A Wide-Bandwidth Inexpensive Current Sensor Based on the Signal Fusion of Tunneling Magnetoresistance and a Current Transformer.

In technology and industrial production, many applications require wide-bandwidth current measurements. In this paper, a signal fusion scheme for a current sensor comprising tunneling magnetoresistance and a current transformer is proposed, achieving a flat frequency response in the DC to MHz range. The measurement principles in different cases of the scheme are introduced, and the total transfer function of the entire scheme is derived by analyzing each section separately. Furthermore, the feasibility and selected parameters of the scheme are verified through a systematic simulation utilizing the MATLAB software. Based on the proposed scheme, a group of principal prototypes are built to experimentally evaluate the bandwidth, amplitude and phase flatness, accuracy, sensitivity, and impulse response. The relative amplitude variation in the passband of the fusion sensor is less than 4%, and the estimated bandwidth of the fusion sensor is close to 17 MHz. The accuracy is better than 0.6%, even when measuring the current at 1 MHz, and the relative standard deviation is 5% when measuring the impulse signal. The sensors developed using this scheme, with a low financial cost, have advantages in many wide-bandwidth current measuring scenarios.

Symmetry-controlled SrRuO3/SrTiO3/SrRuO3magnetic tunnel junctions: spin polarization and its relevance to tunneling magnetoresistance.

Magnetic tunnel junctions (MTJs), that consist of two ferromagnetic electrodes separated by an insulating barrier layer, have non-trivial fundamental properties associated with spin-dependent tunneling. Especially interesting are fully crystalline MTJs where spin-dependent tunneling is controlled by the symmetry group of wave vector. In this work, using first-principles quantum-transport calculations, we explore spin-dependent tunneling in fully crystalline SrRuO3/SrTiO3/SrRuO3(001) MTJs and predict tunneling magnetoresistance (TMR) of nearly 3000%. We demonstrate that this giant TMR effect is driven by symmetry matching (mismatching) of the incoming and outcoming Bloch states in the SrRuO3(001) electrodes and evanescent states in the SrTiO3(001) barrier. We argue that under the conditions of symmetry-controlled transport, spin polarization, whatever definition is used, is not a relevant measure of spin-dependent tunneling. In the presence of diffuse scattering, however, e.g. due to localized states in the band gap of the tunnel barrier, symmetry matching is no longer valid and TMR in SrRuO3/SrTiO3/SrRuO3(001) MTJs is strongly reduced. Under these conditions, the spin polarization of the interface transmission function becomes a valid measure of TMR. These results provide an important insight into understanding and optimizing TMR in all-oxide MTJs.

Spin Transport Modulation of 2D Fe3O4 Nanosheets Driven by Verwey Phase Transition.

Realizing spin transport between heavy metal and two-dimensional (2D) magnetic materials at high Curie temperature (TC) is crucial to advanced spintronic information storage technology. Here, environmentally stable 2D nonlayered Fe3O4 nanosheets are successfully synthesized using a reproducible process and found that they exhibit vortex magnetic domains at room temperature. A Verwey phase transition temperature (TV) of ≈110 K is identified for ≈3 nm thick nanosheet through Raman characterization and spin Hall device measurement of the Pt/Fe3O4 bilayer. The anisotropic magnetoresistance ratio decreases near TV, while both the spin Hall magnetoresistance ratio and spin mixing conductance (Gr) increase at TV. As the temperature approaches 112 K, the anomalous Hall effect ratio tends to become zero. The maximum Gr reaches ≈5 × 1015 Ω-1m-2 due to the clean and flat interface between Pt and 2D nanosheet. The observed spin transport behavior in Pt/Fe3O4 spin Hall devices indicates that 2D Fe3O4 nanosheets possess potential for high-power micro spintronic storage devices applications.

Room temperature spin injection study across semi-crystalline PVDF-HFP thin films in PVDF-HFP/NiFe bilayers and Ag/(NiFe or Co)/PVDF-HFP/NiFe spin valves.

