Magnetic Multilayer Structures Research Articles

Magnetic soliton-based LIF neurons for spiking neural networks (SNNs) in multilayer spintronic devices

Neuromorphic computing, inspired by biological nervous systems, is gaining traction due to its advantages in latency, energy efficiency, and algorithmic complexity compared to traditional artificial neural networks. This has spurred research into artificial synapses and neurons that replicate brain functions. Spintronic-based technologies, particularly domain walls (DWs) and skyrmions (SKs), have shown remarkable potential for brain-inspired computing, facilitating energy-efficient data storage and advancing beyond CMOS computing architectures. Researchers have proposed various DWs- and Sks-based neuromorphic architectures for neurons and synapses. Leveraging magnetic multilayer structures, we propose a magnetic soliton that incorporates both DWs- and Sks-based magnetic tunnel junction (MTJ) device structures to emulate leaky integrate-and-fire (LIF) characteristics. These characteristics are controlled by spin–orbit torque (SOT)-driven motion within ferromagnetic thin films. By strategically placing the reading block and utilizing a combination of SOT and varying demagnetization energy, we achieve modified LIF neuron characteristics in both DW and Sks MTJ devices. The co-action of soliton dynamics across the nanotrack during the application of the current pulse, along with edge repulsion and variations in demagnetization energy, exploits LIF spiking behavior. Theoretical and micromagnetic analyses reveal that the transitory tunable positions of Sks and the total magnetization of the free layer for DWs mimic the membrane potential of biological neurons. Initial studies on multilayer DW-based LIF characteristics showed promise; however, maintaining leaky behavior required a constant negative current, which is energy inefficient. By incorporating the non-volatile properties of skyrmions and adding a chiral Dzyaloshinskii–Moriya interaction term, we further explored LIF dynamics, yielding encouraging results. Our proposed neuron model, implemented in fully connected and convolutional layers, achieves over 95% classification accuracy on the MNIST and Fashion MNIST datasets using a modified spike-based backpropagation method. With nanosecond latency, these spiking neuron devices, when integrated with CMOS, pave the way for high-density, energy-efficient neuromorphic computing hardware.

Effect of metal insertion on the magnetic properties and anomalous Hall effect in MgO/CoFeB/Ta/MgO films

CoFeB-based nano-magnetic multilayers can be applied in magnetic sensors, magnetic random access memory and other logic devices, which have attracted wide attention. The magnetic properties and interface structure of the magnetic multilayers still need further research for future applications. The effects of Pt insertion on the perpendicular magnetic anisotropy (PMA) and the anomalous Hall effect (AHE) of MgO/CoFeB/Pt/Ta/MgO multilayers were studied. It is found that the sample with 0.2 nm Pt insertion at the CoFeB/Ta interface and annealed at 250 °C can transform the films from in-plane magnetic anisotropy to PMA. The effective magnetic anisotropy K eff can reach 1.82 × 106 erg cm−3. The Pt insertion and annealing heat treatment can promote the migration of oxygen from MgO at the top layer to CoFeB/Ta interface and combine with Fe to generate iron oxide, and improve the orbital hybridization of Fe 3d and O 2p, thus successfully inducing the PMA of the film samples. In addition, Pt insertion and annealing treatment can improve the side-jump mechanism and the skew scattering mechanism which contribute to the AHE. This work provides guidance for the application of PMA materials in magnetic sensor and memory devices.

Influence of the Effects of Competition of Various Types of Anisotropy on the Critical Behavior of Magnetic Multilayer Structures

Influence of the Effects of Competition of Various Types of Anisotropy on the Critical Behavior of Magnetic Multilayer Structures

Exploring the Magnetic Properties of Individual Barcode Nanowires using Wide-Field Diamond Microscopy.

