スピントロニクスデバイス応用を目指した原子レベルで滑らかな表面形状をもつ遷移金属酸化物薄膜の合成

京都大学0048新制・課程博士博士(工学)甲第17582号工博第3741号新制||工||1570(附属図書館)30348京都大学大学院工学研究科材料化学専攻(主査)教授 田中 勝久, 教授 平尾 一之, 教授 三浦 清貴学位規則第4条第1項該当Doctor of Philosophy (Engineering)Kyoto UniversityDFA

Anisotropic growth of zinc oxide pillars on silver nanoparticles by oblique angle deposition

We have prepared composites with anisotropic microstructure consisting of silver nanoparticles and zinc oxide pillars using an oblique angle deposition technique, and examined the optical response originating from their anisotropic morphology. The sample was obtained in three steps. First, the assembly of silver nanoparticles was prepared on a silica glass substrate by electron-beam deposition of the silver thin film and subsequent heat treatment. Next, zinc oxide was obliquely grown by using a pulsed laser deposition. Finally the zinc oxide was crystallized by the post annealing to make an array of inclined pillars grown on the top of the silver nanoparticles. The structure and morphology of the composites were elucidated by a combination of X-ray diffraction analysis and transmission electron microscopy. Optical rotation spectroscopy clarifies that the composite shows optical birefringence due to its inclined pillar morphology. The optical rotation spectrum exhibits two peaks, one being associated to the localized surface plasmon resonance of the silver nanoparticles and the other the excitons in the zinc oxide pillars. The present fabrication method is simple and can be applied to obtain anisotropic composites with relatively large dimensions

Photoemission study of itinerant ferromagnet Cr1-δ Te

The electronic structures of the itinerant ferromagnets Cr1-δTe (δ=0.05, 0.25, and 0.375) have been studied by photoemission spectroscopy. The valence-band spectra are compared with the density of states given by band-structure calculations. In spite of the itinerant nature of the d electrons, disagreement between the photoemission spectra and the band-structure calculations exists in the magnitude of the d-band exchange splitting and the spectral weight at the Fermi level and 2-4 eV below it: The occupied d band for δ=0.05 is shifted away from the Fermi level; the observed spectral weight at the Fermi level is significantly suppressed compared with the band-structure calculations for δ=0.05 and 0.375, where the nominal d-electron numbers are close to integers 4 and 3, respectively. Configuration-interaction cluster-model calculations have been made for δ=0.05 and 0.375 to explain the spectral weight distribution in the high-binding-energy (2-4 eV) region in terms of electron-correlation effects. The d-d on-site Coulomb energy is estimated to be significant, U∼2 eV, and nearly equal to or smaller than the charge-transfer energy Δ∼2-3 eV

Room-Temperature Polar Ferromagnet ScFeO₃ Transformed from a High-Pressure Orthorhombic Perovskite Phase

Multiferroic materials have been the subject of intense study, but it remains a great challenge to synthesize those presenting both magnetic and ferroelectric polarizations at room temperature. In this work, we have successfully obtained LiNbO₃-type ScFeO₃, a metastable phase converted from the orthorhombic perovskite formed under 15 GPa at elevated temperatures. A combined structure analysis by synchrotron X-ray and neutron powder diffraction and high-angle annular dark-field scanning transmission electron microscopy imaging reveals that this compound adopts the polar <i>R</i>3<i>c</i> symmetry with a fully ordered arrangement of trivalent Sc and Fe ions, forming highly distorted ScO₆ and FeO₆ octahedra. The calculated spontaneous polarization along the hexagonal <i>c</i>-axis is as large as 100 μC/cm². The magnetic studies show that LiNbO₃-type ScFeO₃ is a weak ferromagnet with <i>T</i>_N = 545 K due to a canted <i>G</i>-type antiferromagnetic ordering of Fe³⁺ spins, representing the first example of LiNbO₃-type oxides with magnetic ordering far above room temperature. A comparison of the present compound and rare-earth orthorhombic perovskites RFeO₃ (R = La–Lu and Y), all of which possess the corner-shared FeO₆ octahedral network, allows us to find a correlation between <i>T</i>_N and the Fe–O–Fe bond angle, indicating that the A-site cation-size-dependent octahedral tilting dominates the magnetic transition through the Fe–O–Fe superexchange interaction. This work provides a general and versatile strategy to create materials in which ferroelectricity and ferromagnetism coexist at high temperatures

Room-Temperature Polar Ferromagnet ScFeO₃ Transformed from a High-Pressure Orthorhombic Perovskite Phase

Multiferroic materials have been the subject of intense study, but it remains a great challenge to synthesize those presenting both magnetic and ferroelectric polarizations at room temperature. In this work, we have successfully obtained LiNbO₃-type ScFeO₃, a metastable phase converted from the orthorhombic perovskite formed under 15 GPa at elevated temperatures. A combined structure analysis by synchrotron X-ray and neutron powder diffraction and high-angle annular dark-field scanning transmission electron microscopy imaging reveals that this compound adopts the polar <i>R</i>3<i>c</i> symmetry with a fully ordered arrangement of trivalent Sc and Fe ions, forming highly distorted ScO₆ and FeO₆ octahedra. The calculated spontaneous polarization along the hexagonal <i>c</i>-axis is as large as 100 μC/cm². The magnetic studies show that LiNbO₃-type ScFeO₃ is a weak ferromagnet with <i>T</i>_N = 545 K due to a canted <i>G</i>-type antiferromagnetic ordering of Fe³⁺ spins, representing the first example of LiNbO₃-type oxides with magnetic ordering far above room temperature. A comparison of the present compound and rare-earth orthorhombic perovskites RFeO₃ (R = La–Lu and Y), all of which possess the corner-shared FeO₆ octahedral network, allows us to find a correlation between <i>T</i>_N and the Fe–O–Fe bond angle, indicating that the A-site cation-size-dependent octahedral tilting dominates the magnetic transition through the Fe–O–Fe superexchange interaction. This work provides a general and versatile strategy to create materials in which ferroelectricity and ferromagnetism coexist at high temperatures