Spin injection across 160 nm thick semi-crystalline Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) is methodically investigated at room temperature in PVDF-HFP/NiFe bilayers and Ag/(NiFe or Co)/PVDF-HFP/NiFe vertical organic spin valves (OSVs) using both the co-planar waveguide ferromagnetic resonance (CPW-FMR: 7-35 GHz) and magnetoresistance (MR) techniques. The structural and microstructural characteristics of PVDF-HFP reveal the formation of mixed non-ferroelectric alpha and ferroelectric beta phases. The spin injection due to the transfer of angular momentum in PVDF-HFP/NiFe is quantified by measuring the spin-mixing conductance (g↑↓) and the enhancement in Gilbert damping (α) parameters from CPW-FMR data. A significant increase inαof 26% andg↑↓of (2.72 ± 0.45) × 1019m-2highlights the efficient spin injection into the PVDF-HFP spacer layer. Further, the MR in OSV structures reveals a room temperature spin injection with a maximum MR of 0.278 ± 0.006% for Ag/Co/PVDF-HFP/NiFe and 0.349 ± 0.039% for the Ag/NiFe/PVDF-HFP/NiFe devices. Furthermore, the spin injection processes are discussed w.r.t to bias voltages, interfaces and microwave frequencies.

Giant Anisotropic Magnetoresistance in Few-Layer α-RuCl3 Tunnel Junctions.

The spin-orbit-assisted Mott insulator α-RuCl3 is proximate to the coveted quantum spin liquid (QSL) predicted by the Kitaev model. In the search for the pure Kitaev QSL, reducing the dimensionality of this frustrated magnet by exfoliation has been proposed as a way to enhance magnetic fluctuations and Kitaev interactions. Here, we perform angle-dependent tunneling magnetoresistance (TMR) measurements on ultrathin α-RuCl3 crystals with various layer numbers to probe their magnetic, electronic, and crystal structures. We observe a giant change in resistance, as large as ∼2500%, when the magnetic field rotates either within or out of the α-RuCl3 plane, a manifestation of the strongly anisotropic spin interactions in this material. In combination with scanning transmission electron microscopy, this tunneling anisotropic magnetoresistance (TAMR) reveals that few-layer α-RuCl3 crystals remain in the high-temperature monoclinic phase at low temperatures. It also shows the presence of a zigzag antiferromagnetic order below the critical temperature TN ≃ 14 K, which is twice the one typically observed in bulk samples with rhombohedral stacking. Our work offers valuable insights into the relation between the stacking order and magnetic properties of this material, which helps lay the groundwork for creating and electrically probing exotic magnetic phases such as QSLs via van der Waals engineering.

Weak antilocalization in the topological semimetal candidate YbAuSb.

We report a study of the magnetic and magnetotransport properties of YbAuSb single crystals, which were grown using the bismuth flux. The x-ray diffraction data indicate that YbAuSb crystallizes in LiGaGe-type hexagonal structure with space groupP63mc. Our magnetic measurements revealed that YbAuSb is nonmagnetic with a divalent state of ytterbium ion. The temperature-dependent electrical resistivity exhibits a metallic behavior. A cusp-like feature in transverse and longitudinal magnetoresistance is observed at the low field regime. This cusp-like feature is attributed to the weak antilocalization (WAL) effect, which is more prominent at low temperatures. The transverse magnetoconductivity in low field region follows semiclassical model∼B, which is consistent with the presence of WAL phenomena. The WAL effect in transverse and longitudinal magnetoconductance is well explained using the modified Hikami-Larkin-Nagaoka and generalized Altshuler-Aronov model, respectively. The Hall resistivity shows a linear field dependence with a positive slope, suggesting hole charge carriers dominate in electrical transport. The calculated carrier density and mobility are in the order of 1020 cm-3and 102 cm2 V-1 s-1, respectively.

Effects of electronic correlation on topological properties of Kagome semimetal Ni3In2S2.

Kagome metals gain attention as they manifest a spectrum of quantum phenomena such as superconductivity, charge order, frustrated magnetism, and allied correlated states of condensed matter. With regard to electronic band structure, several of them exhibit non-trivial topological characteristics. Here, we present a thorough investigation on the growth and the physical properties of single crystals of Ni3In2S2which is established to be a Dirac nodal line Kagome semimetal. Extensive characterization is attained through temperature and field-dependent resistivity, angle-dependent magnetoresistance (MR) and specific heat measurements. The central question we seek to address is the effect of electronic correlations in suppressing the manifestation of topological characteristics. In most metals, the Fermi liquid behaviour is restricted to a narrow range of temperatures. Here, we show that Ni3In2S2follows the Fermi-liquid behaviour up to 86 K. This phenomenon is further supported by a high Kadowaki-Woods ratio obtained through specific heat analysis. Different interpretations of the magneto-transport study reveal that MR exhibits linear behaviour, suggesting the presence of Dirac fermions at lower temperatures. The angle-dependent magneto-transport study obeys the Voigt-Thomson formula. This, on the contrary, implies the classical origin of MR. Thus, the effect of strong electron correlation in Ni3In2S2manifests itself in the anisotropic magneto-transport. Furthermore, the magnetization measurement shows the presence of de-Haas van Alphen oscillations. Calculations of the Berry phase provide insights into the topological features in the Kagome semimetal Ni3In2S2.

