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We successfully produced a dopant‐free oxygen‐defective HfO2−x film that exhibit room‐temperature ferromagnetism via a heat treatment of metallic Hf foil at 1673 K for 10 h under an oxygen partial pressure of 1.0 × 10−4 atm. The sample, which exhibits ∼50% transmission of visible light, demonstrates room‐temperature ferromagnetism, with saturation and residual magnetizations of ∼0.05 emu/g and ∼0.005 emu/g, respectively. The HfO2−x film comprises columnar grains several hundred nanometers wide and several micrometers long. High‐angle annular dark‐field scanning electron microscopy observations indicated that the tetragonal HfO2 structure was distributed in nanodomains within the monoclinic HfO2 grains; that is, the films exhibited a hybridized microstructure. The tetragonal HfO2 structure would be mainly due to the introduction of oxygen deficiencies, which led to the room‐temperature ferromagnetism. [ABSTRACT FROM AUTHOR]

Dopant-free, transparent ZrO 2 with a monoclinic structure was successfully developed through a two-step oxidation process by combining heat treatment in air and under a low oxygen partial pressure, using a Zr metal foil. The transmission of the sample was ~50% at 600 nm (in the region of visible light) and over 60% beyond the near-infrared region. This transparent ZrO 2 with a monoclinic structure comprised equiaxed grains with dimensions of tens of nanometers and columnar grains with lengths of several micrometers. Moreover, it formed a dense film without cracks or voids, having fewer oxygen deficiencies. Figure (a) Appearance and (b) cross-sectional SEM images of black-ZrO 2 annealed at 800 °C for 15 min in air. (c) Appearance and (d) cross-sectional SEM images of transparent-ZrO 2 annealed at 800 °C for 15 min in air, followed by heat-treatment (900 °C for 15 h in PO 2 = 1 × 10−6 atm) Image, graphical abstract [ABSTRACT FROM AUTHOR]

Ferromagnetism can be induced by introducing oxygen vacancies into metallic oxides. The oxygen deficiencies introduced into ZrO 2 samples thus far have been present in unquantifiable concentrations and randomly scattered near the surface. We successfully produced room-temperature-ferromagnetic 9 μm-thick ZrO 2− x films via a stepwise oxidation process consisting of annealing metallic Zr foil in air and under a controlled oxygen partial pressure. The ZrO 2− x films are composed of a periodic structure with a spacing of 3–4 nm; this structure originated from the presence and absence of oxygen deficiencies. The unique structure in the ZrO 2− x phase consisting of the periodic arrangements of oxygen deficiencies can be produced by the stepwise oxidation using Zr metal. Our proposed process should open the door for new applications of defect-driven ferromagnetic films of undoped magnetic oxides with a periodic oxygen-defective structure. Figure (a) A cross-sectional bright-field TEM image of the surface of sample A. (b) The enlarged area B near the surface in (a). (c) Selected-area diffraction pattern for grain C in (b). (d) Diffraction intensity profile for the red-shaped area X - Y in (c). (e) A low-magnification HAADF-STEM image taken along the [011] P 12/ c 1 direction of a monoclinic ZrO 2 phase. (f) High-magnification HAADF-STEM image of the sample shown in (e). [Display omitted] [ABSTRACT FROM AUTHOR]

Black-ZrO 2 thin films were successfully fabricated on the surface of a Zr metal plate by simple oxidation at a controlled temperature in air, under normal atmospheric pressure. The black-ZrO 2 thin films were composed of two types of grains with monoclinic ZrO 2 structures: 1) fine equiaxed grains with sizes of up to a few hundred nanometers from the surface to a depth of about 5 μm; 2) columnar grains, with long and short diameters of several microns and a few hundred nanometers, respectively, in the inner region above the equiaxed grains. The bandgap of the black-ZrO 2 thin films was estimated to be 0.72 eV, based on the Tauc plot using diffuse reflections, which allowed for absorption in the ultraviolet and visible regions of the solar spectrum. [ABSTRACT FROM AUTHOR]

We produced Magnéli phase vanadium oxides with a thickness of approximately 1.0 μm on the surface of vanadium metal foil via a stepwise oxidation process consisting of annealing at 723 K in air and a subsequent heat treatment at 773 K under a low oxygen partial pressure of P O2 = 1.0 × 10−10 atm. The diffuse reflectance of samples with a Magnéli phase vanadium oxide was less than 20%, indicating the absorption of visible and near-infrared lights. Magnéli phase vanadium oxides with the desired vanadium-to-oxygen atomic ratio could be produced by controlling the heat-treatment conditions in our oxidation process. [Display omitted] [ABSTRACT FROM AUTHOR]

A strong-interference film that absorbs visible light by using oxygen-deficient ZrO 2 – x was produced by heat treatment of Zr metal foil under a low partial pressure of oxygen. The oxygen-defective tetragonal ZrO 2 – x film contained nanocrystallites several nanometers in size and its thickness (t) increased from 30 to 140 nm on increasing the holding time during heat treatment. The color of the sample changed depending on the thickness of the oxygen-deficient ZrO 2 – x film from gold (t = 30 nm) through blue (t = 75 nm) to black (t = 140 nm). Thick ZrO 2 – x films (t ≧ 140 nm) absorbed all visible light, leading to the observed black color, whereas in thinner films (t < 140 nm), strong interference resulted in changes in the sample color, for example, from gold to blue. Fig. The appearances of a sample (a) before heating and after heating the Zr metal foil at 1273 K for (b) 1.8ks, (c) 3.6 ks, (d) 5.4 ks under PO2 = 1.0 × 10-10 atm. [Display omitted] [ABSTRACT FROM AUTHOR]

A 3-μm-thick film of Magnéli Ti 4 O 7 was successfully fabricated on the surface of Ti metal foil by a stepwise oxidation process involving annealing at 973 K in air followed by heating to 1173 K under a low oxygen partial pressure. Transmission electron microscopy and high-angle annular dark-field scanning transmission electron microscopy showed that the Magnéli Ti 4 O 7 phase consisted of a period of four layers of equiaxial grains several hundreds of nanometers in size. The black Magnéli Ti 4 O 7 thin film formed by our process absorbed light in the visible and near-infrared regions. Figure (a) Appearance and (b) corresponding XRD patterns of a sample before and after heating. (I) Ti metal foil before heating, and after heating to (II) 973 K 10.8 ks in air, (III) 1173 K with no resting time in air, and two-step heat treatment involving heating at 973 K 10.8 ks in air and subsequent heating to 1173 K with no resting time under (IV) P O2 = 1.0 × 10−5 atm, (V) P O2 = 1.0 × 10−10 atm, respectively. [Display omitted] [ABSTRACT FROM AUTHOR]