Spin-torque Devices Research Articles

Micromagnetic modeling of double spin-torque magnetic tunnel junction devices

Emerging nonvolatile magnetoresistive random access memory exhibits high endurance and long data retention compared to flash memory. Additionally, devices with double spin torque magnetic tunnel junctions (dsMTJ) featuring two magnetic reference layers demonstrate enhanced torques, fast switching, and reduced switching currents. To accurately model these devices, we adopt a coupled spin and charge transport approach allowing to describe spin-transfer torques in metallic spin valves and magnetic tunnel junctions on equal footing. Our findings indicate the critical influence of metallic non-magnetic spacers (NMS) properties separating the free layer from the second reference layer on the switching speed improvement in dsMTJ devices.

Tunable Colossal Anomalous Hall Conductivity in Half-Metallic Material Induced by d-Wave-Like Spin-Orbit Gap.

The anomalous Hall conductivity (AHC) in magnetic materials, resulting from inverted band topology, has emerged as a key adjustable function in spin-torque devices and advanced magnetic sensors. Among systems with near-half-metallicity and broken time-reversal symmetry, cobalt disulfide (CoS2) has proven to be a material capable of significantly enhancing its AHC. In this study, the AHC of CoS2 is empirically assessed by manipulating the chemical potential through Fe- (hole) and Ni- (electron) doping. The primary mechanism underlying the colossal AHC is identified through the application of density functional theory and tight-binding analyses. The main source of this substantial AHC is traced to four spin-polarized massive Dirac dispersions in the kz = 0 plane of the Brillouin zone, located slightly below the Fermi level. In Co0.95Fe0.05S2, the AHC, which is directly proportional to the momentum-space integral of the Berry curvature (BC), reached a record-breaking value of 2507 Ω-1cm-1. This is because the BCs of the four Dirac dispersions all exhibit the same sign, a consequence of the d-wave-like spin-orbit coupling among spin-polarized egorbitals.

Side-jump scattering enhanced spin Hall effect in SrTiO3-implanted Pt

A spin Hall effect (SHE) enables the electrical generation and detection of spin currents for promising applications in spintronics, but heavy metals with low spin Hall angle θSH limit the development of SHE devices. In this work, we have introduced dielectric oxide material SrTiO3 into Pt by magnetron sputtering and measured the θSH on the NiCo/Pt1–x(STO)x heterostructure through spin-torque ferromagnetic resonance. Our results demonstrate that the maximum spin Hall angle in Pt0.98(STO)0.02 is 0.121 ± 0.003, which is approximately twice that of pure Pt (0.064 ± 0.003). Moreover, theoretical analysis has revealed that the spin Hall angle arises from a complementary interplay between intrinsic and extrinsic mechanisms, namely, the strong spin–orbit coupling in Pt for the intrinsic mechanism and side-jump scattering caused by scalar potential and lattice expansion at dielectric impurities for the extrinsic mechanism. This interplay significantly contributes to enhancing the spin Hall angle. This work demonstrates an effective strategy for fabricating high-performance spin Hall materials with low resistivity, large spin Hall angle, and excellent compatibility with semiconductor processes in low-power spin-torque devices.

Controlling Selection Rules for Magnon Scattering in Nanomagnets by Spatial Symmetry Breaking

Nanomagnets are the building blocks of many existing and emergent spintronic technologies. The magnetization dynamics of nanomagnets is often dominated by nonlinear processes, which have been recently shown to have many surprising features and far-reaching implications for applications. Here we develop a theoretical framework uncovering the selection rules for multimagnon processes and discuss their underlying mechanisms. For its technological relevance, we focus on the degenerate three-magnon process in thin elliptical nanodisks to illustrate our findings. We parameterize the selection rules through a set of magnon interaction coefficients which we calculate using micromagnetic simulations. We postulate the selection rules and investigate how they are altered by perturbations that break the symmetry of static magnetization configuration and spatial spin-wave profiles and that can be realized by applying off-symmetry-axis or nonuniform magnetic fields. Our work provides the phenomenological understanding of the mechanics of magnon interaction as well as the formalism for determining the interaction coefficients from simulations and experimental data. Our results serve as a guide to analyze the magnon processes inherently present in spin-torque devices in order to boost their performance or to engineer a specific nonlinear response in a nanomagnet used in a neuromorphic or quantum magnonic application.

