Spin-orbit Torque Switching Research Articles

Investigation of spin–orbit torque switching mechanism in crystalline ferromagnetic semiconductor

We investigated the spin–orbit torque (SOT) switching mechanism of a single layer of crystalline diluted ferromagnetic semiconductor by simulating the current scan hysteresis using the Landau–Lifshitz–Gilbert equation. Our study focuses on the switching of the out-of-plane magnetization component during current scans to provide a detailed understanding of the SOT switching process. The simulation results reveal that the SOT switching strongly depends on the relative strengths of the damping-like torque (DLT) and field-like torque (FLT). Through a systematic analysis, we found that the DLT to FLT ratio required for full SOT switching of the out-of-plane magnetized (GaMn) (AsP) film falls within the range of 0.5–1.0. We also identified a relationship between the DLT to FLT ratio and the linear behavior of the out-of-plane component of magnetization during current scans under a strong in-plane bias field. This suggests that the DLT to FLT ratio of a ferromagnetic film can be directly determined from current scan measurements under a large external field, providing crucial information for developing SOT-based devices.

A Spin‐Orbit Torque Switch at Ferromagnet/Antiferromagnet Interface Toward Stochastic or Memristive Applications via Tailoring Antiferromagnetic Ordering

AbstractAntiferromagnet (AFM) has currently participated in the spin‐orbit torque (SOT) technology due to its great potential to be applied to the field‐free SOT switching and to promote the thermal stability of MRAM. However, the effect of varying AFM ordering on the SOT switching and the associated properties is still not comprehensively understood. This work reports how an AFM ordering modifies the strength of Dzyaloshinskii–Moriya‐interaction (DMI) in a heavy metal (Pt)/FM (Co)/AFM (IrMn) trilayer and its effects on SOT switching. Increasing the AFM ordering reflects the enhanced exchange bias through increasing IrMn thickness appears to significantly reduce the DMI strength of the trilayer. Controlling the IrMn thickness appears to serve as a unique switch to activate memristivity/stochasticity in the devices via tailoring AFM ordering on exchange bias: The strong AFM ordering via increasing IrMn thickness enables to increase the stability of multi‐levels for SOT switching, which promotes the memristivity for neuromorphic application. On the contrary, the weak AFM ordering via reducing IrMn thickness will lead to significant stochasticity for the physically unclonable functionality. This work demonstrates an intrinsic tuning over the AFM ordering will serve as a switch to turn the SOT device into a stochastic/memristive cell to bridge probabilistic and neuromorphic computing.

Field-free spin-orbit switching of perpendicular magnetization enabled by dislocation-induced in-plane symmetry breaking

Current induced spin-orbit torque (SOT) holds great promise for next generation magnetic-memory technology. Field-free SOT switching of perpendicular magnetization requires the breaking of in-plane symmetry, which can be artificially introduced by external magnetic field, exchange coupling or device asymmetry. Recently it has been shown that the exploitation of inherent crystal symmetry offers a simple and potentially efficient route towards field-free switching. However, applying this approach to the benchmark SOT materials such as ferromagnets and heavy metals is challenging. Here, we present a strategy to break the in-plane symmetry of Pt/Co heterostructures by designing the orientation of Burgers vectors of dislocations. We show that the lattice of Pt/Co is tilted by about 1.2° when the Burgers vector has an out-of-plane component. Consequently, a tilted magnetic easy axis is induced and can be tuned from nearly in-plane to out-of-plane, enabling the field-free SOT switching of perpendicular magnetization components at room temperature with a relatively low current density (~1011 A/m2) and excellent stability (> 104 cycles). This strategy is expected to be applicable to engineer a wide range of symmetry-related functionalities for future electronic and magnetic devices.

