Spin-orbit torques (SOTs) are used to manipulate magnetization in several spintronic technologies, including SOT memory (SOT-MRAM), SOT oscillators, neuromorphic computing devices, and SOT-based magnonic logic. SOT oscillators are valuable for studying nonlinear magnetization dynamics in nanoscale ferromagnets. The simplest SOT oscillator is the spin Hall oscillator (SHO), a bilayer of a ferromagnet (FM) and a non-magnetic heavy metal (HM). A charge current in the HM layer generates a spin current that interacts with the FM magnetization, creating a spin Hall torque. When this torque overcomes Gilbert damping, it excites persistent auto-oscillatory magnetization precession, generating a microwave voltage. High output power and low phase noise are desirable for applications. Large-amplitude magnetization dynamics (nearly 90° precession cone angle) have been predicted for easy-plane (EP) FMs with spin current polarized normal to the easy plane. This configuration, an EP-SHO, can operate in zero external fields. The EP-SHO is particularly attractive for creating a magnetic spiking neuron, generating large-amplitude sub-nanosecond output voltage pulses above a threshold current, mimicking neuron behavior. While large-amplitude EP dynamics have been theoretically studied in spin-transfer-torque devices, they hadn't been explored in SOT devices like SHOs until this study. This research presents the experimental realization of a nanowire EP-SHO using a Pt/CoNi superlattice bilayer, achieving easy-plane anisotropy by tuning the CoNi perpendicular magnetic anisotropy (PMA) and magnetic shape anisotropy. The goal is to demonstrate enhanced microwave power generation through this easy-plane configuration and its suitability for applications in wireless communications, neuromorphic computing, and microwave-assisted magnetic recording.

The authors review existing literature on spin-orbit torques (SOTs) and their applications in spintronic devices, including SOT-MRAM, SOT oscillators, and neuromorphic computing. They discuss the basic principles of spin Hall oscillators (SHOs) and the challenges in achieving high output power and low phase noise. The literature review highlights theoretical predictions for large-amplitude magnetization dynamics in easy-plane (EP) ferromagnets and the potential of EP-SHOs for neuromorphic computing applications. Previous work on spin-transfer-torque devices and high-frequency tuning of spin Hall nano-oscillators is also cited. The review emphasizes the lack of experimental exploration of large-amplitude EP dynamics in SOT devices prior to this study and the potential benefits of using CoNi superlattices for their tunable PMA.

The researchers fabricated nanowire EP-SHOs from a Pt (7 nm)/[Co(0.98 nm)|Ni(1.46 nm)]2|Co(0.98 nm) superlattice bilayer deposited by magnetron sputtering. Electron beam lithography and Ar+ ion milling were used to pattern 50 nm wide, 40 µm long nanowires. Ta(5 nm)|Au(40 nm)|Ta(5 nm) leads were attached, with the inter-lead gap defining the active region. Two SHO configurations were compared: a standard SHO (S-SHO) with a large applied magnetic field and the EP-SHO with a near-zero field. The PMA in the CoNi superlattice was tuned via adjusting Co and Ni layer thicknesses, as characterized by broadband ferromagnetic resonance (FMR) measurements. Magnetoresistance was measured as a function of applied field angle. Microwave emission was measured using a standard circuit with a microwave spectrum analyzer, amplifier, and bias tee. The sample was placed in a He flow cryostat at 4.2 K. Power spectral density (PSD), integrated power, and amplitude of resistance oscillations were measured as functions of applied direct current. Micromagnetic simulations using the Mumax3 code were performed to model current-driven magnetization dynamics, considering spin Hall torque, Oersted field, and Joule heating. Material parameters from experimental measurements were used in the simulations, along with a current-dependent PMA to account for Joule heating effects. The simulations calculated the time evolution of magnetization and resistance oscillations. Analytical expressions were used to calculate the demagnetization factors for the nanowire.

The study demonstrated a significant enhancement in microwave power in the easy-plane (EP) SHO configuration compared to the standard SHO (S-SHO) configuration. The EP-SHO generated a maximum integrated power of 217 pW, significantly higher than the 74 pW maximum observed in the S-SHO. The EP-SHO exhibited a non-monotonic frequency dependence on the applied direct current, with a frequency minimum observed near the maximum power output. This minimum is attributed to the tuning of PMA by Joule heating, resulting in a perfect easy-xz-plane anisotropy. Micromagnetic simulations confirmed the experimental results, showing qualitative agreement between the measured and simulated frequency dependence on current. Quantitative agreement was observed in the frequency minimum, which corresponded to the perfect easy-xz-plane anisotropy. The amplitude of resistance oscillations was maximized near the frequency minimum in both experimental and simulation data. Micromagnetic simulations revealed bi-stable behavior in the EP-SHO regime, with the system switching between large-amplitude magnetization oscillations and a static state. This behavior is potentially beneficial for neuromorphic computing applications. The simulations showed that the magnetization precession was primarily limited to the +z half-space due to exchange coupling with the static magnetization outside the active region. The experimental measurements of resistance oscillations, however, displayed a larger amplitude compared to the simulations, potentially due to weak exchange coupling between crystallographic grains in the FM film. Analysis revealed that the SMR contribution to resistance oscillations contributed significantly to the observed output power. The S-SHO showed a blue frequency shift with increasing current in both experiment and simulation, but the experimental frequency was about 1 GHz higher than simulated, possibly due to magnetic edge modification effects.

The experimental results demonstrate that an easy-plane spin Hall oscillator (EP-SHO) configuration significantly enhances microwave power output compared to the standard high-field configuration. This enhancement is attributed to the large-amplitude magnetization dynamics enabled by the easy-plane anisotropy. The observed frequency minimum and its correlation with the maximum power output confirm the critical role of PMA tuning via Joule heating in optimizing the EP-SHO performance. The good agreement between experimental data and micromagnetic simulations validates the simulation model and provides further insight into the underlying physics. The bi-stability observed in the simulations highlights the potential of EP-SHOs for neuromorphic computing applications, while the discrepancy between simulated and measured resistance oscillation amplitudes suggests further refinements to the simulation model, particularly considering inter-grain exchange coupling. The higher experimental frequencies compared to simulations point to the significance of magnetic edge effects, which were not fully captured in the simulations. The overall findings showcase the potential of the EP-SHO as a high-power microwave source for various applications.

This research achieved the first experimental demonstration of an easy-plane spin Hall oscillator (EP-SHO), which operates without a bias magnetic field and generates significantly higher output microwave power compared to standard SHOs. The easy-plane anisotropy was engineered by tuning the nanowire shape anisotropy and interfacial PMA. Micromagnetic simulations provided good qualitative agreement with experimental results. The high output power of the EP-SHO and its potential for bi-stable behavior position it as a promising candidate for spintronic spiking neurons and high-power microwave sources, with the potential for further enhancement through integration with tunneling magnetoresistance readout.

The micromagnetic simulations did not fully capture the effects of weak exchange coupling between crystallographic grains in the FM film, leading to a discrepancy between simulated and measured resistance oscillation amplitudes. Magnetic edge modification, not fully accounted for in the simulations, likely contributed to the difference in frequencies between experimental and simulated data. The study focused on a single device geometry; further research could explore the scalability and performance of EP-SHOs with different geometries and materials. The observed bi-stability in the EP-SHO regime, while potentially useful for neuromorphic computing, can be detrimental to coherent microwave generation and needs further investigation to enhance its stability.