X-ray photoelectron spectroscopy (XPS), ellipsometry, and cathode luminescence were utilized for characterizing oxide films formed on the surfaces of FeCo-V alloys (Permendur) that contained small amounts of aluminum via annealing under a low partial pressure of oxygen. XPS spectra showed that the oxide films were composed primarily of aluminum oxide, which was a result of preferential oxidation of aluminum on the surfaces of the alloys during annealing. The thicknesses of the oxide films were estimated from XPS depth profiles coupled with the sputtering rate of a SO2/Si film, and the thicknesses were compared with those of the oxide films measured via non-destructive ellipsometry. The thicknesses obtained from the depth profiles may have been overestimated because of a difference in sputtering rate between silicon oxide and aluminum oxide. As measured via ellipsometry, the thickness of the aluminum oxide thin film, which was less than 100 nm, increases with the aluminum content in the alloys. Wide XPS spectra from the sample surfaces with different roughness suggested that the aluminum oxide thin films were homogenously formed on the smooth surfaces, whereas metallic elements in the alloy substrates were detected on the rough surfaces. Furthermore, the electrical resistivity of the alloys with the smooth surfaces increases significantly with the aluminum content, which may be attributable to the formation of the aluminum oxide thin film. These results indicate that the addition of a small amount of aluminum and surface smoothness are important for controlling formation of the aluminum oxide thin films with high resistivity in FeCo-V alloys by annealing under a low partial pressure of oxygen.

Fe–Co alloys are attracting attention as magnetostrictive materials for energy harvesting and sensor applications. This work investigated the magnetostriction characteristics and crystal structure of additive-manufactured Fe–Co alloys using directed energy deposition. The additive-manufactured Fe–Co parts tended to exhibit better magnetostrictive performance than the hot-rolled Fe–Co alloy. The anisotropy energy ΔK1 for the Fe–Co bulk, prepared under a power of 300 W (referred to as bulk−300 W), was larger than for the rolled sample. For the bulk−300 W sample in a particular plane, the piezomagnetic constant d was large, irrespective of the direction of the magnetic field. Elongated voids that formed during additive manufacturing changed the magnetostrictive behavior in a direction perpendicular to these voids. Magnetic property measurements showed that the coercivity decreased. Since sensors should be highly responsive, Fe–Co three-dimensional parts produced via additive manufacturing can be applied as force sensors.

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The aluminum oxide thin films formed on the surfaces of FeCo-V alloys doped with a small amount of aluminum were characterized by ellipsometry and electrical resistivity measurements. Aluminum oxide films on alloy surfaces were selectively formed by annealing under a low partial pressure of oxygen. The thickness of the oxide films increased with increasing aluminum content and temperature. However, the electrical resistivity of the oxide thin films did not correlate with the thickness of the oxide films. To clarify the anomaly of the resistivity, micro-beam X-ray photoelectron spectroscopy (XPS) was used to characterize the two-dimensional distribution of elements on the alloy surface. The XPS results show that the oxide films were partially exfoliated in the alloys with a high aluminum content. In particular, the oxide films were likely to be exfoliated or separated in relatively thick oxide films because of their dissimilar properties at vicinities close to the interface between the substrate and aluminum oxide thin films. Therefore, the thickness of the oxide film, and the amount of aluminum, are crucial for controlling the formation of aluminum oxide thin films with high resistivity in FeCo-V alloys.