Magnetization Switching in Atom-Thick Mo Engineered Exchange Bias-Based SOT-MRAM

The manipulation and detection of antiferromagnetic (AFM) in exchange bias (EB)-based MRAM using spin-orbit torque (SOT) holds promise for developing highly reliable and ultrafast spintronic memory devices. However, the high switching current induced by the SOT-induced EB field remains a major drawback. Additionally, the mechanism behind the interaction between the EB field and the SOT remains unclear. To address this issue, we have introduced a thin layer of Mo between the AFM and ferromagnetic-free layers to tune the EB field and study the SOT-induced switching properties. Our findings indicate that when the SOT is dominant during short pulses of a few nanoseconds, Mo insertion can significantly reduce the EB field and decrease the SOT switching current, leading to a reduction in power consumption of these memories. This approach could open up new possibilities for optimizing EB-MRAM and improving our understanding of AFM electronics.

Room temperature energy-efficient spin-orbit torque switching in two-dimensional van der Waals Fe3GeTe2 induced by topological insulators

Two-dimensional (2D) ferromagnetic materials with unique magnetic properties have great potential for next-generation spintronic devices with high flexibility, easy controllability, and high heretointegrability. However, realizing magnetic switching with low power consumption at room temperature is challenging. Here, we demonstrate the room-temperature spin-orbit torque (SOT) driven magnetization switching in an all-van der Waals (vdW) heterostructure using an optimized epitaxial growth approach. The topological insulator Bi2Te3 not only raises the Curie temperature of Fe3GeTe2 (FGT) through interfacial exchange coupling but also works as a spin current source allowing the FGT to switch at a low current density of ~2.2×106 A/cm2. The SOT efficiency is ~2.69, measured at room temperature. The temperature and thickness-dependent SOT efficiency prove that the larger SOT in our system mainly originates from the nontrivial topological origin of the heterostructure. Our experiments enable an all-vdW SOT structure and provides a solid foundation for the implementation of room-temperature all-vdW spintronic devices in the future.

A Comprehensive Study of Temperature and Its Effects in SOT-MRAM Devices.

We employ a fully three-dimensional model coupling magnetization, charge, spin, and temperature dynamics to study temperature effects in spin-orbit torque (SOT) magnetoresistive random access memory (MRAM). SOTs are included by considering spin currents generated through the spin Hall effect. We scale the magnetization parameters with the temperature. Numerical experiments show several time scales for temperature dynamics. The relatively slow temperature increase, after a rapid initial temperature rise, introduces an incubation time to the switching. Such a behavior cannot be reproduced with a constant temperature model. Furthermore, the critical SOT switching voltage is significantly reduced by the increased temperature. We demonstrate this phenomenon for switching of field-free SOT-MRAM. In addition, with an external-field-assisted switching, the critical SOT voltage shows a parabolic decrease with respect to the voltage applied across the magnetic tunnel junction (MTJ) of the SOT-MRAM cell, in agreement with recent experimental data.

Handedness anomaly in a non-collinear antiferromagnet under spin-orbit torque.

Non-collinear antiferromagnets are an emerging family of spintronic materials because they not only possess the general advantages of antiferromagnets but also enable more advanced functionalities. Recently, in an intriguing non-collinear antiferromagnet Mn3Sn, where the octupole moment is defined as the collective magnetic order parameter, spin-orbit torque (SOT) switching has been achieved in seemingly the same protocol as in ferromagnets. Nevertheless, it is fundamentally important to explore the unknown octupole moment dynamics and contrast it with the magnetization vector of ferromagnets. Here we report a handedness anomaly in the SOT-driven dynamics of Mn3Sn: when spin current is injected, the octupole moment rotates in the opposite direction to the individual moments, leading to a SOT switching polarity distinct from ferromagnets. By using second-harmonic and d.c. magnetometry, we track the SOT effect onto the octupole moment during its rotation and reveal that the handedness anomaly stems from the interactions between the injected spin and the unique chiral-spin structure of Mn3Sn. We further establish the torque balancing equation of the magnetic octupole moment and quantify the SOT efficiency. Our finding provides a guideline for understanding and implementing the electrical manipulation of non-collinear antiferromagnets, which in nature differs from the well-established collinear magnets.