The visible magneto-optical response of RE1-xAxMnO3manganites: relationship with the charge component of the material.

Magnetic circular dichroism (MCD) spectroscopy for manganite films of various compositions and morphologies has been studied in the range of 1.2-3.7 eV. The primary focus was on the temperature behavior of the MCD spectra, as well as the magnetization and resistivity of the films. The data obtained were analyzed in comparison with magneto-optical spectroscopy of the Kerr rotation (KR) on both single crystal and thin film of manganites. It has been established that the MCD response at 2.3 eV is typical for manganites transitioning into a conducting state. Consequently, it reflects a change in the band structure of the material. This response is also observed in the KR spectrum of manganites in the range 2.3-2.6 eV below the metal-insulator transition temperature. These findings complement the understanding of the electronic structure of manganites in general. Moreover, they also provide a basis for the search for new functional materials.

Ultimate Charge Transport Regimes in Doping-Controlled Graphene Laminates: Phonon-Assisted Processes Revealed by the Linear Magnetoresistance.

Understanding and controlling the electrical properties of solution-processed 2D materials is key to further printed electronics progress. Here, we demonstrate that the thermolysis of the aromatic intercalants utilized in nanosheet exfoliation for graphene laminates allows for high intrinsic mobility and the simultaneous control of doping type (n- and p-) and concentration over a wide range. We establish that the intraflake mobility is high by observing a linear magnetoresistance of such solution-processed graphene laminates and using it to devolve the interflake tunneling and intralayer magnetotransport. Consequently, we determine the temperature dependencies of the inter- and intralayer characteristics. The intraflake transport appears to be dominated by electron-phonon scattering processes at temperatures T > 20 K, while the interflake transport is governed by phonon-assisted tunneling. In particular, we identify the efficiency of phonon-assisted tunneling as the main limiting factor for electrical conductivity in graphene laminates at room temperature. We also demonstrate a thermoelectric sensitivity of around 50 µV·K-1 in a solution-processed metal-free graphene-based thermocouple.

Improvement in CPP-GMR read head sensor performance using [001]-oriented polycrystalline half-metallic Heusler alloy Co2FeGa0.5Ge0.5 and CoFe bilayer electrode.

A current-perpendicular-to-plane giant magnetoresistive (CPP-GMR) device with a half-metallic electrode is one of the most promising candidates of next-generation read head for hard disk drive. In this study, we fabricate [001]-oriented polycrystalline CPP-GMR devices with the normal ferromagnet (NFM) CoFe/half-metallic ferromagnet (HMFM) Co2FeGa0.5Ge0.5 (CFGG) bilayer electrodes to enhance the magnetoresistance (MR) ratio by large interfacial spin-dependent scattering at the NFM/HMFM interface. The CoFe/CFGG bilayer electrode provides the additional large interfacial spin-dependent scattering and achieves high MR ratio of 22.7% with the CoFe(4.5 nm)/CFGG(2.5 nm) bilayer electrodes, which is almost three(two) times larger than the MR ratio with the single CoFe(CFGG) (7 nm) electrodes. The bias voltage dependent study revealed an additional advantage of increasing the output voltage |ΔV| by using the CoFe/CFGG bilayer due to the improvement of the endurance against spin-transfer torque under high bias current. A maximum output voltage Δ V max of 6.5 mV was obtained with the CoFe(5.5 nm)/CFGG(1.5 nm) electrodes, which is the highest ever reported in the CPP-GMR devices with a uniform metallic spacer including high-quality epitaxial devices.

Large improvement of MR ratio and the highest output voltage has been achieved in the poly-crystalline CPP-GMR with the half-metallic Co₂FeGa_0.5Ge_0.5 and normal ferromagnetic CoFe bilayer electrodes.

High-Resolution Rotation-Measuring System for MEMS Ultrasonic Motors Using Tunneling Magnetoresistance Sensors.

This study proposes a high-resolution rotation-measuring system for miniaturized MEMS ultrasonic motors using tunneling magnetoresistance (TMR) sensors for the first time. Initially, the architecture and principle of the rotation-measuring system are described in detail. Then, the finite element simulation is implemented to determine the miniaturized permanent magnet's residual magnetization, dimensions, and TMR sensor position. Finally, the experiments are implemented to evaluate the performance. Using calibration based on a high-precision servo motor, it is found that the relationship between the output and rotational angle is highly linear and immune to the rotor's out-of-plane movement. Meanwhile, the angle-detecting resolution is higher than 0.1°. After the calibration, the continuous rotation of the MEMS ultrasonic motor is tested. It is found that the angle testing result varies with a period close to 360°, which indicates that the rotation-measuring system has successfully detected the motor's rotation.

