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We have performed detailed magnetic and anisotropic magnetoresistance (AMR) measurements on a magnetic-field-induced topological Weyl semimetal, TbPtBi, exhibiting a weak metamagnetic transition in the antiferromagnetic state. The magnetic torque and temperature-dependent AMR measurements indicate that the spin order dominates the symmetry of AMR curves at temperature below Neel temperature (TN) and low magnetic fields. Remarkably, the AMR with a twofold symmetry gradually dominates the curves as the magnetic fields increase. Moreover, sign change in the AMR effect is confirmed for both T < TN and T > TN at high magnetic fields, which may be related to the magnetic field-induced splitting of the topological band in TbPtBi.

This work reports the enhancement of damping-like and field-like spin–orbit torque (SOT) efficiencies and interfacial spin transparency ( Tin) in the Py/NiO/Pt heterostructure. The SOT efficiencies and Tin are characterized by combining the spin–torque ferromagnetic resonance (ST-FMR) and the spin-pumping (SP) techniques. The inevitable inverse spin Hall voltage contamination induced by SP in the ST-FMR spectrum is extracted and subtracted by combining additional SP measurements, which allows obtaining accurate SOT efficiencies and Tin. The damping-like and field-like SOT efficiencies vary with the NiO insertion layer thickness, which is a result of the change of Tin. The maximum Tin reaches ∼0.82 for a 0.6 nm-thick NiO layer. This work shows that NiO insertion is an effective method for enhancing Tin and, hence, the SOT efficiency.

Magnetic two-dimensional (2D) van der Waals (vdWs) materials are receiving increased attention due to their exceptional properties and potential applications in spintronic devices. Because exchange bias and spin–orbit torque (SOT)-driven magnetization switching are basic ingredients for spintronic devices, it is of pivotal importance to demonstrate these effects in the 2D vdWs material-based magnetic heterostructures. In this work, we employ a vacuum exfoliation approach to fabricate Fe3GeTe2 (FGT)/Ir22Mn78 (IrMn) and FGT/Pt bilayers, which have high-quality interfaces. An out-of-plane exchange bias of up to 895 Oe is obtained in the former bilayer, which is larger than that of the previously studied bilayers. In the latter bilayer, the SOT switching of the perpendicularly magnetized FGT is realized, which exhibits higher SOT-driven switching performance compared to the previously studied FGT/Pt bilayer devices with interfacial oxidation. The minimum of SOT efficiency is further determined to be 0.18 ± 0.04, comparable to the previously reported values for the Pt/Co and Pt/CoFeB bilayers. This work highlights the importance of the high-quality interface for the exchange bias and SOT effect and may pave the way for implementing 2D vdWs in spintronic devices.

Spintronics, that is, the utilization of electron spin to enrich the functionality of microelectronics, has led to the inception of numerous novel devices, particularly magnetic random-access memory (MRAM). Over the last decade, significant effort has been devoted to magnetization manipulation using spin-orbit torque (SOT), which shows great promise for ultrafast and energy-efficient MRAM. In this Perspective, we summarize the latest progress in the study of SOT and highlight some of the technical challenges facing the development of practical SOT devices. After introducing the basic concepts of SOT and its relevance for magnetization switching, we will focus on several methods to realize deterministic SOT switching in the absence of an external field, which is a requirement for practical SOT devices. Additionally, we summarize the materials used in SOT devices. The final section is devoted to the most important recent advances in the application of SOT devices, including SOT-MRAM, spin logic, spin Hall nano-oscillators, and neuromorphic devices.

Gradual magnetization switching driven by spin–orbit torque (SOT) is preferred for neuromorphic computing in a spintronic manner. Here we have applied focused ion beam to selectively illuminate patterned regions in a Pt/Co/MgO strip with perpendicular magnetic anisotropy, soften the illuminated areas, and realize the gradual switching by a SOT-driven nucleation process. It is found that a large in-plane field is helpful to reduce the nucleation barrier, increase the number of nucleated domains and intermediate states during the switching progress, and finally flatten the switching curve. We proposed a phenomenological model for descripting the current dependence of magnetization and the dependence of the number of nucleation domains on the applied current and magnetic field. This study may promote the birth of SOT devices applicable in neuromorphic computing applications.

