Field-assisted growth of one-dimensional ZnO nanostructures with high defect density.

One-dimensional ZnO nanostructures have shown great potential in electronics, optoelectronics and electromechanical devices owing to their unique physical and chemical properties. Most of these nanostructures were grown by equilibrium processes where the defects density is controlled by thermodynamic equilibrium. In this work, flash sintering, a non-equilibrium field-assisted processing method, has been used to synthesize ZnO nanostructures. By applying a high electric field and limiting a low current flow, ZnO nanorods grew uniformly by a vapor-liquid-solid mechanism due to the extreme temperatures achieved near the hot spot. High density basal stacking faults in the nanorods along with ultraviolet excitonic emission and a red emission under room temperature demonstrate the potential of defect engineering in nanostructures via the field-assisted growth method.

Intrinsic Magnetic Order of Chemically Exfoliated 2D Ruddlesden-Popper Organic-Inorganic Halide Perovskite Ultrathin Films.

Thin film fabrication of 2D layered organic-inorganic hybrid perovskites (2D-OIHPs) for spintronic applications has been attempted using solution-based process like Langmuir-Blodgett technique. However, monolayer or few-layered 2D magnets are not yet realized, even though a wide spectrum of 2D Ruddlesden-Popper (RP) OIHPs are known as quasi-2D Heisenberg magnets in bulk compounds. Here, chemical exfoliation by solvent engineering is applied to successfully synthesize large-sized, few unit-cell-thick 2D RP-OIHPs. Comprehensive structural characterization reveals that binary co-solvents with high relative polarity in spin coating technique are the most effective among nine kinds of solvents. Above all, this enables few-layered 2D RP-OIHP ultrathin films sustaining their intrinsic magnetic order. It is found that XY-like magnetic anisotropy driven by Jahn-Teller effect responsible for ferromagnetism in seven-layered (C6 H5 CH2 CH2 NH3 )2 CuCl4 ultrathin films remains very robust, whereas Ising-like dipolar anisotropy responsible for canted antiferromagnetism in ten-layered (C6 H5 CH2 CH2 NH3 )2 MnCl4 ultrathin films is greatly reduced. It is expected that ferromagnetism even at monolayer limit should be possible by means of further sophisticated solvent engineering as long as Jahn-Teller effect is active. The chemical exfoliation using solvent engineering unambiguously can bring about a new breakthrough in the development of 2D RP-OIHP van der Waals magnets for ultrahigh energy-efficient spintronic, opto-spintronic devices.

Asymmetric Hysteresis for Probing Dzyaloshinskii-Moriya Interaction.

The interfacial Dzyaloshinskii-Moriya interaction (DMI) is intimately related to the prospect of superior domain-wall dynamics and the formation of magnetic skyrmions. Although some experimental efforts have been recently proposed to quantify these interactions and the underlying physics, it is still far from trivial to address the interfacial DMI. Inspired by the reported tilt of the magnetization of the side edge of a thin film structure, we here present a quasi-static, straightforward measurement tool. By using laterally asymmetric triangular-shaped microstructures, it is demonstrated that interfacial DMI combined with an in-plane magnetic field yields a unique and significant shift in magnetic hysteresis. By systematic variation of the shape of the triangular objects combined with a droplet model for domain nucleation, a robust value for the strength and sign of interfacial DMI is obtained. This method gives immediate and quantitative access to DMI, enabling a much faster exploration of new DMI systems for future nanotechnology.

Selective Charge Carrier Transport and Bipolar Conduction in an Inorganic/Organic Bulk-Phase Composite: Optimization for Low-Temperature Thermoelectric Performance.

Abundant conducting polymers are promising organic substances for low-temperature thermoelectric applications due to their inherently low thermal conductivities. By introducing a conducting polymer filler (PEDOT:PSS─poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonic acid)) into a representative inorganic thermoelectric matrix (Bi2Te3), a bulk-phase composite (i.e., inorganic matrix/organic filler) for low-temperature thermoelectric applications is proposed. This composite hosts an interfacial energy barrier between the inorganic and organic components, facilitating controlled carrier transport based on its energy level, known as the energy filtering effect, and thus the composite exhibits a highly improved Seebeck coefficient compared to pristine Bi2Te3. The composite also displays a completely different temperature dependence on the Seebeck coefficient from Bi2Te3 due to its distinct bipolar conduction tendency. By regulation of the energy filtering effect and bipolar conduction tendency, the composite undergoes noticeable variations in the Seebeck coefficient, resulting in a significantly enhanced power factor. Furthermore, the composite shows a substantially reduced thermal conductivity compared to Bi2Te3 because it has lower carrier/lattice thermal contributions, possibly attributed to its high carrier/phonon scattering probabilities. Owing to the superior power factor and reduced thermal conductivity, the composite exhibits markedly enhanced thermoelectric performance, achieving a maximum figure of merit of approximately 1.26 at 380 K and an average figure of merit of approximately 1.23 in the temperature range of 323-423 K. The performance of the composite is competitive with previously reported n-type Bi2Te3 binary or ternary analogues. Therefore, the composite is highly expected to be a promising n-type counterpart of p-type Bi2Te3-based alloys for various low-temperature thermoelectric applications.

