Wide Linearity Range 3D Magnetic Sensor and Angular Position Detector Based on a Single FePt Spin-Orbit Torque Device.

Three-dimensional (3D) vector magnetic sensors play a significant role in a variety of industries, especially in the automotive industry, which enables the control of precise position, angle, and rotation of motion elements. Traditional 3D magnetic sensors integrate multiple sensors with their sensing orientations along the three coordinate axes, leading to a large size and inevitable nonorthogonal misalignment. Here, we demonstrate a wide linearity range 3D magnetic sensor utilizing a single L10-FePt Hall-bar device, whose sensitivity is 291 VA-1 T-1 in the z-axis and 27 VA-1 T-1 in the in-plane axis. Based on the spin-orbit torque-dominated magnetization reversal, the linear response of anomalous Hall resistance within a large linear range (±200 Oe) for the x, y, and z components of magnetic fields has been obtained, respectively. Typically, it exhibits a relatively lower magnetic noise level of 7.9 nV at 1 Hz than previous results, improving measurement resolution at the low frequency. Furthermore, we provide a straightforward approach for noncontact angular position detection based on a single Hall-bar device, which shows great potential for application in rotational motion control.

Unconventional polarization fatigue in van der Waals layered ferroelectric ionic conductor CuInP2S6.

Recent progress in two-dimensional ferroelectrics greatly expands the versatility and tunability in van der Waals heterostructure based electronics. However, the switching endurance issue that widely plagues conventional ferroelectrics in practical applications is hitherto unexplored for van der Waals layered ferroelectrics. Herein, we report the observation of unusual polarization fatigue behaviors in van der Waals layered CuInP2S6, which also possesses finite ionic conductivity at room temperature. The strong intertwinement of the short-range polarization switching and long-range ionic movement in conjunction with the van der Waals layered structure gives rise to unique morphological and polarization evolutions under repetitive electric cycles. With the help of concerted chemical, structural, lattice vibrational and dielectric analyses, we unravel the critical role of the synergy of ionic migration and surface oxidation on the anomalous polarization enhancement and the eventual polarization degradation. This work provides a general insight into the polarization fatigue characteristics in ionically-active van der Waals ferroelectrics and delivers potential solutions for the realization of fatigue-free capacitors.

All-Water Etching-Free Electron Beam Lithography for On-Chip Nanomaterials.

Electron beam lithography uses an accelerated electron beam to fabricate patterning on an electron-beam-sensitive resist but requires complex dry etching or lift-off processes to transfer the pattern to the substrate or film on the substrate. In this study, etching-free electron beam lithography is developed to directly write a pattern of various materials in all-water processes, achieving the desired semiconductor nanopatterns on a silicon wafer. Introduced sugars are copolymerized with metal ions-coordinated polyethylenimine under the action of electron beams. The all-water process and thermal treatment result in nanomaterials with satisfactory electronic properties, indicating that diverse on-chip semiconductors (e.g., metal oxides, sulfides, and nitrides) can be directly printed on-chip by an aqueous solution system. As a demonstration, zinc oxide patterns can be achieved with a line width of 18 nm and a mobility of 3.94 cm2 V-1 s-1. This etching-free electron beam lithography strategy provides an efficient alternative for micro/nanofabrication and chip manufacturing.

Self-Enhancing Photoelectrochemical Properties in van der Waals Ferroelectric CuInP2S6 by Photoassisted Acid Hydrolysis.

Transition metal thiophosphate, CuInP2S6 (CIPS), has recently emerged as a potentially promising material for photoelectrochemical (PEC) water splitting due to its intrinsic ferroelectric polarization for spontaneous photocarrier separation. However, the poor kinetics of the hydrogen evolution reaction (HER) greatly limits its practical applications. Herein, we report self-enhancing photocatalytic behavior of a CIPS photocathode due to chemically driven oxygen incorporation by photoassisted acid oxidation. The optimal oxygen-doped CIPS demonstrates a >1 order of magnitude enhancement in the photocurrent density compared to that of pristine CIPS. Through comprehensive spectroscopic and microscopic investigations combined with theoretical calculations, we disclose that oxygen doping will lower the Fermi level position and decrease the HER barrier, which further accelerates charge separation and improves the HER activity. This work may deliver a universal and facile strategy for improving the PEC performance of other van der Waals metal thiophosphates.

