Imaging of magnetization response of soft magnetic thin film using diamond quantum sensors with wide frequency range

In power electronics, a core loss of inductors in the high-frequency range is a bottleneck for the miniaturization of the system. To develop soft magnetic materials with low loss at high frequency, AC magnetic imaging is effective. Here, we developed a diamond quantum imaging with a wide frequency range.

Using nitrogen-vacancy centers, we performed simultaneous imaging of the amplitude and phase of the AC stray field from a CoFeB-SiO₂ thin film, developed for high-frequency inductors. This film has low conductivity derived from the structure of nanomagnetic columns dispersed in an insulator matrix and in-plane uniaxial anisotropy. We imaged a 100-10 kHz AC field by continuous-wave optically-detected magnetic resonance with frequency modulation of the microwave, where the AC signal is down-converted to DC and captured by the camera's speed. The magnetic response correlated with the anisotropy. When the external field was parallel to the easy axis, the phase delayed as frequency increased, implying the loss increased. When perpendicular, the phase didn't delay up to 5 kHz, implying the loss was almost zero. Diamond quantum sensors will help evaluate loss in inductors.