Stripe Domains Research Articles

Impact of Mn doping on the ferroelectric photovoltaic effect in multidomain BiFeO3 thin films under above-bandgap illumination

While recent studies have revealed that chemical doping can enhance the photovoltaic (PV) response of ferroelectrics under light with photon energy (hν) lower than the bandgap energy (E g), its impact under above-bandgap illumination (hν > E g) is not fully understood. In this study, the PV response at hν > E g of Mn-doped BiFeO3 (BFO) thin films with a periodic stripe domain pattern is investigated. The short-circuit photocurrent density (J sc) and open-circuit voltage decrease with increasing Mn content. Analyses of the light-polarization angle dependence of J sc indicate that the PV response arises primarily from the non-centrosymmetric nature of crystal lattices in both the bulk and domain wall (DW) regions. In contrast, the contribution of local electric fields (E local) across DWs which dominates the PV response of undoped BFO films is significantly suppressed by doping. We consider that a screening effect by charged defects leads to a suppressed E local in the Mn-doped films.

Flow patterns and defect dynamics of active nematic liquid crystals under an electric field.

The effects of an electric field on the flow patterns and defect dynamics of two-dimensional active nematic liquid crystals are numerically investigated. We found that field-induced director reorientation causes anisotropic active turbulence characterized by enhanced flow perpendicular to the electric field. The average flow speed and its anisotropy are maximized at an intermediate field strength. Topological defects in the anisotropic active turbulence are localized and show characteristic dynamics with simultaneous creation of two pairs of defects. A laning state characterized by stripe domains with alternating flow directions is found at a larger field strength near the transition to the uniformly aligned state. We obtained periodic oscillations between the laning state and active turbulence, which resembles an experimental observation of active nematics subject to anisotropic friction.

Role of polarization switching and domain patterns in the enhanced piezoelectric characteristics of a Pb-free ferroelectric system

The Pb-free piezoelectric BaTi1−x Sn x O3 system is a promising environmentally friendly ceramic due to its superior piezoresponse properties. The enhanced piezoresponse of this system has a strong correlation with its microscopic and macroscopic characteristics. In this work, the effects of structural and microstructural evolution on the piezoelectric properties of a BaTi0.97Sn0.03O3 (BST) compound are explored. Structural and piezoelectric analysis elucidates that the samples experiencing a field induced tetragonal to orthorhombic phase transformation exhibit better piezoresponse. In addition, the domain configurations have a strong correlation with the synthesis conditions, where the distribution of lamellar and/or stripe types of domain patterns show enhancement in comparison to those of labyrinth type with increase in sintering temperature. Rayleigh analysis confirms that the increase in the irreversible nature of non-180° domain walls can improve the d 33 values of the BST samples. Overall, the results illustrate that the stripe domain configurations and field induced polarization switching from tetragonal to orthorhombic phase favour superior piezoresponse in the samples.

Metallic filament domain dynamics-induced abrupt resistive switching characteristics with small hysteresis in two dimensional-crystalline VO2 films

The domain configurations in vanadium dioxide (VO2) that are interfacing with a substrate play an important role, particularly in the electronic property, due to the interface-dependent strain induction and phase coexistence across the metal-insulator transition (MIT) process. Here, we demonstrate an abrupt resistive switching behavior with a small hysteresis by utilizing the strain-induced metallic filament domains in VO2 films with puzzle-like two-dimensional single crystalline domains (2D-crystalline VO2 films), which are synthesized on a SiO2 layer. We find that an anisotropic tensile strain induced at the 2D-crystalline VO2 films /SiO2 interfaces leads to the preferential formation and propagation of metallic stripe domains along the in-plane growth direction across the MIT. We also demonstrate that based on simultaneous in situ observations of electrical characteristics and phase evolution in the 2D-crystalline VO2 films, the abrupt transition with a narrow hysteresis width (<4 °C) is attributed to the thermally activated and reversible evolution of metallic stripe domains acting as a conducting filament channel within an insulating phase, which is associated with anisotropic strain distribution. Moreover, we show that these experimental results are consistent with the theoretical analysis based on a parallel resistor model. In addition, we further demonstrate thermally-triggered abrupt and reversible resistive switching performance of the 2D-crystalline VO2 film, maintaining stable for 500 switching cycles. These results provide an important strategy to modulate and utilize the unique MIT hysteresis behavior properties through controlling the configuration of metallic and insulating stripe domains in the 2D-crystalline structure of VO2 for the development of high-performance VO2 based electrical devices.