A barcode magnetic nanowire typically comprises a multilayer magnetic structure in a single body with more than one segment type. Interestingly, due to selective functionalization and novel interactions between the layers, it has attracted significant attention, particularly in bioengineering. However, analyzing the magnetic properties at the individual nanowire level remains challenging. Herein, the characterization of a single magnetic nanowire is investigated at room temperature under ambient conditions based on magnetic images obtained via wide-field quantum microscopy with nitrogen-vacancy centers in diamond. Consequently, critical magnetic properties of a single nanowire can be extracted, such as saturation magnetization and coercivity, by comparing the experimental result with that of micromagnetic simulation. This study opens up the possibility for a versatile in situ characterization method suited to individual magnetic nanowires.

Giant magnetoresistance (GMR) spin-valve based magnetic sensor with linear and bipolar characteristics for low current detection

High sensitivity linear, bipolar, and hysteresis-free magnetic field sensor is necessary for low current measurement of the order of 1000 mA with a resolution of 1 mA. Magnetoresistive technology has evolved into popular magnetic sensing technology where magnetic multilayered structures were tailored to obtain linear output characteristics. In this report, a GMR spin-valve structure was used to develop magnetic field sensing elements. Mechanical assembly of the sensing elements in the form of a Wheatstone bridge configuration was made to have the linear and bipolar characteristics of the sensor. Application of the constant DC biasing field perpendicular to the sensitivity axis was made to have fully hysteresis-free output characteristics and the maximum sensitivity was achieved of the order 2 to 2.5 mV/G with a supply of 1 mA current to the bridge circuit. The in-house developed spin valve sensor was then used to measure the current in the range of ± 1 A, having a sensitivity of the order of 2 mV/mA with a resolution down to 1 mA by using non-intrusive current sensing technology. A core-based sensor has a large aperture size that accommodates a current-carrying conductor with an ampacity of 1 kA. The calibrated output shows the sensor can measure the field in the range of ± 0.4 mT with a resolution of ∼ 200 nT and has the potential for floating current measurement application in the battery management system.

Fe-Mn-xCr-Al-Si steel with multi-layer magnetic structure fabricated by oxidation treatment

Fe-Mn-xCr-Al-Si steel with multi-layer magnetic structure fabricated by oxidation treatment

Coupling of terahertz light with nanometre-wavelength magnon modes via spin–orbit torque

Spin-based technologies can operate at terahertz frequencies but require manipulation techniques that work at ultrafast timescales to become practical. For instance, devices based on spin waves, also known as magnons, require efficient generation of high-energy exchange spin waves at nanometre wavelengths. To achieve this, a substantial coupling is needed between the magnon modes and an electro-magnetic stimulus such as a coherent terahertz field pulse. However, it has been difficult to excite non-uniform spin waves efficiently using terahertz light because of the large momentum mismatch between the submillimetre-wave radiation and the nanometre-sized spin waves. Here we improve the light–matter interaction by engineering thin films to exploit relativistic spin–orbit torques that are confined to the interfaces of heavy metal/ferromagnet heterostructures. We are able to excite spin-wave modes with frequencies of up to 0.6 THz and wavelengths as short as 6 nm using broadband terahertz radiation. Numerical simulations demonstrate that the coupling of terahertz light to exchange-dominated magnons originates solely from interfacial spin–orbit torques. Our results are of general applicability to other magnetic multilayered structures, and offer the prospect of nanoscale control of high-frequency signals.

A novel system for measuring effectiveness of magnetic shields in a liquid helium Dewar with a fluxgate magnetometer

A novel system which is specially designed for measuring effectiveness of small-scale magnetic shields in a liquid helium Dewar is introduced in this paper. This system is mainly applied to test the shielding performance of multilayer magnetic shielding structure which is used in the field of single-flux-quantum (SFQ) electronics testing at liquid helium temperature. This system allows using a fluxgate magnetometer to measure the magnetic flux instead of the superconducting quantum interference device (SQUID) method. The fluxgate magnetometer can be put into the system through a circular channel which is connected with the ambient environment. Thus, both the shielding effectiveness and the absolute magnetic flux can be obtained by this system. In addition, this system is designed with the shielding of the interference of environmental magnetic field, which improves the measurement accuracy. By utilizing this system, the magnetic shield effectiveness of a four-layer magnetic shields manufactured with low-temperature permalloy material and superconducting material was measured. The experimental results shows that the axial shielding effectiveness (SE) is 58.3 dB. The comparison between the experimental and theoretical results verified the effectiveness of this system.