Current-induced spin–orbit torques (SOTs) in heavy metal/ferromagnet heterostructures have emerged as an efficient method for magnetization switching with applications in nonvolatile magnetic memory and logic devices. However, experimental realization of SOT switching of perpendicular magnetization requires an additional inversion symmetry breaking, calling for modifications of the conventional SOT heterostructures. In this work, we study SOTs and deterministic switching of perpendicular magnetization by inserting different asymmetric dusting layers at the heavy metal/ferromagnet interface. Similar to the previous works with lateral structural asymmetry, we study the emergence of current-induced perpendicular effective magnetic fields (Hzeff). By examining three different material combinations of heavy metal/dusting layers (W/IrMn, Pt/IrMn, and W/Ta), we shed light on the origins of Hzeff; we show that Hzeff is generically created in all the studied asymmetric structures, has a close correlation with the interfacial magnetic anisotropy, and is independent of the signs of spin Hall angles of the materials. Furthermore, we show that the induction of Hzeff enables field-free deterministic SOT switching of perpendicular magnetization. Our results can be used in designing SOT heterostructures for practical applications in nonvolatile technologies.

The interfacial spin transmission and spin–orbit torques (SOTs) in as-grown and annealed W/Co2FeAl(CFA)/MgO multilayers are studied by the spin-torque ferromagnetic resonance (ST-FMR) technique. The effective spin-mixing conductance of 4.40 × 10 14 Ω − 1 m − 2 and the interfacial spin transparency Tin of 0.47 are obtained for the as-grown samples, which are comparable to those of the widely employed W/CoFeB bilayer. The annealing influence on the W/CFA/MgO multilayer is subsequently studied. The damping-like torque efficiency (ξDL) and field-like torque efficiency (ξFL) are extracted for different annealing temperatures via modulation of damping measurements and the angular dependence of ST-FMR spectra. Importantly, the ξDL value is fairly large (0.3 –0.5) and does not degrade even when the annealing temperature is increased up to 420 °C. Our results indicate that the studied W/CFA/MgO multilayers could be promising candidate materials for designing and developing SOT-driven spintronic devices.

Spin orbit torque-driven magnetization switching in perpendicularly magnetized thin film relies on an extra in-plane magnetic field to break the in-plane magnetic symmetry, which is an obstacle for the integration of spin–orbit torque-based spintronic devices. Here, we propose a simple method to realize the field-free spin–orbit torque-driven magnetization switching by exploiting a tilted magnetic anisotropy, which is caused by the direct coupling of two ferromagnetic layers in the Pt/Co/FeNi/Ta structure. When preparing the sample, a 1000 Oe in-plane magnetic field was applied to ensure the magnetic moment deviating a small angle from the out-of-plane direction to this in-plane field direction. We experimentally demonstrate the deterministic field-free magnetization switching in Pt/Co/FeNi/Ta by the field-like spin–orbit torques when the electric current is applied perpendicular to this in-plane field direction. The switching performance is slightly degraded with the critical switching current density and thermal stability factor, respectively, reaching 6.4 × 106 A/cm2 and 25 due to the slightly decreased spin–orbit torque efficiency and perpendicular magnetic anisotropy with introducing the FeNi layers. Our work paves the way for realizing the field-free magnetization switching by spin–orbit torques.

Here, the spin-torque ferromagnetic resonance signal and the spin Hall magnetoresistance induced by the spin Hall effect of W/Cu/CoFeB heterostructures with different Cu layer thicknesses ( t Cu) have been systemically studied. The effective spin mixing conductance g eff ↑ ↓, the interfacial spin transparency T, and the real spin–orbit torque efficiency ( J s J c ) real show a significant increase compared to the W/CoFeB heterostructure. ( J s J c ) real reaches its maximum of ∼0.54, increased up to ∼50% at the optimized t Cu ∼ 0.52 nm according to our theoretical prediction. More importantly, the intrinsic spin Hall angle of W, θ SH int ∼ 0.79 ± 0.20, has also been obtained after the correction of the inverse spin Hall effect and T. This suggests that the Cu insertion improves the interface quality and, therefore, assists the spin transport in the heterostructures, which potentially improves the performance of next-generation spintronic devices.

