Spin-orbit Torque Research Articles

Numerical study of two-terminal SOT-MRAM

A fully three-dimensional model coupling charge, spin, magnetization, and temperature dynamics is employed to study the switching behavior of the two-terminal spin–orbit torque magnetoresistive random access memory (2T-SOT-MRAM). We demonstrated that free layer (FL) spin-transfer torque (STT) switching is strongly affected by the inclusion of the SOT. The nucleation of the FL magnetization reversal is similar to the conventional three-terminal (3T)-SOT-MRAM. In comparison to the traditional STT-MRAM, the switching times and critical switching voltages are considerably reduced. Furthermore, the full thermal simulations show the significance of the heating process and temperature modeling in MRAM devices as they demonstrate a significantly lower switching time than the one in a constant temperature model.

Microscopic theory of spin-orbit torque and spin memory loss from interfacial spin-orbit coupling

Microscopic theory of spin-orbit torque and spin memory loss from interfacial spin-orbit coupling

Terahertz Néel spin-orbit torques drive nonlinear magnon dynamics in antiferromagnetic Mn2Au

Antiferromagnets have large potential for ultrafast coherent switching of magnetic order with minimum heat dissipation. In materials such as Mn2Au and CuMnAs, electric rather than magnetic fields may control antiferromagnetic order by Néel spin-orbit torques (NSOTs). However, these torques have not yet been observed on ultrafast time scales. Here, we excite Mn2Au thin films with phase-locked single-cycle terahertz electromagnetic pulses and monitor the spin response with femtosecond magneto-optic probes. We observe signals whose symmetry, dynamics, terahertz-field scaling and dependence on sample structure are fully consistent with a uniform in-plane antiferromagnetic magnon driven by field-like terahertz NSOTs with a torkance of (150 ± 50) cm2 A−1 s−1. At incident terahertz electric fields above 500 kV cm−1, we find pronounced nonlinear dynamics with massive Néel-vector deflections by as much as 30°. Our data are in excellent agreement with a micromagnetic model. It indicates that fully coherent Néel-vector switching by 90° within 1 ps is within close reach.

Optimization of growth of large-area SnS thin films and heterostructures for spin pumping and spin-orbit torque

Optimization of growth of large-area SnS thin films and heterostructures for spin pumping and spin-orbit torque

High efficient field-free magnetization switching via exchange bias effect induced by antiferromagnetic insulator interface

Spin–orbit torque induced deterministic magnetization switching typically requires the assistance of an external magnetic field for symmetry breaking. However, achieving field-free switching in perpendicular magnetized layers is crucial for expanding the market of high-density memory. Previous reports have utilized exchange bias, an antiferromagnetic interfacial effect, to realize field-free magnetization switching. However, metallic antiferromagnetic layers will introduce shunting effects that reduce switching efficiency and the Néel vector becomes unstable when current flows through the antiferromagnetic layer. In this study, we achieved the zero-field magnetization switching in NiO/Pt/Co/Pt multilayers. Simulation results demonstrate higher efficiency compared to metallic antiferromagnetic IrMn-based structures. Our findings highlight that the insulator antiferromagnetic can provide an exchange bias field, eliminating the need for an external magnetic field. By avoiding shunting effects, our designed structure offers a more efficient approach for spintronic devices.

Theory for current-induced intrinsic fieldlike spin-orbit torque in topological materials

Theory for current-induced intrinsic fieldlike spin-orbit torque in topological materials

Zero-Frequency Chiral Magnonic Edge States Protected by Nonequilibrium Topology.

Topological bosonic excitations must, in contrast to their fermionic counterparts, appear at finite energies. This is a key challenge for magnons, as it prevents straightforward excitation and detection of topologically protected magnonic edge states and their use in magnonic devices. In this Letter, we show that in a nonequilibrium state, in which the magnetization is pointing against the external magnetic field, the topologically protected chiral edge states in a magnon Chern insulator can be lowered to zero frequency, making them directly accessible by existing experimental techniques. We discuss the spin-orbit torque required to stabilize this nonequilibrium state, and show explicitly using numerical Landau-Lifshitz-Gilbert simulations that the edge states can be excited with a microwave field. Finally, we consider a propagating spin wave spectroscopy experiment, and demonstrate that the edge states can be directly detected.

Enhanced Rashba Spin Orbit Coupling and Magnetic Behavior at Oxide Heterointerfaces by Optical Gating.

Complex oxide heterointerfaces have been a hot research spot due to their rich physical phenomena and broad quantum coherence that respond to multiple external stimuli. Among these external stimuli, light is a very powerful one to manipulate properties such as carrier density and spin characteristics. However, achieving a light-magnetic correlation is in high demand for multifield responding devices, and its intrinsic mechanism remains unclear. Here, by illuminating Nd0.86Sr0.14Al0.86Ni0.14O3-SrTiO3 heterointerfaces using 360 nm light, we observe a series of interesting physical phenomena, like enhanced magnetoresistance (MR). More interestingly, a band splitting and strong Rashba spin-orbit coupling (SOC) effect occur after illumination, accompanied by a magnetic feature and thus leading to an anomalous Hall effect (AHE). Upon optical gating, the magnetism can be caused by Rashba SOC induced spin-orbit torque (SOT). The work will be sure to have great importance in both theoretical studies and all-oxide devices.

