Spin-orbit Torque Switching Research Articles

Current‐Induced Spin–Orbit Torques for Spintronic Applications

Control of magnetization in magnetic nanostructures is essential for development of spintronic devices because it governs fundamental device characteristics such as energy consumption, areal density, and operation speed. In this respect, spin-orbit torque (SOT), which originates from the spin-orbit interaction, has been widely investigated due to its efficient manipulation of the magnetization using in-plane current. SOT spearheads novel spintronic applications including high-speed magnetic memories, reconfigurable logics, and neuromorphic computing. Herein, recent advances in SOT research, highlighting the considerable benefits and challenges of SOT-based spintronic devices, are reviewed. First, the materials and structural engineering that enhances SOT efficiency are discussed. Then major experimental results for field-free SOT switching of perpendicular magnetization are summarized, which includes the introduction of an internal effective magnetic field and the generation of a distinct spin current with out-of-plane spin polarization. Finally, advanced SOT functionalities are presented, focusing on the demonstration of reconfigurable and complementary operation in spin logic devices.

Spin-orbit Torque Switching of Perpendicular Magnetization in Ferromagnetic Trilayers

In ferromagnetic trilayers, a spin-orbit-induced spin current can have a spin polarization of which direction is deviated from that for the spin Hall effect. Recently, magnetization switching in ferromagnetic trilayers has been proposed and confirmed by the experiments. In this work, we theoretically and numerically investigate the switching current required for perpendicular magnetization switching in ferromagnetic trilayers. We confirm that the tilted spin polarization enables field-free deterministic switching at a lower current than conventional spin-orbit torque or spin-transfer torque switching, offering a possibility for high-density and low-power spin-orbit torque devices. Moreover, we provide analytical expressions of the switching current for an arbitrary spin polarization direction, which will be useful to design spin-orbit torque devices and to interpret spin-orbit torque switching experiments.

Single-shot dynamics of spin-orbit torque and spin transfer torque switching in three-terminal magnetic tunnel junctions.

Current-induced spin-transfer torques (STT) and spin-orbit torques (SOT) enable the electrical switching of magnetic tunnel junctions (MTJs) in non-volatile magnetic random access memories. To develop faster memory devices, an improvement in the timescales that underlie the current-driven magnetization dynamics is required. Here we report all-electrical time-resolved measurements of magnetization reversal driven by SOT in a three-terminal MTJ device. Single-shot measurements of the MTJ resistance during current injection reveal that SOT switching involves a stochastic two-step process that consists of a domain nucleation time and propagation time, which have different genesis, timescales and statistical distributions compared to STT switching. We further show that the combination of SOT, STT and the voltage control of magnetic anisotropy leads to reproducible subnanosecond switching with the spread of the cumulative switching time smaller than 0.2 ns. Our measurements unravel the combined impact of SOT, STT and the voltage control of magnetic anisotropy in determining the switching speed and efficiency of MTJ devices.

Efficient Spin-Orbit Torque Switching with Nonepitaxial Chalcogenide Heterostructures.

The spin-orbit torques (SOTs) generated from topological insulators (TIs) have gained increasing attention in recent years. These TIs, which are typically formed by epitaxially grown chalcogenides, possess extremely high SOT efficiencies and have great potential to be employed in next-generation spintronics devices. However, epitaxy of these chalcogenides is required to ensure the existence of the topologically protected surface state (TSS), which limits the feasibility of using these materials in industry. In this work, we show that nonepitaxial BixTe1-x/ferromagnet heterostructures prepared by conventional magnetron sputtering possess giant SOT efficiencies even without TSS. Through harmonic voltage measurement and hysteresis loop shift measurement, we find that the damping-like SOT efficiencies originated from the bulk spin-orbit interactions of such nonepitaxial heterostructures can reach values greater than 100% at room temperature. We further demonstrate current-induced SOT switching in these BixTe1-x-based heterostructures with thermally stable ferromagnetic layers, which indicates that such nonepitaxial chalcogenide materials can be potential efficient SOT sources in future SOT magnetic memory devices.

Efficient Spin-Orbit Torque Switching of the Semiconducting Van Der Waals Ferromagnet Cr2 Ge2 Te6.