Achieving room temperature plasticity in brittle ceramics through elevated temperature preloading.

Ceramic materials with high strength and chemical inertness are widely used as engineering materials. However, the brittle nature limits their applications as fracture occurs before the onset of plastic yielding. There has been limited success despite extensive efforts to enhance the deformability of ceramics. Here we report a method for enhancing the room temperature plastic deformability of ceramics by artificially introducing abundant defects into the materials via preloading at elevated temperatures. After the preloading treatment, single crystal (SC) TiO2 exhibited a substantial increase in deformability, achieving 10% strain at room temperature. SC α-Al2O3 also showed plastic deformability, 6 to 7.5% strain, by using the preloading strategy. These preinjected defects enabled the plastic deformation process of the ceramics at room temperature. These findings suggest a great potential for defect engineering in achieving plasticity in ceramics at room temperature.

3D Printing of CNT- and YSZ-Added Dental Resin-Based Composites by Digital Light Processing and Their Mechanical Properties.

This study demonstrates the successful 3D printing of dental resin-based composites (DRCs) containing ceramic particles using the digital light processing (DLP) technique. The mechanical properties and oral rinsing stability of the printed composites were evaluated. DRCs have been extensively studied for restorative and prosthetic dentistry due to their clinical performance and aesthetic quality. They are often subjected to periodic environmental stress, and thus can easily undergo undesirable premature failure. Here, we investigated the effects of two different high-strength and biocompatible ceramic additives, carbon nanotube (CNT) and yttria-stabilized zirconia (YSZ), on the mechanical properties and oral rinsing stabilities of DRCs. Dental resin matrices containing different wt.% of CNT or YSZ were printed using the DLP technique after analyzing the rheological behavior of slurries. Mechanical properties such as Rockwell hardness and flexural strength, as well as the oral rinsing stability of the 3D-printed composites, were systematically investigated. The results indicated that a DRC with 0.5 wt.% YSZ exhibits the highest hardness of 19.8 ± 0.6 HRB and a flexural strength flexural strength of 50.6 ± 6 MPa, as well as reasonable oral rinsing steadiness. This study provides a fundamental perspective for designing advanced dental materials containing biocompatible ceramic particles.

The Stack Optimization of Magnetic Heterojunction Structures for Next-Generation Spintronic Logic Applications.

Magnetic heterojunction structures with a suppressed interfacial Dzyaloshinskii-Moriya interaction and a sustainable long-range interlayer exchange coupling are achieved with an ultrathin platinum insertion layer. The systematic inelastic light scattering spectroscopy measurements indicate that the insertion layer restores the symmetry of the system and, then, the interfacial Dzyaloshinskii-Moriya interaction, which can prevent the identical magnetic domain wall motions, is obviously minimized. Nevertheless, the strong interlayer exchange coupling of the system is maintained. Consequently, synthetic ferromagnetic and antiferromagnetic exchange couplings as a function of the ruthenium layer thickness are observed as well. Therefore, these optimized magnetic multilayer stacks can avoid crucial issues, such as domain wall tilting and position problems, for next-generation spintronic logic applications. Moreover, the synthetic antiferromagnetic coupling can open a new path to develop a radically different NOT gate via current-induced magnetic domain wall motions and inversions.

Control of crystallization and magnetic properties of CoFeB by boron concentration.

Controlling the crystallinity of CoFeB is the most essential issue for designing various spintronics devices. Here we show the microstructure and magnetic properties of MgO/CoFeB/MgO structures for various boron concentration. We present the effect of boron on the crystallinity of CoFeB into two categories: the critical boron concentration (5 ~ 6%) at which CoFeB crystallizes and the effect of remaining boron (0 ~ 5%) in the crystallized CoFeB. And the trends of the saturation magnetization, exchange stiffness, exchange length, domain wall energy and Gilbert damping constant according to the boron concentration are provided. Abrupt variation of properties near the critical boron concentration (5 ~ 6%) and a noticeable change in the crystallized CoFeB (0 ~ 5%) are confirmed, revealing a clear causal relationship with the structural analysis. These results propose that the crystallization, microstructure, and major magnetic properties of CoFeB are governed by the amount of boron, and emphasize the need for delicate control of boron concentration.