Surface properties, chemical reactivity, and ambient stability of cadmium diarsenide CdAs2, a topological chiral material hosting Kramers-Weyl fermions

Chiral materials showing Kramers-Weyl fermions represent a suitable platform for quantum technology, i.e., for engineering quantum solenoids, spin-torque devices, polarization-sensitive photodetectors based on quantized circular photogalvanic effect, etc. Accordingly, the stability of this class of materials in oxidative environments, such as the ambient atmosphere, should be carefully investigated to succeed in technology transfer. Here, taking as case-study example the well-recognized topological chiral system cadmium diarsenide (CdAs2), we assess its chemical reactivity towards ambient gases (oxygen and water) and air by density functional theory and experiments. The surface of CdAs2 evolves into an oxide skin, but its thickness remains nanometric even after one year in air, as directly imaged by high-resolution transmission electron microscopy. Accordingly, it is evident that future quantum devices based on Kramers-Weyl fermions could be stable in air, as the oxide layer formed on chiral quantum materials only represents a native oxide, which actually protects bulk features, including Kramers-Weyl fermions (correlated to bulk band structure), from degradation in air.

Magnetization switching probability in the dynamical switching regime driven by spin-transfer torque

We study magnetization switching in the dynamical regime for in-plane magnetized systems, useful for field-free spin-orbit torque switching devices. We derive a formula for the switching probability, characterized by the thermal stability factor, critical current density, and the linewidth of ferromagnetic resonance. The formula agrees well with numerical simulations based on the Landau-Lifshitz-Gilbert equation. To study the viability of the theory developed, the switching probability of an in-plane magnetized ferromagnet in three-terminal spin-orbit torque switching devices is measured. We find that the transition width of the switching probability versus current increases with decreasing pulse width. The shape of the probability density, the current derivative of the switching probability, changes with varying pulse width. These characteristics are in good agreement with the theory developed. From the analyses, we show that the dynamical and thermally activated switching regimes can be distinguished simply from the shape of the probability density. The formula therefore provides useful means to analyze the switching probability of two-terminal spin torque and three-terminal spin-orbit torque switching devices.

Robust spin injection via thermal magnon pumping in antiferromagnet/ferromagnet hybrid systems

Robust spin injection and detection in antiferromagnetic thin films is a prerequisite for the exploration of antiferromagnetic spin dynamics and the development of nanoscale antiferromagnet-based spintronic applications. Previous studies have shown spin injection and detection in antiferromagnet/nonmagnetic metal bilayers; however, spin injection in these systems has been found effective at cryogenic temperatures only. Here, we experimentally demonstrate sizable interfacial spin transport in a hybrid antiferromagnet/ferromagnet system, consisting of ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ and permalloy, which remains robust up to the room temperature. We examine our experimental data within a spin diffusion model and find evidence for the important role of interfacial magnon pumping in the signal generation. The results bridge spin-orbitronic phenomena of ferromagnetic metals with antiferromagnetic spintronics and demonstrate an advancement toward antiferromagnetic spin-torque devices.

Two Terminal Perpendicular Spin Transfer Torque Magnetic Random Access Memory Based Stochastic Nature Spin Neuron Device for Spiking Neural Network