Being able to electrically manipulate the magnetic properties in recently discovered van der Waals ferromagnets is essential for their integration in future spintronics devices. Here, the magnetization of a semiconducting 2D ferromagnet, i.e., Cr2 Ge2 Te6 , is studied using the anomalous Hall effect in Cr2 Ge2 Te6 /tantalum heterostructures. The thinner the flakes, hysteresis and remanence in the magnetization loop with out-of-plane magnetic fields become more prominent. In order to manipulate the magnetization in such thin flakes, a combination of an in-plane magnetic field and a charge current flowing through Ta-a heavy metal exhibiting giant spin Hall effect-is used. In the presence of in-plane fields of 20 mT, charge current densities as low as 5 × 105 A cm-2 are sufficient to switch the out-of-plane magnetization of Cr2 Ge2 Te6 . This finding highlights that current densities required for spin-orbit torque switching of Cr2 Ge2 Te6 are about two orders of magnitude lower than those required for switching nonlayered metallic ferromagnets such as CoFeB. The results presented here show the potential of 2D ferromagnets for low-power memory and logic applications.

Compact Modeling and Analysis of Voltage-Gated Spin-Orbit Torque Magnetic Tunnel Junction

Recently, experimental results have demonstrated that perpendicular magnetic tunnel junction (p-MTJ) with the antiferromagnetic(AFM)/ferromagnetic (FM)/oxide structure can achieve field-free spin-orbit torque (SOT) switching since the AFM metal strip can not only generate the SOT, but also provide an exchange bias (HEX), making it suitable for practical applications. However, owing to that the HEX is weak, such field-free SOT switching is incomplete, thus resulting in severe switching reliability. In addition, a large SOT switching current (ISOT) is also required, leading to high switching energy dissipation. In this paper, to address these issues, the voltage-controlled magnetic anisotropy (VCMA) is introduced to assist the SOT switching, and such novel switching method is referred as voltage-gated SOT (VGSOT). First, we develop a physics-based compact model for the three-terminal VGSOT-MTJ device, which includes three modules, i.e., the electrical module, the tunnel magnetoresistance module and the dynamic switching module. Then, the impact of the VCMA effect on the field-free SOT switching is investigated by solving a modified Landau-Lifshitz-Gilbert (LLG) equation with consideration of the VCMA, SOT and HEX. Simulation results show that thanks to the introduction of the VCMA effect, the critical ISOT can be reduced greatly, and the incomplete field-free SOT switching can be completed. With further analysis, we obtain a special switching condition, under which complete SOT field-free switching can be achieved with a shortest path and ultra-low power. Moreover, a novel write pulse scheme is proposed to achieve high speed and reliability.

Spin–Orbit Torque Switching in Low-Damping Magnetic Insulators: A Micromagnetic Study

Magnetic insulators, in particular, rare-earth iron garnets, have low damping compared to metallic ferromagnetic materials due to lack of conduction electrons. In analogy to spin-transfer-torque devices, the low-damping nature is presumed to be an advantage for spintronic applications. We report that perpendicular magnetic anisotropy material with low damping actually does not favor reliable spin–orbit torque (SOT) switching. Increasing damping, introducing interfacial Dzyaloshinskii–Moriya interactions, or field-like torques may help SOT switching in some cases. Having notches in a nanometer-scale element, which is a more realistic size for practical applications, can also improve switching stability.

Ultrafast and energy-efficient spin–orbit torque switching in compensated ferrimagnets

Spin–orbit torque can be used to manipulate magnetization in spintronic devices. However, conventional ferromagnetic spin–orbit torque systems have intrinsic limitations in terms of operation speed due to their inherent magnetization dynamics. Antiferromagnets and ferrimagnets with antiparallel exchange coupling exhibit faster spin dynamics and could potentially overcome these limitations. Here, we report ultrafast spin–orbit torque-induced magnetization switching in ferrimagnetic cobalt-gadolinium (CoGd) alloy devices. Using a stroboscopic pump–probe technique to perform time-resolved measurements, we show that the switching time in the ferrimagnets can be reduced to the subnanosecond regime and a domain wall velocity of 5.7 km s‒1 can be achieved, which is in agreement with analytical modelling and atomistic spin simulations. We also find that the switching energy efficiency in the ferrimagnets is one to two orders of magnitude higher than that of ferromagnets. Time-resolved measurements show that current-induced magnetization switching in ferrimagnetic devices is faster and more energy-efficient than in ferromagnet devices.