Non-equilibrium chiral domain wall dynamics excited by transverse magnetic field pulses.

Non-equilibrium domain wall dynamics on a perpendicularly magnetized nanowire manipulated by the transverse magnetic field pulse are numerically investigated. We systematically observe the large displacements of the chiral domain wall and the domain wall tilting angles generated by Dzyaloshinskii-Moriya interaction during the competition between the precession torque and the magnetic damping process. The magnetic-property-dependent domain wall displacements exhibit that the lower magnetic damping constants and Dzyaloshinskii-Moriya energy densities generate the longer transition times and the significant larger domain wall displacements for the non-equilibrium magnetization dynamics. Compare with the spin-polarized-current-driven domain wall dynamics, the transverse magnetic field pulses guarantee faster domain wall movements without Walker breakdown and lower energy consumptions because it is free from the serious Joule heating issue. Finally, we demonstrate successive chiral domain wall displacements, which are necessary to develop multilevel resistive memristors for next-generation artificial intelligent devices based on magnetic domain wall motions.

High-strength nanocrystalline intermetallics with room temperature deformability enabled by nanometer thick grain boundaries.

Although intermetallics are attractive for their high strength, many of them are often brittle at room temperature, thereby severely limiting their potential as structural materials. Here, we report on a previously unidentified deformable nanocrystalline CoAl intermetallics with Co-rich thick grain boundaries (GBs). In situ micropillar compression studies show that nanocrystalline CoAl with thick GBs exhibits ultrahigh yield strength, exceeding 4.5 gigapascals. Unexpectedly, nanocrystalline CoAl intermetallics also show prominent work hardening to a flow stress of 5.7 gigapascals up to 20% compressive strain. Transmission electron microscopy studies show that deformation induces abundant dislocations inside CoAl grains with thick GBs, which accommodate plastic deformation. Molecular dynamics simulations reveal that the Co-rich thick GBs play a vital role in promoting nucleation of dislocations at the Co/CoAl interfaces, thereby enhancing the plasticity of the intermetallics. This study provides a perspective to promoting the plasticity of intermetallics via the introduction of thick GBs.

Structure-induced ferromagnetic stabilization in free-standing hexagonal Fe(1.3)Ge nanowires.

Single-crystalline free-standing hexagonal Fe(1.3)Ge nanowires (NWs) are synthesized for the first time using a chemical vapor transport process without using any catalyst. Interestingly, Fe(1.3)Ge NWs are found to be ferromagnetic at room temperature, while bulk Fe(1.3)Ge has the lower critical temperature of 200 K. We perform first-principles density functional calculations and suggest that the observed strong ferromagnetism is attributed to the reduced distances between Fe atoms, increased number of Fe-Fe bonds, and the enhanced Fe magnetic moments. Both experimental and theoretical studies show that the magnetic moments are enhanced in the NWs, as compared to bulk Fe(1.3)Ge. We also modulate the composition ratio of as-grown iron germanide NWs by adjusting experimental conditions. It is shown that uniaxial strain on the hexagonal plane also enhances the ferromagnetic stability.

Ceramic Material Processing Towards Future Space Habitat: Electric Current-Assisted Sintering of Lunar Regolith Simulant.

In situ utilization of available resources in space is necessary for future space habitation. However, direct sintering of the lunar regolith on the Moon as structural and functional components is considered to be challenging due to the sintering conditions. To address this issue, we demonstrate the use of electric current-assisted sintering (ECAS) as a single-step method of compacting and densifying lunar regolith simulant JSC-1A. The sintering temperature and pressure required to achieve a relative density of 97% and microhardness of 6 GPa are 700 °C and 50 MPa, which are significantly lower than for the conventional sintering technique. The sintered samples also demonstrated ferroelectric and ferromagnetic behavior at room temperature. This study presents the feasibility of using ECAS to sinter lunar regolith for future space resource utilization and habitation.

Effect of Hyaluronic Acid Filler Injection on the Interdental Papilla in a Mouse Model of Open Gingival Embrasure.

