Koji Fujita

Kyoto University, Kioto, Kyōto, Japan

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Publications (125)238.17 Total impact

  • [show abstract] [hide abstract]
    ABSTRACT: We have prepared amorphous xFeO(100 ? x)SiO2 (x = 38.2, 54.8, and 67.5 in mol %) thin films by a pulsed laser deposition method and examined their magnetic properties. The temperature dependence of zero-field-cooled magnetization manifests a cusp-like peak, i.e., spin-freezing in a low temperature region. The spin-freezing temperature depends on the frequency of ac magnetic field, and the dependence is explainable in terms of the scaling law. The aging-memory effect characteristic of spin glass is also observed. Furthermore, superparamagnetic behavior is found in the variation of magnetization with magnetic field at high temperatures. These observations indicate that there exist magnetic clusters of Fe2+ ions in the amorphous FeO?SiO2 thin films and that the intercluster interactions bring about the superspin glass transition at low temperatures.
    Japanese Journal of Applied Physics 04/2014; 53(5S1):05FB11. · 1.07 Impact Factor
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    ABSTRACT: A novel silver-containing perovskite, AgCu3V4O12, was synthesized under high-pressure and high-temperature conditions. It crystallizes in an A-site-ordered perovskite structure (space group Im3̅), in which silver ions occupy the 12-coordinated A sites forming regular icosahedra, and exhibits metallic behavior. Bond-valence-sum calculations and X-ray photoemission spectroscopy reveal that Ag ions are present in the mixed-valence state, most likely attributable to the coexistence of Ag(+) and Ag(3+), unlike the case of well-known perovskite-type AgNbO3 and AgTaO3 containing only Ag(+) ions. We discuss metallic conduction in relation to electronic structure calculations.
    Inorganic Chemistry 12/2013; 52(24):13824-13826. · 4.59 Impact Factor
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    ABSTRACT: Remarkable magneto-optical properties of a new isolator material, that is, europium sulfide nanocrystals with gold (EuS-Au nanosystem), has been demonstrated for a future photo-information technology. Attachment of gold particles that exhibit surface plasmon resonance leads to amplification of the magneto-optical properties of the EuS nanocrystals. To construct the EuS-Au nanosystems, cubic EuS and spherical Au nanocrystals have been joined by a variety of organic linkers, that is, 1,2-ethanedithiol (EDT), 1,6-hexanedithiol (HDT), 1,10-decanedithiol (DDT), 1,4-bisethanethionaphthalene (NpEDT), or 1,4-bisdecanethionaphthalene (NpDDT) . Formation of these systems was observed by XRD, TEM, and absorption spectra measurements. The magneto-optical properties of the EuS-Au nanosystem have been characterized by using Faraday rotation spectroscopy. The Faraday rotation angle of the EuS-Au nanosystem is dependent on the Au particle size and interparticle distance between EuS and Au nanocrystals. Enhancement of the Faraday rotation of EuS-Au nanosystems was observed. The spin configuration in the excited state of the EuS-Au nanosystem was also investigated using photo-assisted electron paramagnetic resonance.
    Chemistry 09/2013; · 5.93 Impact Factor
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    ABSTRACT: A novel cubic perovskite MnCu3V4O12 has been synthesized at a high pressure and high temperature of 12 GPa and 1373 K. This compound crystallizes in the A-site-ordered perovskite structure (space group Im3̅) with lattice constant a = 7.26684(10) Å at room temperature. The most notable feature of this compound lies in the fact that the Mn(2+) ion is surrounded by 12 equidistant oxide ions to form a regular icosahedron; the situation of Mn(2+) is unprecedented for the crystal chemistry of an oxide. An anomalously large atomic displacement parameter Uiso= 0.0222(8) Å(2) is found for Mn(2+) at room temperature, indicating that the thermal oscillation of the small Mn(2+) ion in a large icosahedron is fairly active. Magnetic susceptibility and electric resistivity measurements reveal that 3d electrons of Mn(2+) ions are mainly localized, while 3d electrons in Cu(2+) and V(4+) ions are delocalized and contribute to the metallic conduction.
    Inorganic Chemistry 09/2013; · 4.59 Impact Factor
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    ABSTRACT: Magnetic, optical, and magneto-optical properties have been examined for aluminoborosilicate glasses with high concentration of divalent europium ions. An analysis on temperature dependence of magnetic susceptibility yields positive Weiss temperatures, indicating that magnetic moments of 4f electrons of Eu2+ interact ferromagnetically with each other in the glasses. The glasses show large Faraday rotation angles in the visible range; glass with a composition of 58.0EuO·12.0Al2O3·20.0B2O3·10.0SiO2 (mol%) has a Verdet constant of -1.03 min/Oe cm (-300 rad/T m) at 633 nm. Wavelength dependence of the Verdet constant is interpreted in terms of Eu2+ 4f7 to 4f65d1 electronic transitions based on the Van Vleck-Hebb theory.
    Optical Materials 09/2013; 35(11):1997-2000. · 1.92 Impact Factor