In recent years, research on neuromorphic chips of spin-based devices capable of low-power, ultra-high-speed switching operation has been actively conducted. There were reports of spin-based neuromorphic devices using spin-transfer-torque (STT), which are assisted by an external magnetic field [1] and spin-orbit-torque (SOT) spin neurons via heavy metal that have a high spin Hall angle. [2-4] Since these spin neurons operate with pulse bias, they are suitable for calculating spiking neural network (SNN), which is a low-power artificial neural network (ANN) that operates similar to the human brain. However, the previously announced devices required an external magnetic field or implemented in a three-terminal method (16F2), making fabrication impossible for high density and low-power devices. In this study, a 300 nm-scale 2-terminal (4F2) perpendicular STT-based (p-STT) spin neuron device capable of switching operation without an external magnetic field was fabricated to demonstrate stochastic neuron nature. Furthermore, an SNN was constructed using fitting data and an MNIST pattern recognition test was conducted.As shown in Fig. 1-(a), a 300 nm pattern process was processed via E-beam lithography of a double MgO p-STT magnetic tunnel junction (MTJ) with a tunnel-magnetoresistance (TMR) ratio of 158.13%. Figure 1-(b) is a transmission electron microscopy (TEM) image of a 300 nm scale p-STT spin based neuron. Figure 1-(c) is a DC sweep R-V graph of a spin neuron. It was confirmed that the resistance was 985 Ω in parallel state (LRS) and 2137 Ω in anti-parallel state (HRS) relatively, and the difference of resistance was 1114 Ω. To measure the stochastic characteristics depending on the set voltage, the set voltage pulse amplitude was varied from -0.61 mV to -0.695 mV with pulse width of 250 μs. It was confirmed that switching is impossible below -0.61 V, and switching occurs with one pulse above -0.695 V. In addition, as a result of the measurement the switching probability depending on the set voltage pulse was fitted with a sigmoid function. Furthermore, we performed a pattern recognition simulation assuming that p-STT neurons were applied in a single-layer SNN consisting of 784 input layers and 100 output layers. In this simulation, the images used for pattern recognition were MNIST handwritten digit images with a size of 28X28 pixels, 60000 images were used for training, and 10000 images not used for training were used for testing. In our presentation, we will review in detail the stochastic nature of the p-STT neuron depending on the spike amplitude and SNN pattern recognition.This research was supported by National R&D Program through the National Research Foundation of Korea(NRF) funded by Ministry of Science and ICT(2021M3F3A2A01037733) Reference [1] Kondo, Kei, et al. "A two-terminal perpendicular spin-transfer torque based artificial neuron." Journal of Physics D: Applied Physics 51.50 (2018): 504002.[2] Kurenkov, Aleksandr, et al. "Artificial neuron and synapse realized in an antiferromagnet/ferromagnet heterostructure using dynamics of spin–orbit torque switching." Advanced Materials 31.23 (2019): 1900636.[3] Ostwal, Vaibhav, et al. "Spin-torque devices with hard axis initialization as Stochastic Binary Neurons." Scientific reports 8.1 (2018): 1-8.[4] Reidy, Brendan, and Ramtin Zand. "SOT-MRAM based Sigmoidal Neuron for Neuromorphic Architectures." arXiv preprint arXiv:2006.01238 (2020). Figure 1

Magnetic reversal and pinning in a perpendicular zero-moment half-metal

Compensated ferrimagnets are promising materials for fast spintronic applications based on domain wall motion as they combine the favourable properties of ferromagnets and antiferromagnets. They inherit from antiferromagnets immunity to external fields, fast spin dynamics and rapid domain wall motion. From ferromagnets they inherit straightforward ways to read out the magnetic state, especially in compensated half metals, where electrons flow in only one spin channel. Here, we investigate domain structure in compensated half-metallic Mn2Ru0.5Ga films and assess their potential in domain wall motion-based spin-electronic devices. Our focus is on understanding and reducing domain wall pinning in unpatterned epitaxial thin films. Two modes of magnetic reversal, driven by nucleation or domain wall motion, are identified for different thin film deposition temperatures $(T_{dep})$. The magnetic aftereffect is analysed to extract activation volumes $(V^*)$, activation energies $(E_A)$, and their variation $({\Delta}E_A)$. The latter is decisive for the magnetic reversal regime, where domain wall motion dominated reversal (weak pinning) is found for ${\Delta}E_A<0.2$ eV and nucleation dominated reversal (strong pinning) for ${\Delta}E_A>0.5$ eV. A minimum ${\Delta}E_A=28$ meV is found for $T_{dep}=290{\deg}$C. Prominent pinning sites are visualized by analysing virgin domain patterns after thermal demagnetization. In the sample investigated they have spacings of order 300 nm, which gives an upper limit of the track-width of spin-torque domain-wall motion-based devices.