Sub-Nanometer Thick Wafer-Size NiO Films with Room-Temperature Ferromagnetic Behavior.

Adding ferromagnetism into semiconductors attracts much attentions due to its potential usage of magnetic spins in novel devices, such as spin field-effect transistors. However, it remains challenging to stabilize their ferromagnetism above room temperature. Here we introduce an atomic chemical-solution strategy to grow wafer-size NiO thin films with controllable thickness down to sub-nanometer scale (0.92 nm) for the first time. Surface lattice defects break the magnetic symmetry of NiO and produce surface ferromagnetic behaviors. Our sub-nanometric NiO thin film exhibits the highest reported room-temperature ferromagnetic behavior with a saturation magnetization of 157 emu/cc and coercivity of 418 Oe. Attributed to wafer size, the easily-transferred NiO thin film is further verified in a magnetoresistance device. Our work provides a sub-nanometric platform to produce wafer-size ferromagnetic NiO thin films as atomic layer magnetic units in future transparent magnetoelectric devices.

Kondo effect and superconductivity in niobium with iron impurities.

Kondo effect is an interesting phenomenon in quantum many-body physics. Niobium (Nb) is a conventional superconductor important for many superconducting device applications. It was long thought that the Kondo effect cannot be observed in Nb because the magnetic moment of a magnetic impurity, e.g. iron (Fe), would have been quenched in Nb. Here we report an observation of the Kondo effect in a Nb thin film structure. We found that by co-annealing Nb films with Fe in Argon gas at above 400 [Formula: see text]C for an hour, one can induce a Kondo effect in Nb. The Kondo effect is more pronounced at higher annealing temperature. The temperature dependence of the resistance suggests existence of remnant superconductivity at low temperatures even though the system never becomes superconducting. We find that the Hamann theory for the Kondo resistivity gives a satisfactory fitting to the result. The Hamann analysis gives a Kondo temperature for this Nb-Fe system at [Formula: see text] 16 K, well above the superconducting transition onset temperature 9 K of the starting Nb film, suggesting that the screening of the impurity spins is effective to allow Cooper pairs to form at low temperatures. We suggest that the mechanism by which the Fe impurities retain partially their magnetic moment is that they are located at the grain boundaries, not fully dissolved into the bcc lattice of Nb.

Two-dimensional magnetic interplay in the tensile-strained LaCoO3 thin films.

High-quality epitaxial LaCoO3 (LCO) thin films have been deposited on SrTiO3 (STO) substrates with pulsed laser deposition (PLD). We find that the LCO films undergo a typical ferromagnetic-paramagnetic (FM-PM) phase transition at ∼80 K. To understand the nature of magnetic phase transition, various methods, including the modified Arrott plot and critical isotherm analysis, were used to determine the critical exponents, which are ß = 0.754(1) with a Curie temperature TC = 79.8(8) K and γ = 1.52(2) with TC = 79.9(2) K. The reliability of these critical exponents was confirmed using the Widom scaling relation and the scaling hypothesis. Further analysis revealed that the spin coupling within the LCO films exhibits two-dimensional (2D) long-range magnetic interaction and the magnetic exchange distance decays as J(r) ∼r-(3.46). Our critical behavior analysis may shed new light on the further understanding of the origin of FM and the relatively fixed TC in LCO thin films.

Single-Stimulus-Induced Modulation of Multiple Optical Properties.