Spin orbit torques in ferrimagnetic GdFeCo with various compositions

Compositional dependence of spin–orbit torque (SOT) of the bilayer comprised of Ta and ferrimagnetic GdFeCo was investigated. Critical current density of SOT switching Jsw of the GdFeCo/Ta bilayers did not vary with Gd composition x, and were found to exhibit roughly Jsw = 11 MA cm−2. Two orthogonal components of SOT, damping-like torque τDL and field-like torque τFL were estimated by measuring harmonic Hall resistance under in-plane fields parallel and perpendicular to the AC current, respectively. The absolute values of SOT, ∣τ DL∣ and ∣τFL∣, were confirmed to be roughly constant within 22 ≤ x ≤ 28. On the other hand, the sign of τFL changed across the compensation composition. These results suggest that the injected spin current is considered to exert a torque to the transition metal FeCo moment rather than to the rare earth Gd moment.

Write Margin Analysis of Spin–Orbit Torque Switching Using Field-Assisted Method

Switching dynamics of perpendicular magnetic tunnel junction (MTJ) driven by spin–orbit torque (SOT) are investigated by the Landau–Lifshitz–Gilbert (LLG)-based physical model considering the temperature dependence. The field-assisted switching method is proposed to develop the reliable sub-ns writing of SOT-magnetoresistive random-access memory (SOT-MRAM) by removing the plateau time. The conventional method of SOT-MTJ requires the large write current, leading to the area increase of access transistors and the penalty of memory density. The write current and the switching time of SOT-MTJ can be efficiently reduced at the same time by our field-assisted method using the enhanced magnetic field. The magnetic field can be provided by the Co magnetic hard mask above the MTJ. Considering an SOT-MRAM array, the surrounded Co metals have the insignificant influence of stray magnetic field on an MTJ at the center. With the write time of 0.2 ns, the 60% reduction of write current is achieved by our field-assisted method compared to the conventional method. The required write current for the SOT switching decreases with the increasing temperature due to the lowering of thermal stability factor. The write Shmoo plots are further analyzed to calculate the write current margin at the various working temperature. The write time of 0.2 ns exhibits the narrow margin of write current ( $2.6~\mu \text{A}$ ) in the temperature range from 25 °C to 85 °C, while the write time of 0.8 ns has the wide write margin of $102~\mu \text{A}$ . The switching behavior and the write margin are also sensitive to the magnetic field.

A Novel Compound Synapse Using Probabilistic Spin–Orbit-Torque Switching for MTJ-Based Deep Neural Networks

Analog electronic nonvolatile memories mimicking synaptic operations are being explored for the implementation of neuromorphic computing systems. Compound synapses consisting of ensembles of stochastic binary elements are alternatives to analog memory synapses to achieve multilevel memory operation. Among the existing binary memory technologies, magnetic tunneling junction (MTJ)-based magnetic random access memory (MRAM) technology has matured to the point of commercialization. More importantly for this work, stochasticity is natural to the MTJ switching physics, e.g., devices referred to as p-bits that mimic binary stochastic neurons. In this article, we experimentally demonstrate a novel compound synapse that uses stochastic spin–orbit-torque (SOT) switching of an ensemble of nanomagnets that are located on one shared spin Hall effect (SHE) material channel, i.e., tantalum. By using a properly chosen pulse scheme, we are able to demonstrate linear potentiation and depression in the synapse, as required for many neuromorphic architectures. In addition to this experimental effort, we also performed circuit simulations on an SOT-MRAM-based $784\times 200\times 10$ deep belief network (DBN) consisting of p-bit-based neurons and compound synapses. MNIST pattern recognition was used to evaluate the system performance, and our findings indicate that a significant reduction in recognition error rates can be achieved by improving the linearity of the potentiation and depression curves using an incremental pulse scheme.

Absence of Evidence of Electrical Switching of the Antiferromagnetic Néel Vector.

Much theoretical and experimental attention has been focused on the electrical switching of the antiferromagnetic (AFM) Néel vector via spin-orbit torque. Measurements employing multiterminal patterned structures of Pt/AFM show recurring signals of the supposedly planar Hall effect and magnetoresistance, implying AFM switching. We show in this Letter that similar signals have been observed in structures with and without the AFM layer, and of an even larger magnitude using different metals and substrates. These may not be the conclusive evidence of spin-orbit torque switching of AFM, but the thermal artifacts of patterned metal structure on substrate. Large current densities in the metallic devices, beyond the Ohmic regime, can generate unintended anisotropic thermal gradients and voltages. AFM switching requires unequivocal detection of the AFM Néel vector before and after SOT switching.