Translational Boundaries as Incipient Ferrielectric Domains in Antiferroelectric PbZrO_{3}.

In the archetypal antiferroelectric PbZrO_{3}, antiparallel electric dipoles cancel each other, resulting in zero spontaneous polarization at the macroscopic level. Yet in actual hysteresis loops, the cancellation is rarely perfect and some remnant polarization is often observed, suggesting the metastability of polar phases in this material. In this work, using aberration-corrected scanning transmission electron microscopy methods on a PbZrO_{3} single crystal, we uncover the coexistence of the common antiferroelectric phase and a ferrielectric phase featuring an electric dipole pattern of ↓↑↓. This dipole arrangement, predicted by Aramberri etal. to be the ground state of PbZrO_{3} at 0K, appears at room temperature in the form of translational boundaries. The dual nature of the ferrielectric phase, both a distinct phase and a translational boundary structure, places important symmetry constraints on its growth. These are overcome by sideways motion of the boundaries, which aggregate to form arbitrarily wide stripe domains of the polar phase embedded within the antiferroelectric matrix.

Pattern Formation by Electric-Field Quench in a Mott Crystal.

The control of the Mott phase is intertwined with the spatial reorganization of the electronic states. Out-of-equilibrium driving forces typically lead to electronic patterns that are absent at equilibrium, whose nature is however often elusive. Here, we unveil a nanoscale pattern formation in the Ca2RuO4 Mott insulator. We demonstrate how an applied electric field spatially reconstructs the insulating phase that, uniquely after switching off the electric field, exhibits nanoscale stripe domains. The stripe pattern has regions with inequivalent octahedral distortions that we directly observe through high-resolution scanning transmission electron microscopy. The nanotexture depends on the orientation of the electric field; it is nonvolatile and rewritable. We theoretically simulate the charge and orbital reconstruction induced by a quench dynamics of the applied electric field providing clear-cut mechanisms for the stripe phase formation. Our results open the path for the design of nonvolatile electronics based on voltage-controlled nanometric phases.

Periodic Ferroelectric Stripe Domains in α-In2Se3 Nanoflakes Grown via Reverse-Flow Chemical Vapor Deposition.

The two-dimensional (2D) layered semiconductor α-In2Se3 has aroused great interest in atomic-scale ferroelectric transistors, artificial synapses, and nonvolatile memory devices due to its distinguished 2D ferroelectric properties. We have synthesized α-In2Se3 nanosheets with rare in-plane ferroelectric stripe domains at room temperature on mica substrates using a reverse flow chemical vapor deposition (RFCVD) method and optimized growth parameters. This stripe domain contrast is found to be strongly correlated to the stacking of layers, and the interrelated out-of-plane (OOP) and in-plane (IP) polarization can be manipulated by mapping the artificial domain structure. The acquisition of amplitude and phase hysteresis loops confirms the OOP polarization ferroelectric property. The emergence of striped domains enriches the variety of the ferroelectric structure types and novel properties of 2D In2Se3. This work paves a new way for the controllable growth of van der Waals ferroelectrics and facilitates the development of novel ferroelectric memory device applications.