Stimuli-responsive smart optical materials hold great promise for applications in active optics, display, sensing, energy conversion, military camouflage, and artificial intelligence. However, their applications are greatly restricted by the difficulty of tuning different optical properties within the same material, especially by a single stimulus. Here, magnetic modulations of multiple optical properties are demonstrated in a crystalline colloidal array (CCA) of magnetic nanorods. Small-angle X-ray scattering studies reveal that these nanorods form an unusual monoclinic crystal in concentrated suspensions. The CCA exhibits optical anisotropy in the form of a photonic bandgap and birefringence, thus enabling magnetic tuning of the structural color and transmittance at a rate of 50 Hz. As a proof-of-concept, it is further demonstrated that the fabrication of a multifunctional device for display, anticounterfeiting, and smart-window applications based on this multiple magneto-optical effect. The study not only provides a new model system for understanding colloidal assembly, but also opens up opportunities for new applications of smart optical materials for various purposes.

Influence of CNTs on the Crystalline Microstructure and Ferroelectric Behavior of P(VDF-TrFE).

We investigate the effect of carbon nanotubes (CNTs) on the crystalline microstructure and ferroelectric behavior of polyvinylidene fluoride- co-trifluoroethylene (P(VDF-TrFE)). X-ray analysis suggests that CNT can act as a template and direct the chain orientation of P(VDF-TrFE) crystals. In the presence of CNTs, the molecular chain axis ( c axis) of the ß-phase crystal is oriented parallel to the long axis of CNTs. Moreover, we find that this templating effect did not cause a polymorph transition. For P(VDF-TrFE)/CNT composites, the crystallinity of P(VDF-TrFE) is slightly decreased. The orientation of the c axis induced by the templating effect of CNTs has a significant impact on the ferroelectric behavior of P(VDF-TrFE). As compared to a pure P(VDF-TrFE) film, the remnant polarization of the P(VDF-TrFE)/CNT composite is enhanced. Correspondingly, the piezoelectric property of the P(VDF-TrFE)/CNT composite shows a significant enhancement.

Local-Curvature-Controlled Non-Epitaxial Growth of Hierarchical Nanostructures.

Site-selective growth on non-spherical seeds provides an indispensable route to hierarchical complex nanostructures that are interesting for diverse applications. However, this has only been achieved through epitaxial growth, which is restricted to crystalline materials with similar crystal structures and physicochemical properties. A non-epitaxial growth strategy is reported for hierarchical nanostructures, where site-selective growth is controlled by the curvature of non-spherical seeds. This strategy is effective for site-selective growth of silica nanorods from non-spherical seeds of different shapes and materials, such as α-Fe2 O3 , NaYF4 , and ZnO. This growth strategy is not limited by the stringent requirements of epitaxy and is thus a versatile general method suitable for the preparation of hierarchical nanostructures with controlled morphologies and compositions to open up a verity of applications in self-assembly, nanorobotics, catalysis, electronics, and biotechnology.

Gram-scale synthesis of superparamagnetic Fe3O4 nanocrystal clusters with long-term charge stability for highly stable magnetically responsive photonic crystals.

We report the gram-scale hydrothermal synthesis of superparamagnetic Fe3O4 CNCs with superior long-term charge stability, which are suitable building blocks for magnetically responsive photonic crystals (MRPCs) with widely, rapidly and reversibly tunable diffractions across the visible and near IR range, as well as long-term stability of photonic performances. The scalable synthesis of Fe3O4 CNCs with excellent long-term colloidal stability will facilitate their wide application in photonics, biomedicine, catalysis and other areas.

Dielectric relaxation, resonance and scaling behaviors in Sr3Co2Fe24O41 hexaferrite.