Giant Magnetoresistance Biosensors in Biomedical Applications.

The giant magnetoresistance (GMR) effect has seen flourishing development from theory to application in the last three decades since its discovery in 1988. Nowadays, commercial devices based on the GMR effect, such as hard-disk drives, biosensors, magnetic field sensors, microelectromechanical systems (MEMS), etc., are available in the market, by virtue of the advances in state-of-the-art thin-film deposition and micro- and nanofabrication techniques. Different types of GMR biosensor arrays with superior sensitivity and robustness are available at a lower cost for a wide variety of biomedical applications. In this paper, we review the recent advances in GMR-based biomedical applications including disease diagnosis, genotyping, food and drug regulation, brain and cardiac mapping, etc. The GMR magnetic multilayer structure, spin valve, and magnetic granular structure, as well as fundamental theories of the GMR effect, are introduced at first. The emerging topic of flexible GMR for wearable biosensing is also included. Different GMR pattern designs, sensor surface functionalization, bioassay strategies, and on-chip accessories for improved GMR performances are reviewed. It is foreseen that combined with the state-of-the-art complementary metal-oxide-semiconductor (CMOS) electronics, GMR biosensors hold great promise in biomedicine, particularly for point-of-care (POC) disease diagnosis and wearable devices for real-time health monitoring.

Design and Shimming Method of Low Length-to-Interdiameter Ratio Halbach Magnet

The conventional Halbach permanent magnet usually has a high length to inter diameter ratio to gain a higher magnetic field homogeneity but can cause an increase in volume and weight, limiting the application in miniaturization. This paper proposes a novel design and shimming method for the Halbach magnet to have not only a lower length to inter diameter ratio but also a better homogeneity. In this paper, a multi-layer magnetic ring structure was first designed through theoretical calculation and simulation optimization, which compensated for the magnetic field inhomogeneity. Then, a passive shimming method was applied which simulated the first-order and second-order terms of magnetic field inhomogeneity using small magnetic blocks whose magnetization direction was consistent with the main magnetic field. This method has high shimming efficiency and can avoid the demagnetization of the magnetic blocks caused by their magnetization directions against the direction of the main magnetic field, especially when the intensity of the magnetic field is close to or beyond the remanence of the magnetic blocks. The magnetic field intensity of the proposed magnet is 1.08 T and the homogeneity reaches close to the sub-ppm. The performance of the magnet was verified in the portable MRI system combined with a set of homemade low-power gradient coils. MRI images of phantoms and okra reached a resolution of 156 μm (pixel size 156 μm, matrix 128 × 128, FOV 20 × 20 mm2), which shows the potential for wide application of the proposed design and shimming method.

Multilayer magnetic shielding for testing a superconducting single-flux-quantum circuit chip in liquid helium dewar

In order to reduce the influence of environmental magnetic field on the test of superconducting single-flux-quantum (SFQ) circuit chip in a liquid helium dewar, a multilayer magnetic shielding structure is designed to provide a clean magnetic field lower than 50 nT and achieve a shielding effectiveness higher than 60 dB. The magnetic shielding structure consists of three cylindrical layers with uniform 1 mm thickness. The first inner layer is made of NbTi which works as a strong diamagnet shell, while the outer two layers are made of permalloy which has high permeability at helium temperature. The superconducting quantum interference device (SQUID) was used to test the shielding performance of the structure under low frequency (0.1–100 Hz) magnetic interference at helium temperature. The results show that the radial and axial shielding effectiveness (SE) are about 80 dB and 68 dB, respectively, indicating that the multilayer structure has good shielding performance and can be used in the testing system of superconducting single-flux-quantum circuit chip.

