In order to adapt to the growing demands for smaller and more energy–efficient electrical equipment in recent years, the magnetic powder cores used in the passive elements that compose the circuit systems of such equipment must exhibit high magnetic saturation flux and low losses, while ensuring linearity in excitation response without magnetic saturation. Therefore, high saturation magnetization soft magnetic powder is suitable as magnetic powder core materials. Furthermore, in order to suppress inter–particle eddy current losses, forming an insulating shell on the particles is effective. Additionally, finer the particle size to minimize intra–particle eddy current losses would enable further loss minimization. Thus, high saturation magnetization soft magnetic nanoparticles with insulating shells are promising. In order to derive high saturation magnetization soft magnetic properties, body centered cubic (bcc) Fe–based alloys which is containing small amounts of solid solution elements for decline magnetic crystalline anisotropy are suitable. However, nanoparticle fabrication of Fe–based alloys involves the problem, including the risk of ignition, combustion, and explosion due to exposure or impact in air. Therefore, this study proposes Fe–based alloy nanoparticle clusters that can be safely handled in air. Focusing on crystal structure changes, hydrogen reduction via gas–solid reaction of Si or Al containing magnetite was employed as a synthesis method. The obtained particles were found to be clusters of Fe–based alloy crystallines, with cluster sizes in the micron range comparable to the size of magnetite. Magnetite containing small amounts of Al enabled smaller crystalline sizes while achieving higher saturation magnetization. Magnetite containing small amounts of Si allowed for higher saturation magnetization and low coercivity.

  Accurate characterization of the high-frequency magnetic response of particle-based materials is often hindered by demagnetizing effects and a low signal-to-noise (SN) ratio when the effective magnetic volume under excitation is small. To overcome this limitation, we propose a direct-conduction method in which an alternating current is conducted through needle-shaped magnetic particles packed in a micro-gap between microstrip conductors, thereby confining the alternating magnetic flux within the particles. This configuration effectively suppresses the perpendicular component of the magnetic field and enables clear observation of ferromagnetic resonance (FMR) behavior without requiring a large sample volume. The method combines broadband S-parameter measurements with finite-element modeling to establish the relationship between the measured impedance and magnetic parameters. The real part of the relative permeability is obtained by matching the measured equivalent inductance with the FEM-derived L–μ correspondence, while the loss component is evaluated from the resistance-to-reactance ratio. Using this framework, Fe–Cr–Co needle-particle assemblies were characterized in terms of complex permeability. The proposed method achieved an SN ratio comparable to that of the conventional short-terminated microstrip line while requiring only about 1/85 of the sample volume, demonstrating its effectiveness for high-sensitivity magnetic characterization of micro- and nano-scale particle assemblies.

  High-sensitivity magnetic field sensors are crucial for applications such as biomedical diagnostics and electromagnetic compatibility (EMC) assessments. This study presents an approach to enhance the sensitivity of high-frequency driven thin-film magnetic sensors by improving impedance matching through the introduction of a 10 μm slit in the magnetic layer. The sensors with and without the slit were evaluated under weak AC magnetic fields at frequencies of 500 kHz and 109 Hz. Employing the carrier suppression circuit effectively reduced phase noise. When a 109 Hz, 4.81 × 10^-6 T AC magnetic field was applied to the slit-patterned sensor, the carrier component was suppressed by approximately 60 dB, resulting in a signal-to-noise (SN) ratio improvement of 40 dB. The minimum detectable magnetic fields achieved with slit-patterned sensor were 2.18 × 10^-12 T/Hz^1/2 at 500 kHz and 1.19 × 10⁻¹⁰ T/Hz^1/2 at 109 Hz. These results demonstrate that improved impedance matching, combined with carrier suppression, significantly enhances the sensitivity of room-temperature-operating high-frequency driven thin-film magnetic sensors, thereby simplifying the measurement of weak magnetic fields.