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Spin-Hall nano-oscillators are a promising class of microwave spintronic devices with potential applications in RF $/$ microwave communication and neuromorphic computing. The nano-constriction spin-Hall nano-oscillators (NC-SHNO) have relatively high power, narrow linewidth, and low drive current. Several synchronization schemes e.g. arrays of spin-wave coupled oscillators have been proposed for more stable operation and higher output power. For such arrays, it is crucial to have good oscillator stability and small device-to-device variability. Here, a micromagnetic simulation technique is proposed that includes realistic material properties and hence enables variability and modal stability to be investigated. It is demonstrated, using both measurements and simulation, that the presence of physical grains in the free magnetic layer can induce multiple oscillation modes or frequency sidebands. Our investigation could help in the development of more stable NC-SHNOs that would enable oscillator arrays with stronger synchronization. [ABSTRACT FROM AUTHOR]

The modulation bandwidth (${f}_{{BW}}$) is a critical figure-of-merit for wireless communication applications of spin torque nano-oscillators (STNOs) as it determines the maximum data rate. Although both theory and previous experiments have shown that ${f}_{{BW}}$ in STNOs is governed by the amplitude relaxation frequency ${f}_{{\mathrm {p}}}$ , we here demonstrate, using single-shot time-resolved measurements of a magnetic tunnel junction based STNO, that it can be many times larger under strong modulation. The behavior is qualitatively reproduced in macrospin simulations. Our results show that ${f}_{{BW}}$ of STNOs is not as limiting a factor for future wireless applications as previously believed. [ABSTRACT FROM AUTHOR]

Spin–orbit torque (SOT) is an emerging technology that enables the efficient manipulation of spintronic devices. The initial processes of interest in SOTs involved electric fields, spin–orbit coupling, conduction electron spins, and magnetization. More recently, interest has grown to include a variety of other processes that include phonons, magnons, or heat. Over the past decade, many materials have been explored to achieve a larger SOT efficiency. Recently, holistic design to maximize the performance of SOT devices has extended material research from a nonmagnetic layer to a magnetic layer. The rapid development of SOT has spurred a variety of SOT-based applications. In this article, we first review the theories of SOTs by introducing the various mechanisms thought to generate or control SOTs, such as the spin Hall effect, the Rashba-Edelstein effect, the orbital Hall effect, thermal gradients, magnons, and strain effects. Then, we discuss the materials that enable these effects, including metals, metallic alloys, topological insulators, 2-D materials, and complex oxides. We also discuss the important roles in SOT devices of different types of magnetic layers, such as magnetic insulators, antiferromagnets, and ferrimagnets. Afterward, we discuss device applications utilizing SOTs. We discuss and compare three- and two-terminal SOT-magnetoresistive random access memories (MRAMs); we mention various schemes to eliminate the need for an external field. We provide technological application considerations for SOT-MRAM and give perspectives on SOT-based neuromorphic devices and circuits. In addition to SOT-MRAM, we present SOT-based spintronic terahertz generators, nano-oscillators, and domain-wall and skyrmion racetrack memories. This article aims to achieve a comprehensive review of SOT theory, materials, and applications, guiding future SOT development in both the academic and industrial sectors. [ABSTRACT FROM AUTHOR]

Emerging spin-torque nano oscillators (STNOs) and spin-Hall nano oscillators (SHNOs) are potential candidates for microwave applications. Recent advances in three-terminal magnetic tunnel junction (MTJ)-based SHNOs opened the possibility to develop more reliable and well-controlled oscillators, thanks to individual spin Hall-driven precession excitation and read-out paths. To develop hybrid systems by integrating three-terminal SHNOs and CMOS circuits, an electrical model able to capture the analog characteristics of three-terminal SHNOs is needed. This model needs to be compatible with current electric design automation (EDA) tools. This work presents a comprehensive macrospin-based model of three-terminal SHNOs able to describe the dc operating point, frequency modulation, phase noise, and output power. Moreover, the effect of voltage-controlled magnetic anisotropy (VCMA) is included. The model shows good agreement with experimental measurements and could be used in developing hybrid three-terminal SHNO/CMOS systems. [ABSTRACT FROM AUTHOR]

