We have developed an experimental technique to investigate a nonlinear magnetization dynamics resonantly excited by strong microwave fields. In this technique, a magnetization switching was observed when a sub-nanosecond wide pulsed field was applied to the ferromagnetic resonance-excited ferromagnets. Such a cooperative switching with simultaneous application of microwave- and pulsed-fields can evaluate an effective energy barrier for magnetization switching in various nonlinear magnetization dynamics. In this review, we show the experimental results on (1) the temporal evolution of the energy barrier in the transient of magnetization precession growth and (2) the observation of saddle-node bifurcation in the microwave-excited nonlinear magnetization dynamics.

Among the various permanent magnets reported to date, FeNdB exhibits the highest magnetic performance, but reducing the use of the rare-earth elements is necessary to avoid exhaustion of rare-earth elements. The key to improve the performance of permanent magnets is to increase magnetization and coercivity. FeCo has a very large magnetization, but has a very small coercivity caused by the extremely small magnetic anisotropy due to the cubic crystal structure, so that FeCo has long been unsuitable for permanent magnets. However, recent reports have revealed that the transformation of the crystal structure from cubic to tetragonal causes large magnetic anisotropy. In 2017, the author reported that a large coercivity can be obtained by fabricating tetragonal FeCo nanodots. This article reviews recent progress of rear-earth free permanent magnets especially for the tetragonal FeCo based alloys.

Recent topics of magnetic domain observation technique of nano-scaled magnetic materials were summarized and reported. Magnetic properties of nano-scaled magnetic materials are highly dependent on the magnetic domain structure inside their magnetic materials. Therefore, an observation technique of the magnetic domain structure of magnetic materials is an extremely important field of technology to improve the performance of various devices. Spatial resolution of the domain observation and the observation technique in high magnetic fields were described.

This paper describes static and high frequency magnetic properties of Fe_100-xGa_x polycrystalline films (x=18.5, 24.9, and 33.4) with various thickness between 5-100 nm. The saturation magnetostriction (λ_s) for each x become lower than those of Fe-Ga single crystal films and Fe-Ga bulk alloy regardless of the film thickness, which may be attributed to the crystalline orientation in these films. On the other hand, for every x, the relationship between the in-plane effective damping constant (α_//) and the film thickness is almost same. Their values become higher than those of other Fe-binary alloy thin film with soft magnetic properties, which may be attributed to the coexistence of both intrinsic and extrinsic damping. These results suggest that the reduction of α_// is one of the most important for the high-frequency device applications of Fe_100-xGa_x polycrystalline films with various film thickness.

We introduce FeCo-BaF and FeCo-SiN nanogranular films exhibiting giant Faraday effect. These films have a nanocomposite structure, in which nanometer-sized FeCo ferromagnetic granules are dispersed in BaF₂ or Si₃N₄ matrix. FeCo-BaF film has a good figure of merit of Faraday effect from the crystalized BaF₂ matrix. FeCo-SiN film has a large refraction index from the nature of Si₃N₄ matrix. These properties contribute to the practical application of nanogranular films to optical devices.

This contribution reports mechanical characteristic together with magnetic properties of PLD (Pulsed Laser Deposition)-fabricated isotropic Nd-Fe-B thick-films on Si substrates with each glass under layer. An enhancement in the thickness of a PLD-made glass buffer-layer enabled us to reduce the stress between a Si substrate and a film magnet. Moreover, the adoption of a glass buffer-layer was effective to enhance the values of residual magnetic polarization and (BH)_max comparing those of a Nd-Fe-B film magnet with almost the same thickness on a Si substrate with a 500-nm thick thermal oxide layer.

