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Spin-based neuromorphic computing (NC) is investigated as a power-efficient enabler for artificial intelligence [1-3]. Hence, magnetic tunnel junction (MTJ) based domain wall devices are investigated as synthetic synapses and neurons to mimic their biological counterparts [2-3]. While a biological neuron resets itself after firing, such a function has not been demonstrated experimentally in spintronic systems [3]. The self-reset feature of neurons reduces the power consumption and the complexity of NC architecture. In this work, we present a domain wall (DW) based artificial neuron that automatically resets without applying a reset pulse.&lt;br/&gt;At first, we simulated a synthetic antiferromagnetic (SAF) DW device where the free layer (CoFeB) is driven by spin-transfer-torque (STT) to achieve integrate function. Leakage function was achieved when the antiferromagnetic coupling (AFC) between the free and pinned layers pushes the DW towards the starting position automatically (Fig. 1). The left quarter region of the free layer is ferromagnetically coupled (FC) to the pinned layer so that as DW returns (during reset), it is stabilized at the initial position (Fig. 2(a) and Fig. 2(b)). Experimentally, the integrate and leakage functions are also achieved as illustrated in Fig. 2(d) and Fig. 2(e), suggesting that an SAF-based neuron is viable for NC.

![](https://assets.underline.io/uploads/markdown_image/1/image/4f911e30d06059db93a1e69c895660a8.jpg)

Fig.1 Proposed design of the artificial neuron. Domain wall is moved forward by STT and (after firing), reset backward by SAF coupling.


![](https://assets.underline.io/uploads/markdown_image/1/image/c297c876d46646d24593590c9f897b4d.jpg)

Fig. 2 (a and b) DW position (free layer) vs time graphs for different AFC strength in the simulations. (c, d and e) Kerr images of the fabricated SAF neuron device with magnetic field in the -z direction. (c) No current applied and device is in antiparallel configuration, (d) current is applied, pushing the DW and achieving the integrate function, and (e) current is removed and the device automatically resets, indicating the leakage function.

2022 Joint MMM-INTERMAG

Synthetic antiferromagnetic domain wall device as artificial neuron for neuromorphic computing

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Spin-based neuromorphic computing (NC) is investigated as a power-efficient enabler for artificial intelligence [1-3]. Hence, magnetic tunnel junction (MTJ) based domain wall devices are investigated as synthetic synapses and neurons to mimic their biological counterparts [2-3]. While a biological neuron resets itself after firing, such a function has not been demonstrated experimentally in spintronic systems [3]. The self-reset feature of neurons reduces the power consumption and the complexity of NC architecture. In this work, we present a domain wall (DW) based artificial neuron that automatically resets without applying a reset pulse. At first, we simulated a synthetic antiferromagnetic (SAF) DW device where the free layer (CoFeB) is driven by spin-transfer-torque (STT) to achieve integrate function. Leakage function was achieved when the antiferromagnetic coupling (AFC) between the free and pinned layers pushes the DW towards the starting position automatically (Fig. 1). The left quarter region of the free layer is ferromagnetically coupled (FC) to the pinned layer so that as DW returns (during reset), it is stabilized at the initial position (Fig. 2(a) and Fig. 2(b)). Experimentally, the integrate and leakage functions are also achieved as illustrated in Fig. 2(d) and Fig. 2(e), suggesting that an SAF-based neuron is viable for NC.

![](https://assets.underline.io/uploads/markdown_image/1/image/4f911e30d06059db93a1e69c895660a8.jpg)

Fig.1 Proposed design of the artificial neuron. Domain wall is moved forward by STT and (after firing), reset backward by SAF coupling.


![](https://assets.underline.io/uploads/markdown_image/1/image/c297c876d46646d24593590c9f897b4d.jpg)

Fig. 2 (a and b) DW position (free layer) vs time graphs for different AFC strength in the simulations. (c, d and e) Kerr images of the fabricated SAF neuron device with magnetic field in the -z direction. (c) No current applied and device is in antiparallel configuration, (d) current is applied, pushing the DW and achieving the integrate function, and (e) current is removed and the device automatically resets, indicating the leakage function.