We have carried out detailed Landau–Lifshitz-Gilbert simulations of pairs of serially connected spin torque oscillators (STOs) in parallel with a resistive load. To study the impact of realistic process variations on STO synchronization we let the two STOs have different in-plane anisotropy fields (Hk). The simulation also provides for a time delay (τ). We construct a phase diagram of the STO synchronization as a function of Hk and direct current (Idc) at different τ. The phase diagram turns out to be quite rich with different types of synchronized precession modes. While the synchronized state is originally very sensitive to STO process variations and can only sustain up to 4% Hk variation, the addition of a small time delay dramatically improves its robustness and allows as much as 145% Hk variation in the entire out-of-plane precession regime. [ABSTRACT FROM AUTHOR]

A single nanoconstriction spin-Hall nano-oscillator (NC-SHNO) in out-of-plane fields is presented as a nonlinear amplitude and frequency modulator operated by radio-frequency (RF) current modulation. The current modulation was carried out in different NC-SHNO nonlinearity regimes corresponding to negative, zero, and positive values of df/dI in order to investigate the device response to an 80 MHz modulating current. Our study shows that current modulation of SHNOs can be quantitatively predicted by a nonlinear frequency and amplitude modulation (NFAM) model using the values of df/dI and d2 f/dI2 extracted from the free-running frequency f versus current I profile. The NFAM model reproduces the asymmetric sideband amplitude as well as the red and blue shift of the frequency in excellent agreement with the experimental results. The ability to predict the modulation process is a necessary benchmark in designing SHNO modulators for future integrated microwave circuits. [ABSTRACT FROM PUBLISHER]

Broadband ferromagnetic resonance (FMR) spectroscopy is used to study the temperature (T) and dopant-concentration dependence of the magnetodynamic properties of Permalloy (Py = Ni80Fe20) and Py100-xMx films, where the dopant M = Pt, Au, and Ag. The saturation magnetization (MS) and Gilbert damping constant (α) are determined from the uniform FMR mode, while the spin wave stiffness (D) is extracted using the first perpendicular standing spinwave mode. The temperature dependence of D is best described by a T2 law, which suggests a noticeable effect of the itinerant character of the electrons. The spin wave stiffness is also estimated using Bloch's law and the two methods are compared. The results strongly imply that not only spin wave and Stoner excitations, but also other mechanisms contribute to the reduction of MS. The damping increases with T for all samples, but the enhancement is most pronounced for Py doped with 30 at.% Au. [ABSTRACT FROM PUBLISHER]

This paper reviews the state of the art in spin-torque and spin-Hall-effect-driven nano-oscillators. After a brief introduction to the underlying physics, the authors discuss different implementations of these oscillators, their functional properties in terms of frequency range, output power, phase noise, and modulation rates, and their inherent propensity for mutual synchronization. Finally, the potential for these oscillators in a wide range of applications, from microwave signal sources and detectors to neuromorphic computation elements, is discussed together with the specific electronic circuitry that has so far been designed to harness this potential. [ABSTRACT FROM PUBLISHER]

Time-averaged images of the magnetization within single-layer spin-transfer oscillators have been obtained using the holography with extended reference by autocorrelation linear differential operator technique. Transport measurements on a Pd(5)-Cu(20)-Ni81Fe19(7)-Cu(2)-Pd(2) (in nanometers) stack with a 100 nm diameter nanocontact reveal the presence of vortex dynamics. Magnetic images of the device for injected current values of 24 and −24 mA suggest that a vortex has been ejected from the nanocontact and become pinned at the edge of the region that is visible through the Au mask. [ABSTRACT FROM PUBLISHER]

Magnetic tunnel junction (MTJ) spin torque oscillators (STOs) have shown the potential to be used in a wide range of microwave and sensing applications. To evaluate the potential uses of MTJ STO technology in various applications, an analytical model that can capture MTJ STO’s characteristics, while enabling system- and circuit-level designs, is of great importance. An analytical model based on macrospin approximation is necessary for these designs since it allows implementation in hardware description languages. This paper presents a new macrospin-based, comprehensive, and compact MTJ STO model, which can be used for various MTJ STOs to estimate the performance of MTJ STOs together with their application-specific integrated circuits. To adequately present the complete model, this paper is divided into two parts. In Part I, the analytical model is introduced and verified by comparing it against measured data of three different MTJ STOs, varying the angle and magnitude of the magnetic field, as well as the DC biasing current. The proposed analytical model is suitable for being implemented in Verilog-A and used for efficient simulations at device, circuit, and system levels. In Part II, the full Verilog-A implementation of the analytical model with accurate phase noise generation is presented and verified by simulations. [ABSTRACT FROM AUTHOR]