Observation of Moment-Dependent and Field-Driven Unidirectional Magnetoresistance in CoFeB/InSb/CdTe Heterostructures.

Magnetoresistance effects are crucial for understanding the charge-spin transport as well as propelling the advancement of spintronic applications. Here, we report the coexistence of magnetic-moment-dependent (MD) and magnetic-field-driven (FD) unidirectional magnetoresistance (UMR) effects in CoFeB/InSb/CdTe heterostructures. The strong spin-orbital coupling of InSb and the matched impedance at the CoFeB/InSb interface warrant a distinct MD-UMR effect at room temperature, while the interaction between the in-plane magnetic field and the Rashba effect at the InSb/CdTe interface induces the marked FD-UMR signal that dominates the high-field region. Moreover, owning to different spin scattering mechanisms, these two types of non-reciprocal charge transports show opposite polarities with respect to the magnetic field direction, which further enables an effective phase modulation of the angular-dependent magnetoresistance. The demonstration of the tunable UMR response validates our CoFeB/InSb/CdTe system as a suitable integrated building block for multifunctional spintronic memory and sensor designs.

Daily Fungal Cell-Free DNA Testing to Assess Clinical Status during Candida krusei Fungemia.

We present a case of a man immunocompromised due to myelodysplastic syndrome with Candida krusei fungemia who had a rising cell-free DNA (cfDNA) giant magnetoresistance (GMR) signal when tested daily using plasma blood samples. With the rise in GMR signal paralleling the development of skin lesions in this patient, we conclude that cfDNA can be used to indicate uncontrolled infection and thus help monitor response to therapy. This index patient provides evidence that an invasive fungal infection requires both direct antifungal therapy and an intact immune system to control the infection. This biosensing platform has been simplified to potentially serve as a point-of-care test, setting it apart by overcoming the three common barriers of cfDNA testing: complexity, cost, and time.

Quantum Transport through a Quantum Dot Coupled to Majorana Nanowire and Two Ferromagnets with Noncollinear Magnetizations.

We study the electron tunneling (ET) and local Andreev reflection (AR) processes in a quantum dot (QD) coupled to the left and right ferromagnetic leads with noncollinear ferromagnetisms. In particular, we consider that the QD is also side-coupled to a nanowire hosting Majorana bound states (MBSs) at its ends. Our results show that when one mode of the MBSs is coupled simultaneously to both spin-up and spin-down electrons on the QD, the height of the central peak is different from that if the MBS is coupled to only one spin component electrons. The ET and AR conductances, which are mediated by the dot-MBS hybridization, strongly depend on the angle between the left and right magnetic moments in the leads. Interaction between the QD and the MBSs will result in sign change of the angle-dependent tunnel magnetoresistance. This is very different from the case when the QD is coupled to regular fermonic mode, and can be used for detecting the existence of MBSs, a current challenge in condensed matter physics under extensive investigations.

Half-Metallic Transport and Spin-Polarized Tunneling through the van der Waals Ferromagnet Fe4GeTe2.

We examine the coherent spin-dependent transport properties of the van der Waals (vdW) ferromagnet Fe4GeTe2 using density functional theory combined with the nonequilibrium Green's function method. Our findings reveal that the conductance perpendicular to the layers is half-metallic, meaning that it is almost entirely spin-polarized. This property persists from the bulk to a single layer, even under significant bias voltages and with spin-orbit coupling. Additionally, using dynamical mean field theory for quantum transport, we demonstrate that electron correlations are important for magnetic properties but minimally impact the conductance, preserving almost perfect spin-polarization. Motivated by these results, we then study the tunnel magnetoresistance (TMR) in a magnetic tunnel junction consisting of two Fe4GeTe2 layers with the vdW gap acting as an insulating barrier. We predict a TMR ratio of ∼500%, which can be further enhanced by increasing the number of Fe4GeTe2 layers in the junction.

Optimization of Magnetic Tunnel Junction Structure through Component Analysis and Deposition Parameters Adjustment.

In this work, we focus on a detailed study of the role of each component layer in the multilayer structure of a magnetic tunnel junction (MTJ) as well as the analysis of the effects that the deposition parameters of the thin films have on the performance of the structure. Various techniques including atomic force microscopy (AFM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were used to investigate the effects of deposition parameters on the surface roughness and thickness of individual layers within the MTJ structure. Furthermore, this study investigates the influence of thin films thickness on the magnetoresistive properties of the MTJ structure, focusing on the free ferromagnetic layer and the barrier layer (MgO). Through systematic analysis and optimization of the deposition parameters, this study demonstrates a significant improvement in the tunnel magnetoresistance (TMR) of the MTJ structure of 10% on average, highlighting the importance of precise control over thin films properties for enhancing device performance.