Magnetic domain-dependent ultrafast optical demagnetization in stripe domain films

We investigated femto- and picosecond-time magnetization dynamics in a ferromagnetic Ni80Fe20 film with varying thicknesses (wedge-shaped film). We observed that the thickness gradient strongly affects the magnetic moment distribution, causing a magnetization reorientation from in-plane to out-of-plane, and formatting a stripe domain (SD) at the thicker end of the wedge. The magnetization dynamics measurements reveal that the part of the film displaying SDs follows a substantially faster demagnetization and magnetization recovery and smaller magnetization quenching compared to the in-plane domain film. The experiments and micromagnetic simulations support that the decrease in relaxation time is caused by a magnetic anisotropy of the films introduced by SD formation. Our results point out that the micromagnetic structure plays an important role in the magnetization dynamics in ferromagnetic films after optically triggered demagnetization.

Rotatable anisotropy in exchange bias bilayer and stripe domains films with uniaxial anisotropy

We report the rotatable anisotropy in three magnetic film systems, including in-plane magnetized FeNi film, FeNi/FeMn bilayer and FeNi stripe domains (SDs) structure film. Omnidirectional ferromagnetic resonance driven by rotatable anisotropy can be achieved in these films. We have found that the in-plane magnetized FeNi films have a negligible rotatable anisotropy of about 1 ∼ 2 Oe. Rotatable anisotropy fields of 6.1 Oe and 5.5 Oe are found for isotropic and anisotropic FeNi/FeMn bilayers, which comes from the exchange coupling at the interface of FM and AFM layer. This magnitude is smaller than the value found in thick FeNi film where a big rotatable anisotropy field of 87 Oe is present stemming from its domain structure. In addition, the uniaxial anisotropy could tailor the resonance frequency from 0.78 and 2.28 GHz for in-plane magnetized FeNi thin film, and 1.3 to 2.4 GHz for FeNi SDs films. Furthermore, a conclusion that a uniaxial anisotropy has no influence on rotatable anisotropy in three magnetic films systems, was yielded.

Néel Skyrmion Bubbles in La0.7Sr0.3Mn1–xRuxO3 Multilayers

Ferromagnetic La0.7Sr0.3Mn1-xRuxO3 epitaxial multilayers with controlled variation of the Ru/Mn content were synthesized to engineer canted magnetic anisotropy and variable exchange interactions, and to explore the possibility of generating a Dzyaloshinskii-Moriya interaction. The ultimate aim of the multilayer design is to provide the conditions for the formation of domains with nontrivial magnetic topology in an oxide thin film system. Employing magnetic force microscopy and Lorentz transmission electron microscopy in varying perpendicular magnetic fields, magnetic stripe domains separated by Néel-type domain walls as well as Néel skyrmions smaller than 100 nm in diameter were observed. These findings are consistent with micromagnetic modeling, taking into account a sizable Dzyaloshinskii-Moriya interaction arising from the inversion symmetry breaking and possibly from strain effects in the multilayer system.

Magnetic skyrmion nucleation via current injection in confined nanotrack with modified perpendicular anisotropy region

Magnetic skyrmions, as spintronic information carriers, are promising for next-generation spin logic and memory devices. For such skyrmion-based devices, effective control of skyrmion nucleation and controllable motion in the nanotrack are of great importance. The ion irradiation process can modify magnetic properties, such as perpendicular magnetic anisotropy (PMA) and Dzyaloshinskii–Moriya interaction (DMI), at the nanoscale, which can be used to reduce the design complexity of devices. In this study, a nanoregion without PMA in the nanotrack is adopted as a skyrmion nucleation seed and a current-driven highly efficient, in-line, and on-demand skyrmion nucleation schematic is presented. A key factor for realizing this concept is that the disappearance of PMA and the existence of DMI induce magnetization tilts and create a chiral perpendicular stripe domain within the nucleation region. This stripe domain allows the effective control of the spin transfer torque, and it is ejected from the PMA-modified region and propelled into the nanotrack, forming a stable skyrmion. Our proposed device allows the controlled nucleation and propagation of a series of skyrmions, which allows binary information to be written in a controlled manner, consequently, yielding simple devices with two terminals. This study provides an efficient route for designing tunable skyrmionics-mechanic memory devices.

