3 research outputs found

    Nanometer-size hard magnetic ferrite exhibiting high optical-transparency and nonlinear optical-magnetoelectric effect

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    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.UTokyo Research掲載「世界最小ハードフェライト磁石の開発に成功」 URI: http://www.u-tokyo.ac.jp/ja/utokyo-research/research-news/the-worlds-smallest-hard-ferrite-magnet.htmlUTokyo Research "The world\u27s smallest hard ferrite magnet" URI: http://www.u-tokyo.ac.jp/en/utokyo-research/research-news/the-worlds-smallest-hard-ferrite-magnet.htm

    Direct Observation of Chemical Conversion from Fe<sub>3</sub>O<sub>4</sub> to ε‑Fe<sub>2</sub>O<sub>3</sub> by a Nanosize Wet Process

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    ε-iron oxide (ε-Fe<sub>2</sub>O<sub>3</sub>) has drawn attention from the viewpoints of high-density magnetic recording and high-frequency millimeter wave absorption. To date, chemical conversion from Fe<sub>3</sub>O<sub>4</sub> (magnetite) to ε-Fe<sub>2</sub>O<sub>3</sub> under wet process conditions have been difficult. Herein, we report that ε-Fe<sub>2</sub>O<sub>3</sub> could be obtained from Fe<sub>3</sub>O<sub>4</sub> using a nanosize wet process. In the present method, 10 or 16 nm sized Fe<sub>3</sub>O<sub>4</sub> nanocrystals are used as the precursor. Fe<sub>3</sub>O<sub>4</sub> nanocrystals are embedded in a silica matrix and subsequently sintered around 1000 °C in air, resulting in the chemical conversion from Fe<sub>3</sub>O<sub>4</sub> to ε-Fe<sub>2</sub>O<sub>3</sub> being confirmed. In the case of 10 nm sized Fe<sub>3</sub>O<sub>4</sub> precursors, the sample consists of 16% ε-Fe<sub>2</sub>O<sub>3</sub> and 84% γ-Fe<sub>2</sub>O<sub>3</sub>, whereas in the case of 16 nm sized Fe<sub>3</sub>O<sub>4</sub> precursors, the sample consists of 24% ε-Fe<sub>2</sub>O<sub>3</sub> and 76% γ-Fe<sub>2</sub>O<sub>3</sub>. The magnetic hysteresis loops of the samples are theoretically predicted using the large hysteresis loop of ε-Fe<sub>2</sub>O<sub>3</sub> and the magnetization curve of super-paramagnetic γ-Fe<sub>2</sub>O<sub>3</sub>. The experimental and predicted hysteresis loops agree well. First-principles calculations suggest that Fe<sub>3</sub>O<sub>4</sub> nanocrystals between 8 and 43 nm transform directly to ε-Fe<sub>2</sub>O<sub>3</sub>. Due to the strict size condition, the chemical conversion from Fe<sub>3</sub>O<sub>4</sub> to ε-Fe<sub>2</sub>O<sub>3</sub> is the first to be observed by a wet process. The nanosize wet process from Fe<sub>3</sub>O<sub>4</sub> to ε-Fe<sub>2</sub>O<sub>3</sub> should accelerate the development of highly functional hard magnetic ferrite ε-Fe<sub>2</sub>O<sub>3</sub>
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