Manipulation of perpendicular magnetic anisotropy and spin–orbit torque switching behavior in ferrimagnetic D022-Mn3Ga based multilayers

Perpendicularly magnetized ferrimagnets have shown potential application in nonvolatile, high-density, and ultrafast spintronic devices. Achieving this requires tuning the material performance of ultrathin ferrimagnetic films. Here, we present an experimental demonstration of tunable perpendicular magnetic anisotropy (PMA) and spin–orbit torque (SOT) switching behavior in a ferrimagnetic Heusler alloy D022-Mn3Ga based multilayers by introducing lattice-matching underlayers. It is found that the highly (001) orientated crystalline structure is well maintained, whereas coercivity, anisotropy field, and PMA constant rely heavily on the specific underlayer. Owing to the strong out-of-plane exchange coupling, the PMA constant of Co2MnSi/D022-Mn3Ga is significantly enhanced by four times and reaches 12.1 Merg/cm3. Moreover, the SOT measurements suggest that Fe/D022-Mn3Ga and Co2MnSi/D022-Mn3Ga exhibit analog and abrupt switching dynamics, respectively. This contrasting SOT switching behavior is attributed to different magnetization switching mechanisms, verified by the direct imaging of the nanoscale magnetic domain. These results provide a deep insight into the manipulation of not only fundamental magnetic properties but also SOT switching of ferrimagnetic Heusler alloys.

Efficient spin–orbit torque switching in perpendicularly magnetized CoFeB facilitated by Fe2O3 underlayer

Spin–orbit torque (SOT) is recognized as an effective way to manipulate magnetization in spintronic devices. For the low-power consumption and high-endurance requirements of future computer architectures, reducing the critical SOT switching current density and improving SOT efficiency are crucial, especially in the perpendicularly magnetized structures. Here, we have conducted a comprehensive study on improving the SOT efficiency of the Ta/CoFeB structure with a perpendicular magnetic anisotropy by inserting an oxide insulating layer Fe2O3 as the bottom layer. We found that only a 1–5 nm thickness of Fe2O3 significantly reduces the SOT critical switching current by 70% and enhances the spin Hall angle of Ta. The spin Hall angle increases from 0.078 for pure Ta/CoFeB to 0.13 for Fe2O3/Ta/CoFeB, and both types of spin–orbit torques, damping-like and field-like torques, are significantly enhanced. It is suggested that the atomic diffusion of O from the Fe2O3 underlayer leads to the partial oxidization of the Ta layer as well as the Ta/CoFeB interfaces, accounting for the observed enhanced SOT efficiency. Our results provide a reliable method to improve the SOT performance in perpendicularly magnetized structures by inserting the oxide underlayer using magnetron sputtering, in favor of its potential real-world application in spintronic devices.

Visualization of out-of-plane spin generation in mirror symmetry broken Co

Generating out-of-plane spins in sputtered materials holds immense potential for achieving field-free spin–orbit torque switching in practical applications and mass production. In this work, we present the detection of out-of-plane spins from single-layer ferromagnetic Co layers, which are visualized through helicity-dependent photomapping techniques. Our experiments have shown that out-of-plane spins are dependent on the magnetization direction, current density, and Co thickness. Our findings indicate that amorphous sputtered Co can be a promising candidate as an out-of-plane spin source material for industrial massive production.

Voltage-gated field-free spin–orbit torque switching in Pt/Co/Ir/MgO wedged structures

The ability to efficiently manipulate magnetization is of great significance for practical applications of spin–orbit torque (SOT) devices. In this study, we report the voltage-controlled, field-free SOT switching in perpendicular magnetized Pt/Co/Ir/MgO structures with wedge iridium interlayers. The insertion of a thin iridium interlayer at ferromagnet/oxide can significantly reduce the perpendicular magnetic anisotropy depending on the Ir thickness. The wedging of the iridium layer breaks lateral structural symmetry, resulting in deterministic switching without the assistance of in-plane magnetic fields. In such a structure, the SOT critical switching currents are remarkably decreased by 29% when a positive 6 V gate voltage is applied. Further quantitative analysis shows that multiple factors contribute to the decrease in switching currents, including a 23% reduction in magnetic anisotropy energy, a reduction in nucleation field, and a minor enhancement in damping-like torque under gate voltage. Moreover, the probabilistic hindrance that gate voltage poses to field-free switching is revealed by the decrease in current-induced perpendicular effective fields from symmetry-breaking. Our research shows that energy-efficient SOT switching can be controlled by gating and offers insight into the mechanism behind voltage-gated SOT switching.

