二次元コバルト-オクタシアノタングステン錯体の光磁性現象

学位の種別: 課程博士審査委員会委員 : （主査）東京大学教授 大越 慎一, 東京大学教授 山内 薫, 東京大学教授 佃 達哉, 東京大学教授 合田 圭介, 東京大学准教授 山野井 慶徳University of Tokyo(東京大学

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

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

Magnetic lotus root based on a cyanido-bridged Co–W metal assembly

An octacyanidometalate-based nanochannel magnet, Co₇[W­(CN)₈]₄Cl₂·29H₂O, is reported. Single-crystal synchrotron X-ray diffraction measurements reveal that this compound has a three-dimensional cyanido-bridged network based on {Co₅W₄}_n columns situated along the <i>c</i>-axis. The channels resemble a lotus root and have a remarkably large diameter of 1.4 nm, which corresponds to a 44% vacancy. Although the channel size is very large, the compound exhibits ferromagnetic ordering with a Curie temperature of 29 K and coercive field of 5500 Oe. Such a ferromagnetic lotus root can adsorb large molecules and may realize host–guest chemistry in ferromagnets

Phonon-Mode Calculation, Far- and Mid-Infrared, and Raman Spectra of an ε‑Ga_0.5Fe_1.5O₃ Magnet

Gallium-substituted epsilon iron oxide (ε-Ga_0.5Fe_1.5O₃) has drawn attention because its millimeter wave absorption frequency meets the millimeter wave radar frequency for automobiles. We report the phonon modes of ε-Ga_0.5Fe_1.5O₃, which has an orthorhombic structure in the <i>Pna</i>2₁ space group. First-principles phonon-mode calculations show that ε-Ga_0.5Fe_1.5O₃ has 117 optical phonon modes (fundamental vibrations) with symmetries of A₁, A₂, B₁, and B₂ as well as three acoustic phonon modes. The phonon density of states demonstrates that the movements of Fe and Ga contribute to the phonon modes in the lower energy region of 20–370 cm^–1, while the movements of O contribute to the phonon modes in the higher energy region of 370–720 cm^–1. We directly measure the optical phonon modes of ε-Ga_0.48Fe_1.52O₃ nanoparticles using far- and mid-infrared (IR) and Raman spectroscopies, which agree well with those obtained by first-principles phonon-mode calculations. Additionally, the thermodynamic parameters of the internal energy (<i>U</i>), the vibrational entropy (<i>S</i>_vib), and the Helmholtz energy (<i>A</i>) are calculated and understood through the investigation of the phonon modes. Heat capacity measurements confirm that the observed thermodynamic parameters are consistent with the predicted values