The magnetic domain structure on the (001) plane of an Fe-16.1at% Ga alloy single crystal grown by the Czochralski method was observed by magneto-optic Kerr effect microscopy. Under zero magnetic field, four different domain phases with the magnetization direction in four kinds of magnetic easy directions on the (001) plane were mainly obtained. In other words, the magnetic domain structure under zero magnetic field was composed of straight 90° domain walls and stair-like 180° domain walls. When a magnetic field was applied in the [010] direction, 180° domain walls primarily moved, whereas 90° domain walls hardly moved. A striped domain structure composed of 90° domain walls was formed at an applied magnetic field of about 560 Oe, 180° domain walls having almost disappeared. A single domain structure was obtained by applying a magnetic field of about 1980 Oe. It should be noted that the magnetostriction in the [010] direction on the (001) plane was only about 10 ppm at the applied magnetic field of about 560 Oe and reached a saturation value of about 153 ppm at the applied magnetic field of about 1980 Oe. Therefore, the above-mentioned magnetostrictive behaviors correspond to evolution of the observed magnetic domain structure. Consequently, the characteristic evolution of the magnetic domain structure of an Fe-Ga alloy single crystal was successfully revealed.

The magnetic flux density change ΔB caused by the inverse magnetostrictive effect is key for achieving high-performance vibration power generation. As the magnetization curve of the polycrystalline Fe–47.6 at. % Co–2.3 at. % V (Fe–Co–V) alloy became easier to magnetize by applying tensile stresses, the value of ΔB estimated from the magnetization curves depended significantly on magnetic fields. Hence, the vibration power generation of a U-shaped unimorph device using a polycrystalline Fe–Co–V alloy core was demonstrated under various bias magnetic fields. As a result of bias magnetic field adjustment, the open-circuit voltage induced by the vibration of the device improved to ∼7.0 V. Such superior performance is attributable to a large ΔB of 1.1 T in the Fe–Co–V alloy core. Therefore, adjustment of bias magnetic fields is essential for obtaining large ΔB in Fe–Co–V alloys, which are promising inverse magnetostrictive materials for high-performance vibration power generation.

Iron-gallium (Fe-Ga) alloys have emerged as promising materials due to their notable magnetostriction and ductility properties, making them potential candidates for use in advanced devices. To better understand the mechanism involved in the magnetostriction of Fe-Ga alloys, we have employed a combination of fast Fourier transformation (FFT) and unsupervised machine learning techniques. Specifically, we have analyzed the features obtained from the principal component analysis (PCA) decomposition and found a strong correlation between these features and the physical components of the material. By separating the contribution of the magnetostriction in the extracted features, we were able to identify the region where the magnetostriction exhibits a significant increase. Our results show the complex relationship between the magnetostriction and the underlying physical structure of Fe-Ga alloys, and have the potential to inform the development of new materials with improved magnetostrictive properties.

Acquiring the quantitative state-of-charge (SOC) distribution of a lithium-ion battery cathode is important to improve the performance of lithium-ion batteries for the application to large-scale devices. The conventional method to obtain SOC distribution, i.e., X-ray absorption spectroscopy (XAS), requires a synchrotron radiation facility, references that are difficult to prepare, and an investigation of the XAS spectra of the references. Herein, we demonstrate that laser-induced breakdown spectroscopy (LIBS) can acquire a quantitative SOC of a charged lithium-ion battery cathode with high accuracy by comparing the SOC values of charged cathodes containing Li₂NaV₂(PO₄)₃ as an active material determined by LIBS, XAS, and a charge–discharge curve. We perform LIBS measurements in a reduced argon atmosphere of 1000 Pa and use a calibration curve obtained by measuring the emission intensities at 610.4 nm of the pristine cathode. The SOCs of the charged cathodes agree well with the average SOCs estimated from the charge–discharge curves, whereas those determined by XAS do not. Therefore, LIBS is more suitable to acquire a quantitative SOC distribution of a lithium-ion battery cathode than XAS because LIBS is a laboratory-scale measurement and requires a single simple reference in addition to its high accuracy.

The crystal structure and magnetic properties of Cu _x Co _1-x Fe ₂ O ₄ samples synthesized by a solid-phase reaction method at various annealing temperatures (Ta = 1073-1223 K) were investigated. Tetragonal distortion due to the Jahn-Teller effect of Cu ²⁺ ions was observed in the x = 0.8 sample synthesized at Ta = 1223 K. As the magnetic anisotropy constant is enhanced by such distortion, the Cu concentration dependence of the coercivity value Hc exhibits a maximum. A decrease in Ta caused the enhancement of tetragonal distortion and the reduction of crystallite size. As a result, the largest value of H _c = 2100 Oe was obtained for the x = 0.8 sample synthesized at T _a = 1073 K. It is clear that the control of both the Cu concentration and annealing temperatures during synthesis is important to obtain large coercivity in Cu _x Co _1-x Fe ₂ O ₄ .