Development and Comprehensive Evaluation of TMR Sensor-Based Magnetrodes.

Due to their compact size and exceptional sensitivity at room temperature, magnetoresistance (MR) sensors have garnered considerable interest in numerous fields, particularly in the detection of weak magnetic signals in biological systems. The "magnetrodes", integrating MR sensors with needle-shaped Si-based substrates, are designed to be inserted into the brain for local magnetic field detection. Although recent research has predominantly focused on giant magnetoresistance (GMR) sensors, tunnel magnetoresistance (TMR) sensors exhibit a significantly higher sensitivity. In this study, we introduce TMR-based magnetrodes featuring TMR sensors at both the tip and midsection of the probe, enabling detection of local magnetic fields at varied spatial positions. To enhance detectivity, we designed and fabricated magnetrodes with varied aspect ratios of the free layer, incorporating diverse junction shapes, quantities, and serial arrangements. Utilizing a custom-built magnetotransport and noise measurement system for characterization, our TMR-based magnetrode demonstrates a limit of detection (LOD) of 300pT/Hz at 1 kHz. This implies that neuronal spikes can be distinguished with minimal averaging, thereby facilitating the elucidation of their magnetic properties.

Synthesis of Few-Layer Hexagonal Boron Nitride for Magnetic Tunnel Junction Application.

Hexagonal boron nitride (hBN), a wide-gap two-dimensional (2D) insulator, is an ideal tunneling barrier for many applications because of the atomically flat surface, high crystalline quality, and high stability. Few-layer hBN with a thickness of 1-2 nm is an effective barrier for electron tunneling, but the preparation of few-layer hBN relies on mechanical exfoliation from bulk hBN crystals. Here, we report the large-area growth of few-layer hBN by chemical vapor deposition on ferromagnetic Ni-Fe thin films and its application to tunnel barriers of magnetic tunnel junction (MTJ) devices. Few-layer hBN sheets mainly consisting of two to three layers have been successfully synthesized on a Ni-Fe catalyst at a high growth temperature of 1200 °C. The MTJ devices were fabricated on as-grown hBN by using the Ni-Fe film as the bottom ferromagnetic electrode to avoid contamination and surface oxidation. We found that trilayer hBN gives a higher tunneling magnetoresistance (TMR) ratio than bilayer hBN, resulting in a high TMR ratio up to 10% at ∼10 K.

Voltage-Driven Fluorine Motion for Novel Organic Spintronic Memristor.

Integrating tunneling magnetoresistance (TMR) effect in memristors is a long-term aspiration because it allows to realize multifunctional devices, such as multi-state memory and tunable plasticity for synaptic function. However, the reported TMR in different multiferroic tunnel junctions is limited to 100%. This work demonstrates a giant TMR of -266% in La0.6Sr0.4MnO3(LSMO)/poly(vinylidene fluoride)(PVDF)/Co memristor with thin organic barrier. Different from the ferroelectricity-based memristors, this work discovers that the voltage-driven florine (F) motion in the junction generates a huge reversible resistivity change up to 106% with nanosecond (ns) timescale. Removing F from PVDF layer suppresses the dipole field in the tunneling barrier, thereby significantly enhances the TMR. Furthermore, the TMR can be tuned by different polarizing voltage due to the strong modification of spin-polarization at the LSMO/PVDF interface upon F doping. Combining of high TMR in the organic memristor paves the way to develop high-performance multifunctional devices for storage and neuromorphic applications.

Observation of Linear Magnetoresistance in MoO2.

Magnetoresistance, the change in resistance with applied magnetic fields, is crucial to the magnetic sensor technology. Linear magnetoresistance has been intensively studied in semimetals and semiconductors. However, the air-stable oxides with a large linear magnetoresistance are highly desirable but remain to be fully explored. In this paper, we report the direct observation of linear magnetoresistance in polycrystalline MoO2 without any sign of saturation up to 7 T under 50 K. Interestingly, the linear magnetoresistance reaches as large as 1500% under 7 T at 2 K. The linear field dependence is in great contrast to the parabolic behavior observed in single-crystal MoO2, probably due to phonon scattering near the grain boundaries. Our results pave the way to comprehending magneto-transport behavior in oxides and their potential applications in magnetic sensors.