Spin-torque Nano-oscillators Research Articles

Enhancement of microwave frequency via heterogeneous magneto resistance tilted polarizer based spin torque nano oscillator with the inclusion of Oersted field

Here, we presented a heterogeneous Magneto Resistance Tilted Polarizer-based Spin Torque Nano Oscillator [MRTP-STNO] and theoretically included Current Induced Oersted Field [CIOF] via the macro-spin domain. The dynamics of magnetization provoked by Spin Transfer Torque [STT] are studied by solving Landau-Lifshitz-Gilbert-Slonczewski [LLGS] equation. In this device, β is the independent tilt angle of the fixed layer. In contrast, θ is the angle between the free layer magnetization and the easy axis of the device, respectively. Angles β and θ can be varied from 10° to 90° as an increment of 10°. The maximum obtained frequency of the device of about 215 GHz, and PSD of 1.7 μW/mA2/GHz with CIOF as 50 kA/m. We suggest the device is an ideal candidate for ultra High-Frequency and unveils a new path for foreseeable spintronic devices.

Modeling and Evaluation of Echo-State Networks Using Spin Torque Nano-Oscillators

Modeling and Evaluation of Echo-State Networks Using Spin Torque Nano-Oscillators

Tilted magnetic anisotropy-tailored spin torque nano-oscillators for neuromorphic computing

Spin torque nano-oscillators (STNOs) hold significant promise for communication and bio-inspired computing applications. However, their modulation capability is constrained by a dilemma between frequency window and linewidth reduction, particularly in hypercritical conditions like the presence of an external magnetic field. This poses a notable challenge in the practical application of STNOs. Here, we report a unique type of all-electrical compact STNOs that employ the tilted magnetic anisotropy (TMA), which can efficiently promote the linewidth Δf reduction and precisely modulate oscillation frequency ranging from 495 to 556 MHz. The developed STNOs consist of a ferromagnetic reference layer with tunable TMA, wherein the spin transfer torque along the tilted spin polarization direction elaborates a self-oscillation of magnetic moments in the free layer without application of magnetic field. The free layer equips in a magnetic droplet oscillation mode, and the oscillation frequency can be modulated either synergistically or independently by varying the current intensity and/or the TMA angle. Nevertheless, the TMA angle primarily governs the deformation of the magnetic droplet and the corresponding oscillation frequency and linewidth. Moreover, a unique 4 × 4 STNO array with optimized input current and TMA configuration is proposed to execute the reservoir computing hardware training based on nonlinear dynamic oscillation phase-coupling characteristics, promising a diverse synchronization map with high kernel quality and low generation rank for highly reliable pattern classification implementation. The developed STNOs possess a simple structure, nonlinearity, high frequency tunability, and compatibility with CMOS processes, enabling them a fundamental component for large-scale integration of advanced hardware in neuromorphic computing.

Field-free domain wall spin torque nano-oscillators with multimodal real-time modulation and high-quality factor

We report on a magnetic domain wall (DW)-based spin torque nano-oscillator (STNO) with real-time multimodal frequency modulation characteristics by engineering the synergistic effect of shape anisotropy, Dzyaloshinskii-Moriya interaction (DMI), and spin-transfer torque. The achieved manageable oscillation and precession of magnetic DW motivate us to implement such attributes into the developed STNO under a low current density of ∼107 A/cm2, which demonstrates outstanding functionality of multimodal real-time modulation ranging from few GHz and sub-THz with corresponding few tens and >104 quality factor (fSTNO/Δf) in absence of external magnetic field. Furthermore, the dynamic process of DW motion and magnetic moment precession under different modes has been revealed systematically, and the frequency dependence of various physical parameters including damping constant, uniaxial anisotropy constant, saturation magnetization, exchange stiffness, and DMI constant has also been summarized in detail. Such a DW-based device enriches the STNO family and promises great potential with a broad spectrum of applications in microwave generators and neuromorphic computing.