Magnetic domain structure changes during a demagnetization process of 2-17 type Sm-Co magnets with different amounts of carbon and oxygen were observed using a Kerr effect microscope. Moreover, the effects of the excess carbon and excess oxygen compositions on the demagnetization process of the magnets were investigated. As a result, it is found that in the Sm-Co magnets with high carbon and oxygen composition, the nucleation of reversal magnetic domains inside grains during the demagnetization process easily occurs at a lower demagnetization field strength. The nucleation of the reversal magnetic domain in grains causes a decrease in a squareness ratio of a demagnetization curve.

In this study, the inverter noise propagation on rolling stock under dynamic conditions was investigated using a multi-channel time-domain measurement system. Measurement results revealed a dependence on train speed of the noise that propagates between car body and rails that was not found in the inverter common-mode current. Since this difference is presumably caused by the bogie bearings, a special measurement of bogie bearings under dynamic conditions was conducted using roller rigs and a 2-port vector network analyzer. The results clarified the dependence of the electromagnetic noise on speed that was observed during measurements with the train in operation. We then prepared an equivalent circuit on the basis of the measurement results.

The step response of an ESD generator in the air discharge mode, which reflects the model-specific attributes, is stable and reproducible due to the lack of sparking. It has the potential to be a candidate for calibration requirements to enhance the reproducibility of the test results by air/contact discharge testing methods specified in IEC 61000-4-2. At present, however, it is unclear how the step response involves the current waveforms of air discharge and contact discharge. In this study, to clarify the relationship between the step response and the current waveforms of air and contact discharges, a formula of step response to the air discharge of an ESD generator is given based on the admittance seen from the discharge electrode of the generator. The step responses are calculated for three different types of commercially available ESD generators along with the corresponding frequency spectra, which are compared with the measured current waveforms of both air discharges and contact discharges, to reveal the attributes peculiar to the generators in relation to their ringing waveforms. Furthermore, the air discharge current is analyzed from the falling waveform of the spark voltage obtained from the Rompe-Weizel spark resistance law, and its calculation formula consisting of the step response and the spark current is derived. Comparison between the calculated and measured discharge current waveforms exhibits that the derived formula well represents the whole waveform of the air discharge current measured for each of the generators, and generally agrees with the first current peak waveform being determined by the spark length and the circuit properties specific to the generator.

It is necessary to enhance DC electrical insulation performance in power equipment such as DC gas insulated switchgears (DC-GISs) for the introduction of DC power transmission systems. In gas-solid DC composite insulation systems, the gas conductivity is strongly affected by the sources for the charge carriers such as the electrode roughness and partial discharges. In particular, two types of conductivity conditions are considered; (1) the higher conductivity value in SF₆ gas than that in solid insulator, that is, gas-conductivity-dominant (GCD) condition under which the source of charge carrier such as partial discharges and electron emission due to surface roughness. (2) the higher conductivity value in solid insulator than that in SF₆ gas, that is, solid-conductivity-dominant (SCD) condition under which natural ionization and charge injection from electrode. In this paper, the electric field distributions in SF₆ gas around epoxy-spacer in DC-GISs were calculated at DC switch-on, DC steady-state and DC polarity reversal (DC-PR) conditions, while changing the parameters of the spacer angle and thickness under both GCD and SCD conditions. Accumulated surface charge on solid insulator and resultant DC electric field transition were investigated in consideration of the difference between GCD and SCD conditions. In addition, we discussed the relationship between the accumulated charge density under DC steady-state conditions and the electric field distribution at DC-PR condition. As a result, in the case of GCD conditions, the electric field is concentrated in epoxy-spacer under DC steady-state conditions and the location of maximum electric field changes with time due to significant accumulated charge on epoxy-spacer. On the other hand, in the case of SCD conditions, DC electric field distribution around epoxy-spacer hardly changes due to slightly accumulated charges. In addition, in the case of GCD conditions, the significantly accumulated charges under DC steady-state conditions can be a critical parameter in determining the maximum electric field at DC-PR conditions. From the view points of the amount of accumulated charge under DC steady-state conditions and electric field stress at DC-PR conditions, electric field distributions under GCD conditions can be more distorted than that under SCD conditions.