Direct Observation of Magnetic Bubble Lattices and Magnetoelastic Effects in van der Waals Cr2Ge2Te6

AbstractFerromagnetic van der Waals (vdW) materials are of large current interest for the fundamental study of low‐dimensional magnetism and for potential applications in multilayer heterostructures. Cr2Ge2Te6 (CGT) is particularly exciting because it is a ferromagnetic semiconductor with tunable electronic and magnetic properties. Controlling the magnetic domain structure of CGT is a requirement for understanding its novel interface physics and for tuning behavior for potential devices. Herein, cryo‐Lorentz transmission electron microscopy is performed in the temperature range of 12–50K to directly image the magnetic domain structures in CGT. A rich phase diagram of domain structures including stripe domains, magnetic bubble lattices of mixed‐chirality, and topologically‐protected lattices of homochiral magnetic bubbles is observed. The types and chiralities of the bubbles can be controlled by topographical changes in the CGT flakes. Additionally, it is observed that in‐plane strain and magnetoelastic coupling can align and organize both bubble lattices and stripe domains. This study provides insights into creating and controlling complex magnetic domain structures for integration into multilayer heterostructures and for future studies of 2D magnetism.

The Effect of Size and Strain on Micro Stripe Magnetic Domain Structure of CoFeB Thin Films

The prerequisite for flexible magnetic electronic devices is the knowledge of the preparation technology of flexible magnetic films and the evolution of the film properties under strain. In this work, CoFeB amorphous ferromagnetic films with stripe domains were prepared on flexible polyimide (PI) substrates by oblique sputtering. The results show that oblique sputtering induces the formation of columnar crystal structure in CoFeB films, which increases the perpendicular magnetic anisotropy of the films, thus leading to the appearance of stripe magnetic domain structures. On this basis, the CoFeB films with stripe domains were processed on a microscopic scale to investigate the size effect and strain regulation on the microscopic domain structure of the magnetic films. The characterization of the magnetic domain structure shows that the stripe domain contrast is reduced by the striped structure prepared by lithography. The triangular, circular and ring patterns deflect the alignment of the stripe domain to different degrees. The experimental results show that the deflection of the stripe domains is caused by the anisotropy of the shapes produced by the different patterns and that the size of the microstructure needs to be close to the period of the stripe domains for the size effect to be significant. In addition, the strain-induced magnetoelastic anisotropy effectively rotates the orientation of the stripe domains, and the variation in domain contrast demonstrates that tensile/compressive strains vary the magnitude of the out-of-plane stray field of the film. Our results provide some insight into the modulation of the physical properties of flexible magnetic films.

Room-Temperature Intrinsic Ferromagnetic Chromium Tellurium Compounds with Thickness-Tunable Magnetic Texture.

2D ferromagnetic chromium tellurides exhibit intriguing spin configurations and high-temperature intrinsic ferromagnetism, providing unprecedented opportunities to explore the fundamental spin physics and build spintronic devices. Here, a generic van der Waals epitaxial approach is developed to synthesize the 2D ternary chromium tellurium compounds with thicknesses down to mono-, bi-, tri-, and few-unit cells(UC). The Mn0.14 Cr0.86 Te evolves from intrinsic ferromagnetic behavior in bi-UC, tri-UC, and few-UC to temperature-induced ferrimagnetic behavior as the thickness increases, resulting in a sign reversal of the anomalous Hall resistance. Temperature- and thickness-tunable labyrinthine-domain ferromagnetic behaviors are derived from the dipolar interactions in Fe0.26 Cr0.74 Te and Co0.40 Cr0.60 Te. Furthermore, the dipolar-interaction-induced stripe domain and field-induced domain wall (DW) motion velocity are studied, and multibit data storage is realized through an abundant DW state. The magnetic storage can function in neuromorphic computing tasks, and the pattern recognition accuracy can reach up to 97.93%, which is similar to the recognition accuracy of ideal software-based training (98.28%). Room-temperature ferromagnetic chromium tellurium compounds with intriguing spin configurations can significantly promote the exploration of the processing, sensing, and storage based on 2D magnetic systems.