Granular vortex spin-torque nano oscillator for reservoir computing

In this paper, we investigate the granularity in the free layer of the magnetic tunnel junctions (MTJ) and its potential to function as a reservoir for reservoir computing where grains act as oscillatory neurons while the device is in the vortex state. The input of the reservoir is applied in the form of a magnetic field which can pin the vortex core into different grains of the device in the magnetic vortex state. The oscillation frequency and MTJ resistance vary across different grains in a non-linear fashion making them great candidates to be served as the reservoir's outputs for classification objectives. Hence, we propose an experimentally validated area-efficient single granular vortex spin-torque nano oscillator (GV-STNO) device in which pinning sites work as random reservoirs that can emulate neuronal functions. We harness the nonlinear oscillation frequency and resistance exhibited by the vortex core granular pinning of the GV-STNO reservoir computing system to demonstrate waveform classification.

Microwave frequency tunability via heterogeneous Oersted field tilted polarizer based spin torque nano oscillator

Here, we have devised a heterogeneous-based Tilted Polarizer [TP] Spin Torque Nano Oscillator [STNO] along with the consideration of the Current Induced Oersted Field [CIOF]. The magnetization precession dynamics governed by Spin Transfer Torque [STT] are studied by solving Landau-Lifshitz-Gilbert-Slonczewski [LLGS] equation numerically. Here, β refers to an independent tilt angle of the pinned layer, and θ refers to an angle between the free layer magnetization and the easy axis of the device, respectively. The device emits a maximum frequency of about 77.5 GHz and PSD of 1.3 µW/mA2/GHz with 50 kA/m as the strength of CIOF. The author sparks that the modeled OE TP STNO device applies to High-Frequency applications and paves the way for forthcoming spintronic devices in the future.

Weighted spin torque nano-oscillator system for neuromorphic computing

Neuromorphic computing is a promising strategy to overcome fundamental limitations, such as enormous power consumption, by massive parallel data processing, similar to the brain. Here we demonstrate a proof-of-principle implementation of the weighted spin torque nano-oscillator (WSTNO) as a programmable building block for the next-generation neuromorphic computing systems (NCS). The WSTNO is a spintronic circuit composed of two spintronic devices made of magnetic tunnel junctions (MTJs): non-volatile magnetic memories acting as synapses and non-linear spin torque nano-oscillator (STNO) acting as a neuron. The non-linear output based on the weighted sum of the inputs is demonstrated using three MTJs. The STNO shows an output power above 3 µW and frequencies of 240 MHz. Both MTJ types are fabricated from a multifunctional MTJ stack in a single fabrication process, which reduces the footprint, is compatible with monolithic integration on top of CMOS technology and paves ways to fabricate more complex neuromorphic computing systems.

Binary frequency shift keying modulation in spin torque oscillators with synthetic antiferromagnetic layer

Antiferromagnets exhibit ultrafast magnetization precession, which has the potential to enable the development of terahertz spin torque nano-oscillators. By utilizing perpendicularly magnetized magnetic nanopillars with a synthetic antiferromagnetic (SAF) free layer, we have demonstrated through theoretical and numerical analysis that stable out-of-plane precession states can be achieved by applying current and an electric field. In the case of small current, the two magnetic layers of the SAF are in antiparallel alignment and rotate around the z-axis with the precession frequency decreasing as the current strength increases. When the current-induced spin torque is strong enough to fully overcome the antiferromagnetic coupling, the SAF free layer is driven into a scissor-like precession state around the z-axis with the frequency increasing with current. By selecting the appropriate combination of the current and electric field, the magnetization precession orbits and precession frequencies can be adjusted. These controllable procession orbits with tunable frequencies and fixed magnetization precession amplitude may be a promising candidate for implementing binary frequency shift keying modulation techniques.

Sub micro-accelerometer based on spintronic technology: A design optimization

Here we propose a spintronic based accelerometer that exploits the coupling of mechanical with spintronic components to develop a compact sub micro-accelerometer in which the external acceleration is obtained via measurement of the voltage variation in a spin torque diode. The solution is based on three spintronic devices, two spin torque diodes and a spin torque nano oscillator, that enables a robust and reliable sensing and read-out mechanisms thanks to voltage differential measurements. The results confirm the potential of mechanical-spintronic accelerometers to become an alternative to current state of the art technology.