The rapid development of the magnetic tunnel junction (MTJ) spin torque oscillator (STO) technology demands an analytical model to enable building MTJ STO-based circuits and systems so as to evaluate and utilize MTJ STOs in various applications. In Part I of this paper, an analytical model based on the macrospin approximation has been introduced and verified by comparing it with the measurements of three different MTJ STOs. In Part II, the full Verilog-A implementation of the proposed model is presented. To achieve a reliable model, an approach to reproducing the phase noise generated by the MTJ STO has been proposed and successfully employed. The implemented model yields a time domain signal, which retains the characteristics of operating frequency, linewidth, oscillation amplitude, and DC operating point, with respect to the magnetic field and applied DC current. The Verilog-A implementation is verified against the analytical model, providing equivalent device characteristics for the full range of biasing conditions. Furthermore, a system that includes an MTJ STO and CMOS RF circuits is simulated to validate the proposed model for system- and circuit-level designs. The simulation results demonstrate that the proposed model opens the possibility to explore STO technology in a wide range of applications. [ABSTRACT FROM AUTHOR]

Magnetic coupling in trilayer films of FeNi/Cu/FeCo deposited on Si/SiO2 substrates have been studied. While the thicknesses of the FeNi and FeCo layers were kept constant at 100Å, the thickness of the Cu spacer was varied from 5 to 50Å. Both hysteresis loop and ferromagnetic resonance results indicate that all films are ferromagnetically coupled. Micromagnetic simulations well reproduce the ferromagnetic resonance mode positions measured by experiments, enabling the extraction of the coupling constants. Films with a thin Cu spacer are found to be strongly coupled, with an effective coupling constant of 3erg/cm² for the sample with a 5Å Cu spacer. The strong coupling strength is qualitatively understood within the framework of a combined effect of Ruderman-Kittel-Kasuya-Yosida and pinhole coupling, which is evidenced by transmission electron microscopy analysis. The magnetic coupling constant surprisingly decreases exponentially with increasing Cu spacer thickness, without showing an oscillatory thickness dependence. This is partially connected to the substantial interfacial roughness that washes away the oscillation. The results have implications on the design of multilayers for spintronic applications. [ABSTRACT FROM AUTHOR]

We present a comprehensive review of the most recent advances in nanocontact spin torque oscillators (NC-STOs). NC-STOs are highly tunable, with both applied magnetic field and dc, broadband microwave signal generators. As opposed to the nanopillar geometry, where the lateral cross section of the entire device has been confined to a typically <100~nm diameter, in NC-STOs, it is only the current injection site that has been laterally confined on top of an extended magnetic film stack. Three distinct material combinations will be discussed: 1) a Co/Cu/NiFe pseudospin valve (PSV) where both the Co and NiFe have a dominant in-plane anisotropy; 2) a Co/Cu/ [Co/Ni]4 orthogonal PSV where the Co/Ni multilayer has a strong perpendicular anisotropy; and 3) a single NiFe layer with asymmetric non-magnetic Cu leads. We explore the rich and diverse magnetodynamic modes that can be generated in these three distinct sample geometries. [ABSTRACT FROM AUTHOR]

Magnetic multilayer (ML) structures comprising a perpendicular magnetic anisotropy (PMA) layer coupled to an in-plane magnetic anisotropy (IMA) layer are promising materials for zero/low field operating spin-torque oscillators and bit patterned recording media. The magnetization tilt angle can be easily tuned by varying the IMA layer thickness due to the competition between PMA and IMA layers. To explore the underlying magnetization reversal mechanism and to further understand the control of tilt angle and uniformity of the magnetization, the IMA (NiFe, Co, and CoFeB)/PMA (Co/Pd MLs) exchange spring systems are systematically studied. Experimental data obtained from magnetometry show good agreement with 1-D micromagnetic simulations, allowing us to design tunable exchange coupled spring as a function of IMA thickness. [ABSTRACT FROM AUTHOR]