Magnetic anisotropy in Co/phosphorene heterostructure

Magnetic anisotropy (MA) in magnetic multilayer structures plays a critical role in the design of future spintronic and magnetic devices. This property is more important for structures with a thickness of few angstroms, such as few-layer two-dimensional materials. Here, we determined the variation in the interface MA in mono- and bi-layer Co/phosphorene heterostructures via first-principles density functional theory calculations. We found that the MA of these structures was comparable to those of ferromagnet/heavy-metal interfaces, despite the small spin-orbit coupling strength of phosphorus atoms. The hybridization of the metal d and the phosphorene p orbitals is well manifested in the projected density of states of these systems. The strain dependence of the magnetic anisotropy energy (MAE) in bilayer Co/phosphorene heterostructures under uniaxial strains along zigzag and armchair directions of the phosphorene layer was investigated, indicating that the MAE of the system increases by more than 80% by applying the strain. More specifically, by utilizing the uniaxial tensile strain along the zigzag direction of this structure, we found that MA could vary from perpendicular to in-plane directions. Our findings indicate that interlayer hybrid bonds in two-dimensional layered materials can be a promising approach to tailoring the MA property.

The new features of generalized order parameter localization in multi-layered magnetic structures

The new types of nonlinear localized state of generalized order parameter in the three-layered structure of band antiferromagnets with congruent sections of the Fermi surface are obtained. The nonlinear effects inside the narrow plane defects between the layers representing the local magnetization in the direction of polarization of the spin density wave are described by the nonlinear source function. The variation of the Ginsburg–Landau functional near the Lifshitz point leads to nonlinear Schrödinger equation (NSE) with cubic nonlinear terms and self-consistent nonlinear potential modeling the plane defects. The dispersion equations of two type stationary nonlinear localized states are derived. The solutions of dispersion equations in explicit analytical form in dependence on characteristics of plane defects and the distance between them are found and analyzed. The localized states of order parameter existing only near the plane defects with nonlinear response are obtained.

MXene-based accordion 2D hybrid structure with Co9S8/C/Ti3C2Tx as efficient electromagnetic wave absorber

Owing to their unique structure and high dielectric loss, two-dimensional transition metal carbides and nitrides (MXenes) are widely used in electromagnetic wave absorption. The novel Co9S8/C/Ti3C2Tx hybrid material with a multilayer structure was successfully fabricated by hydrothermal and annealing processes. The unique multilayered hybrid material is assembled from Ti3C2Tx film layers, Co9S8 particles, and carbon flakes. The Co9S8 particles were uniformly anchored on Ti3C2Tx and carbon flakes. The Co9S8/C/Ti3C2Tx-100 hybrid material has demonstrated impressive electromagnetic absorption performance after the multi-component introduction and interface design were optimized. Besides, the hybrid material exhibits the maximum reflection loss of −50.07 dB at 7.6 GHz under a matched thickness of 2.51 mm, and the effective absorption bandwidth (RL ≤ -10 dB) is covered 4.24 GHz. The synergistic effect of dielectric and magnetic loss, interfacial polarization, and multilayer structure was beneficial to the improvement of electromagnetic wave absorption performance. This research provides a simple demonstration of the design and synthesis of high-performance MXenes-based absorbing materials.

Non-equilibrium phenomena in magnetic multilayer nanostructures and aging in magnetoresistance

A Monte Carlo simulation of the non-equilibrium behavior of multilayer magnetic structures Co/Cu(100)/Co and Pt/Co/Cu(100)/Co/Pt characterizing different types of magnetic anisotropy is realized. Simulation of transport properties gives possibility to reveal a nontrivial aging effects in the magnetoresistance of these structures and influence of initial states on two-time dependence of magnetoresistance.

