Development of nanometer-sized magnetic particles exhibiting a large coercive field (Hc) is in high demand for densification of magnetic recording. Herein, we report a single-nanosize (i.e., less than ten nanometers across) hard magnetic ferrite. This magnetic ferrite is composed of ε-Fe2O3, with a sufficiently high Hc value for magnetic recording systems and a remarkably high magnetic anisotropy constant of 7.7 × 106 erg cm−3. For example, 8.2-nm nanoparticles have an Hc value of 5.2 kOe at room temperature. A colloidal solution of these nanoparticles possesses a light orange color due to a wide band gap of 2.9 eV (430 nm), indicating a possibility of transparent magnetic pigments. Additionally, we have observed magnetization-induced second harmonic generation (MSHG). The nonlinear optical-magnetoelectric effect of the present polar magnetic nanocrystal was quite strong. These findings have been demonstrated in a simple iron oxide, which is highly significant from the viewpoints of economic cost and mass production.

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The phase transition between gamma-trititanium-pentoxide (γ-Ti3O5) and delta-trititanium-pentoxide (δ-Ti3O5) was clarified from both experimental and theoretical viewpoints. With decreasing temperature, the monoclinic I2/c crystal structure of γ-Ti3O5 was found to switch to a monoclinic P2/a crystal structure of δ-Ti3O5 due to lowering of symmetry. Electrical conductivity (σ) measurement shows that γ-Ti3O5 behaves like a metallic conductor with a σ value of 4.7 S cm–1 at 320 K, while δ-Ti3O5 shows a semiconductive property with a σ value of 2.5 × 10–5 S cm–1 at 70 K. Optical measurement also supports that γ-Ti3O5 is a metallic conductor, while δ-Ti3O5 is a semiconductor with a band gap of 0.07 eV. First-principles calculations show that γ-Ti3O5 is a metallic conductor, and the energy state on the Fermi energy is composed of the 3d orbital of Ti and 2p orbital of O with one-dimensional linkage along the crystallographic c-axis. On the contrary, δ-Ti3O5 has a band gap, and the energy state around the Fermi energy is split into the valence band and the conduction band, which are assigned to the lower and upper Hubbard bands, respectively. Thus, the phase transition between γ-Ti3O5 and δ-Ti3O5 is caused by breaking of a one-dimensionally conducting pathway due to a Mott–Hubbard metal–insulator phase transition.

We show that the barrier profile of in situ grown AlO x tunnel barriers strongly depends on the material choices of the oxide-metal interface. By doing transport measurements on Al and Nb-based metal-oxide-metal tunnel junctions in a wide temperature range and using the phenomenological Simmons' model, we obtain barrier parameters that are qualitatively consistent with the values obtained from the first-principles calculations. The latter suggest that the formation of metal-induced gap states originating from the hybridization between the metallic bands and Al 2 O 3 conduction band is responsible for the tunnel barrier modification. These findings are important for nanoelectronic devices containing tunnel junctions with a thin insulating layer.