Field-free spin-orbit torque switching in interlayer exchange coupled Co/Ta/CoTb

This study investigates a T-type field-free spin-orbit torque device with an in-plane magnetic layer coupled to a perpendicular magnetic layer via a non-magnetic spacer. The device utilizes a Co/Ta/CoTb structure, in which the in-plane Co layer and the perpendicular CoTb layer are ferromagnetically (FM) coupled through the Ta spacer. ‘T-type’ refers to the magnetization arrangement in the FM/spacer/FIM structure, where the magnetization in FM is in-plane, while in FIM, it is out-of-plane. This configuration forms a T-shaped arrangement for the magnetization of the two magnetic layers. Additionally, ‘interlayer exchange coupling (IEC)’ denotes the interaction between the two magnetic layers, which is achieved by adjusting the material and thickness of the spacer. Our results show that an in-plane effective field from the IEC enables deterministic current-induced magnetization switching of the CoTb layer. The field-driven and the current-driven asymmetric domain wall motion are observed and characterized by magneto-optic Kerr effect measurements. The functionality of multistate synaptic plasticity is demonstrated by understanding the relationship between the anomalous Hall resistance and the applied current pulses, indicating the potential for the device in spintronic memory and neuromorphic computing.

Field-free spin-orbit torque switching assisted by in-plane unconventional spin torque in ultrathin [Pt/Co

Electrical manipulation of magnetization without an external magnetic field is critical for the development of advanced non-volatile magnetic-memory technology that can achieve high memory density and low energy consumption. Several recent studies have revealed efficient out-of-plane spin-orbit torques (SOTs) in a variety of materials for field-free type-z SOT switching. Here, we report on the corresponding type-x configuration, showing significant in-plane unconventional spin polarizations from sputtered ultrathin [Pt/Co]N, which are either highly textured on single crystalline MgO substrates or randomly textured on SiO2 coated Si substrates. The unconventional spin currents generated in the low-dimensional Co films result from the strong orbital magnetic moment, which has been observed by X-ray magnetic circular dichroism (XMCD) measurement. The x-polarized spin torque efficiency reaches up to −0.083 and favors complete field-free switching of CoFeB magnetized along the in-plane charge current direction. Micromagnetic simulations additionally demonstrate its lower switching current than type-y switching, especially in narrow current pulses. Our work provides additional pathways for electrical manipulation of spintronic devices in the pursuit of high-speed, high-density, and low-energy non-volatile memory.

Robust spin torque switching of noncollinear antiferromagnet Mn3Sn

Electrical switching of topological antiferromagnetic states in Mn3Sn thin films has been a subject of active investigation. However, dependences of switching behaviors on the film thickness, external field, and crystal orientation remain to be fully explored, which motivate the present study. In this work, (112̄0)-orientated Mn3Sn thin films are fabricated on sapphire substrates, in which a large anomalous Hall effect over a wide temperature range (270–400 K) can be identified. The current-induced spin–orbit torques (SOTs) are utilized to electrically manipulate the topological antiferromagnetic states in Mn3Sn/Pt bilayers. The robust SOT switching can be realized in Mn3Sn films with thicknesses up to 100 nm and with in-plane fields up to 1200 mT. Furthermore, SOT switching behaviors that are independent of the choice of crystal orientations are clearly revealed. Our results could be useful for implementing Mn3Sn films for efficient and stable antiferromagnetic spintronics.