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The 15th Joint MMM-INTERMAG Conference (2022 Joint) is sponsored jointly by AIP Publishing and the IEEE Magnetics Society. Members of the international scientific and engineering communities interested in recent developments in fundamental and applied magnetism are invited to attend and contribute to the technical sessions. The technical program will include invited and contributed papers in oral and poster sessions, invited symposia, a plenary session, and an evening session, with about 1500 presentations overall. This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

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Controlling and switching magnetization by the process of current-induced torque has a considerable advantage in low power consumption for a memory device and spin logic device applications. For a heavy metal (HM) / ferromagnet(FM) / heavy metal (HM) based perpendicular magnetic anisotropy system with broken normal space inversion symmetry (SIA), current-induced magnetization reversal (CIMR) takes place using spin-orbit torque (SOT) with the assistance of an in-plane (IP) magnetic field. For a similar HM/FM/HM-based PMA system having broken lateral symmetry, also known as the quasi-PMA system, the CIMR occurs without the assistance of the IP field. The lateral SIA [1-4] helps in the CIMR process by producing an effective out-of-plane (OOP) field which does not depend on the direction of the IP field and initial magnetization. Thus, the quasi-PMA system has a massive significance for magnetic field-free CIMR. Pt/CoFeB/Pt based quasi-PMA heterostructure has been chosen for CIMR study with the optimal bottom layer Pt thickness for which the effect of lateral SIA is maximum and consists of a ferromagnetic layer with in-plane magnetic anisotropy (IMA) at the top layer of the heterostructure. The IMA ferromagnetic layer [5] contributes to the CIMR process by producing the stray field. Thus, by designing a heterostructure to combine the SOT originated by spin-Hall effect, the stray field by IMA ferromagnetic layer, and the lateral SIA effect, we have enhanced the SOT efficiency of the system to reduce the critical current density (Jc) for CIMR both under the absence and the presence of the different IP fields. The switching phase diagram (SPD) for this device has been shown in Fig. 1. We have achieved Jc as low as (8.65 ± 0.20) x 1010 A/m2 and (3.35 ± 0.17) x 1010 A/m2 at zero bias and 60 mT, respectively. The device has massive potential in low-power magnetic data storage application.

![](https://assets.underline.io/uploads/markdown_image/1/image/d095b538536a799fbce2047bef1a8a64.jpg)

Fig.1. Current-induced switching phase diagram for the device Ta(2)/Pt(3)/CoFeB(0.6)/Pt(0.5)/Ta(1.5)/CoFeB(2.5)/Pt(1)

Improving the spin-orbit torque efficiency in Pt/CoFeB/Pt based perpendicularly magnetized system to decrease the critical current density for current-induced magnetization switching

Application of electric fields (E-fields) on perpendicular magnetic tunnel junctions (p-MTJs) is a promising solution for improving energy efficiency in spintronics based memory and logic systems [1]. Many studies on E-field effects on p-MTJs have focused on voltage-controlled magnetic anisotropy (VCMA) of the p-MTJs free layer to reduce the energy required for p-MTJs to switch magnetization states [2]. In this approach, VCMA temporarily decreases the energy barrier separating the two magnetization states and bidirectional switching is achieved using either an external magnetic field or a spin-polarized current. Another approach is to utilize voltage controlled exchange coupling (VCEC) between two ferromagnetic layers in synthetic antiferromagnetic (SAF) structures. Recently, bidirectional switching through VCEC has been demonstrated on p-MTJs with bottom dual-SAF free layers [3]. Benchmarking on MRAM technologies has shown VCEC-MRAM to have much lower energy dissipation than spin-orbit torque (SOT) MRAM [4]. However, SOT-MRAM has faster write speeds since SOT allows for ultra-fast (≤ 400ps) switching [5]. In this work, we explore the potential for SOT-assisted VCEC switching on p-MTJs with bottom dual-SAF free layers fabricated on bilayer spin hall channels. Fig. 1 shows that the SOT current can tune the coercivity of the p-MTJ, which illustrates that SOT can assist in VCEC switching. Fig. 2 shows that a voltage applied to the MTJ causes either a positive or negative shift in its stray field (center of the field switching curve), which indicates that voltage influences interlayer exchange coupling in the SAF free layer, thus demonstrating VCEC. Since the coercivity of the p-MTJs can be tuned with both VCEC and SOT, dual-biasing of SOT+VCEC can possibly generate stochastic signals for probabilistic-bits. By utilizing the effects of both VCEC and SOT, we introduce a potential solution for low-energy and ultra-fast MRAM technology as well as more novel computing paradigms such as probabilistic spin logic.