Nonlinear dynamics of magnetization evolution in orthogonal spin torque devices: Phases and classification

The magnetization evolution of the free layer in an orthogonal spin torque device is studied based on a macrospin model. The trajectory of the magnetization vector under various conditions has shown rich nonlinear properties. The phase diagram is obtained in the parameter spaces of current density and the polarization distribution (the ratio of polarization of in-plane to out-of-plane layers), where two critical currents and three phases are found. These dynamic phases can be classified according to their nonlinear behaviors, which are different in terms of limit cycles and limit points. The classification is meaningful to design ultra-fast spin torque devices under different dynamic conditions toward various applications, such as in memory and oscillators.

Resonant Enhancement of Exchange Coupling for Voltage-Controlled Magnetic Switching

We propose an electric-field-controlled mechanism for magnetization switching assisted solely by the interlayer-exchange coupling (IEC) between the fixed and the free magnets, which are separated by two oxide barriers sandwiching a spacer material known for exhibiting large IEC. The basic idea relies on the formation of a quantum-well (QW) within the spacer material and controlling the transmission coefficient across the structure with an electric-field via the resonant tunneling phenomena. Using non-equilibrium Green's function (NEGF) method, we show that the structure can exhibit a bias-dependent oscillatory IEC that can switch the free magnet to have either a parallel or an antiparallel configuration with respect to the fixed magnet, depending on the sign of the IEC. Such bi-directional switching can be achieved with the same voltage polarity but different magnitudes. With proper choice of the spacer material, the current in the structure can be significantly reduced. Due to the conservative nature of the exerted torque by the IEC, the switching threshold of the proposed mechanism is decoupled from the switching speed, while the conventional spin-torque devices exhibit a trade-off due to the non-conservative nature of the exerted torque.

(Invited) Ferromagnetism and Exchange Coupling with Fe-Doped WSe2

The incorporation of magnetic dopants is a way to realize magnetism in semiconductors. The 2D material WSe2 is a semiconductor with a bandgap of 1.3 eV, and Fe-doped WSe2 is predicted to have room temperature, long-range ferromagnetism. Moreover, WSe2 has been extensively studied for applications in electronic devices, and can be prepared by deposition methods that are compatible with semiconductor fabrication processes. 2D magnetic films are particularly intriguing in technologies that rely on exchange coupling since the efficiency of the exchange with the magnet is inversely proportional to the magnet thickness 1/tFM. Chalcogenide-based TMD magnets can also have ideal interfaces with chalcogenide-based topological insulators (like Bi2Se3), enabling efficient spin torque devices and potentially realizing the quantum Hall effect without an external magnetic field.In this work, we demonstrate the growth of Fe-doped WSe2 and report on its up to room temperature ferromagnetic properties. The evolution of Fe-doped WSe2 films under different doping levels indicates that strain from the impurity atom itself coupled with strain imparted by the substrate can drive phase separation at high Fe concentrations. We find that suppressing that film strain helps promote more Fe incorporation (and higher Curie temperatures).Cr2O3 has only 0.2% lattice mismatch with WSe2 and is a magnetoelectric that can couple with ferromagnetic films. By using seed-layer techniques, we successfully prepared layered Fe-WSe2 on Cr2O3 and demonstrate magnetic coupling between the two, a potentially promising route toward realizing electric field control of 2D magnets. Magnetic measurements show a clear hysteresis loop at 100 K in a 2 Tesla cooling field, from which a large negative bias field of 600 Oe is resolved. The bias field vs temperature mesurements, where a negative bias field emerges below 250 K, indicates the existence of ferromagnetic exchange coupling in this heterostructure.This work is supported in part by NEWLIMITS, a center in nCORE, a Semiconductor Research Corporation (SRC) program sponsored by NIST through award number 70NANB17H041. This work is also supported by the National Science Foundation under awards 1917025 and 1921818.