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    ABSTRACT: A SPASER, short for surface plasmon amplification by stimulated emission of radiation, is key to accessing coherent optical fields at the nanoscale. Nevertheless, the realization of a SPASER in the visible range still remains a great challenge because of strong dissipative losses. Here, we demonstrate that room-temperature SPASER emission can be achieved by amplifying longitudinal surface plasmon modes supported in gold nanorods as plasmon nanocavities and utilizing laser dyes to supply optical gain for compensation of plasmon losses. By choosing a particular organic dye and adjusting the doping level, the resonant wavelength of the SPASER emission can be tuned from 562 to 627 nm with a spectral linewidth narrowed down to 5 ~ 11 nm. This work provides a versatile route towards SPASERs at extended wavelength regimes.
    Nano Letters 08/2013; · 13.03 Impact Factor
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    ABSTRACT: Random lasing properties of dye solutions suspended with gold–silica core–shell nanoparticles are investigated. The core–shell architecture allows adjustment of plasmon coupling strength between metal and optical gain media by varying the thickness of dielectric shell. Consequently, multiple aspects of random lasers can be controlled, such as mode interactions, lasing spikiness, and pump threshold. The results show that bare gold nanoparticles give rise to the most profound lasing spikes that are evenly separated in resonant wavelength. The lowest threshold is observed when the shell thickness is ~14.6 nm. The experimental observations are interpreted in terms of resonant coupling between metal nanoparticles and fluorophores, localization of pump light and lasing modes at the surface of metal, as well as plasmonic modifications of absorption and pump rates of fluorophores. The results suggest that metal–dielectric core–shell nanostructures can serve as promising candidates towards development of well-controlled random lasers.
    Advanced Optical Materials. 08/2013; 1(8).
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    ABSTRACT: Amorphous Eu2TiO4 and EuTiO3 have been studied by a combination of the Faraday effect in the visible region and polarization-dependent x-ray absorption spectroscopy at the Eu M4,5 and L2,3 edges to examine the role of Eu 4f-5d exchange interactions on the ferromagnetic behavior. The bulk-sensitive x-ray absorption spectra (XAS) for Eu L2,3 edges show that most of the europium ions are present as the divalent state in the amorphous Eu2TiO4 and EuTiO3. The Eu M4,5 edge x-ray magnetic circular dichroism (XMCD) signals, measured for the amorphous Eu2TiO4, dramatically increase upon cooling through the Curie temperature (16 K) determined by a superconducting quantum interference device (SQUID) magnetometer. Sum-rule analysis of the XMCD at Eu M4,5 edges measured at 10 K yields a 4f spin magnetic moment of 6.6μB per Eu2+ ion. These results confirm that the ferromagnetic properties exclusively arise from 4f spins of Eu2+. In addition, for both the amorphous Eu2TiO4 and EuTiO3, the temperature and magnetic-field dependence of Eu L2,3 edge XMCD signals can be scaled with the corresponding magnetization measured by SQUID, indicating that the 5d magnetic polarization of Eu2+ is involved in the process to cause the ferromagnetic interaction between Eu2+ ions. We further discuss the origin of ferromagnetism in the amorphous system on the basis of the energy diagram of Eu 4f and 5d levels deduced from the Faraday effect in the visible region. From the wavelength dependence of Faraday rotation angles of the amorphous EuO-TiO2 system in comparison with those of the divalent Eu chalcogenides as reported previously, it is found that the magnitude of crystal-field splitting of Eu 5d levels in the former is on the same order as that in the latter, which explains an enhanced ferromagnetic exchange interaction between Eu 4f and 5d states.
    Physical Review B 07/2013; 88(2). · 3.77 Impact Factor
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    ABSTRACT: Lattice volume expansion or amorphization renders EuTiO3 ferromagnetic, although the stable phase of crystalline EuTiO3 is an antiferromagnet. The lattice volume expansion is induced into the crystalline EuTiO3 thin film by utilizing the lattice mismatch between the thin film and a substrate. The magnetization at low temperatures monotonically increases with an increase in lattice volume for the crystalline EuTiO3 thin film, coincident with the results of calculations based on the hybrid Hartree–Fock density functional approach. The ferromagnetic interaction between Eu2+ ions is enhanced by the amorphization as well; the amorphous EuTiO3 thin film becomes a ferromagnet, and the Curie temperature is higher for amorphous Eu2TiO4 than for its crystalline counterpart. The phenomenon, that is, the volume expansion- and amophization-induced ferromagnetism, is explained in terms of the competition between ferromagnetic and antiferromagnetic interactions among Eu2+ ions.
    Journal of Materials Research 04/2013; 28(08):1031-1041. · 1.71 Impact Factor