The perpendicular magnetic anisotropy (PMA), current-induced spin–orbit torques (SOTs), and Dzyaloshinskii–Moriya interaction (DMI) in the as-grown W or Ta/Ir22Mn78(IrMn)/CoFeB/MgO stacks with varying IrMn layer thicknesses were investigated. The in-plane magnetized W/CoFeB/MgO sample becomes perpendicularly magnetized after inserting the IrMn layer without the requirement of the annealing process. The effective magnetization fields 4πMeff show a nonmonotonic dependence on the IrMn layer thickness, which reaches the maximum in magnitude at a thickness of tIrMn = 0.75 nm. The SOT effective fields corresponding to damping-like and field-like torques decrease with the insertion layer thickness. Moreover, the variation of the IrMn layer thickness leads to the change of the DMI in magnitude and sign change from positive (favoring right-handed chirality) to negative (favoring left-handed chirality). The realization of changing the PMA, SOTs, and DMI by inserting the IrMn layer provides more flexibility in the design of spintronic devices.

We study the magnetic domain pattern in a ferrimagnetic CoTb thin film using Lorentz transmission electron microscopy. The evolution of domain patterns is captured at different temperatures and external fields. Magnetic dipole skyrmions and bubbles can be created simultaneously by increasing the temperature of the sample and applying appropriate out-of-plane magnetic fields. By tuning the external magnetic fields, it is found that the skyrmions are annihilated at a higher magnetic field compared to bubbles. Moreover, we also find that the in-plane magnetic field influences the formation and the shape of skyrmions and bubbles. Further, micromagnetic simulations are also consistent with the experimental observations. Our findings provide insights into the magnetic field stability of skyrmions.

Two-dimensional layered materials with remarkable thermoelectric coefficients are promising candidates for sustainable thermopower batteries. Here, we investigate the anomalous Nernst effect and anomalous Hall effect in the polycrystalline Fe 3 GeTe 2 thin films. While its Seebeck coefficient and anomalous Hall angle (or magnetization) monotonously increase and decrease with temperature, respectively, the anomalous Nernst coefficient of the Fe 3 GeTe 2 films exhibits a peak value of 0.28 μV K–1 T–1 at 150 K, which is the compromised outcome between the enhanced Seebeck effect and the gradually weakened magnetism with elevating temperature. A noticeable anomalous Nernst effect observed in Fe 3 GeTe 2 sheds light on the low-temperature application of two-dimensional layered materials in spin-caloritronics.

The current-induced spin-orbit torques (SOTs) in the perpendicularly magnetized Ir22Mn78/Co20Fe60B20/MgO structures are investigated. The damping- and field-like torques are characterized using a harmonic technique. The spin Hall angle of Ir22Mn78 is determined to be θSHE = +0.057 ± 0.002. The SOT-driven magnetization switching is also demonstrated with the assistance of an external in-plane field. Furthermore, the magneto-optical Kerr effect imaging experiments show that the magnetization switching is realized through domain nucleation and domain wall motion. These results may promise potential practical applications in high-performance SOT devices based on the antiferromagnetic materials.

We study the magnetic properties of W/Co40Fe40B20 (CoFeB)/MgO films using the spin-torque ferromagnetic resonance (ST-FMR) technique. This study takes the advantage of the spin Hall magnetoresistance (SMR) for generating an oscillating resistance, which is one of the necessary requirements for obtaining mixing voltage in the ST-FMR technique. We have measured both the as-grown and the annealed samples with different CoFeB layer thicknesses, which include the in-plane and out-of-plane magnetic anisotropies. The spectra for these two types of anisotropies show distinct signatures. By analyzing the ST-FMR spectra, we extract the effective anisotropy field for both types of samples. In addition, we investigate the influence of CoFeB thickness and annealing on the Gilbert damping constant. Our experiments show that by taking advantage of SMR, the ST-FMR measurement acts as an effective tool with high sensitivity for studying the magnetic properties of ultrathin magnetic films.