Fabrication of spin torque devices by using quasi-antiferromagnetic materials

The devices we use on a day-to-day basis require a substantial amount of energy to power, but even more energy is left unused and goes to waste. What if our devices were (at least nearly) completely energy efficient? This would help pave the way towards a greener and more energy smart future. This concept is something one team of Japanese researchers is working on, by using quasi antiferromagnetic (AFM) materials which work successfully at the nanoscale. Professor Hiroma Yuasa is based at Kyushu University, Japan, where her laboratory is working on spintronics research. Currently, her focus includes spin current physics and spin torque in artificial magnetic structures, including the applications of these, such as in energy harvesting and ultra-low power consumption devices, which could help in achieving a greener future.

Orbital torque: Torque generation by orbital current injection

We propose a mechanism of torque generation by injection of an orbital current, which we call $\textit{orbital torque}$. In a magnetic bilayer consisting of a nonmagnet (NM) and a ferromagnet (FM), we consider a situation where the spin-orbit coupling (SOC) is present only in the FM. Although the SOC is absent in the NM, the orbital Hall effect can arise in the NM. When the resulting orbital Hall current is injected to the FM, the SOC of the FM converts the orbital angular momentum into spin, which exerts torque to the magnetization of the FM. Remarkably, even for small SOC strength comparable to that of $3d$ FMs, the orbital torque can be comparable to the spin torque induced by the spin Hall effect of the NM with strong SOC. This provides a way to experimentally probe the OHE and opens a new venue to achieving spin-torque devices based on light elements that exhibit gigantic orbital response. Experimental implications are discussed.

Giant nonlinear damping in nanoscale ferromagnets.

Magnetic damping is a key metric for emerging technologies based on magnetic nanoparticles, such as spin torque memory and high-resolution biomagnetic imaging. Despite its importance, understanding of magnetic dissipation in nanoscale ferromagnets remains elusive, and the damping is often treated as a phenomenological constant. Here, we report the discovery of a giant frequency-dependent nonlinear damping that strongly alters the response of a nanoscale ferromagnet to spin torque and microwave magnetic field. This damping mechanism originates from three-magnon scattering that is strongly enhanced by geometric confinement of magnons in the nanomagnet. We show that the giant nonlinear damping can invert the effect of spin torque on a nanomagnet, leading to an unexpected current-induced enhancement of damping by an antidamping torque. Our work advances the understanding of magnetic dynamics in nanoscale ferromagnets and spin torque devices.

Enhancing Spin-Orbit Torque by Strong Interfacial Scattering From Ultrathin Insertion Layers

Increasing dampinglike spin-orbit torque (SOT) is both of fundamental importance for enabling new research into spintronics phenomena and also technologically urgent for advancing low-power spin-torque memory, logic, and oscillator devices. Here, we demonstrate that enhancing interfacial scattering by inserting ultra-thin layers within a spin Hall metals with intrinsic or side-jump mechanisms can significantly enhance the spin Hall ratio. The dampinglike SOT was enhanced by a factor of 2 via sub-monolayer Hf insertion, as evidenced by both harmonic response measurements and current-induced switching of in-plane magnetized magnetic memory devices with the record low critical switching current of ~73 uA (switching current density of 3.6x10^6 A/cm^2). This work demonstrates a very effective strategy for maximizing dampinglike SOT for low-power spin-torque devices.

A cryogenic spin-torque memory element with precessional magnetization dynamics

We present a study of precessional magnetization switching in orthogonal spin-torque spin-valve devices at low temperatures. The samples consist of a spin-polarizing layer that is magnetized out-of-the film plane and an in-plane magnetized free and reference magnetic layer separated by non-magnetic metallic layers. We find coherent oscillations in the switching probability, characterized by high speed switching (~200 ps), error rates as low as 10−5 and decoherence effects at longer timescales (~1 ns). Our study, which is conducted over a wide range of parameter space (pulse amplitude and duration) with deep statistics, demonstrates that the switching dynamics are likely dominated by the action of the out-of-plane spin polarization, in contrast to in-plane spin-torque from the reference layer, as has been the case in most previous studies. Our results demonstrate that precessional spin-torque devices are well suited to a cryogenic environment, while at room temperature they have so far not exhibited coherent or reliable switching.