Domain wall based spin torque nano-oscillator in Z-type magnetic nanowire with perpendicular magnetic anisotropy

A domain wall based spin torque nano-oscillator is a potential device for microwave generation because of its tunability of frequency, operation at room temperature, and integration with complementary metal–oxide–semiconductors. Here, we proposed and numerically demonstrated a spin nano-oscillator with domain wall steady oscillation in Z-type magnetic nanowires with perpendicular magnetic anisotropy by micromagnetic simulations. We find that the dynamical behavior of domain walls at Z-junctions is highly affected by the dimension of Z-junctions and current density. Three kinds of behaviors are found, i.e., damped oscillation, steady oscillation, and collapse. The observed steady oscillation can be used as spin torque nano-oscillators with a working frequency can approach 20 GHz. Our findings could enrich the investigation of spin torque nano-oscillators at high frequencies for novel microwave emitters.

High-frequency oscillations in a spin-torque nano-oscillator due to bilinear coupling

High-frequency oscillations in a spin-torque nano-oscillator due to bilinear coupling

Dynamical modes in a vertical nanocontact-based spin Hall nano-oscillator

Spin-torque nano-oscillators (STNOs) with high nonlinear tunability can serve as nanoscale spin-wave emitters for high-speed magnon-based logic circuits and neural networks for promising new energy-efficiency computing. Although various STNOs have been modeled theoretically and characterized experimentally, there is a sustained effort to improve their coherence and nonlinear tunability. By using numerical simulation, here we explore the evolution rule of spectral characteristics and nonlinearities of the emitted coherent spin waves in a type of spin Hall nano-oscillator (SHNO) with a perpendicular magnetic anisotropy (PMA) coefficient ${K}_{u}$, which is based on a vertical nanocontact fabricated on an extended heavy metal/ferromagnet bilayer. There exist three distinct dynamic regimes with varying ${K}_{u}$. At the easy-plane anisotropy-dominated in-plane magnetized regime, the SHNO exhibits coexistence of the nonlinear self-localized spin-wave bullet mode with a significantly negative nonlinearity coefficient $\ensuremath{\aleph}$ and a secondary high-frequency quasilinear edge mode at higher current density. The latter excitation is supported by the energy transfer from the former via nonlinear coupling rather than the injected spin currents located in the center nanogap of the device. When the easy-plane shape anisotropy is effectively compensated by PMA, only a single quasipropagating spin-wave mode with a near-zero $\ensuremath{\aleph}$ is excited at lower current density, consistent with minimization of nonlinear damping due to the suppression of nonlinear mode coupling. For the out-of-plane magnetized SHNOs with ${K}_{u}$ above the demagnetization energy, a well-defined propagating spin-wave mode with a positive $\ensuremath{\aleph}$, characterized by a significant frequency blueshift and a rightward elongated spatial profile perpendicular to the in-plane component of the applied magnetic field, is observed. This detailed evolution diagram of auto-oscillating dynamics on the PMA coefficient ${K}_{u}$ provides a practical approach to selectively excite dynamical modes and optimize the spectral characteristics of the SHNO for applications in microwave technology and spin-wave logic.

Angular Dependent Auto-Oscillations by Spin-Transfer and Spin-Orbit Torques in Three-Terminal Magnetic Tunnel Junctions

Spintronic oscillators are promising candidates for neuromorphic computing due to their true miniaturization, non-linearity and synchronization properties. However, spin torque nano-oscillators are excited by current-induced spin-transfer torque which may cause high power consumption and reliability problems. Spin Hall nano-oscillators can realize higher energy efficiency, while their relatively low power emission limits their further applications. Here, we demonstrate three-terminal magnetic tunnel junction based spin torque nano-oscillators excited by the combination of spin-transfer and spin-orbit torques under different angle configurations. Thanks to the large spin Hall angle (-0.278) of W, the ratio between threshold current density of spin-transfer and spin-orbit torques for auto-oscillations can reach 0.6, indicating their high energy efficiency. Furthermore, we observe that the ratio has a sine function dependence on the angle between the spin-polarization directions of spin-transfer and spin-orbit torques. Our work could benefit the development of high-efficiency spintronic oscillators and their large network designs.