We study the heavy metal layer thickness dependence of the current-induced spin-orbit torque (SOT) in perpendicularly magnetized Hf|CoFeB|MgO multilayer structures. The damping-like (DL) current-induced SOT is determined by vector anomalous Hall effect measurements. A non-monotonic behavior in the DL-SOT is found as a function of the thickness of the heavy-metal layer. The sign of the DL-SOT changes with increasing the thickness of the Hf layer in the trilayer structure. As a result, in the current-driven magnetization switching, the preferred direction of switching for a given current direction changes when the Hf thickness is increased above ~7 nm. Although there might be a couple of reasons for this unexpected behavior in DL-SOT, such as the roughness in the interfaces and/or impurity based electric potential in the heavy metal, one can deduce a roughness dependence sign reversal in DL-SOT in our trilayer structure. © 2016 Author(s).

We study spin-orbit-torque (SOT)-driven magnetization switching in perpendicularly magnetized Ta/Mo/Co40Fe40B20 (CoFeB)/MgO films. The thermal tolerance of the perpendicular magnetic anisotropy (PMA) is enhanced, and the films sustain the PMA at annealing temperatures of up to 430 °C, due to the ultra-thin Mo layer inserted between the Ta and CoFeB layers. More importantly, the Mo insertion layer also allows for the transmission of the spin current generated in the Ta layer due to spin Hall effect, which generates a damping-like SOT and is able to switch the perpendicular magnetization. When the Ta layer is replaced by a Pt layer, i.e., in a Pt/Mo/CoFeB/MgO multilayer, the direction of the SOT-induced damping-like effective field becomes opposite because of the opposite sign of spin Hall angle in Pt, which indicates that the SOT-driven switching is dominated by the spin current generated in the Ta or Pt layer rather than the Mo layer. Quantitative characterization through harmonic measurements reveals that the large SOT effective field is preserved for high annealing temperatures. This work provides a route to applying SOT in devices requiring high temperature processing steps during the back-end-of-line processes.

Compared with current-controlled magnetization switching in a perpendicular magnetic tunnel junction (MTJ), electric field- or voltage-induced magnetization switching reduces the writing energy of the memory cell, which also results in increased memory density. In this work, an ultra-thin PZT film with high dielectric constant was integrated into the tunneling oxide layer to enhance the voltage-controlled magnetic anisotropy (VCMA) effect. The growth of MTJ stacks with an MgO/PZT/MgO tunnel barrier was performed using a combination of sputtering and atomic layer deposition techniques. The fabricated MTJs with the MgO/PZT/MgO barrier demonstrate a VCMA coefficient, which is ∼40% higher (19.8 ± 1.3 fJ/V m) than the control sample MTJs with an MgO barrier (14.3 ± 2.7 fJ/V m). The MTJs with the MgO/PZT/MgO barrier also possess a sizeable tunneling magnetoresistance (TMR) of more than 50% at room temperature, comparable to the control MTJs with an MgO barrier. The TMR and enhanced VCMA effect demonstrated si...

We study voltage-controlled magnetic anisotropy (VCMA) and other magnetic properties in annealed Mo|CoFeB|MgO layered structures. The interfacial perpendicular magnetic anisotropy (PMA) is observed to increase with annealing over the studied temperature range, and a VCMA coefficient of about 40 fJ/V-m is sustained after annealing at temperatures as high as 430 °C. Ab initio electronic structure calculations of interfacial PMA as a function of strain further show that strain relaxation may lead to the increase of interfacial PMA at higher annealing temperatures. Measurements also show that there is no significant VCMA and interfacial PMA dependence on the CoFeB thickness over the studied range, which illustrates the interfacial origin of the anisotropy and its voltage dependence, i.e., the VCMA effect. The high thermal annealing stability of Mo|CoFeB|MgO structures makes them compatible with advanced CMOS back-end-of-line processes, and will be important for integration of magnetoelectric random access memory into on-chip embedded applications.