The dielectric properties of Z-type hexaferrite Sr3Co2Fe24O41 (SCFO) have been investigated as a function of temperature from 153 to 503 K between 1 and 2 GHz. The dielectric responses of SCFO are found to be frequency dependent and thermally activated. The relaxation-type dielectric behavior is observed to be dominating in the low frequency region and resonance-type dielectric behavior is found to be dominating above 10(8) Hz. This frequency dependence of dielectric behavior is explained by the damped harmonic oscillator model with temperature dependent coefficients. The imaginary part of impedance (Z″) and modulus (M″) spectra show that there is a distribution of relaxation times. The scaling behaviors of Z″ and M″ spectra further suggest that the distribution of relaxation times is temperature independent at low frequencies. The dielectric loss spectra at different temperatures have not shown a scaling behavior above 10(8) Hz. A comparison between the Z″ and the M″ spectra indicates that the short-range charges motion dominates at low temperatures and the long-range charges motion dominates at high temperatures. The above results indicate that the dielectric dispersion mechanism in SCFO is temperature independent at low frequencies and temperature dependent at high frequencies due to the domination of resonance behavior.

Vertical Interface Induced Dielectric Relaxation in Nanocomposite (BaTiO3)1-x:(Sm2O3)x Thin Films.

Vertical interfaces in vertically aligned nanocomposite thin films have been approved to be an effective method to manipulate functionalities. However, several challenges with regard to the understanding on the physical process underlying the manipulation still remain. In this work, because of the ordered interfaces and large interfacial area, heteroepitaxial (BaTiO3)1-x:(Sm2O3)x thin films have been fabricated and used as a model system to investigate the relationship between vertical interfaces and dielectric properties. Due to a relatively large strain generated at the interfaces, vertical interfaces between BaTiO3 and Sm2O3 are revealed to become the sinks to attract oxygen vacancies. The movement of oxygen vacancies is confined at the interfaces and hampered by the misfit dislocations, which contributed to a relaxation behavior in (BaTiO3)1-x:(Sm2O3)x thin films. This work represents an approach to further understand that how interfaces influence on dielectric properties in oxide thin films.

Vertical-interface-manipulated conduction behavior in nanocomposite oxide thin films.

Vertically aligned nanocomposites with vertical interfaces are a novel concept that show powerful advantages over conventional nanocomposites with lateral interfaces. However, significant obstacles to a systematic understanding of vertical interfaces still remain. Here, heteroepitaxial (BaTiO3)0.5:(Sm2O3)0.5 nanocomposite thin films have been fabricated and the conduction behaviors have been investigated. A spontaneous phase ordering with clear vertical interfaces has been found in the composite films. Because of the structural discontinuity as well as a large strain generated at the interfaces, the vertical interfaces are revealed to become the sinks to attract oxygen vacancies. The accumulated oxygen vacancies contributed to a largely reduced leakage current and a different leakage mechanism in the composite films compared to that of the pure BaTiO3 film. The present work represents a methodology to manipulate functionalities by designing configuration of the interfaces in oxide thin films.

Oxygen-vacancy-induced antiferromagnetism to ferromagnetism transformation in Eu0.5Ba0.5TiO3₋δ multiferroic thin films.

Oxygen vacancies (V(O)) effects on magnetic ordering in Eu0.5Ba0.5TiO3₋δ (EBTO3₋δ) thin films have been investigated using a combination of experimental measurements and first-principles density-functional calculations. Two kinds of EBTO3₋δ thin films with different oxygen deficiency have been fabricated. A nuclear resonance backscattering spectrometry technique has been used to quantitatively measure contents of the V(O). Eu0.5Ba0.5TiO3 ceramics have been known to exhibit ferroelectric (FE) and G-type antiferromagnetic (AFM) properties. While, a ferromagnetic (FM) behavior with a Curie temperature of 1.85 K has been found in the EBTO3₋δ thin films. Spin-polarized Ti(3+) ions, which originated from the V(O), has been proven to mediate a FM coupling between the local Eu 4f spins and were believed to be responsible for the great change of the magnetic ordering. Considering the easy formation of V(O), our work opens up a new avenue for achieving co-existence of FM and FE orders in oxide materials.