Steady-state skyrmion oscillations in a spin valve (SV) nanopillar based on hybrid polarizer: A skyrmion-based spin torque nano-oscillator (STNO)

The skyrmion-based spin torque nano-oscillators (STNOs) have received a lot of interest due to their prospects as microwave signal generators. These oscillators are accessible by achieving consistent circular spinning of the skyrmion in a circular disk, caused by the spin transfer torque (STT) effect. However, the Magnus force effect, that relates to the non-trivial topological part of the skyrmion and driven on by the higher current densities, leads the skyrmion to accumulate in the center or annihilate toward the edge of the device. To counterbalance the Magnus force, we propose a skyrmion-based STNO model built on a circular nanopillar-shaped spin valve having a hybrid polarizer, which is divided into two regions, each with a different polarization angle. Using Thiele equation-based analytical framework in parallel with micromagnetic simulations, we illustrate that the proposed hybrid polarizer system is intended to ensure steady circular motion of the skyrmion without skyrmion annihilation or accumulation, by strengthening the boundary-induced force. Moreover, we also investigate the current density and polarizer parameters effect on skyrmion oscillation frequency. The results show that the frequency that can be limited for a skyrmion-based STNO with a hybrid polarizer is up to 3.35 GHz, which is higher in terms of magnitude than that of the homogeneous polarizer-based STNO. Our findings provide an insight into the phenomenon of skyrmion dynamics in a circular nanopillar-shaped spin valve with hybrid polarizer, which may open up new avenues for overcoming the frequency limit of the skyrmion-based STNOs.

Magnetic field detection using spin-torque nano-oscillator combined with magnetic flux concentrator

Spin-torque nano-oscillators (STNO) are studied in terms of the Landau–Lifshitz–Gilbert (LLG) equation. The effect on the limit of detectivity of an STNO concerning externally applied magnetic fields is studied with micromagnetic models by placing adjacent magnetic flux concentrators (MFCs) at different distances from the nanopillar to analyze the effect on the induced auto-oscillations and magnetization dynamics. Perpendicular STNO structures allow for different detectivities with respect to externally applied magnetic fields depending on the distance from the MFCs to the nanopillar. The optimal design of an STNO combined with MFCs is proposed to improve the limit of detectivity, where the STNO consists of two out-of-plane (OP) ferromagnetic (FM) layers separated by a MgO insulating nonmagnetic (NM) thin film, and the MFCs positioned in the vicinity of the STNO are made of permalloy. The time evolution of the free-layer magnetization is governed by the Landau–Lifshitz–Gilbert (LLG) equation. The auto-oscillations induced within the free-layer averaged magnetization are provoked by externally applied magnetic fields. In addition, the DC current-driven auto-oscillations in the STNO structure are studied as a function of the externally applied magnetic field strength, with and without MFCs. The suppression of the DC current-driven auto-oscillations is observed due to the damping effect generated by the MFCs positioned at varying distances with respect to the STNO. By placing MFCs adjacent to the STNO, the lowest detectable magnetic field strength is enhanced from 10 (μT) to 10 (nT). Therefore, it is concluded that MFCs improve the sensitivity of STNO to externally applied magnetic fields thanks to the damped magnetization dynamics. The results presented in this work could inspire the optimal design of STNO and MFC-based ultra-low magnetic field sensors based on nanoscale oscillators and spintronic diodes.