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    ABSTRACT: Novel EuS nanocrystals containing paramagnetic Mn(II), Co(II) or Fe(II) ions have been reported as advanced semiconductor materials with effective optical rotation under magnetic field, Faraday rotation. The EuS nanocrystals with transition metal ions, EuS:M nanocrystals, were prepared by the reduction of the Eu(III) dithiocarbamate complex (tetraphenylphosphonum tetrakis (diethyldithiocarbamate) europium (III)) with transition metal complexes at 300°C. The EuS:M nanocrystals thus prepared were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), inductively coupled plasma atomic emission spectroanalysis (ICP-AES) and a superconducting quantum interference device magnetometer (SQUID). Enhanced Faraday rotations of the EuS:M nanocrystals were observed around 550 nm, and their enhanced spin polarization was estimated using electron paramagnetic resonance (EPR) measurements. In this report, the magneto-optical relationship between the Faraday rotation efficiency and spin polarization is discussed.
    Journal of the American Chemical Society 01/2013; · 10.68 Impact Factor
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    ABSTRACT: The spaser, a quantum amplifier of surface plasmons by stimulated emission of radiation, is recognized as a coherent light source capable of confining optical fields at subwavelength scale. The control over the directionality of spasing has not been addressed so far, especially for a single-particle spasing nanocavity where optical feedback is solely provided by a plasmon resonance. In this work we numerically examine an asymmetric spaser - a resonant system comprising a dielectric core capped by a metal semishell. The proposed spaser emits unidirectionally along the axis of the semishell; this directionality depends neither on the incident polarization nor on the incident angle of the pump. The spasing efficiency of the semishell-capped resonator is one order of magnitude higher than that in the closed core-shell counterpart. Our calculations indicate that symmetry breaking can serve as a route to create unidirectional, highly intense, single-particle, coherent light sources at subwavelength scale.
    Scientific Reports 01/2013; 3:1241. · 2.93 Impact Factor
  • Physica Status Solidi C Current Topics. 12/2012; 9(12):2533-2536.
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    ABSTRACT: We have prepared a three-dimensional magnetophotonic crystal and observed modulated Faraday rotation around its photonic band gap. The magnetophotonic crystal was prepared by the self-assembly of highly monodispersed silica spheres, which were heavily impregnated with maghemite (γ−Fe2O3) nanoparticles. The slab sample with a thickness of 10 μm shows a clear photonic band gap centered at λ = 620 nm in optical transmittance. Faraday rotation is notably modified inside the photonic band gap.
    Applied Physics Letters 10/2012; 101(15). · 3.79 Impact Factor
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    ABSTRACT: A comparative study of the crystal and electronic structure and magnetism of divalent europium perovskite oxides EuMO(3) (M = Ti, Zr, and Hf) has been performed on the basis of both experimental and theoretical approaches playing complementary roles. The compounds were synthesized via solid-state reactions. EuZrO(3) and EuHfO(3) have an orthorhombic structure with a space group Pbnm at room temperature contrary to EuTiO(3), which is cubic at room temperature. The optical band gaps of EuZrO(3) and EuHfO(3) are found to be about 2.4 and 2.7 eV, respectively, much larger than that of EuTiO(3) (0.8 eV). On the other hand, the present compounds exhibit similar magnetic properties characterized by paramagnetic-antiferromagnetic transitions at around 5 K, spin flop at moderate magnetic fields lower than 1 T, and the antiferromagnetic nearest-neighbor and ferromagnetic next-nearest-neighbor exchange interactions. First-principles calculations based on a hybrid Hartree-Fock density functional approach yield lattice constants, band gaps, and magnetic interactions in good agreement with those obtained experimentally. The band gap excitations are assigned to electronic transitions from the Eu 4f to Mnd states for EuMO(3) (M = Ti, Zr, and Hf and n = 3, 4, and 5, respectively).
    Inorganic Chemistry 04/2012; 51(8):4560-7. · 4.59 Impact Factor
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    ABSTRACT: We have prepared optically birefringence materials consisting of an isotropic core of metal nanoparticle and an anisotropic shell of amorphous oxide. The sample shows an enhanced optical birefringence in a wavelength-selective way. The sample was prepared by depositing amorphous iron oxide thin films on top of the silver nanoparticles using the oblique deposition technique. This results in ellipsoidal shell of amorphous iron oxide surrounding a silver nanoparticle. The form birefringence appears because of the anisotropic shape of shells; the refractive index for the light polarized parallel to the elongation direction of ellipsoid is different from that for the light polarized perpendicularly. Moreover, the rotation of polarization plane is significantly enhanced at around the wavelength of localized surface plasmon resonance (LSPR). The difference in refractive index between two optical axes is as large as 0.34 for a 600 nm light, which is more than twice of typical birefringence crystal calcite (0.14 for visible light). It is speculated that the anisotropic shell induces the dependence of LSPR wavelength on the polarization direction of the incident light, which causes the polarization dependence of refractive index through the Kramers-Kronig relation.