Challenges in electrical detection of spin-orbit torque in Ir20Mn80/Pt hetero-structures

Manipulation of antiferromagnetic sublattice orientations, a key challenge in spintronic device applications, requires unconventional methods such as current induced torques including Spin Transfer Torque (STT) and Spin-Orbit Torque (SOT). In order to observe the deviation of the Néel vector from the anisotropy axis, one of the simplest approaches is the electrical detection, whereby one monitors the change in resistance as a function of applied current. In this work, we have investigated the conditions under which an ultra-thin metallic antiferromagnet, Ir20Mn80 becomes susceptible to SOT effects by studying antiferromagnetic layer structure and thickness dependence in antiferromagnetic metal (Ir20Mn80)/heavy metal (Pt) superlattices. Our electrical measurements reveal that in bilayer structures there exists a shallow range of Ir20Mn80 thicknesses (∼1–2 nm) for which SOT driven control of spins is apparent, whereas for lower thicknesses incomplete sublattice formation and for higher thicknesses improved thermal stability prohibits vulnerability to spin currents. Furthermore, in multilayers, structural changes in Ir20Mn80 layer quenches local torques due to stronger (111) magnetocrystalline anisotropy. These results suggest that an exhaustive optimization of the antiferromagnet parameters is crucial for the successful deployment of spintronic devices.

Picosecond spin-orbit torque-induced coherent magnetization switching in a ferromagnet.

Electrically controllable nonvolatile magnetic memories show great potential for the replacement of conventional semiconductor-based memory technologies. Here, we experimentally demonstrate ultrafast spin-orbit torque (SOT)-induced coherent magnetization switching dynamics in a ferromagnet. We use an ultrafast photoconducting switch and a coplanar strip line to generate and guide a ~9-picosecond electrical pulse into a heavy metal/ferromagnet multilayer to induce ultrafast SOT. We then use magneto-optical probing to investigate the magnetization dynamics with sub-picosecond resolution. Ultrafast heating by the approximately 9 picosecond current pulse induces a thermal anisotropy torque which, in combination with the damping-like torque, coherently rotates the magnetization to obtain zero-crossing of magnetization in ~70 picoseconds. A macro-magnetic simulation coupled with an ultrafast heating model agrees well with the experiment and suggests coherent magnetization switching without any incubation delay on an unprecedented time scale. Our work proposes a unique magnetization switching mechanism toward markedly increasing the writing speed of SOT magnetic random-access memory devices.

Giant Spin-Orbit Torque in Sputter-Deposited Bi Films.

Bismuth (Bi) has the strongest spin-orbit coupling among non-radioactive elements and is thus a promising material for efficient charge-to-spin conversion. However, previous electrical detections have reported controversial results for the conversion efficiency. In this study, an optical detection of a spin-orbit torque is reported in a Bi/CoFeB bilayer with a polycrystalline texture of (012) and (003). Taking advantage of the optical detection, spin-orbit torque is accurately separated from the Oersted field and achieves a giant damping-like torque efficiency of +0.5, verifying efficient charge-to-spin conversion. This study also demonstrates a field-like torque efficiency of -0.1. For the mechanism of the charge-to-spin conversion, the bulk spin Hall effect and the interface Rashba-Edelstein effect are considered.

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.

Numerical analysis of voltage-controlled magnetization switching operation in magnetic-topological-insulator-based devices

We theoretically investigate influences of electronic circuit delay, noise, and temperature on write-error-rate (WER) in voltage-controlled magnetization switching operation of a magnetic-topological-insulator-based device by means of the micromagnetic simulation. This device realizes magnetization switching via spin–orbit torque (SOT) and voltage-controlled magnetic anisotropy (VCMA), which originate from the 2D-Dirac electronic structure. We reveal that the device operation is extremely robust against circuit delay and signal-to-noise ratio. We demonstrate that the WER on the order of ∼10−4 or below is achieved around room temperature due to steep change in VCMA. Also, we show that the larger SOT improves thermal stability factor. This study provides a next perspective for developing voltage-driven spintronic devices with ultra-low power consumption.

Tilted Magnetic Anisotropy with In‐Plane Broken Symmetry in Ru‐Substituted Manganite Films

AbstractControlling the magnetic anisotropy of materials is important in a variety of applications including magnetic memories, spintronic sensors, and skyrmion‐based devices. Ru‐substituted La0.7Sr0.3MnO3 (Ru‐LSMO) is an emerging material, showing tilted magnetic anisotropy (TMA) and possible nontrivial magnetic topologies. Here anisotropic in‐plane magnetization is reported in moderately compressed Ru‐LSMO films, coexisting with TMA. This combination is attractive for technological applications, such as spin‐orbit torque (SOT) based devices and other spintronic applications. A microstructural analysis of films of this material is presented, and Ru single ion anisotropy and strain‐induced structural mechanisms are found to be responsible for both the in‐plane anisotropy and the TMA. The manifestation of these properties in a correlated oxide with Curie temperature near room temperature highlights an attractive platform for technological realization of SOT and other spintronic devices. Illustrating the mechanisms behind these properties provides the necessary engineering space for harnessing these phenomena for practical devices.