Influence of magnetic structure on the performance of twisted skyrmion-based nano-oscillator

The spin torque nano-oscillator (STNO), a nanosize microwave signal generator, have caught the attention of a number of researchers due to its obvious advantages. Recently a chiral bulk material with twisted skyrmion has been discovered in studies with different helicity degrees. In this work, we design a new STNO based on twisted skyrmion existing in free layers of magnetic tunnel junction structure. We first investigate the effect of the magnetic moment of fixed layer on the twisted skyrmion and frequency of STNO. Although the magnetic moment of fixed layer does not affect the state of the twisted skyrmion but affects the precession frequency of STNO. Later, the current, external magnetic field and Dzyaloshinskii–Moriya interaction strength are changed to regulate the oscillation frequency of STNO. Our result may be favorable for the design of new twisted skyrmion-based STNO.

Field-Free High-Frequency Exchange-Spring Spin-Torque Nano-Oscillators.

Spin-torque nano-oscillators (STNOs) are a type of nanoscale microwave auto-oscillators utilizing spin-torque to generate magnetodynamics with great promise for applications in microwaves, magnetic memory, and neuromorphic computing. Here, we report the first demonstration of exchange-spring STNOs, with an exchange-spring ([Co/Pd]-Co) reference layer and a perpendicular ([Co/Ni]) free layer. This magnetic configuration results in high-frequency (>10 GHz) microwave emission at a zero magnetic field and exchange-spring dynamics in the reference layer and the observation of magnetic droplet solitons in the free layer at different current polarities. Our demonstration of bipolar and field-free exchange-spring-based STNOs operating over a 20 GHz frequency range greatly extends the design freedom and functionality of the current STNO technology for energy-efficient high-frequency spintronic and neuromorphic applications.

Comparison of Magnetic Droplet Nucleation Boundaries in Spin-torque Nano-oscillators with Different Fixed Layers

Comparison of Magnetic Droplet Nucleation Boundaries in Spin-torque Nano-oscillators with Different Fixed Layers

Field-like torque-induced tunable zero-field spin-torque nano-oscillator

The spin-torque nano-oscillator (STNO), which is a novel type of nano-sized microwave oscillator driven by direct current, is considered as a promising candidate for future radio frequency (RF) transceivers owing to its scalability, nanoscale size and high frequency tunability. However, the potential application of STNO is limited because its stable oscillation requires an external magnetic field. In this work, the influences of the field-like torque and applied current intensity on the stable oscillation of STNO with a perpendicularly magnetized free layer are studied theoretically based on the macrospin model (also known as the single-spin or single-domain model) and the Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation in the absence of magnetic field. It is demonstrated numerically that a stable oscillation of STNO can be observed when the ratio between the field-like torque and the spin torque is a negative value and larger than a certain value that depends on the damping coefficient and the current intensity, whose physical mechanism can be understood by the energy balance equation. Moreover, the frequency of stable oscillation of STNO can be modulated by the ratio between the field-like torque and the spin torque and also by the current intensity. Particularly, the larger the absolute value of the ratio between the field-like torque and the spin torque and the smaller the applied current intensity (above the critical current intensity), the more conducive it is to suppressing the formation of second and third oscillation frequencies, thereby enhancing the STNO’s “single-frequency” feature. Our findings provide a theoretical scheme for realizing a frequency tunable zero-field STNO, which may be useful for designing future RF transceivers.

Dynamics of magnetic system formed by two semi-skyrmions in circular nano-pillars with a perpendicular anisotropy

Magnetic skyrmion has the advantages of stable topology and small volume. Many researchers choose different materials or build double free layers for using skyrmions in spin torque nano-oscillators capable of producing GHz frequencies. In this paper, the dynamics of the two semi-skyrmions in a circular nano-pillar with perpendicular magnetic anisotropy free layer and a spin polarizer are studied using micromagnetic simulation. The oscillation frequency of two semi-skyrmions is more than two times higher that of the single semi-skyrmion. In addition, we also explore the influences of different parameters (current density, damping coefficient, anisotropy constant, and temperature) on the motion of two semi-skyrmions. The results show that damping coefficient and exchange interaction constant have the most pronounced influence on the oscillation frequency of the system.