![](https://assets.underline.io/uploads/markdown_image/1/image/c46c24b4bba3f3242ec62765e893ac77.jpg)

Fig. 1. Anomalous hall resistance versus field measurements at SOT currents ranging from 1mA to 2.25mA.

![](https://assets.underline.io/uploads/markdown_image/1/image/98c15c49ed5dd8b81e43a340598f2316.jpg)

Fig. 2. Tunneling magneto-resistance (TMR) vs. field measurements at VCEC voltages of +1.1V and -1.1V.

Spin-orbit torque assisted switching with voltage controlled exchange coupling

Magnetic tunnel junctions have been implemented in a broad range of applications[1] due to high sensitivity, excellent reliability and low power consumption. This work discusses the implementation of a vertical packed strategy[2] applied to magnetic tunnel junctions. A multi- TMR device was designed to bring higher signal with reduced noise and compact footprint by increasing the number of sensors vertically within a single sensor[fig.1] or an arrays of sensors per level. A CoFeB/MgO/CoFeB based magnetic tunnel junction was selected and the microfabrication process was established to achieve a functional multi-level TMR device, using a 300 nm thick SiN layer to isolate consecutive levels. Then a common electrical contact is created to connect all levels in a parallel configuration to reduce the resistance. The sensor stack presents an initial RxA of 60 kΩ.µm2 and a TMR=120% deposited by a N3600/N8800 tool at INESC MN. Here we show results for a three-level device microfabricated with a total of 600 sensors within an area of 0.12 mm2. The impact on the sensor transfer curve of topography and misalignments across levels are discussed. Although accumulated roughness reduces the TMR signals, thus, the sensitivity, the magnetostatic coupling is negligible due to optimized level separation, which successfully mitigates the stray fields. Furthermore, a magnetic field of 71 nT/Hz1/2 at 35 Hz was measured on unshielded environment [fig.2]. This value is obtained from converting the voltage output into field units using a sensitivity of 3V/T. Also, the electrical robustness of the multi-TMR devices was evaluated through the breakdown voltage showing similar robustness compared with the arrays in-plane.This work shows an ambitious architecture design for TMR sensor integration in applications requiring small size and low magnetic field detection levels. Comparing with series of sensors achieving 20 nT/Hz1/2 at 1 Hz on 10mm2[3] or 115 pT/Hz1/2 at 200 Hz occupying an area of 4.5x5.5 mm2[4], the proposed geometry is at least 2 orders of magnitude more compact.

![](https://assets.underline.io/uploads/markdown_image/1/image/6c9bba3f2d0ab6cfa2916b7f4e455072.jpg)

Scheme of vertically packed array (arrows represent current flow).

![](https://assets.underline.io/uploads/markdown_image/1/image/4552efa2d071d8bb23eac98f05a1196d.jpg)

Output of device subjected to variable current amplitude applied to the external coil at 35 Hz.

Multi-TMR device with a compact footprint for low magnetic field detection at room temperature

Rare earth iron garnets (REIGs) are magnetic insulators that can be grown as nanometer thick films with perpendicular magnetic anisotropy (PMA), making them attractive for studies on ultrafast spin wave dynamics. PMA has been achieved in epitaxial films of several REIG systems on garnet substrates due to magnetoelastic anisotropy from lattice mismatch strain1-2. Polycrystalline REIG films as thin as 22nm can also be grown on silicon with PMA due to thermal expansion mismatch between the film and substrate following a high temperature anneal3. However, it is of interest to develop ultrathin polycrystalline garnet on silicon with PMA for spintronics devices integrated with CMOS technology. Films below ~20nm thickness crystallize poorly and require a garnet underlayer to provide lattice templating that promotes crystallization. Previous research shows this templating effect still holds when a Pt layer is present between the garnet film and the substrate4. Here we report on the growth and properties of 10nm thin polycrystalline dysprosium iron garnet (DyIG) and dysprosium yttrium iron garnet (Y:DyIG) films on Si (100) substates using an underlayer of a non-magnetic garnet, Gd3Ga5O12, coated with ~1.5nm of Pt to prevent interdiffusion. This enabled the growth of ultrathin polycrystalline DyIG films directly on Si with bulk-like magnetization and PMA. Films were grown by pulsed laser deposition and later crystallized by a rapid thermal anneal (750 °C/5 min). Thermal mismatch creates a tensile strain in the films, which combined with the negative magnetostriction coefficients of DyIG overcomes the shape anisotropy and results in PMA. X-ray diffraction shows single phase pure garnet and atomic force microscopy of the films indicates sub-nanometer surface roughness. Vibrating sample magnetometry (VSM) measurements of a 10nm DyIG/Pt/GGG/Si film showed PMA with a saturation magnetization of 15 emu/cc and a coercivity of ~300 Oe. The spin Hall magnetoresistance and spin mixing conductance of Pt/DyIG/Pt/GGG/Si will be compared with that of Pt/REIG single crystal films.

