Y. Taguchi

RIKEN, Вако, Saitama, Japan

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Publications (264)695.77 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: The magnetic phase diagram of a polar ferrimagnet CaBaFe4O7 with a magnetic easy axis has been investigated by measurements of magnetization, specific heat, and magnetoelectricity. A ferrimagnetic transition takes place at TC1=275 K within the orthorhombic phase followed by a second magnetic transition at TC2=211 K. Below TC2, successive metamagnetic transitions occur for magnetic fields applied perpendicular to the easy axis, implying a sequential emergence of magnetic states which are neither collinear nor coplanar. The observation of the static magnetoelectric effect was limited to temperatures below 120 K due to the conducting nature of the crystals at higher temperatures. The magnitude of the ferroelectric polarization shows large changes between the different field-induced magnetic phases. The low-field state is characterized by a large linear magnetoelectric coefficient of αcc=39 ps/m, while a gigantic polarization change of ΔP=850 μC/m2 is observed for μoH=14 T applied along the easy axis.
    No preview · Article · Jan 2016
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    ABSTRACT: Spin excitations were studied by absorption spectroscopy in CaBaCo4O7 which is a type-I multiferroic compound with the largest magnetic-order induced ferroelectric polarization ({\Delta}P=17mC/m2) reported, so far. We observed two optical magnon branches: a solely electric dipole allowed one and a mixed magnetoelectric resonance. The entangled magnetization and polarization dynamics of the magnetoelectric resonance gives rise to unidirectional light absorption, i.e. that magnon mode absorbs the electromagnetic radiation for one propagation direction but not for the opposite direction. Our systematic study of the magnetic field and temperature dependence of magnon modes provides information about the energies and symmetries of spin excitations, which is required to develop a microscopic spin model of CaBaCo4O7.
    Preview · Article · Dec 2015 · Physical Review B
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    ABSTRACT: Magnetic skyrmions, swirling nanometric spin textures, have been attracting increasing attention by virtue of their potential applications for future memory technology and their emergent electromagnetism. Despite a variety of theoretical proposals oriented towards skyrmion-based electronics (that is, skyrmionics), few experiments have succeeded in creating, deleting and transferring skyrmions, and the manipulation methodologies have thus far remained limited to electric, magnetic and thermal stimuli. Here, we demonstrate a new approach for skyrmion phase control based on a mechanical stress. By continuously scanning uniaxial stress at low temperatures, we can create and annihilate a skyrmion crystal in a prototypical chiral magnet MnSi. The critical stress is merely several tens of MPa, which is easily accessible using the tip of a conventional cantilever. The present results offer a new guideline even for single skyrmion control that requires neither electric nor magnetic biases and consumes extremely little energy.
    Preview · Article · Oct 2015 · Nature Communications
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    ABSTRACT: We have performed time-resolved (TR) core-level and valence-band angle-resolved (AR) photoemission spectroscopy (PES) to investigate ultrafast dynamics of an electron-doped topological insulator Cu0.17Bi2Se3. The Bi 5d5/2 line was composed of a single peak and exhibited broadening upon both heating and pumping, which we interpreted as a change of phonon temperature (Tp), in the surface region. The electronic dynamics and electron temperature (Te), on the other hand, were determined with near-EF TRARPES. The transient temperature deduced from core-level TRPES shows a similar behavior with Te deduced from near-EF TRARPES. This similar behavior of Tp and Te can be reproduced not by a simple two-temperature model but by a modified one, although we cannot exclude a possibility that core-level broadening also reflects Te.
    No preview · Article · Sep 2015 · Physical Review B
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    ABSTRACT: Topologically stable matters can have a long lifetime, even if thermodynamically costly, when the thermal agitation is sufficiently low. A magnetic skyrmion lattice (SkL) represents a unique form of long-range magnetic order that is topologically stable, and therefore, a long-lived, metastable SkL can form. Experimental observations of the SkL in bulk crystals, however, have mostly been limited to a finite and narrow temperature region in which the SkL is thermodynamically stable; thus, the benefits of the topological stability remain unclear. Here, we report a metastable SkL created by quenching a thermodynamically stable SkL. Hall-resistivity measurements of MnSi reveal that, although the metastable SkL is short-lived at high temperatures, the lifetime becomes prolonged (>> 1 week) at low temperatures. The manipulation of a delicate balance between thermal agitation and the topological stability enables a deterministic creation/annihilation of the metastable SkL by exploiting electric heating and subsequent rapid cooling, thus establishing a facile method to control the formation of a SkL.
