T. Nagai

Hokkaido University, Sapporo, Hokkaidō, Japan

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Publications (111)208.13 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Spin transition and substitution of Fe3+ in Fe3+AlO3-bearing MgSiO3 perovskite (Pv) and post-perovskite (PPv) were examined up to 200 GPa and 165 GPa, respectively, at room temperature by X-ray emission spectroscopy (XES) and XRD. The results of XES and XRD indicate that in Pv high spin (HS) Fe3+ at the dodecahedral (A) site replaces Al at the octahedral (B) site and becomes low spin (LS) between 50 and 70 GPa with pressure, while in PPv LS Fe3+ occupies the B-site and Al occupies the A-site above 80-100 GPa. The Fe3+-Al coupled substitution seems to be at work in both Pv and PPv. Combining these results on Fe3+ with the recent first-principles calculations on Fe2+ in Pv and PPv, the spin transition and substitution of iron in pyrolitic lower mantle minerals are proposed. Further, their effects on iron-partitioning among the lower mantle minerals are discussed.
    Physics of The Earth and Planetary Interiors 01/2014; · 2.38 Impact Factor
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    ABSTRACT: Crystal structures of high-pressure polymorphs of Ca(OD)2, (a) at room temperature (phase II′) and (b) at high temperature (phase II), were obtained from in situ neutron diffraction measurements.
    Journal of Solid State Chemistry. 01/2014; 218:95–102.
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    ABSTRACT: The distorted CuO6 octahedron in La2CuO4 was studied using aberration-corrected scanning transmission electron microscopy at atomic resolution. The near-edge structure in the oxygen K-edge electron energy-loss spectrum was recorded as a function of the position of the electron probe. After background subtraction, the measured spectrum image was processed using a recently developed inversion process to remove the mixing of signals on the atomic columns due to elastic and thermal scattering. The spectra were then compared with first-principles band structure calculations based on the local-density approximation plus on-site Coulomb repulsion (LDA + U) approach. In this article, we describe in detail not only anisotropic chemical bonding of the oxygen 2p state with the Cu 3d state but also with the Cu 4p and La 5d/4f states. Furthermore, it was found that buckling of the CuO2 plane was also detectable at the atomic resolution oxygen K-edge. Lastly, it was found that the effects of core-hole in the O K-edge were strongly dependent on the nature of the local chemical bonding, in particular, whether it is ionic or covalent.
    Journal of Applied Physics 08/2013; 114(8). · 2.21 Impact Factor
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    ABSTRACT: Experiments using laser-heated diamond anvil cells combined with synchrotron X-ray diffraction and SEM–EDS chemical analyses have confirmed the existence of a complete solid solution in the MgSiO3–MnSiO3 perovskite system at high pressure and high temperature. The (Mg, Mn)SiO3 perovskite produced is orthorhombic, and a linear relationship between the unit cell parameters of this perovskite and the proportion of MnSiO3 components incorporated seems to obey Vegard’s rule at about 50 GPa. The orthorhombic distortion, judged from the axial ratios of a/b and $ \sqrt{2}\,a/c, $ 2 a / c , monotonically decreases from MgSiO3 to MnSiO3 perovskite at about 50 GPa. The orthorhombic distortion in (Mg0.5, Mn0.5)SiO3 perovskite is almost unchanged with increasing pressure from 30 to 50 GPa. On the other hand, that distortion in (Mg0.9, Mn0.1)SiO3 perovskite increases with pressure. (Mg, Mn)SiO3 perovskite incorporating less than 10 mol% of MnSiO3 component is quenchable. A value of the bulk modulus of 256(2) GPa with a fixed first pressure derivative of four is obtained for (Mg0.9, Mn0.1)SiO3. MnSiO3 is the first chemical component confirmed to form a complete solid solution with MgSiO3 perovskite at the P–T conditions present in the lower mantle.