High-density N\xe9el-type magnetic skyrmion phase stabilized at high temperature

The discovery of a thermally stable, high-density magnetic skyrmion phase is a key prerequisite for realizing practical skyrmionic memory devices. In contrast to the typical low-density Néel-type skyrmions observed in technologically viable multilayer systems, with Lorentz transmission electron microscopy, we report the discovery of a high-density homochiral Néel-type skyrmion phase in magnetic multilayer structures that is stable at high temperatures up to 733 K (≈460 °C). Micromagnetic simulations reveal that a high-density skyrmion phase can be stabilized at high temperature by deliberately tuning the magnetic anisotropy, magnetic field, and temperature. The existence of the high-density skyrmion phase in a magnetic multilayer system raises the possibility of incorporating chiral Néel-type skyrmions in ultrahigh-density spin memory devices. Moreover, the existence of this phase at high temperature shows its thermal stability, demonstrating the potential for skyrmion devices operating in thermally challenging modern electronic chips.

Synthetic chiral magnets promoted by the Dzyaloshinskii–Moriya interaction

The ability to engineer the interactions in assemblies of nanoscale magnets is central to the development of artificial spin systems and spintronic technologies. Following the emergence of the Dzyaloshinskii–Moriya interaction (DMI) in thin film magnetism, new routes have been opened to couple the nanomagnets via strong chiral interactions, which is complementary to the established dipolar and exchange coupling mechanisms. In this Perspective, we review recent progress in the engineering of synthetic magnets coupled by the interlayer and intralayer DMI. We show how multilayer chiral magnetic structures and two-dimensional synthetic antiferromagnets, skyrmions, and artificial spin systems can be realized by simultaneous control of the DMI and magnetic anisotropy. In addition, we show that, with the combination of DMI and current-induced spin–orbit torques, field-free switching of synthetic magnetic elements is obtained as well as all-electric domain wall logic circuits.

INFLUENCE OF INITIAL STATES ON TWO-TIME DEPENDENCE OF MAGNETORESISANCE
 IN MULTILAYER STRUCTURES WITH DIFFERENT TYPES OF ANISOTROPY

The Monte Carlo study of initial states influence on non-equilibrium slow dynamics and two-time dependence of magnetoresistance in multilayer magnetic structures Co/Cu(100)/Co и Pt/Co/Cu(100)/Co/Pt with different types of anisotropy is presented. It was revealed nontrivial aging effects in the magnetoresistance and essential influence of initial states on the magnetoresistance.

Detection of electron-phonon coupling in two-dimensional materials by light scattering

Electron-phonon coupling affects the properties of two-dimensional (2D) materials significantly, leading to a series of novel phenomena. Inelastic light scattering provides a powerful experimental tool to explore electron-phonon interaction in 2D materials. This review gives an overview of the basic theory and experimental advances of electron-phonon coupling in 2D materials detected by Raman and Brillouin scattering, respectively. In the Raman scattering part, we review Raman spectroscopy studies of electron-phonon coupling in graphene, transition metal disulfide compounds, van der Waals heterostructures, strongly correlated systems, and 2D magnetic materials. In the Brillouin scattering part, we extensively introduce Brillouin spectroscopy in non-van der Waals 2D structures, including temperature sensors for phonons and magnons, interfacial Dzyaloshinsky-Moriya interaction and spin torque in multilayer magnetic structures, as well as exciton-polariton in semiconductor quantum well.

Determination of fine magnetic structure of magnetic multilayer with quasi antiferromagnetic layer by using polarized neutron reflectivity analysis

We carried out polarized neutron reflectivity (PNR) analysis to determine the fine magnetic structure of magnetic multilayers with quasi-antiferromagnetic (quasi-AFM) layers realized by 90-deg coupling using two Co90Fe10 layers, and quantitatively evaluated the magnetization of quasi-AFM layers. Two types of samples with different buffer layers, Ru buffer and a NiFeCr buffer, were investigated and the average angles between the respective magnetization of the two Co90Fe10 layers were estimated to be +/− 39 degrees and +/− 53 degrees. In addition, less roughness was found in the NiFeCr buffer sample resulting stronger 90-deg coupling. A perfect quasi-AFM is expected to be realized by a flat interface of the magnetic multilayer.