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Improving energy conversion efficiency in power electronics is vital for a sustainable society, with wide-bandgap semiconductors like GaN and SiC power devices offering advantages due to their high-frequency capabilities. However, energy losses in passive components at high frequencies hinder efficiency and miniaturization. This underscores the need for advanced soft magnetic materials with lower energy losses.

In a study in Communications Materials, a research team led by Professor Mutsuko Hatano from the School of Engineering, Institute of Science, Tokyo, Japan, has developed a novel method for analyzing such losses by simultaneously imaging the amplitude and phase of alternating current (AC) stray fields, which are key to understanding hysteresis losses.

Using a diamond quantum sensor with nitrogen-vacancy (NV) centers and developing two protocols—qubit frequency tracking (Qurack) for kHz and quantum heterodyne (Qdyne) imaging for MHz frequencies—they realized wide-range AC magnetic field imaging. This study was carried out in collaboration with Harvard University and Hitachi, Ltd.

The researchers conducted a proof-of-principle wide-frequency-range magnetic field imaging experiment by applying an AC current to a 50-turn coil and sweeping the frequency from 100 Hz to 200 kHz for Qurack and 237 kHz to 2.34 MHz for Qdyne. As expected, the uniform AC Ampere magnetic field's amplitude and phase were imaged using NV centers with high spatial resolution (2–5 µm), validating both measurement protocols.

Using this innovative imaging system, the team could simultaneously map the amplitude and phase of stray magnetic fields from the CoFeB–SiO₂ thin films, which have been developed for high-frequency inductors. Their findings revealed that these films exhibit near-zero phase delay of up to 2.3 MHz, indicating negligible energy losses along the hard axis. Moreover, they observed that energy loss depends on the material's magnetic anisotropy—when magnetization is driven along the easy axis, phase delay increases with frequency, signifying higher energy dissipation.

Overall, the results showcase how quantum sensing can be used to analyze soft magnetic materials operating at higher frequencies, which is considered to be a major challenge in developing highly efficient electronic systems. Notably, the capacity to resolve domain wall motion, one of the magnetization mechanisms strongly related to energy losses, is a pivotal step, leading to important practical advances and optimizations in electronics.

Looking forward, the researchers hope to further improve the proposed techniques in various ways. "The Qurack and Qdyne techniques used in this study can be enhanced by engineering improvements," says Hatano. "Qurack's performance can be enhanced by adopting high-performance signal generators to extend its amplitude range, whereas optimizing spin coherence time and microwave control speed would broaden Qdyne's frequency detection range."

"Simultaneous imaging of the amplitude and phase of AC magnetic fields across a broad frequency range offers numerous potential applications in power electronics, electromagnets, non-volatile memory, and spintronics technologies," remarks Hatano. "This success contributes to the acceleration of quantum technologies, particularly in sectors related to sustainable development goals and well-being."