    Physics and Chemistry of Minerals 07/2013; · 1.30 Impact Factor
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    ABSTRACT: Fabrication of high quality InP nanopillar structures, with the help of self-masking properties and using the electron cyclotron resonance ion etching techniques, can be a easy demanding and one step large scale production method compared to the traditional, expensive and multi-step complicated methods. In this paper regular arrays of crystalline and high aspect ratio InP nanopillars were fabricated by low energy electron cyclotron resonance Ar+ ion irradiation technique. Several scanning electron microscopy images were utilized to investigate the width, height, and orientation of these nanopillars. The average width and length of these nano-pillars were about 50 nm and 500 nm, respectively. Cross-sectional high resolution transmission electron microscopy studies revealed that these nanopillars are crystalline in nature. Photoluminescence measurements also revealed the crystalline nature as well as the enhancement in PL intensity due to the large surface area of the nanopillars.
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    ABSTRACT: We have observed the response of spin arrangements to external magnetic fields in a multiorbital Mott insulator, ferromagnetic insulating manganite La7/8Sr1/8MnO3, by low-temperature in situ Lorentz microscopy. Magnetic fields normal to the plane of the thin-plate sample continuously change the width of domains in the serpentine-like domain structure, eventually giving rise to nanoscale elliptical magnetic bubbles of ∼200 nm major diameter at 3.6 kOe. The formation of these bubbles implies large magnetic anisotropy related to the orbital ordering and suggests the possibility of manipulating the bubbles using an electric field.
    Applied Physics Letters 10/2012; 101(16). · 3.79 Impact Factor
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    ABSTRACT: The skeletal texture and crystal morphology of the massive reef-building coral Porites lobata were observed from the nano- to micrometer scale using an analytical transmission electron microscope (ATEM). The skeletal texture consists of centers of calcification (COCs) and fiber area. Fiber areas contain bundles of needle-like aragonite crystals that are elongated along the crystallographic c-axis and are several hundred nanometers to one micrometer in width and several micrometers in length. The size distribution of aragonite crystals is relatively homogeneous in the fibers. Growth lines are observed sub-perpendicular to the direction of aragonite growth. These growth lines occur in 1-2μm intervals and reflect a periodic contrast in the thickness of an ion-spattered sample and pass through the interior of some aragonite crystals. These observations suggest that the medium filled in the calcification space maintains a CaCO(3)-supersaturated state during fiber growth and that a physical change occurs periodically during the aragonite crystals of the fiber area.
    Journal of Structural Biology 10/2012; · 3.36 Impact Factor
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    ABSTRACT: The manipulation of spin textures with electric currents is an important challenge in the field of spintronics. Many attempts have been made to electrically drive magnetic domain walls in ferromagnets, yet the necessary current density remains quite high (~10(7) A cm(-2)). A recent neutron study combining Hall effect measurements has shown that an ultralow current density of J~10(2) A cm(-2) can trigger the rotational and translational motion of the skyrmion lattice in MnSi, a helimagnet, within a narrow temperature range. Raising the temperature range in which skyrmions are stable and reducing the current required to drive them are therefore desirable objectives. Here we demonstrate near-room-temperature motion of skyrmions driven by electrical currents in a microdevice composed of the helimagnet FeGe, by using in-situ Lorentz transmission electron microscopy. The rotational and translational motions of skyrmion crystal begin under critical current densities far below 100 A cm(-2).
    Nature Communications 08/2012; 3:988. · 10.02 Impact Factor
  • Microscopy and Microanalysis 07/2012; 18(S2):1032-1033. · 2.50 Impact Factor
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    ABSTRACT: Hybridization state mapping with atomic resolution was demonstrated using electron energy loss spectroscopy combined with scanning transmission electron microscopy. The O 2p states hybridized with Sn 5s and Cu 3d in a layered double perovskite La2CuSnO6 were individually distinguished by significant contrast differences in the oxygen K-edge energy-loss near-edge structure. The anisotropic oxygen intensity in the distorted CuO6 octahedron resulting from the Jahn-Teller effect could also be observed. The localized Cu 3d hole in the bc plane was indirectly imaged in real space using two-dimensional oxygen mapping.