Electrical Control of Perpendicular Magnetic Anisotropy and Spin‐Orbit Torque‐Induced Magnetization Switching

AbstractVoltage‐driven oxygen ion migration in ferromagnetic metal/oxide heterostructures offers a highly effective means to tailor emergent interfacial functionalities. In heterojunctions with a core structure of Pt/Co/CoO/TiO2 (TaOx), it is demonstrated that exchange coupling of magnetic moments across the Co/CoO interface provides an extra source to stabilize the perpendicular magnetic anisotropy (PMA). Moreover, the strength of this interfacial coupling can be reversibly controlled through voltage‐driven oxygen ion migration at the Co/CoO interface, resulting in electrical‐field‐controllable PMA. In combination with the spin current generated from Pt, it is revealed that the spin‐orbit torque (SOT) switching of the perpendicular magnetization of Co can be turned ON/OFF by electrical field. Tunable PMA and SOT switching makes heavy metal/ferromagnetic metal/antiferromagnetic oxide heterojunctions a promising candidate to future voltage‐controlled, ultralow‐power, and high‐density spintronics devices.

Micromagnetic analysis and optimization of spin-orbit torque switching processes in synthetic antiferromagnets

Based on micromagnetic simulations, we show that it is possible to achieve spin–orbit torque field-free switching of a synthetic antiferromagnet comprised of two ferromagnetic layers with perpendicular magnetic anisotropy, sitting on top of a conventional antiferromagnet. Field-free magnetization reversal is propelled by the competing exchange fields and spin torques. Although some antiferromagnetic coupling is necessary to switch both ferromagnetic layers, strong Ruderman-Kittel-Kasuya-Yosida inhibits the switching process due to the strong repelling forces experienced by both FM layers. The switching happens through domain nucleation and propagation and is aided by Dzyaloshinskii–Moriya interactions. The overall heterostructure is applicable in conjunction with a magnetic tunnel junction, where the free layer is comprised of the proposed synthetic antiferromagnet.

Ferromagnetic resonance of perpendicularly magnetized Tm3Fe5O12/Pt heterostructures

Broadband ferromagnetic resonance is used to investigate magnetization dynamics, damping, interfacial spin transport, and perpendicular magnetic anisotropy (PMA) of (111)-oriented epitaxial thin films of the ferrimagnetic insulator Tm3Fe5O12 (TmIG) on substrates of (111)-oriented Gd3Ga5O12. A PMA field of ∼162 mT is found at 350 K, in the temperature range where spin–orbit torque switching was previously reported [Avci et al., Nat. Mater. 16, 309–314 (2017)]. A Landé g-factor of 1.56 strongly supports large intrinsic spin–orbit coupling due to the presence of the heavy rare earth Tm. Gilbert damping coefficients α are compared for three samples: a 28 nm thin TmIG film (α ∼ 0.014), a TmIG (28 nm)/Pt (6 nm) bilayer (α ∼ 0.022), and a TmIG (28 nm)/Cu (3 nm)/Pt (6 nm) trilayer (α ∼ 0.024). Applying the spin pumping formalism, we find that the real part of the effective interfacial spin mixing conductance Geff↑↓ = 5.7 × 1014 Ω−1 m−2 is comparable to that of well-studied garnet/Pt interfaces. Our work strengthens the candidacy of TmIG for spintronics applications requiring PMA in insulating thin films.

Tunable damping-like and field-like spin-orbit-torque in Pt/Co/HfO2 films via interfacial charge transfer

The spin–orbit-torque (SOT) consists of dampinglike torque (DLT) and fieldlike torque (FLT), where the combined effects of these two torques need further consideration for efficient SOT switching. Here, the tunable correlation between the DLT and FLT is investigated in Pt/Co/HfO2 multilayers with different annealing temperatures (Ta). With increasing Ta, the FLT decreases monotonously, while both the sign and the magnitude of DLT are changed. Interfacial analysis results reveal that the tunable correlation of them is strongly dependent on the interfacial electron structure between the Co and HfO2 layer. The interfacial charge transfer between the Co, O, and Hf atoms could modify interfacial spin–orbit coupling and the crystal electric field (ECEF), which promotes the interface-generated SOT. This work demonstrates an effective method to tune the correlation of the two SOT components, a desirable feature which will be beneficial for the design of SOT-based devices.

