts, while others do not. Additionally, in monoclinic systems, domain wall planes are shown to exist that are not perfectly permissible but are very close to permissible, thus, these walls were not found by prior analytical methods. The visualization of strain compatibility of all the domain walls makes it easy to see precisely on which planes domain walls are expected or not expected and how the domain walls change their orientation under the effect of external stimulus. Such an analysis can also be used to investigate the relative and changing energetics of nonideal domain walls in systems under thermal, compositional, electrical, and mechanical stimuli.

Theory of domain-wall magnetoresistance in metallic antiferromagnets

We develop a theory to compute the domain-wall magnetoresistance (DWMR) in antiferromagnetic (AFM) metals with different spin structures. In the diffusive transport regime, the DWMR can be either {\it negative} or positive depending on the domain-wall orientation and spin structure. In contrast, when the transport is in the ballistic regime, the DWMR is always positive, and the magnitude depends on the width and orientation of the domain wall. Our results pave the way of using electrical measurements for probing the internal spin structure in antiferromagnetic metals.

Temperature dependence of three-dimensional domain wall arrangement in ferroelectric K0.9Na0.1NbO3 epitaxial thin films

The three-dimensional arrangement and orientation of domain walls in ferroelectric K0.9Na0.1NbO3/(110)NdScO3 epitaxial thin films were investigated at different temperatures both experimentally by means of piezoresponse force microscopy and three-dimensional x-ray diffraction and theoretically by three-dimensional phase-field simulations. At room temperature, a well-ordered herringbone-like domain pattern appears in which there is a periodic arrangement of a1a2/MC monoclinic phases. Four different types of domain walls are observed, which can be characterized by out-of-plane tilt angles of ±45° and in-plane twist angles of ±21°. For the orthorhombic high-temperature phase, a periodic a1/a2 stripe domain pattern with exclusive in-plane polarization is formed. Here, two different types of domain walls are observed, both of them having a fixed out-of-plane domain wall angle of 90° but distinguished by different in-plane twist angles of ±45°. The experimental results are fully consistent with three-dimensional phase-field simulations using anisotropic misfit strains. The qualitative agreement between the experiment and the theory applies, in particular, to the wide phase transition range between about 180 °C and 260 °C. In this temperature range, a complex interplay of coexisting monoclinic a1a2/MC and orthorhombic a1/a2 phases takes place.

Orientation and internal structure of domain walls in ferromagnetic films with anisotropic Dzyaloshinskii-Moriya interaction

We present an analytical study of domain-wall internal structure and orientation in a ferromagnetic film with out-of-plane anisotropy and the anisotropic interfacial Dzyaloshinskii-Moriya interaction (iDMI). The term “anisotropic” means that the iDMI constant is different for different in-plane directions. The interplay between the magnetostatic interaction and the iDMI in a domain wall defines both its structure and orientation. In the case of the isotropic iDMI there is no preferable orientation of the DW in the film plane leading to formation of labyrinth domain structure with random shapes of the domains. In the case of the anisotropic iDMI an oriented domain stripe structure and zigzag domains appear. Depending on system parameters, either the Bloch domain walls, or the Néel ones or the hybrid canted walls realize in the magnetic film. The spatial orientation of the DW and the orientation of the magnetization rotation plane in the DW are intimately related by a simple linear ratio. The analytical results are corroborated by micromagnetic simulations.

Resonant nanodiffraction x-ray imaging reveals role of magnetic domains in complex oxide spin caloritronics.

Spin electronic devices based on crystalline oxide layers with nanoscale thicknesses involve complex structural and magnetic phenomena, including magnetic domains and the coupling of the magnetism to elastic and plastic crystallographic distortion. The magnetism of buried nanoscale layers has a substantial impact on spincaloritronic devices incorporating garnets and other oxides exhibiting the spin Seebeck effect (SSE). Synchrotron hard x-ray nanobeam diffraction techniques combine structural, elemental, and magnetic sensitivity and allow the magnetic domain configuration and structural distortion to be probed in buried layers simultaneously. Resonant scattering at the Gd L2 edge of Gd3Fe5O12 layers yields magnetic contrast with both linear and circular incident x-ray polarization. Domain patterns facet to form low-energy domain wall orientations but also are coupled to elastic features linked to epitaxial growth. Nanobeam magnetic diffraction images reveal diverse magnetic microstructure within emerging SSE materials and a strong coupling of the magnetism to crystallographic distortion.