Order parameter dynamics in Mn3Sn driven by DC and pulsed spin–orbit torques

We numerically investigate and develop analytic models for both the DC and pulsed spin–orbit-torque (SOT)-driven response of order parameter in single-domain Mn3Sn, which is a metallic antiferromagnet with an anti-chiral 120° spin structure. We show that DC currents above a critical threshold can excite oscillatory dynamics of the order parameter in the gigahertz to terahertz frequency spectrum. Detailed models of the oscillation frequency vs input current are developed and found to be in excellent agreement with the numerical simulations of the dynamics. In the case of pulsed excitation, the magnetization can be switched from one stable state to any of the other five stable states in the Kagome plane by tuning the duration or the amplitude of the current pulse. Precise functional forms of the final switched state vs the input current are derived, offering crucial insights into the switching dynamics of Mn3Sn. The readout of the magnetic state can be carried out via either the anomalous Hall effect or the recently demonstrated tunneling magnetoresistance in an all-Mn3Sn junction. We also discuss possible disturbance of the magnetic order due to heating that may occur if the sample is subject to large currents. Operating the device in a pulsed mode or using low DC currents reduces the peak temperature rise in the sample due to Joule heating. Our predictive modeling and simulation results can be used by both theorists and experimentalists to explore the interplay of SOT and the order dynamics in Mn3Sn and to further benchmark the device performance.

Electrical control of 180° domain walls in an antiferromagnet

We demonstrate the reversible current-induced motion of 180° antiferromagnetic domain walls in a CuMnAs device. By controlling the magnitude and direction of the current pulse, the position of a domain wall can be switched between three distinct pinning sites. The domain wall motion is attributed to a field-like spin–orbit torque that induces the same sense of rotation on each magnetic sublattice, owing to the crystal symmetry of CuMnAs. Domain wall motion is observed for current densities down to ≈2.5×1010 A/m2 at room temperature.

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.

Light modulated magnetism and spin–orbit torque in a heavy metal/ferromagnet heterostructure based on van der Waals-layered ferroelectric materials

This study presents experimental evidence of the substantial modulation of ferromagnetism and spin–orbit torque (SOT) efficiency in a Ta/Pt/Co/Ta/In2Se3 heterostructure with perpendicular magnetic anisotropy through light irradiation. An increase in SOT efficiency of ∼20% and a reduction in critical switching current density of ∼52% were observed. The significant modulation primarily originates from the photostrictive effect of a van der Waals-layered ferroelectric In2Se3 layer, which is assisted by thermal effects under light irradiation. The present research provides a potential approach to modulating magnetism and SOT efficiency for energy-efficient spintronic devices.

Spin-orbit torque magnetic tunnel junction based on 2-D materials: Impact of bias-layer on device performance

Two-dimensional (2D) material-based Spin-orbit torque (SOT) Magnetic tunnel junction (MTJ) has drawn a lot of research interest due to its potential for low power logic and memory applications. In this study, we employed a Bias-layer (BL) on top of the Topological insulator (TI) based SOT-MTJ structure to achieve field-free deterministic switching. A systematic micromagnetic simulation is carried out to examine the impact of variations in the BL-width (BW) and BL-distance (BLD) from the Free-layer (FL) on the performance parameters of the device. The stray field produced by the BL has a non-uniform profile; it declines with increase inBLD from the FL and improves with as BW increases. We demonstrate that by engineering the BLD and BW, the critical current density reduces to 3.9 × 109 A/m2, switching speed improves by 0.14 ns and power dissipation density reduces to 6.98 × 1014 Watt/m3as compared to the conventional SOT-MTJ with an external magnetic field of 100 mH.

Impact of inherent energy barrier on spin-orbit torques in magnetic-metal/semimetal heterojunctions.

Spintronic devices are based on heterojunctions of two materials with different magnetic and electronic properties. Although an energy barrier is naturally formed even at the interface of metallic heterojunctions, its impact on spin transport has been overlooked. Here, using diffusive spin Hall currents, we provide evidence that the inherent energy barrier governs the spin transport even in metallic systems. We find a sizable field-like torque, much larger than the damping-like counterpart, in Ni81Fe19/Bi0.1Sb0.9 bilayers. This is a distinct signature of barrier-mediated spin-orbit torques, which is consistent with our theory that predicts a strong modification of the spin mixing conductance induced by the energy barrier. Our results suggest that the spin mixing conductance and the corresponding spin-orbit torques are strongly altered by minimizing the work function difference in the heterostructure. These findings provide a new mechanism to control spin transport and spin torque phenomena by interfacial engineering of metallic heterostructures.