Spin-orbit torque (SOT) nano-oscillators, particularly those utilizing the spin Hall effect (SHE), offer a promising route to compact, current-controlled microwave signal generation. These devices, typically consisting of a ferromagnetic (FM)/nonmagnetic (NM) bilayer, leverage the spin-orbit interaction in the NM layer to generate a pure spin current that exerts a torque on the magnetization of the FM layer, inducing auto-oscillations. The resulting magnetization dynamics are then converted into a microwave voltage through the anisotropic magnetoresistance (AMR) of the FM layer. However, the inherently low AMR in thin-film ferromagnets severely limits the output power of these spin Hall oscillators (SHOs), typically to a few picowatts. This limitation significantly hinders their potential applications in areas such as microwave-assisted magnetic recording, neuromorphic computing, and chip-to-chip wireless communication. The simplicity of fabrication for SOT oscillators, involving a single e-beam lithography and etching step, makes them attractive compared to the more complex spin-transfer torque oscillators based on magnetic tunnel junctions (MTJs). This work addresses the critical issue of low output power in SOT oscillators by introducing a novel design that leverages the significantly larger magnetoresistance effect – giant magnetoresistance (GMR) – to enhance the conversion efficiency of magnetization dynamics into a microwave signal. By incorporating a ferromagnetic reference layer, the researchers aim to demonstrate a substantial increase in output power without compromising the structural simplicity that is a key advantage of SOT-based devices.

Existing literature extensively explores the fundamental principles and applications of spin-orbit torque nano-oscillators. Studies have demonstrated the generation of auto-oscillations in FM/NM bilayers driven by spin Hall torques, and characterized the resulting microwave emission using AMR as the detection mechanism. However, the consistent limitation of low output power, typically on the order of a few picowatts, has been a significant obstacle to widespread adoption. The research community has explored various approaches to improve oscillator performance, including materials optimization, device geometry modifications, and techniques to enhance the interaction between multiple oscillators to improve coherence. The use of magnetic tunnel junctions (MTJs) with their high tunneling magnetoresistance (TMR) has also been explored as a means to increase output power in three-terminal device designs. However, these MTJ-based oscillators require more complex fabrication processes compared to the simpler bilayer structures, offsetting some of their advantages. This paper builds on this existing body of work by proposing and demonstrating an alternative approach utilizing CIP-GMR to address the output power limitations.

The researchers fabricated two types of nano-oscillators: a giant magnetoresistance spin Hall oscillator (GMR SHO) and a conventional anisotropic magnetoresistance spin Hall oscillator (AMR SHO) for comparison. The GMR SHO device comprised a nanowire structure consisting of an antiferromagnetic (AFM)/FM/NM/FM/Pt multilayer. The AFM layer provided exchange bias to pin the magnetization of the bottom FM layer, while the top FM layer was free to oscillate under the influence of the spin Hall torque generated in the Pt layer. The NM layer acted as a spacer, and the entire structure was designed to exhibit CIP-GMR. The AMR SHO device had a similar structure but lacked the bottom FM layer and the AFM layer, relying on AMR for microwave detection. Nanowire devices were patterned using e-beam lithography and Ar ion milling. Magnetoresistance measurements were performed at 4.2 K to characterize the GMR and AMR ratios. Microwave emission measurements were carried out using a microwave spectrum analyzer and a low-noise amplifier. Measurements involved varying the applied direct current (I_dc) and the angle of an applied magnetic field (θ) with respect to the nanowire axis to explore the effects on oscillation frequency, power, and linewidth. The angular dependence of microwave emission was studied to investigate the influence of both SHT efficiency and the angular dependence of the magnetoresistance effects (GMR and AMR) on output power. Data analysis focused on extracting parameters such as critical current for auto-oscillation onset, microwave power spectral density (PSD), oscillation frequency, and linewidth. A detailed comparison of the GMR SHO and AMR SHO performances, including output power, critical current, oscillation frequency, and linewidth, was conducted to assess the effectiveness of the CIP-GMR approach. The power calculations accounted for impedance mismatch between the device and the standard 50 Ω microwave measurement setup.

The experimental results demonstrated a significant enhancement in microwave output power using the CIP-GMR approach. The GMR SHO device generated a maximum microwave power exceeding 1 nW, a substantial improvement compared to the AMR SHO, which produced only a few picowatts. The GMR SHO exhibited two distinct auto-oscillatory modes, identified as a bulk spin-wave mode and an edge spin-wave mode, with the edge mode dominating the output power. The critical current for auto-oscillation onset was higher in the GMR SHO. The spectral linewidth of the GMR SHO was considerably larger than that of the AMR SHO, attributed to higher ohmic heating, interaction between the two modes, and nonlinear frequency shift. Analysis of the bias current dependence of integrated microwave power showed a monotonic increase for the GMR SHO up to the highest tested current, while the AMR SHO exhibited a peak followed by a decrease at higher currents, due to nonlinear magnon scattering. The angular dependence study revealed that the GMR SHO's output power was maximized when the magnetization was perpendicular to the nanowire axis, a condition that simultaneously optimized both SHT efficiency and CIP-GMR. In contrast, the AMR SHO exhibited a more complex angular dependence, reflecting a trade-off between SHT efficiency and the angular dependence of AMR. The experimental findings strongly support the hypothesis that the incorporation of a ferromagnetic reference layer and the utilization of CIP-GMR provide a powerful strategy for enhancing the output power of SOT nano-oscillators.

The results demonstrate the successful implementation of CIP-GMR to overcome the longstanding limitation of low output power in SOT nano-oscillators. The substantial increase in power observed in the GMR SHO compared to the AMR SHO underscores the significant contribution of the GMR effect. The observation of two distinct auto-oscillatory modes and their different characteristics highlight the complex interplay of spin-wave dynamics and the device geometry. The wider linewidth of the GMR SHO, while undesirable for some applications, may be mitigated by techniques such as synchronization of multiple oscillators. The higher critical current in the GMR SHO is a potential trade-off, but the substantial increase in output power outweighs this limitation in many applications. These findings suggest a new avenue for designing high-performance SOT-based microwave sources, potentially paving the way for the realization of practical applications that were previously infeasible due to power limitations.

This study successfully demonstrated the significant enhancement of microwave output power in spin-orbit torque nano-oscillators through the integration of a current-in-plane giant magnetoresistance spin valve. The GMR SHO device exhibited output power exceeding 1 nW, a considerable improvement over conventional AMR-based designs. The results highlight the synergistic optimization of spin Hall torque efficiency and magnetoresistance for maximizing power output. Future research could focus on optimizing the GMR ratio, tuning free layer magnetization and damping, and exploring novel SOT sources to further improve device performance. This work contributes substantially to the development of high-power, compact microwave sources for diverse applications.

While the study demonstrates a significant enhancement in output power, certain limitations should be considered. The larger linewidth observed in the GMR SHO compared to the AMR SHO might be a constraint for applications requiring high spectral purity. The study was conducted at a cryogenic temperature (4.2 K), and further investigation is needed to evaluate the performance at room temperature. Moreover, the long-term stability and reliability of the device under continuous operation need to be assessed. Finally, although the device demonstrates significant power enhancement, it remains a proof-of-concept, and further optimization is necessary to fully realize its potential.