Misfit strain-temperature phase diagram of multi-domain structures in (111)-oriented ferroelectric PbTiO3 films

High-index-oriented epitaxial ferroelectric films have recently attracted great attention due to their emergent functional properties related to the multi-domain structures. Here, the equilibrium phases and domain structures in (111)-oriented ferroelectric PbTiO3 films were investigated via phase-field simulations and the misfit strain-temperature equilibrium phase diagram was constructed. It was found that the epitaxial strain can effectively control the orientations of polarization vectors, which uniquely determine the domain variant patterns, domain wall orientations and densities. In particular, two types of tetragonal-like domain patterns can form under different strains with their distinct structural characteristics, ferroelectric properties and thermal conductivities. These results provide guidance for interpreting and understanding experimental data as well as designing of (111)-oriented ferroelectric films with specified domain structures and properties.

Domain wall tuned superconductivity in superconductor–ferromagnet bilayers

Due to the opposite natures of superconducting and ferromagnetic order parameters, hybrid bilayers systems have gained huge attention in past decades. Here we report manipulation of superconductivity by the orientation of the domain wall in such superconducting–ferromagnetic bilayers based on the phase-field model. The phase of the primary order parameter in the Ginzburg–Landau model was used to describe the vortex dynamics path in the superconductor. Vortices were observed to follow the available path within domain centers perpendicular to the applied current, under the Lorentz force. Applied current was observed to follow the spatial path described by the ferromagnetic domain walls in the superconductors, where in the absence of such clear paths current was observed to form leakage channels from within the domain centers. Carrier concentration, supercurrent density, and energy components in the superconductor were observed to strongly depend on the orientation of the domain wall in current-carrying superconductors. The variation in carrier concentration, which defines the resistivity of the superconductor, was observed to depend strongly on the orientation of the domain wall with respect to the direction of applied current and was observed to have about 30% variation for 0° to 56° (and 124°–180°) domain wall orientations. This huge variation in carrier concentration and other parameters controlled by domain wall orientation have potential applications in sensors, controls and communication.

Energetics and Structure of Domain Wall Networks in Minimally Twisted Bilayer Graphene under Strain

The parameters of the triangular domain wall network in bilayer graphene with a simultaneously twisted and biaxially stretched bottom layer are studied using the two-chain Frenkel-Kontorova model. It is demonstrated that if the graphene layers are free to rotate, they prefer to stay co-aligned upon stretching the bottom layer and the regular triangular network of tensile domain walls is formed upon the commensurate-incommensurate phase transition. If the angle between the layers is fixed, the regular triangular network of shear domain walls is observed at zero elongation of the bottom layer. Upon stretching the bottom layer, however, the domain walls transform into the tensile ones and the size of the commensurate domains decreases. We also show that the parameters of the isosceles triangular domain wall network in twisted bilayer graphene under shear strain can be determined through purely geometrical considerations. Experimental analysis of the orientation of domain walls and period of the triangular network would, on the one hand, contribute to understanding of the interlayer interaction of graphene layers, and, on the other hand, serve for detection of relative strains and rotation between the layers. Vice versa external strains can be used to control the parameters of the triangular domain wall network and, therefore, electronic properties of twisted bilayer graphene.

Imaging anisotropic vortex dynamics in FeSe

Strong vortex pinning in FeSe could be useful for technological applications and could provide clues about the coexistence of superconductivity and nematicity. To characterize the pinning of individual, isolated vortices, we simultaneously apply a local magnetic field and image the vortex motion with scanning SQUID susceptibility. We find that the pinning is highly anisotropic: the vortices move easily along directions that are parallel to the orientations of twin domain walls and pin strongly in a perpendicular direction. These results are consistent with a scenario in which the anisotropy arises from vortex pinning on domain walls and quantify the dynamics of individual vortex pinning in FeSe.

Controlled piezotronic properties on recoverable energy storage density in rare-earth ions doped epitaxial PZT thin films

The present study describes the influence of rare-earth (RE = La, Eu, Dy and Ho) ions on recoverable energy storage density in PLD grown epitaxial Pb(ZrxTi1−x)O3 (PZT) thin films on SRO/LSAT (0 0 1) hetrostructures. Special attention has been paid to remove the pyrochlore phase, which is a prominent unwanted phase in ferroelectric specimens with rare earth dopants. The evidence of single crystallinity of the hetrostructures was confirmed from x-ray diffraction. The presence of single zone points in the Kikuchi patterns in electron back scattering diffraction reveals the epitaxial nature of rare earth doped PZT thin films. In addition, an x-ray photoelectron spectroscopy experiment was carried out to determine the oxidation states after doping the rare-earth ions in the PZT sample. The enhancement of spontaneous polarization in the donor (La, Eu) doped PZT films compared to the pure one is attributed to easy orientation and mobility of domain walls. It is found that the piezoelectric coefficient (d33) is more (130 pm V−1) for La doped PZT films (La:PZT), compared to the other rare earth doped PZT thin films. In our results, we found asymmetric polarization versus electric field hysteresis loop and energy storage efficiency () values of 30% were recorded in the case of pure PZT film and the value decreased to 28% with Ho doping, however, a high value of 46% could be realized in La:PZT.