Exploiting GHz-scale ferromagnetic dynamics is a promising route to advanced quantum sensing and computing, with nitrogen-vacancy (NV) centers in diamond proving to be a versatile tool for detecting this magnetic phenomena. Despite progress, a comprehensive understanding of domain wall (DW) dynamics using NV centers remains elusive. Now a new study by Jeffrey Rable and colleagues bridges this gap by demonstrating the off-resonant detection of GHz-scale oscillations of a single transverse DW pinned in a ferromagnetic nanowire, using spin relaxometry measurements of NV centers in strategically placed nanodiamonds.

Rable et al.'s research is an experimental study that meticulously combines micromagnetic simulations with advanced nanofabrication techniques to explore the interaction between NV centers and DW dynamics. The team fabricated semicircular permalloy (NiFe) nanowires with a central notch defect to pin the DWs, ensuring precise control over their oscillations. The researchers employed a pick-and-place protocol to position nanodiamonds containing NV centers near the DWs, enabling the detection of magnetic fields generated by DW oscillations. This methodological approach is significant as it allows for the direct observation of DW behavior at GHz frequencies, which is crucial for potential quantum applications.

The study involved two devices with different notch sizes, where pulsed optically detected magnetic resonance measurements were conducted to detect the presence and characteristics of DW oscillations. The researchers observed discrepancies between their experimental results and simulations, hypothesizing that factors such as edge roughness and local mechanical strain induced by the nanodiamond might influence DW dynamics. Despite these challenges, Rable et al. successfully demonstrated that the features observed in their pulsed magnetic resonance measurements were indeed due to the presence of pinned DWs.

Summarizing their findings, Rable and colleagues have shown that NV center-based detection can reveal localized GHz-scale DW dynamics within ferromagnets. Their results indicate that off-resonant detection is likely due to broadband AC noise generated by DW oscillations at higher frequencies. The study concludes with promising implications: resonant coupling between NV centers and DWs could significantly enhance qubit control in quantum computing applications by reducing π pulse times through increased driving field strength.

Looking forward, this work paves the way for using current-driven DW oscillations as nanoscale microwave generators or amplifiers in quantum technologies. The potential for resonant NV-DW coupling could lead to more efficient qubit manipulation methods, contributing to the development of advanced quantum sensing devices and potentially revolutionizing quantum information processing.

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