Several experiments report the presence of finite jumps in the properties of spin–torque oscillators at room temperature, such as oscillation frequency and power as functions of current or field. On the basis of micromagnetic simulations, this paper links those experimental discontinuities to the changes in the curve slope numerically observed in the absence of thermal effects. Our numerical results show the key ingredient triggering this behavior is the presence of abrupt changes in the oscillation axis of the magnetization precession. We also predict that by fixing the bias point of the oscillator near those critical regions, it is possible to observe hysteretic synchronization. This result should be a key point in the design of nanoscale on-chip phase-locked loop receivers with improved sensitivity. [ABSTRACT FROM PUBLISHER]

We demonstrate numerically and analytically that a nano-pillar spin torque oscillator (STO), operating either with in-plane or out-of-plane free-layer precession, locks to a microwave field (Hac) having the same frequency as the STO. By varying the spatial direction of the microwave field, we further show the preferred phase shift (\Delta\Phi0) between the STO and Hac can be tuned in a linear fashion. We explain this phenomenon by using a magnetic-energy-based analysis. Our results provide a way to synchronize serially connected STOs by tuning the phase shift of each individual STO with external microwave field, which may enhance the locking efficiency, the locking range, and the output power of the serially connected STOs. [ABSTRACT FROM PUBLISHER]

We demonstrate spin transfer torque (STT) driven microwave signal generation, from about 250 MHz to above 3 GHz, in single permalloy layers underneath a nano-contact with diameter of 100 nm. The threshold current for signal generation is found to be strongly hysteretic, the microwave signal shows a number of harmonics, zero-field operation is straightforward, and the microwave frequency increases quasi-linearly with drive current. All observations are consistent with STT driven motion of a vortex-antivortex pair nucleated by the Oersted field underneath the nano-contact. While the generated power is about 10 dB smaller than the best GMR based nano-contact spin torque oscillators, the linewidth of 6–100 MHz is of the same order. [ABSTRACT FROM AUTHOR]

Spin-torque oscillators (STO) hold promise for multi-octave frequency operation at very high frequencies and modulation speeds. STO operation is typically demonstrated using large electromagnets and probe stations, and has so far not been packaged with a portable form factor. For STOs to be utilized in real applications, a smaller packaging solution is needed. We integrate STOs with packages originally developed for YIG oscillators, modified to incorporate permanent magnets and to achieve a compact portable oscillator based on the STO. [ABSTRACT FROM AUTHOR]

The fabrication and subsequent analysis of continuously graded anisotropy films are discussed. During deposition, a compositional gradient is first achieved by varying the Cu concentration from Cu-rich (Fe53Pt47)70Cu30 to Cu-free Fe53Pt47. The anisotropy gradient is then realized after thermal post-annealing, and by utilizing the strong composition dependence of the low-anisotropy (A1) to high-anisotropy (L10) ordering temperature. The magnetic properties are investigated by surface sensitive magneto-optical Kerr effect and alternating gradient magnetometer (AGM) measurements. AGM first-order reversal curve (FORC) measurements are employed in order to provide a detailed analysis of the reversal mechanisms, and therefore the induced anisotropy gradient. At low annealing temperatures, the FORC measurements clearly indicate the highly coupled reversal of soft and hard phases. However, significant interdiffusion results in virtually uniform films at elevated annealing temperatures. Additionally, the A1 to L10 ordering process is found to depend on the film thickness. [ABSTRACT FROM AUTHOR]

We demonstrate analytically and numerically, that a thin film of an antiferromagnetic (AFM) material, having biaxial magnetic anisotropy and being driven by an external spin-transfer torque signal, can be used for the generation of ultra-short “Dirac-delta-like” spikes. The duration of the generated spikes is several picoseconds for typical AFM materials and is determined by the inplane magnetic anisotropy and the effective damping of the AFM material. The generated output signal can consist of a single spike or a discrete group of spikes (“bursting”), which depends on the repetition (clock) rate, amplitude, and shape of the external control signal. The spike generation occurs only when the amplitude of the control signal exceeds a certain threshold, similar to the action of a biological neuron in response to an external stimulus. The “threshold” behavior of the proposed AFM spike generator makes possible its application not only in the traditional microwave signal processing but also in the future neuromorphic signal processing circuits working at clock frequencies of tens of gigahertz. [ABSTRACT FROM AUTHOR]