    Optics Express 11/2011; 19(23):23581-9. · 3.55 Impact Factor
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    ABSTRACT: Measurements of fundamental magnetic properties including not only dc and ac susceptibilities but also magnetic aging effects have been performed for aluminoborate glasses with high concentrations of iron and rare-earth R3+ ions (R=Sm, Gd, and Tb) in order to give an insight into the magnetic structures and interactions in amorphous oxides containing both 3d transition metal and 4f rare-earth ions, which manifest magnetic interactions that differ from each other. We demonstrate that the antiferromagnetic interactions between iron and rare-earth ions as well as those between iron ions play a significant role for their magnetic properties, while those between rare-earth ions are of little importance. Most of the rare-earth ions remain paramagnetic even below the spin-freezing temperatures under the strong molecular field caused by the spin-glass freezing of the iron ions, as in the case of rare-earth garnet ferrites.
    Physical review. B, Condensed matter 10/2011;
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    ABSTRACT: We have synthesized hierarchically porous Y3Al5O12 (YAG) ceramics doped with trivalent cerium (Ce3+) ions (YAG:Ce) by the sol–gel process and examined their optical properties including scattering strength. Starting from the ionic precursors, i.e., AlCl3·6H2O, YCl3·6H2O, CeCl3·7H2O, and poly(ethylene oxide) (PEO) dissolved in a mixture of water and ethanol, monolithic wet gels are synthesized using propylene oxide as a geletion initiator. PEO induces phase separation parallel to the gelation in the sol–gel system to form bicontinuous morphologies consisting of gel- and fluid-phases. The wet gels thus obtained are subjected to either evaporative or supercritical drying, and then heat-treated to crystallize the gel skeletons to form YAG phase. The drying process critically affects the nanostructures of YAG skeletons after heat treatment; the supercritical drying results in skeletons consisting of fine grains (<50 nm), while evaporative drying yields skeletons with larger grains. The scattering strength of the heat-treated samples is evaluated by using coherent backscattering measurements and scattering-based hole burning, both of which clarify that the samples prepared via evaporative drying scatter visible light much more strongly than that prepared via supercritical drying.
    08/2011;
  • 06/2011: pages 181 - 189; , ISBN: 9781118144077
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    ABSTRACT: Photoreactive random media containing silver nanoparticles (Ag NPs) have been fabricated, and hole burning effects based on the interference of multiply scattered light have been investigated. Through the analysis of hole profiles, transport mean free path ℓ is estimated for the light with the wavelength of 585 nm. We find that ℓ in the samples embedded with Ag NPs is much shorter compared with that in a medium containing the same amount of titania (TiO2) NPs, indicating that Ag NPs scatter 585 nm lightwaves much stronger than TiO2 NPs because of their larger scattering cross sections due to the localized surface plasmon resonance.
    Applied Physics Letters 03/2011; 98(12):121917-121917-3. · 3.79 Impact Factor
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    ABSTRACT: We experimentally demonstrate the capability of tailoring lasing resonance properties by manipulating the coupling between surface plasmons and photons in random lasing media composed of metallic-dielectric core-shell nanoparticles and organic dyes. It is revealed that core-shell nanoparticle-based systems exhibit optical feedback features distinctive from those containing pure metallic nanoparticles, provided that the scattering strength is weak enough. The pump threshold increases with an increment in the shell thickness, which can provide a direct proof that the local field enhancement plays a central role in the emergence of coherent feedback. The anomalous behavior in both threshold and optical feedback is discussed in terms of the modification of fluorescent properties of fluorophores close to metallic surface.
    Nano Letters 02/2011; 11(3):1374-8. · 13.03 Impact Factor

Publication Stats

204 Citations
337 Downloads
238.17 Total Impact Points

Institutions

  • 1997–2014
    • Kyoto University
      • • Department of Material Chemistry
      • • Department of Materials Science and Engineering
      • • Fukui Institute for Fundamental Chemistry
      Kioto, Kyōto, Japan
  • 2009–2010
    • Japan Science and Technology Agency (JST)
      Edo, Tōkyō, Japan
  • 2002
    • Kyoto Institute of Technology
      • Department of Chemistry and Materials Technology
      Kioto, Kyōto, Japan