We study the effect of the oxide layer on current-induced perpendicular magnetization switching properties in Hf|CoFeB|MgO and Hf|CoFeB|TaOx tri-layers. The studied structures exhibit broken in-plane inversion symmetry due to a wedged CoFeB layer, resulting in a field-like spin-orbit torque (SOT), which can be quantified by a perpendicular (out-of-plane) effective magnetic field. A clear difference in the magnitude of this effective magnetic field (H z F L) was observed between these two structures. In particular, while the current-driven deterministic perpendicular magnetic switching was observed at zero magnetic bias field in Hf|CoFeB|MgO, an external magnetic field is necessary to switch the CoFeB layer deterministically in Hf|CoFeB|TaOx. Based on the experimental results, the SOT magnitude (H z F L per current density) in Hf|CoFeB|MgO (-14.12 Oe/107 A cm-2) was found to be almost 13× larger than that in Hf|CoFeB|TaOx (-1.05 Oe/107 A cm-2). The CoFeB thickness dependence of the magnetic switching behavior, and the resulting H z F L generated by in-plane currents are also investigated in this work. © 2015 AIP Publishing LLC.

We demonstrate strain-induced modulation of perpendicular magnetic anisotropy (PMA) in (001)-oriented [Pb(Mg1/3Nb2/3)O3](1−x)-[PbTiO3]x (PMN-PT) substrate/Ta/CoFeB/MgO/Ta structures using ferromagnetic resonance (FMR). An in-plane biaxial strain is produced by applying voltage between the two surfaces of the PMN-PT substrate, and is transferred to the ferromagnetic CoFeB layer, which results in tuning of the PMA of the CoFeB layer. The strain-induced change in PMA is quantitatively extracted from the experimental FMR spectra. It is shown that both first and second-order anisotropy terms are affected by the electric field, and that they have opposite voltage dependencies. A very large value of the voltage-induced perpendicular magnetic anisotropy modulation of ∼7000 fJ/V·m is obtained through this strain-mediated coupling. Using this FMR technique, the magnetostriction coefficient λ is extracted for the ultrathin 1.1 nm Co20Fe60B20 layer, and is found to be 3.7 × 10−5, which is approximately 4 times larger...

Conventional semi-infinite analytical solutions of correlation diffusion equation may lead to errors when calculating blood flow index (BFI) from diffuse correlation spectroscopy (DCS) measurements in tissues with irregular geometries. Very recently, we created an algorithm integrating a Nth-order linear model of autocorrelation function with the Monte Carlo simulation of photon migrations in homogenous tissues with arbitrary geometries for extraction of BFI (i.e., αDB ). The purpose of this study is to extend the capability of the Nth-order linear algorithm for extracting BFI in heterogeneous tissues with arbitrary geometries. The previous linear algorithm was modified to extract BFIs in different types of tissues simultaneously through utilizing DCS data at multiple source-detector separations. We compared the proposed linear algorithm with the semi-infinite homogenous solution in a computer model of adult head with heterogeneous tissue layers of scalp, skull, cerebrospinal fluid, and brain. To test the capability of the linear algorithm for extracting relative changes of cerebral blood flow (rCBF) in deep brain, we assigned ten levels of αDB in the brain layer with a step decrement of 10% while maintaining αDB values constant in other layers. Simulation results demonstrate the accuracy (errors

We study the current-driven perpendicular magnetization switching in Ta/CoFeB(wedge)/[TaOx or MgO/TaOx] devices with a lateral structural asymmetry introduced by a varying CoFeB thickness. In these devices, an in-plane current can generate a field-like torque and its corresponding effective magnetic field ( HzFL) is out-of-plane, which can deterministically switch perpendicular magnetization at zero magnetic field. Experimental results indicate that the method used for breaking lateral structural symmetry greatly affects the resulting field-like torque, and that the gradient of perpendicular anisotropy, resulting from the CoFeB thickness variation, is not by itself sufficient to give rise to the current-induced HzFL. Analysis of the oxidation gradient at the CoFeB/TaOx interface indicates that the oxidation gradient may play a more important role than the gradient of magnetic anisotropy for the generation of HzFL. For practical applications, the demonstration of perpendicular magnetization switching in Ta...