![](https://assets.underline.io/uploads/markdown_image/1/image/d9868548b1779ee257badc65ac17c2d0.jpg)

In plane and out of plane VSM hysteresis loops of 10nm DyIG/Pt/GGG/Si film with PMA.

![](https://assets.underline.io/uploads/markdown_image/1/image/14b77bc84f7b6309f7c23e005e96455a.jpg)

Grazing incidence X-ray diffraction of 10nm/Pt/GGG/Si film.

Integration of 10 nm thick Dysprosium Iron Garnet Films on Silicon

Magnetic antiskyrmions (ASk) are topological magnetic textures that can be stabilized in ultrathin disks by anisotropic Dzyaloshinskii-Moriya interaction (DMI)1. In addition to their recent experimental observation2,3, they have attracted a lot of attention and their dynamic behaviour has been studied in thin nanowires1 and confined nanodisks4. To further our understanding we investigated the resonant modes of magnetic antiskyrmions in antiferromagnetic-exchange coupled thin disks, i.e. in a bilayer SAF, for different microwave excitations. Their response is, as expected, similarly rich to the dynamic behaviour of skyrmions in SAF5, and the spectra revealed several gyrotropic and breathing modes for in and out of plane excitation, respectively. By analysing the trajectories of the antiskyrmion guiding centre in the two disks as well as the spatial distribution of the spectral power we have identified a variety of coupled modes in such a system. Furthermore, we show that these modes can be controlled by tuning the strength of the interlayer exchange coupling between the two disks.

Antiskyrmion excitation modes in antiferromagnetic-exchange coupled disks

Magnetic Hopfions are three-dimensional topological solitons which are considered as a promising candidate for future spintronics devices and non-volatile magnetic storage. These 3D spin textures are found to be stabilized in frustrated, chiral magnets or materials hosting target skyrmions [1-3]. In this work, we have investigated the stability of Hopfions and their spin-wave behavior in individual nanostructures of thin films of Co/Pt with interfacial DMI. We have performed micro-magnetic simulations to understand in details the process of nucleation and annihilation of Hopfions in nanostrutcures in presence of an external magnetic field. Our results demonstrate a narrow region in the hysteresis loop where Hopfions are stabilized (Fig.1). The spin-wave dynamics in this topological structures were studied by perturbing the system with an external transient square field pulse of duration 60 ps and amplitude of 3 mT applied along the x-axis (in the plane of the hopfion). We identified a fundamental spin-wave mode which is excited at about 2 GHz. The nature of this mode is investigated by carrying out fast fourier transform (FFT) at each cell of the discretized geometry. The profile of the mode is investigated by analyzing the image created by plotting the normalized mode-power as well as the phase at the corresponding locations of the nanostructures. Our results suggest that the mode exhibits maximum power in the region of the Hopfions where spins have strong z-components. Furtermore, the excited mode appears to be independent of the external magnetic field.

![](https://assets.underline.io/uploads/markdown_image/1/image/e34473752509d96cae2821cba32de829.jpg)

Fig. 1. (a) Field dependece of the normalized magnetization in Co/Pt nanodisc. Inset shows the spin configuration of hopifion stabilized in the highlighted region of the magnetization vs field curve, (b) Spin wave spectra calculated in individual hopfion stabilized in Co/Pt nanodisk exhibiting interfacial DMI. Inset shows the power profile of the excited fundamental mode at 2.03 GHz.