Spin-torque devices with hard axis initialization as Stochastic Binary Neurons

Employing the probabilistic nature of unstable nano-magnet switching has recently emerged as a path towards unconventional computational systems such as neuromorphic or Bayesian networks. In this letter, we demonstrate proof-of-concept stochastic binary operation using hard axis initialization of nano-magnets and control of their output state probability (activation function) by means of input currents. Our method provides a natural path towards addition of weighted inputs from various sources, mimicking the integration function of neurons. In our experiment, spin orbit torque (SOT) is employed to “drive” nano-magnets with perpendicular magnetic anisotropy (PMA) -to their metastable state, i.e. in-plane hard axis. Next, the probability of relaxing into one magnetization state (+mi) or the other (−mi) is controlled using an Oersted field generated by an electrically isolated current loop, which acts as a “charge” input to the device. The final state of the magnet is read out by the anomalous Hall effect (AHE), demonstrating that the magnetization can be probabilistically manipulated and output through charge currents, closing the loop from charge-to-spin and spin-to-charge conversion. Based on these building blocks, a two-node directed network is successfully demonstrated where the status of the second node is determined by the probabilistic output of the previous node and a weighted connection between them. We have also studied the effects of various magnetic properties, such as magnet size and anisotropic field on the stochastic operation of individual devices through Monte Carlo simulations of Landau Lifshitz Gilbert (LLG) equation. The three-terminal stochastic devices demonstrated here are a critical step towards building energy efficient spin based neural networks and show the potential for a new application space.

Efficient Injection and Detection of Out-of-Plane Spins via the Anomalous Spin Hall Effect in Permalloy Nanowires.

We report a novel mechanism for the electrical injection and detection of out-of-plane spin accumulation via the anomalous spin Hall effect (ASHE), where the direction of the spin accumulation can be controlled by manipulating the magnetization of the ferromagnet. This mechanism is distinct from the spin Hall effect (SHE), where the spin accumulation is created along a fixed direction parallel to an interface. We demonstrate this unique property of the ASHE in nanowires made of permalloy (Py) to inject and detect out-of-plane spin accumulation in a magnetic insulator, yttrium iron garnet (YIG). We show that the efficiency for the injection/detection of out-of-plane spins can be up to 50% of that of in-plane spins. We further report the possibility to detect spin currents parallel to the Py/YIG interface for spins fully oriented in the out-of-plane direction, resulting in a sign reversal of the nonlocal magnon spin signal. The new mechanisms that we have demonstrated are highly relevant for spin torque devices and applications.

Ion irradiation-induced easy-cone anisotropy in double-MgO free layers for perpendicular magnetic tunnel junctions

We have used the ferromagnetic resonance in the X-band (9.37 GHz) to investigate the effect of 400 keV Ar+ irradiation on the perpendicular magnetic anisotropy (PMA) and Gilbert damping parameter, α, of double-MgO free layers designed for application in perpendicular magnetic tunnel junctions. The samples comprised a MgO/Fe72Co8B20/X(0.2 nm)/Fe72Co8B20/MgO layer stack, where X stands for an ultrathin Ta or W spacer. Samples with two different total FeCoB layer thicknesses, tFCB = 3.0 nm and tFCB = 2.6 nm, were irradiated with ion fluences ranging from 1012 cm−2 to 1016 cm−2. The effective first-order PMA field, BK1, decreased nearly linearly with the logarithm of the fluence for both FeCoB thicknesses and spacer elements. The decrease in BK1, which is likely caused by an ion-induced intermixing at the FeCoB/MgO interfaces, resulted in a reorientation of the magnetization of the free layers with tFCB = 2.6 nm, initially exhibiting a perpendicular easy-axis anisotropy. For intermediate fluences, 1013 cm−2 and 1014 cm−2, easy-cone states with different cone angles could be induced in the free layer with a W spacer. Importantly, no corresponding increase in the Gilbert damping was observed. This study shows that ion irradiation can be used to tune the easy-cone anisotropy in perpendicular magnetic tunnel junctions, which is interesting for spintronic applications such as spin-torque magnetic memory devices, oscillators, and sensors.