Stacking domain morphology in epitaxial graphene on silicon carbide

Terrace-sized, single-orientation graphene can be grown on top of a carbon buffer layer on silicon carbide by thermal decomposition. Despite its homogeneous appearance, a surprisingly large variation in electron transport properties is observed. Here, we employ Aberration-Corrected Low-Energy Electron Microscopy (AC-LEEM) to study a possible cause of this variability. We characterize the morphology of stacking domains between the graphene and the buffer layer of high-quality samples. Similar to the case of twisted bilayer graphene, the lattice mismatch between the graphene layer and the buffer layer at the growth temperature causes a moir\'e pattern with domain boundaries between AB and BA stackings. We analyze this moir\'e pattern to characterize the relative strain and to count the number of edge dislocations. Furthermore, we show that epitaxial graphene on silicon carbide is close to a phase transition, causing intrinsic disorder in the form of co-existence of anisotropic stripe domains and isotropic trigonal domains. Using adaptive geometric phase analysis, we determine the precise relative strain variation caused by these domains. We observe that the step edges of the SiC substrate influence the orientation of the domains and we discuss which aspects of the growth process influence these effects by comparing samples from different sources.

Performance Enhancement of a Spin-Wave-Based Reservoir Computing System Utilizing Different Physical Conditions

We numerically study how to enhance reservoir computing performance by thoroughly extracting the spin-wave device potential for higher-dimensional information generation. The reservoir device has a 1-input exciter and 120-output detectors on the top of a continuous magnetic garnet film for spin-wave transmission. For various nonlinear and fading-memory dynamic phenomena distributing in the film space, small in-plane magnetic fields are used to prepare stripe domain structures and various damping constants at the film sides and bottom are explored. The ferromagnetic resonant frequency and relaxation time of spin precession clearly characterizes the change in spin dynamics with the magnetic field and damping constant. The common input signal for reservoir computing is a 1-GHz cosine wave with random 6-valued amplitude modulation. A basic 120-dimensional reservoir output vector is obtained from time-series signals at the 120-output detectors under each of three magnetic field conditions. Then, 240- and 360-dimensional reservoir output vectors are also constructed by concatenating two and three basic ones, respectively. In nonlinear autoregressive moving average (NARMA) prediction tasks, the computational performance is enhanced as the dimension of the reservoir output vector becomes higher and a significantly low prediction error is achieved for the tenth-order NARMA task using the 360-dimensional vector and optimum damping constant. The results are clear evidence that the collection of diverse output signals efficiently increases the dimensionality of the integrated reservoir state vector (i.e. reservoir-state richness) and thereby contributes to high computational performance. This paper demonstrates that performance enhancement through various configuration settings is a practical approach for on-chip reservoir computing devices with small numbers of real output nodes.

Visualizing the Effect of Oxidation on Magnetic Domain Behavior of Nanoscale Fe3GeTe2 for Applications in Spintronics

Magnetic van der Waals (vdW) materials offer an opportunity to design heterostructures that will lead to exotic functionalities that arise from interfacial interaction. In addition to coupling to different vdW materials, the naturally oxidized surface layer of a vdW material also forms a heterostructure with its bulk film, giving rise to intriguing phenomena. Here, we directly observe the impact of oxidation on the magnetic domains, namely, magnetic stripe domain and skyrmions, in a nanoscale Fe3GeTe2 flake using cryo Lorentz transmission electron microscopy. After the Fe3GeTe2 is exposed to ambient conditions, partial oxidation leads to an increase in the density of skyrmions even under zero magnetic field. Complete oxidation leads to a loss of the magnetic domain structure. We observe a gradual change in Fe3GeTe2 from single crystal to amorphous as the oxidation increases. The oxidized Fe3GeTe2 primarily consists of iron oxide, which could be antiferromagnetic in nature. We hypothesize that the interfacial interaction between these surface antiferromagnetic oxides and the bulk ferromagnetic Fe3GeTe2, as well as the effect of interfacial roughness, leads to the increase in Néel skyrmion creation. This work opens a path to harness controlled oxidation as a build block to create dense skyrmion lattices without the need for an external magnetic field, leading to potential future applications in spintronic devices.