Magntic Hopfions in Co/Pt nanostructures and their spin-wave behavior

Sequences of geometrically confined magnetic domain walls (DWs) in micrometer long, flat wires currently hold good promises as a possible system for designing new magnetic memories cell devices. Due to the strong shape anisotropy, narrow and flat rectangular wires (FRW), made of soft ferromagnetic (FM) materials, are prone to have the magnetization oriented along the wire edge. Such wires may turn into possible platforms to hold sequences of DWs, provided a periodic sequence of pinning centers might be created along the wire. A bilayer consisting of an FRW exchange coupled to a two-sublattices vicinal antiferromagnetic (AF) substrate fulfills the main requirements to construct a periodic sequence of pinned DWs.1,2 This may lead to a corresponding arrangement in the FM film, which may turn structured with alternating domains separated DWs centered at the step’s edges. We presently report a theoretical study of spin-polarized current domain wall (DW) release, considering a vicinal AF substrate with a small miscut angle so that there is no interaction between adjacent DWs. We consider a DW separating two opposite domains in a FM (Py or Fe) nanostrip coupled to a two-sublattice uniaxial AF substrate. In the absence of electric current, the DW center is on the interface step defect. We consider a spin-polarized current applied along the easy axis AF. The spin-transfer torque is restricted to the DW region. Our results indicate that the electric current density threshold value (J*) to detach the interface pinned DWs increases as the interface exchange energy increases. Furthermore, J* is smaller for Py/AF nanostrips than for Fe/AF nanostrips with the same geometrical dimensions. For instance, for an interface field value of 200 Oe, considering a Py nanostrip consisting of 125 nm long, 25 nm wide, and 10 nm thick domains separating the DWs, we have found that, for small values of electric current density, the DW center moves from the interface step defect, contrary to the electric current direction, for around 27.5 nm, until J*=29x107 A/cm2. The corresponding values for an Fe nanostrip with the same dimensions and interface field strength are 15 nm and J*=34x107 A/cm2.

Domain wall depinning from FM/AF interface defects by spin-polarized current

Magnetic nanowires (MNWs) are increasingly important for applications in memory [1], biolabels [2], and nanowarming of cryopreserved organs [3]. Object Oriented Micromagnetic Framework (OOMMF) was used to visualize moments inside MNWs. The magnetic properties of MNWs were investigated as a function of nanowire shape. Experimental measurements of magnetic nanowires (MNW) often reveal features that are difficult to explain due to statistical shearing of features and interaction fields in large arrays [4]. Measurements of individual nanowires can be used to remove interaction field effects [5], and other measurements such as first-order reversal curves (FORC) [6] and the projection method [7] often attempt to statistically separate bulk effects. For nickel MNWs with fixed length of 3 μm, the coercivity and the remanence are negatively correlated to the diameter of the wire (Fig.1). When the diameter is increased to 200 nm, a ‘wasp waist’ appeared in the hysteresis loop, Fig.2. (solid line) The reversal process is three-folded. First, since there is a large area for curling in 200nm-diameter nanowires, the magnetic moments at the tips of the MNW fall from pointing along z-axis to aligning in the xy-plane and in the form of vortex. Next, the outer shell of moments turns from in-plane to the negative z-axis. Finally, the axial core of the MNW switches and the wire is fully saturated in the negative z-direction. Interestingly, at Hc , the two vortex walls converged in the center of the MNW and a three-dimensional vortex(hedgehog) [8] was observed using a 2D cross-sectional view, Fig.2 (b). Next, when flat-tipped cylinders were replaced with ellipsoids so that the tips of wire had infinitesimally small areas where vortex was no longer supported, the coercivity of the nanowires significantly increased (Fig.1 and 2, dashed line)

![](https://assets.underline.io/uploads/markdown_image/1/image/e49db041cb0a1b0259e8e6a56d35c4b1.jpg)

Fig.1 Hysteresis loops for nickel MNWs with diameter of 30 nm(black), 50 nm(red) and 100 nm(blue) in cylinder shape (solid line) and ellipsoid shape (dashed line).