    Applied Physics Letters 04/2012; 100(16). · 3.79 Impact Factor
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    ABSTRACT: There are still large discrepancies among the previous reports on the spin transition of iron in Mg–perovskite (Pv). To alleviate this problem, we examined the spin state of Fe3 + in Mg0.85Fe3 +0.15Al0.15Si0.85O3 Pv up to 200 GPa by X-ray emission spectroscopy (XES) and X-ray diffraction (XRD). The gradual decrease of the high spin (HS) ratio of Fe3 + by low temperature annealing of the samples above ~ 60 GPa in XES and the change of the trend of unit cell volumes with pressure by annealing at 50–60 GPa in XRD indicate that Fe3 + occupies the A-site and is HS below ~ 50 GPa but above 50–60 GPa it gradually replaces Al at the B-site through cation exchange reaction by annealing and becomes low spin (LS), while Fe3 + remaining at the A-site is HS up to 200 GPa. This means that the spin state of Fe3 + depends on Fe3 + occupancies between the A- and B-sites and these Fe3 + occupancies are strongly controlled by the synthesis condition and annealing temperature of the samples through the cation exchange reaction. The present results combined with the previous reports indicate that in Al-bearing Mg–Pv in the lower mantle Fe2 + occupies the A-site and remains HS for the whole lower mantle, while Fe3 + occupies the A-site and is HS below ~ 50 GPa but above 50–60 GPa it replaces Al at the B-site and becomes LS, on the assumption that spin transition pressure of Fe2 + at the A-site is higher than that of Fe3 + at the same site.
    Earth and Planetary Science Letters 02/2012; s 317–318:407–412. · 4.72 Impact Factor
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    ABSTRACT: Pressure-induced high spin (HS) - low spin (LS) transitions of iron in the lower mantle minerals are the key issue to understand the mineral behaviors and dynamics of the lower mantle. However, the spin transitions of iron in Mg-perovskite (Pv) and post-Mg-perovskite (PPv) are still unclear because of the large discrepancies of the results for Pv and very limited reports for PPv. These problems seem to originate partly from the poor quality and/or characterization of the samples for Pv and the experimental difficulty for PPv. We examined the spin states of ferric iron in Al-bearing Pv (up to 200 GPa) and PPv (up to165 GPa) by X-ray emission spectroscopy at room temperature and XRD, using the well qualified samples of the composition Mg0.85Fe3+0.15Al0.15Si0.85O3 loaded in a diamond anvil cell. The reason to have chosen this composition is because the recent studies indicate that Fe3+ is more dominant than Fe2+ and the coupled substitution, Mg + Si → Fe3+ + Al, occurs in lower mantle Pv and PPv. The results indicate that in Pv Fe3+ occupies the 8-coordinated (A) site and is HS below ~50 GPa but above 50-60 GPa it replaces Al at the 6-coordinated (B) site by annealing and becomes LS, while Fe3+ remaining at the A site is HS up to 200 GPa. Meanwhile in PPv Fe3+ occupies the B site and is LS at 165 GPa and it becomes HS between 80 - 40 GPa during decompression from 165 GPa without annealing. We think that the cation exchange reaction of Fe3+ and Al between the A and B sites plays an important role in the spin transition of Fe3+. From these results combined with the recent reports, the spin states of iron in lower mantle Pv and PPv are estimated to be as follows, when we assume that the spin transition pressure of Fe2+ is higher than that of Fe3+ at the same site. In Pv Fe2+ occupies the A site and is HS for the whole lower mantle, while Fe3+ occupies the A site and is HS below ~50 GPa but above 50-60 GPa it replaces Al at the B site and becomes LS. Meanwhile in PPv Fe2+ is at the A site and HS and Fe3+ is at the B site and LS in the whole lowermost mantle. In the presentation, the effects of these spin transitions of iron on the Pv-PPv transition pressure and iron partitioning between the iron-bearing phases in the lower mantle are also discussed.