The Modulated Magnetic Domain Structure in the La0.67Sr0.33MnO3 Ferromagnetic Films by Strain Engineering

Novel magnetic domain structures attract much attention due to their potential applications in the high performance of spintronic devices. Herein, the magnetic domains of La0.67Sr0.33MnO3 (LSMO) thin films, which epitaxially grown on (100)-, (110)-, and (111)-oriented (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) substrates, are systematically investigated. The (100)- and (110)-LSMO films show a stripe domain pattern. After in-plane magnetic field saturation, the direction of the stripe domains in (100)-LSMO no longer rotates with the change of magnetic field, while the stripe domains in (110)-LSMO rotate continuously. The (111)-LSMO films produce randomly distributed bubble-like magnetic domains, which gradually connect into strips with the increase of the external magnetic field, and the stability of the magnetic domain in (111)-LSMO is better than the ones in (100)- and (110)-LSMO under a magnetic field. In addition, it is found that with the increase of thickness, the random domains in (111)-LSMO films are gradually transformed into mazy domains. These phenomena can be understood by the regulable strain anisotropy and the magnetic anisotropy. Herein, an effective way is suggested to modify the magnetization anisotropy and magnetic domain because spin polarization depends strongly on the crystal surface orientation as well as epitaxial strain.

Physical properties of Cr-doped epitaxial BiFeO3 thin films influenced by ferroelectric domain structures

We investigated the physical properties of Cr-doped epitaxial BiFeO3 (CBFO) thin films influenced by ferroelectric domain structures. The epitaxial CBFO thin films with Cr concentrations of 0, 2, 4, 6, and 8% were deposited on Nb-doped single crystal SrTiO3 substrates by pulsed laser deposition. The epitaxial CBFO thin films with Cr concentrations of 0, 2, and 4% had highly c-oriented crystallinity and the epitaxial CBFO thin films with Cr concentrations of 6% and 8% exhibited mixed crystallinity of c-oriented crystallinity and (101)-oriented crystallinity. The CBFO thin film with a Cr concentration of 4% showed the highest ferroelectric polarization, and the ferroelectric polarizations of the CBFO thin films with a Cr concentration of more than 4% were reduced by the formation of (101)-oriented grains. On the other hand, the piezoelectric properties, leakage current characteristics, and ferromagnetic properties of CBFO thin films were simultaneously improved by increasing the Cr concentration. Terraces indicating layer-by-layer growth were observed on the surface of the CBFO thin films, and the edges of the terraces formed on the surface of the CBFO thin films were blurred when the Cr concentration increased. The ferroelectric domain structures of the CBFO thin films observed with a piezoelectric response microscope changed from a stripe domain structure to a mosaic domain structure when the Cr concentration exceeded 6%.

Facile Control of Ferroelastic Domain Patterns in Multiferroic Thin Films by a Scanning Tip Bias.

BiFeO3, known as the "holy grail of all multiferroics", provides an appealing platform for exploration of multifield coupling physics and design of functional devices. Many fantastic properties of BiFeO3 are regulated by its ferroelastic domain structure. However, a facile programable control on the ferroelastic domain structure in BiFeO3 remains challenging and our understanding on the existing control strategies is also far from complete. This work reports a facile control of ferroelastic domain patterns in BiFeO3 thin films under area scanning poling by exploiting the tip bias as the control parameter. Combining scanning probe microscopy experiments and simulations, we found that BiFeO3 thin films with pristine 71° rhombohedral-phase stripe domains exhibit at least four switching pathways solely by controlling the scanning tip bias. As a result, one can readily write mesoscopic topological defects into the films without the necessity to change the tip motion. The correlation between conductance of the scanned region and the switching pathway is further investigated. Our results extend the current understanding on the domain switching kinetics and the coupled electronic transport properties in BiFeO3 thin films. The facile voltage control of ferroelastic domains should facilitate the development of configurable electronic and spintronic devices.