![](https://assets.underline.io/uploads/markdown_image/1/image/c86cfaf97d6b8d26bb6cd72e78d6f7a2.jpg)

Fig.2 (a) Hysteresis loop for 200 nm nickel MNWs with cylinder shape (solid line) and ellipsoid shape (dashed line). (b) The corresponding 2D and 3D moment map

Magnetic Properties Affected by the Size and Shape of Magnetic Nanowires

The antiferromagnetic (AFM) medium is an ideal candidate for emitting terahertz (THz) electromagnetic (EM) waves, which have extraordinary potential application in information technology and biological medicine [1–4]. In this work, we report the theoretical and numerical investigation about the emission of terahertz (THz) spin waves from a moving AFM DW under the magnetic anisotropy energy gradient (dEa/dx). The DW is accelerated by dEa/dx that can be generated using the technique displayed in Fig. 1: A DC voltage applied across the wedge-shaped insulating layer generates dEa/dx, which triggers the AFM DW to move towards the end with a lower magnetic anisotropy energy [5]. Based on an atomistic spin model, we have found that the DW velocity exceeds the relativistic limit and keeps increasing, which is accompanied by the broadening of the DW width. This is different from a traditional relativistic AFM DW motion with an upper limit of the DW velocity and Lorentz contraction [6]. The DW acceleration effectuates the DW precession. When the frequency of the DW precession is greater than the frequency gap of the spin wave propagating in the AFM medium, a continuous THz spin wave is resonantly emitted. The frequency of the spin wave keeps increasing with the decrease of the anisotropy energy constant, which seems to be contrary to the AFM spin wave obeying the relativistic-like dynamics. Our results demonstrate that obeying the relativistic dynamics is not necessary for the spin-wave emission from a moving AFM DW.

![](https://assets.underline.io/uploads/markdown_image/1/image/b3d5ff6329dec082c7c8fd30cb4693b4.jpg)

Fig.1. Schematic of calculation model: emission of THz spin wave from a moving DW triggered by a voltage-induced gradient of magnetic anisotropy energy (the emission of the spin wave is accompanied by an increase of the DW width due to the reduction of the magnetic anisotropy energy).

Resonant spin-wave emission from the moving AFM DW without Lorentz invariance

Magnetic domain walls (DWs), as a novel information carrier, provide a reliable technique for modern information processing. In previous study, to control the movement of magnetic DWs, different external excitations, such as magnetic field, spin transfer torque, spin-orbit torque, electric field, and spin waves, etc., have been used as the driving force for the DW motion. In this work, we combine electromechanical and micromagnetic simulations to explore a promising way to manipulate the perpendicular magnetic anisotropy (PMA) DW motion in a given direction using voltage-controlled gradient strain. In particular, we show the voltage induced gradient strain driven the DW motion more complicated than simple uniform linear motion, especially for DW motion in a finite length nano-stripe. As shown schematically in Fig. 1(a), the proposed device consists of a ferromagnetic (CoFeB)/heavy metal (FM/HM) nanostrip deposited on a piezoelectric substrate. The initial spin configurations with different i-DMI coefficient are obtained by relax the DW along the y direction. Fig. 1(b) shows the snapshots of magnetization with various i-DMI coefficient when simulated 10 ns after the strain gradient applied. As shown in Fig. 1(b), the direction of the DW will parallel to the x direction due to the i-DMI and form a Neel type DW, while a Bloch type DW formed without i-DMI. From the micromagnetic simulation results shown in Fig. 1(b), along with the DW displacement, DWs are found to tilt away from the y-axis, which agrees with the experimental observations. From these magnetization images, we can also directly see an increment of DW tilting angel as the i-DMI constant increase. These results indicates that strain gradient induced PMA DW motion can be used to develop next generation artificial intelligent memory and logic devices.

![](https://assets.underline.io/uploads/markdown_image/1/image/66bfbae9388ee43210a60dde3c0ce6ad.jpg)

Fig. 1 (a) Schematic view of the investigated strain driven domain wall in a nanostripe with voltage-controlled strain gradient. (b) Snapshots of the magnetization pattern with different i-DMI coefficient.

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Improving the spin-orbit torque efficiency in Pt/CoFeB/Pt based perpendicularly magnetized system to decrease the critical current density for current-induced magnetization switching

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Synthetic antiferromagnetic domain wall device as artificial neuron for neuromorphic computing