    AGU Fall Meeting Abstracts. 12/2011;
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    ABSTRACT: Regular arrays of InP nano pillars have been fabricated by low energy Electron Cyclotron Resonance (ECR) Ar+ ion irradiation on InP(111) surface. Several scanning electron microscopy (SEM) images have been utilized to invetsigate the width, height, and orientation of these nano pillars on InP(111) surfaces. The average width and length of these nano-pillars are about 50 nm and 500 nm, respectively. The standing angle with respect to the surface of the nano-pillars depend on the incidence angle of the Ar ion irradiation during the fabrication process. Interestingly, the growth direction of the nano pillars are along the reflection direction of the ion beam and the standing angles are nearly same as the ion incidence angle with the surface normal. This nano-pillas are easily transferred from the InP surface to double sided carbon tape without any damage. High Resolution Transmission Electron Microscopy (HRTEM) study of single nano-pillar reveals that this nano-pillar are almost crystalline in nature except 2-4 nm amorphous layer on the outer surface. The transmission electron microscopy combined with energy-dispersive x-ray spectroscopy (TEM-EDS) analysis of these nano pillars exhibit that the ratio of In and P is little higher compared to the bulk InP.
    04/2011;
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    ABSTRACT: The El Niño/Southern Oscillation (ENSO) system during the Pliocene warm period (PWP; 3-5 million years ago) may have existed in a permanent El Niño state with a sharply reduced zonal sea surface temperature (SST) gradient in the equatorial Pacific Ocean. This suggests that during the PWP, when global mean temperatures and atmospheric carbon dioxide concentrations were similar to those projected for near-term climate change, ENSO variability--and related global climate teleconnections-could have been radically different from that today. Yet, owing to a lack of observational evidence on seasonal and interannual SST variability from crucial low-latitude sites, this fundamental climate characteristic of the PWP remains controversial. Here we show that permanent El Niño conditions did not exist during the PWP. Our spectral analysis of the δ(18)O SST and salinity proxy, extracted from two 35-year, monthly resolved PWP Porites corals in the Philippines, reveals variability that is similar to present ENSO variation. Although our fossil corals cannot be directly compared with modern ENSO records, two lines of evidence suggest that Philippine corals are appropriate ENSO proxies. First, δ(18)O anomalies from a nearby live Porites coral are correlated with modern records of ENSO variability. Second, negative-δ(18)O events in the fossil corals closely resemble the decreases in δ(18)O seen in the live coral during El Niño events. Prior research advocating a permanent El Niño state may have been limited by the coarse resolution of many SST proxies, whereas our coral-based analysis identifies climate variability at the temporal scale required to resolve ENSO structure firmly.
    Nature 03/2011; 471(7337):209-11. · 38.60 Impact Factor
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    ABSTRACT: MgSiO3 dominant perovskite is believed to be the most abundant constituent mineral in the Earth’s lower mantle. Generally minerals form solid solutions and their nature should affect on physical properties of minerals. In this paper, we will introduce our recent studies about incorporation mechanism of FeAlO3 component into MgSiO3 perovskite and its crystal chemistry.
    Nihon Kessho Gakkaishi. 01/2011; 53(1):8-12.
  • M. Nagao, T. Nagai, T. Hara, K. Kimoto
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    ABSTRACT: The charge order (CO) domains of dimensionally controlled manganites Pr1−xCaxMnO3 and Pr1−xCa1+xMnO4 (x=0.5, 0.6 and 0.67), which have three-dimensional (3D) and two-dimensional (2D) Mn–O networks, respectively, have been studied by transmission electron microscopy (TEM). Although the electron diffraction data show similar dependences of the modulation wave vector on hole doping x, there are distinctive differences between the 3D and 2D systems in terms of the CO domain sizes. In the 2D system, the TEM images show that the domain size is almost constant irrespective of hole doping x. On the other hand, in the 3D system, the domain size of the incommensurate CO for x=0.6 is much smaller than those of the commensurate CO for x=0.5 and 0.67. Namely, in the 3D system, the CO states are strongly influenced by the incommensurability for the parent lattice. This difference indicates that the dimension of the Mn–O network plays a crucial role in the CO domain and suggests that the electron–lattice coupling of the 3D system is stronger than that of the 2D system.