Symmetry breaking for current-induced magnetization switching

Electromagnetic phenomena, such as magnetization switching, are guided by parity and time-reversal symmetries. Magnetic field and magnetization are time-odd axial vectors. Therefore, the magnetic field can switch magnetization reversibly. In contrast, the electric field is a time-even polar vector that cannot directly switch magnetization. For magnetic recording, an electrical coil-generated local magnetic field is used to switch the magnetic bit. However, in order to integrate the magnetic functionality, e.g., nonvolatile magnetic memory with high speed and low energy consumption, into the chip, it is essential to implement the magnetization switching by an electrical current, where the current induces other axial vectors through spin-transfer torque or spin–orbit torque (SOT). As an energy-efficient tool of magnetization switching, current-induced SOT has been intensively studied for the past decade, which holds great promise in the next generation of magnetic memories and magnetic logic devices [A. Manchon et al., Rev. Mod. Phys. 91, 035004 (2019); X. Han et al., Appl. Phys. Lett. 118, 120502 (2021); C. Song et al., Prog. Mater. Sci. 118, 100761 (2021); Q. Shao et al., IEEE Trans. Magn. 57, 21076639 (2021); J. Ryu et al., Adv. Mater. 32, 1907148 (2020); Y. Cao et al., iScience 23, 101614 (2020)]. In this review, we will first give the basic principle of the symmetry considerations for current-induced magnetization switching. Then, different methods to break the mirror symmetry for deterministic SOT switching will be discussed, together with examples that contain recent progress. In the end, we will give a discussion on the challenges and perspectives of the symmetry designs for SOT, which aim to inspire future fundamental studies and device applications.

Field-Free Magnetization Switching in A1 CoPt Single-Layer Nanostructures for Neuromorphic Computing

Current-induced field-free magnetization switching in single-layer films has been widely investigated for the application of spin–orbit torque (SOT) drivers in low-power, high-density, and nano-sized memory. In this paper, we report field-free SOT switching with threefold angle dependence in A1 disordered single-layer CoPt using the MgO (111) substrate. It is found that the magnetization could not switch in the absence of an external in-plane magnetic field when the current was flowing in the direction of the mirror symmetry high-symmetry axis ([11–2]). While along the direction of the low-symmetry axis ([1–10]) which destroyed the mirror symmetry to the greatest extent, field-free switching could be performed, and the switching ratio was up to 30%. This was mainly attributed to the composition gradient-induced in-plane inversion asymmetry and the low symmetry at the interface of Co/Pt broken out-of-plane inversion. Furthermore, CoPt devices with nanoscale thickness exhibited stable multistate magnetic switching behavior without an external magnetic field, and simultaneously nonlinear synaptic characteristics were obtained. A neural network was established, in which the synaptic weights between the hidden layer and the output layer are updated by using the resistance state of SOT synaptic devices, and the network could achieve about 90.54% recognition accuracy when applied to MNIST’s handwritten digital dataset. This work confirmed that a field-free SOT switch could be realized in single-layer CoPt, which provided theoretical guidance and data support for spin devices.

Field-free spin-orbit torque switching via out-of-plane spin-polarization induced by an antiferromagnetic insulator/heavy metal interface

Manipulating spin polarization orientation is challenging but crucial for field-free spintronic devices. Although such manipulation has been demonstrated in a limited number of antiferromagnetic metal-based systems, the inevitable shunting effects from the metallic layer can reduce the overall device efficiency. In this study, we propose an antiferromagnetic insulator-based heterostructure NiO/Ta/Pt/Co/Pt for such spin polarization control without any shunting effect in the antiferromagnetic layer. We show that zero-field magnetization switching can be realized and is related to the out-of-plane component of spin polarization modulated by the NiO/Pt interface. The zero-field magnetization switching ratio can be effectively tuned by the substrates, in which the easy axis of NiO can be manipulated by the tensile or compressive strain from the substrates. Our work demonstrates that the insulating antiferromagnet based heterostructure is a promising platform to enhance the spin-orbital torque efficiency and achieve field-free magnetization switching, thus opening an avenue towards energy-efficient spintronic devices.