    Preview · Article · Jun 2015
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    ABSTRACT: Skyrmions, topologically protected vortex-like nanometric spin textures in magnets, have been attracting increasing attention for emergent electromagnetic responses and possible technological applications for spintronics. In particular, metallic magnets with chiral and cubic/tetragonal crystal structure may have high potential to host skyrmions that can be driven by low electrical current excitation. However, experimental observations of skyrmions have so far been limited to below room temperature for the metallic chiral magnets, specifically for the MnSi-type B20 compounds. Toward technological applications, it is crucial to transcend this limitation. Here we demonstrate the formation of skyrmions with unique spin helicity both at and above room temperature in a family of cubic chiral magnets: beta-Mn-type Co-Zn-Mn alloys with a different chiral space group from that of B20 compounds. Lorentz transmission electron microscopy (LTEM), magnetization, and small angle neutron scattering (SANS) measurements unambiguously reveal the formation of a skyrmion crystal under the application of magnetic field (H<~1 kOe) in both thin- plate (thickness<150 nm) and bulk forms.
    Preview · Article · Mar 2015 · Nature Communications
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    ABSTRACT: We present a study on the modification of the electronic structure and hole-doping effect for the layered dichalcogenide WSe_2 with a multi-valley band structure, where Ta is doped on the W site along with a partial substitution of Te for its lighter counterpart Se. By means of band-structure calculations and specific-heat measurements, the introduction of Te is theoretically and experimentally found to change the electronic states in WSe_2. While in WSe_2 the valence-band maximum is located at the Gamma point, the introduction of Te raises the bands at the K point with respect to the Gamma point. In addition, thermal-transport measurements reveal a smaller thermal conductivity at room temperature of W_1-xTa_xSe_1.6Te_0.4 than reported for W_1-xTa_xSe_2. However, when approaching 900 K, the thermal conductivities of both systems converge while the resistivity in W_1-xTa_xSe_1.6Te_0.4 is larger than in W_1-xTa_xSe_2, leading to comparable but slightly smaller values of the figure of merit in W_1-xTa_xSe_1.6Te_0.4.
    Preview · Article · Feb 2015 · Physical Review B
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    ABSTRACT: A synchrotron x-ray diffraction study of metal-insulator transitions in W-doped ${\text{VO}}_{2} ({\text{V}}_{1$-${}x}{\text{W}}_{x}{\text{O}}_{2})$ thin films has been carried out. The insulating phase for $x$\le${}0.07$ exhibits cell-doubling with the V dimerization similar to bulk ${\text{VO}}_{2}$, while the insulating phase for $x$\ge${}0.11$ does not. This result suggests that the electronic structure of the $x$\ge${}0.11$ insulators should be different from that of the $x$\le${}0.07$ ones and bulk-insulating phase of ${\text{VO}}_{2}$. The temperature and $x$ dependence of superlattice reflection as observed casts doubt about the direct relationship between the dimerization of V ions and metal-insulator transition. The temperature dependence of the electrical resistivity rather implies the Mott-Anderson localization nature of the insulating phases. X-ray-induced persistent phase transitions are observed at low temperatures in each insulating phase in the vicinity of the boundary to the metallic phase regardless of the difference in the electronic structure. Gradual peak shift suggests that the x-ray irradiation produces nanometer metallic regions.
    No preview · Article · Feb 2015 · Physical Review B
  • Yoichi Nii · A. Kikkawa · Y. Taguchi · Y. Tokura · Y. Iwasa
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    ABSTRACT: We observe the elastic stiffness and ultrasonic absorption of a Skyrmion crystal in the chiral-lattice magnet MnSi. The Skyrmion crystal lattice exhibits a stiffness 3 orders of magnitude smaller than that of the atomic lattice of MnSi, being as soft as the flux line lattice in type-II superconductors. The observed anisotropic elastic responses are consistent with the cylindrical shape of the Skyrmion spin texture. Phenomenological analysis reveals that the spin-orbit coupling is responsible for the emergence of anisotropic elasticity in the Skyrmion lattice.