Strong magnetoresistance modulation by Ir insertion in a Ta/Ir/CoFeB trilayer

The spin-orbit torque (SOT) switching of trilayers with two heavy-metal layers on the same side of the ferromagnetic metal layer was studied, realizing tunable spin Hall angle, domain-wall motion, and Dzyaloshinskii-Moriya interaction. However, systematic research on the magnetoresistance in such structures is still lacking. In this work, we investigate the anisotropic magnetoresistance (AMR) and the spin Hall magnetoresistance (SMR) by inserting an ultrathin Ir layer (${t}_{\mathrm{Ir}}\ensuremath{\le}1.4\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$) into Ta/CoFeB. The Ir layer with the thickness larger than 0.4 nm can transform the AMR from positive to negative, which is attributed to the electronic structure of Ir. This process realizes the interfacial modulation of AMR, which is generally considered as a bulk property. The SMR ratio decreases first and then increases with increasing Ir thickness, producing the minimum and maximum at ${t}_{\mathrm{Ir}}=0.3\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ and ${t}_{\mathrm{Ir}}=0.9\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$, respectively, which reflects the ultrasmall spin-diffusion length (0.5 nm) and strong spin-memory loss in Ir. Further analyses combined with the SOT switching measurements unravel the existence of the anomalous Hall magnetoresistance, implying the non-negligible spin accumulation due to the anomalous Hall effect of the ferromagnetic metal. The combination of a large negative AMR and comparatively smaller SMR results in a negative planar Hall resistance. Our findings enrich the understanding of the magnetoresistances of heavy-metal/ferromagnetic metal trilayer systems.

Field-Free Spin-Orbit Torque Switching of Perpendicular Magnetization by the Rashba Interface.

Current-induced spin-orbit torques (SOTs) enable efficient electrical manipulation of the magnetization in heterostructures with a perpendicular magnetic anisotropy through the Rashba effect or spin-Hall effect. However, in conventional SOT-based heterostructures, an in-plane bias magnetic field along the current direction is required for the deterministic switching. Here, we report that the field-free SOT switching can be achieved by introducing a wedged oxide interface between a heavy metal and a ferromagnet. The results demonstrate that the field-free SOT switching is determined by a current-induced perpendicular effective field (Hzeff) originating from the interfacial Rashba effect due to the lateral structural symmetry-breaking introduced by the wedged oxide layer. Furthermore, we show that the sign and magnitude of Hzeff exhibit a significant dependence on the interfacial oxygen content, which can be controlled by the inserted oxide thickness. Our findings provide a deeper insight into the field-free SOT switching by the interfacial Rashba effect.

Spin‐Orbit Torque: Tuning Effective Spin Hall Angles via Oxygen Vacancies in Multiferroic BiFeO3‐Based Heterostructures (Adv. Electron. Mater. 10/2019)

In article number 1900435, Yong Jiang, Jun Miao, and co-workers present changes in spin-orbit torque switching and effective spin Hall angle in BiFeO3-based heterostructures with opposite polarization fields, which can be attributed to the distribution of oxygen vacancies. Possible applications are demonstrated in logic devices.

Field-free spin-orbit torque switching through domain wall motion

Deterministic current-induced spin-orbit torque (SOT) switching of magnetization in a heavy transition metal/ferromagnetic metal/oxide magnetic heterostructure with the ferromagnetic layer being perpendicularly-magnetized typically requires an externally-applied in-plane field to break the switching symmetry. We show that by inserting an in-plane magnetized ferromagnetic layer CoFeB underneath the conventional W/CoFeB/MgO SOT heterostructure, deterministic SOT switching of the perpendicularly-magnetized top CoFeB layer can be realized without the need of in-plane bias field. Kerr imaging study further unveils that the observed switching is mainly dominated by domain nucleation and domain wall motion, which might limit the potentiality of using this type of multilayer stack design for nanoscale SOT-MRAM application. Comparison of the experimental switching behavior with micromagnetic simulations reveals that the deterministic switching in our devices cannot be explained by the stray field contribution of the in-plane magnetized layer, and the roughness-caused N\'eel coupling effect might play a more important role in achieving the observed field-free deterministic switching.