Field-free magnetization switching in CoPt induced by noncollinear antiferromagnetic Mn3Ga

The magnetic spin Hall effect (MSHE) may produce the out-of-plane spin current with $z$-direction polarization $({\mathbit{\ensuremath{\sigma}}}_{\mathbf{z}})$, thus allowing the field-free switching of perpendicular magnetization in ferromagnetic films. Here, we fabricate the epitaxial $L{1}_{1}\text{\ensuremath{-}}\mathrm{CoPt}/D{0}_{19}\text{\ensuremath{-}}\mathrm{M}{\mathrm{n}}_{3}\mathrm{Ga}$ bilayers on MgO(111) substrates and characterize their magnetic and electrical transport properties. In the $L{1}_{1}\text{\ensuremath{-}}\mathrm{CoPt}/D{0}_{19}\text{\ensuremath{-}}\mathrm{M}{\mathrm{n}}_{3}\mathrm{Ga}$ bilayers, we observe the spin-orbit torque switching of perpendicular magnetization under zero external field. The magnetization switching ratio and polarity are specifically related to the N\'eel vector direction of the antiferromagnetic ${\mathrm{Mn}}_{3}\mathrm{Ga}$ as well as the crystalline orientations which the pulse current is applied along. The field-free switching in the bilayers is attributed to the ${\mathbit{\ensuremath{\sigma}}}_{\mathbf{z}}$ spin current generated by the MSHE, which is driven by the reduced symmetry of ${\mathrm{Mn}}_{3}\mathrm{Ga}$ film with noncollinear spin structure.

Magnetization switching process by dual spin–orbit torque in interlayer exchange-coupled systems

We report current-induced magnetization switching in Pt/Co/Ir/Co/Pt multilayers with different Ir layer thicknesses (tIr), where the perpendicularly magnetized Co layers are coupled ferromagnetically or antiferromagnetically through an interlayer exchange coupling and are sandwiched by the Pt spin Hall layers. The domain structures formed during switching vary depending on the magnetization alignment, i.e., a ferromagnetically coupled or antiferromagnetically coupled configuration. These results clarify the macroscopic picture of switching process for interlayer exchange-coupled systems. The local picture of the switching process is also examined by a numerical calculation based on a macrospin model, which reveals the switching dynamics triggered by dual spin–orbit torques for both antiferromagnetically and ferromagnetically coupled cases. The numerical calculation shows that the dual spin–orbit torques from the two Pt layers effectively act on the two Co layers not only for the antiferromagnetically coupled case but also for the ferromagnetically coupled one. Our findings deepen the understanding of the switching mechanism in a magnetic multilayer and provide an avenue to design spintronic devices with more efficient spin–orbit torque switching.

Unusual spin–orbit torque switching in perpendicular synthetic antiferromagnets with strong interlayer exchange coupling

Synthetic antiferromagnet (SAF) is an outstanding system for controlling magnetic coupling via tuning the layer thickness and material composition. Here, we control the interlayer exchange coupling (IEC) in a perpendicularly magnetized SAF Pt/Co/Ir/CoFeB/MgO multilayer, which is tuned by varying the nonmagnetic layer Ir thickness and the magnetic layer Co thickness. And we study the spin–orbit torque (SOT) driven magnetization switching of the SAF. In the SAF with a weak IEC, the SOT-driven switching behavior is similar to that of a single ferromagnet system, which is dominated by the external magnetic field. In contrast, in the SAF with an ultra-strong IEC, the saturation magnetic field is large than 50 kOe, and the SOT-driven switching behavior is decided by the effective magnetic field. The effective field is correlated to the external magnetic field, the IEC field, magnetic moments of CoFeB and Co, and magnetic anisotropy. These results may advance the understanding of SOT switching of perpendicular SAFs and promote the applications of SAFs with low stray fields and lower power in spintronic devices.