    Physica B Condensed Matter 01/2011; 406(17):3192-3195. · 1.28 Impact Factor
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    ABSTRACT: The pressure responses of portlandite and the isotope effect on the phase transition were investigated at room temperature from single-crystal Raman and IR spectra and from powder X-ray diffraction using diamond anvil cells under quasi-hydrostatic conditions in a helium pressure-transmitting medium. Phase transformation and subsequent peak broadening (partial amorphization) observed from the Raman and IR spectra of Ca(OH)2 occurred at lower pressures than those of Ca(OD)2. In contrast, no isotope effect was found on the volume and axial compressions observed from powder X-ray diffraction patterns. X-ray diffraction lines attributable to the high-pressure phase remained up to 28.5GPa, suggesting no total amorphization in a helium pressure medium within the examined pressure region. These results suggest that the H–D isotope effect is engendered in the local environment surrounding H(D) atoms. Moreover, the ratio of sample-to-methanol–ethanol pressure medium (i.e., packing density) in the sample chamber had a significant effect on the increase in the half widths of the diffraction lines, even at pressures below the hydrostatic limit of the pressure medium. KeywordsPortlandite–Isotope effect–Phase transition–Hydrostaticity–Hydrogen bonding
    Physics and Chemistry of Minerals 01/2011; 38(10):777-785. · 1.30 Impact Factor
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    ABSTRACT: Synchrotron X-ray diffraction study for solid CO2 at 40–100 GPa and around 2,000 K has shown that the diffraction pattern of the high pressure phase CO2-V is consistently interpreted in terms of a tetragonal uni_t cell (Z = 4, a = 3.584 Å, c = 5.908 Å at 50GPa). A β-cristobalite structure (space group I2d) gives a good account of our data qualitatively. Isothermal molar volume (300K) of the CO2-V in the present study is smaller than that indexed as a tridymite structure proposed by previous studies at any pressures.
    Journal of Physics Conference Series 04/2010; 215(1):012015.
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    ABSTRACT: Self-organizing domain structures of charge-orbital-ordered (COO) and ferro-orbital-ordered (FOO) phases were observed for tetragonal layered manganites Nd1−xSr1+xMnO4 (x=0.8 and 0.82) by low-temperature transmission electron microscopy. We revealed a marked temperature variation of coexisting COO domains with equivalent structural distortions and orbital ordering patterns at x=0.8. The observed competition of coexisting FOO domains at x=0.82 also clearly reflects the self-organizing nature of eg electrons. Furthermore, a checkered structure, in which COO domains were reproduced at the intersections of FOO domains, was found in the transition from the COO phase to the FOO phase at x=0.8. The observations suggest that strain energy affects the self-organizing behavior in the orbital ordering.
    Physical review. B, Condensed matter 02/2010; 81(6). · 3.77 Impact Factor
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    ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
    ChemInform 01/2010; 31(13).

Publication Stats

375 Citations
208.13 Total Impact Points

Institutions

  • 2005–2014
    • Hokkaido University
      • • Department of Natural History Sciences
      • • Faculty of Science
      • • Department of Earth and Planetary Sciences
      • • Graduate School of Science
      Sapporo, Hokkaidō, Japan
  • 2002–2013
    • National Institute for Materials Science
      • Advanced Materials Laboratory
      Tsukuba, Ibaraki, Japan
  • 2008–2012
    • RIKEN
      • Strong Correlation Interface Research Group
      Вако, Saitama, Japan
  • 1997–2010
    • Osaka University
      • • Department of Earth and Space Science
      • • Graduate School of Science
      Suika, Ōsaka, Japan
  • 2000–2002
    • Waseda University
      • Department of Earth Science, Resources and Environmental Engineering
      Tokyo, Tokyo-to, Japan
  • 2001
    • Stony Brook University
      • Department of Geosciences
      Stony Brook, NY, United States