The black triangle resulting from interdental papilla (IDP) loss is associated with poor aesthetics and difficulty in pronunciation and food impaction. There is limited knowledge of gingival tissue inflammatory response to hyaluronic acid (HA) filler injection, a minimally invasive IDP reconstruction method. This study aimed to examine the morphological and histological changes in IDP and the inflammatory cytokine localization to the IDP post-HA filler injection using an open gingival embrasure (OGE) mouse model. Mice from the control, sham, and OGE groups were attached with reference, inactive, and activated wires for 5 days, respectively. The degree of IDP loss was determined based on the spring-papilla distance (SPD). Morphological and histological changes in the OGE group injected with phosphate-buffered saline (PBS) or HA fillers were examined on days 2 and 7 post-injection. Immunohistochemical analysis was performed to determine the localization patterns of tumor necrosis factor (TNF)-α, interleukin (IL)-1ß, IL-6, myeloperoxidase (MPO), and Ki67. Five days post-wire attachment, the control and OGE groups exhibited a significantly higher SPD than the sham group (p < 0.0167). The SPD of the HA filler injection group was significantly lower than that of the PBS injection group on days 2, 4, and 7 post-injection (p < 0.05). The IDP of the OGE group was wide and flat. HA filler was stable in the connective tissue underlying the epithelial tissue even on day 7 post-injection. TNF-α, IL-1ß, IL-6, MPO, and Ki67 were highly localized to the connective tissue surrounding the filler on day 2, which decreased on day 7 post-injection. Thus, HA filler can safely and successfully reconstruct the IDP in cases of OGE.

Nanoscale stacking fault-assisted room temperature plasticity in flash-sintered TiO2.

Ceramic materials have been widely used for structural applications. However, most ceramics have rather limited plasticity at low temperatures and fracture well before the onset of plastic yielding. The brittle nature of ceramics arises from the lack of dislocation activity and the need for high stress to nucleate dislocations. Here, we have investigated the deformability of TiO2 prepared by a flash-sintering technique. Our in situ studies show that the flash-sintered TiO2 can be compressed to ~10% strain under room temperature without noticeable crack formation. The room temperature plasticity in flash-sintered TiO2 is attributed to the formation of nanoscale stacking faults and nanotwins, which may be assisted by the high-density preexisting defects and oxygen vacancies introduced by the flash-sintering process. Distinct deformation behaviors have been observed in flash-sintered TiO2 deformed at different testing temperatures, ranging from room temperature to 600°C. Potential mechanisms that may render ductile ceramic materials are discussed.

High temperature deformability of ductile flash-sintered ceramics via in-situ compression.

Flash sintering has attracted significant attention as its remarkably rapid densification process at low sintering furnace temperature leads to the retention of fine grains and enhanced dielectric properties. However, high-temperature mechanical behaviors of flash-sintered ceramics remain poorly understood. Here, we present high-temperature (up to 600 °C) in situ compression studies on flash-sintered yttria-stabilized zirconia (YSZ). Below 400 °C, the YSZ exhibits high ultimate compressive strength exceeding 3.5 GPa and high inelastic strain (~8%) due primarily to phase transformation toughening. At higher temperatures, crack nucleation and propagation are significantly retarded, and prominent plasticity arises mainly from dislocation activity. The high dislocation density induced in flash-sintered ceramics may have general implications for improving the plasticity of sintered ceramic materials.

Thickness dependence of the interfacial Dzyaloshinskii-Moriya interaction in inversion symmetry broken systems.

In magnetic multilayer systems, a large spin-orbit coupling at the interface between heavy metals and ferromagnets can lead to intriguing phenomena such as the perpendicular magnetic anisotropy, the spin Hall effect, the Rashba effect, and especially the interfacial Dzyaloshinskii-Moriya (IDM) interaction. This interfacial nature of the IDM interaction has been recently revisited because of its scientific and technological potential. Here we demonstrate an experimental technique to straightforwardly observe the IDM interaction, namely Brillouin light scattering. The non-reciprocal spin wave dispersions, systematically measured by Brillouin light scattering, allow not only the determination of the IDM energy densities beyond the regime of perpendicular magnetization but also the revelation of the inverse proportionality with the thickness of the magnetic layer, which is a clear signature of the interfacial nature. Altogether, our experimental and theoretical approaches involving double time Green's function methods open up possibilities for exploring magnetic hybrid structures for engineering the IDM interaction.

Spin-orbit torque in a bulk perpendicular magnetic anisotropy Pd/FePd/MgO system.

Spin-orbit torques, including the Rashba and spin Hall effects, have been widely observed and investigated in various systems. Since interesting spin-orbit torque (SOT) arises at the interface between heavy nonmagnetic metals and ferromagnetic metals, most studies have focused on the ultra-thin ferromagnetic layer with interface perpendicular magnetic anisotropy. Here, we measured the effective longitudinal and transverse fields of bulk perpendicular magnetic anisotropy Pd/FePd (1.54 to 2.43 nm)/MgO systems using harmonic methods with careful correction procedures. We found that in our range of thicknesses, the effective longitudinal and transverse fields are five to ten times larger than those reported in interface perpendicular magnetic anisotropy systems. The observed magnitude and thickness dependence of the effective fields suggest that the SOT do not have a purely interfacial origin in our samples.