Spin Hall nano-oscillators (SHNOs) are promising devices for high-frequency applications due to their ease of fabrication and emission characteristics. Their oscillation frequencies and spin wave modes are dependent on applied magnetic fields and DC currents, with the Oersted field (H_Oe) significantly influencing high-frequency emissions. Direct measurement of magnetization properties in the operational state is crucial, and Magnetic Force Microscopy (MFM) offers a potential approach. This research aimed to develop a method combining MFM with electrical and microwave access to enable imaging of operational SHNOs. The challenge lies in the limited space available under the MFM head, requiring careful design of the experimental setup to accommodate both MFM scanning and the necessary electrical/microwave connections. The successful combination of these techniques would provide unprecedented insight into the dynamic behavior of SHNOs during operation and advance the understanding and development of spintronic devices. The importance of this research stems from the potential to directly visualize and analyze the magnetization dynamics of SHNOs, providing critical information for optimizing their performance and exploring novel applications.

Previous research has extensively studied SHNOs, exploring their fabrication, oscillation characteristics, and potential applications. However, directly imaging the magnetization dynamics of SHNOs during operation has remained a significant challenge. While various techniques exist for characterizing SHNOs, a method combining high-resolution spatial imaging with electrical and microwave measurements was lacking. The existing literature on MFM primarily focuses on static magnetic imaging, with limited exploration of its application to dynamic nanoscale systems, particularly in the context of SHNOs' high-frequency oscillations. This study addresses this gap by integrating MFM with a customized microwave probe station, enabling simultaneous imaging and electrical characterization of operational SHNOs.

The researchers fabricated nanoconstriction-based SHNO NiFe/Pt bilayers with constriction widths ranging from 80 to 300 nm. These devices were designed with longer waveguides than conventional designs to accommodate the MFM system's spatial limitations. COMSOL Multiphysics® was used to simulate current density and the induced Oersted field (H_Oe) in the devices. A custom MFM system was developed by integrating a microwave probe station into an MFP-3D-SA MFM system. A specialized microwave probe with an extended coaxial line was designed to ensure stable contact with the device waveguides during MFM scanning. A variable field module (VFM2) was incorporated to apply in-plane magnetic fields. High-resolution MFM probes were fabricated. The experimental setup allowed for simultaneous MFM scanning, electrical biasing, and microwave signal measurement. MFM and AFM measurements were performed on SHNOs under various applied magnetic fields and DC currents. The spatial profile of the Oersted field was imaged using MFM. The fabrication process involved magnetron sputtering of permalloy and Pt layers onto a sapphire substrate, followed by e-beam lithography and argon ion milling to create nanoconstrictions. Optical lithography and lift-off processes defined the coplanar waveguides for electrical contact. Electrical measurements used a high-frequency bias-T, a low-noise amplifier, and a spectrum analyzer.

The COMSOL simulations showed the spatial distribution of current density and the resulting Oersted field within the SHNO devices. The experimental MFM measurements successfully imaged the spatial profile of the operational SHNOs. The dark areas in the MFM images correlated with regions of high current density, consistent with the simulated H_Oe distribution. The observed features were not attributed to magnetization oscillations due to the significant difference between the MFM tapping frequency (kHz range) and the SHNO oscillation frequency (3-9 GHz). The MFM images showed variations in the spatial profile of the Oersted field under different applied magnetic fields and DC currents (Figs. 4 & 5). The results demonstrated the feasibility of using the combined MFM and microwave probe station for imaging the spatial profile of an operational magnetic nanodevice, showing strong correlation between simulations and experimental findings. The key finding is the successful development and demonstration of this novel method for imaging operational SHNOs, providing a new tool for investigating spintronic devices.

The successful imaging of the spatial profile of operational SHNOs using MFM validates the developed method. The correlation between the MFM images and the simulated Oersted field distribution strongly supports the interpretation of the observed features as related to current-induced magnetic fields. The inability to directly resolve the GHz oscillations with the current setup highlights a limitation related to the MFM scanning frequency; however, this doesn't diminish the significance of visualizing the macroscopic effect of the high-frequency dynamics. The method provides valuable insights into the spatial distribution of current and its impact on the local magnetic environment within the SHNO. This research opens up new avenues for studying the spatiotemporal dynamics of spintronic devices. The observed correlation between simulation and experiment is very significant and provides strong validation for the proposed methodology.

This paper successfully demonstrated a new method for imaging operational SHNOs using MFM with integrated microwave probing. The method allows for simultaneous electrical and microwave characterization alongside high-resolution magnetic imaging. While the current system's limitations prevent direct visualization of GHz magnetization oscillations, future work with higher-frequency MFM systems is anticipated to provide more detailed insights. The presented method provides a valuable tool for studying the dynamic behavior of magnetic nanodevices, advancing the understanding and development of spintronics.

The current experimental setup's MFM scanning frequency limits the ability to directly resolve the high-frequency magnetization oscillations within the SHNOs. The results are qualitative rather than quantitative, preventing direct comparison of different MFM scans. Future work could address these limitations by using faster tapping mode MFM and incorporating quantitative analysis techniques.