    No preview · Article · Dec 2014 · Physical Review Letters
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    D Choudhury · T Suzuki · Y Tokura · Y Taguchi
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    ABSTRACT: Magnetocaloric effect is the phenomenon that temperature change of a magnetic material is induced by application of a magnetic field. This effect can be applied to environmentally-benign magnetic refrigeration technology. Here we show a key role of magnetic-field-induced structural instability in enhancing the magnetocaloric effect for MnCo1-xZnxGe alloys (x = 0-0.05). The increase in x rapidly reduces the martensitic transition temperature while keeping the ferromagnetic transition around room temperature. Fine tuning of x around x = 0.03 leads to the concomitant structural and ferromagnetic transition in a cooling process, giving rise to enhanced magnetocaloric effect as well as magnetic-field-induced structural transition. Analyses of the structural phase diagrams in the T-H plane in terms of Landau free-energy phenomenology accounts for the characteristic x-dependence of the observed magnetocaloric effect, pointing to the importance of the magnetostructural coupling for the design of high-performance magnetocalorics.
    Full-text · Article · Dec 2014 · Scientific Reports
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    ABSTRACT: We have investigated the dynamics of spin-state crossover (SSC) for perovskite ${\mathrm{LaCoO}}_{3}$ through optical phonons by means of infrared and inelastic x-ray spectroscopy. Upon thermally induced SSC, anomalously dispersionless Co-O bond stretching phonons coupled to the thermally excited spin state have been identified. The enhanced spin-state fluctuation irrespective of the presence of a clear charge gap suggests the emergence of complex spin-state disproportionation involving low-, intermediate-, and high-spin states due to the strong correlation among thermally activated spin states.
    No preview · Article · Aug 2014 · Physical Review B
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    ABSTRACT: Magnetic, structural, and magnetocaloric properties have been investigated for a solid-solution system, Fe1-xMnxV2O4 (0.0≤x≤1.0) with a spinel structure. As orbital-active Fe2+ ions are partially substituted with orbital-inactive Mn2+ ions, various interactions, such as the Jahn-Teller interaction, spin-orbit coupling, and the exchange interaction, compete with each other, giving rise to a rich magnetic and structural phase diagram. The magnetocaloric effect exhibits two peaks as a function of temperature for x ≤0.9, associated with a higher-temperature ferrimagnetic transition, and with a lower-temperature concomitant spin-canting and orbital-ordering (mostly lattice-structural) transition of the V site. The large magnetocaloric effect as observed in MnV2O4 can thus be attributed to the sum of the entropy changes upon the merged phase transition at TN1~TN2.
    No preview · Article · Feb 2014 · Physical Review B
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    ABSTRACT: The coupling between spins and electric dipoles governs magnetoelectric phenomena in multiferroics. The dynamical magnetoelectric effect, which is an inherent attribute of the spin excitations in multiferroics, drastically changes the optical properties of these compounds compared with conventional materials where light-matter interaction is expressed only by the dielectric permittivity or magnetic permeability. Here we show via polarized terahertz spectroscopy studies on multiferroic Ca2CoSi2O7, Sr2CoSi2O7 and Ba2CoGe2O7 that such magnetoeletric spin excitations exhibit quadrochroism, that is, they have different colours for all the four combinations of the two propagation directions (forward or backward) and the two orthogonal polarizations of a light beam. We demonstrate that one-way transparency can be realized for spin-wave excitations with sufficiently strong optical magnetoelectric effect. Furthermore, the transparent and absorbing directions of light propagation can be reversed by external magnetic fields. This magnetically controlled optical-diode function of magnetoelectric multiferroics may open a new horizon in photonics.
    Full-text · Article · Feb 2014 · Nature Communications
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    ABSTRACT: We have investigated the charge transport and low-energy dynamics originating from the antiferromagnetic (AF) domain wall (DW) in pyrochlore-type Nd2Ir2O7 with the all-in all-out type AF ground state by means of the magneto-resistance measurement and the terahertz time-domain spectroscopy. The resistivity is much lower in the multi-magnetic-domain state than in the nearly single-magnetic-domain one. Furthermore, the terahertz optical conductivity spectrum unravels a Drude-like response only in the multidomain state; the Drude-like absorption is strongly suppressed with increasing field and irreversibly disappears above 5 T. These observations suggest that the AF DW is highly conductive with a minimal damping constant of 2 meV, contrary to the fully-gapped (45 meV) AF insulating state of bulk. We discuss the nature of metallic states at AF DWs and their possible relevance to the gapless edge state inherent to the Weyl semimetal.
    No preview · Article · Jan 2014 · Physical Review B
  • X Z Yu · Y Tokunaga · Y Kaneko · W Z Zhang · K Kimoto · Y Matsui · Y Taguchi · Y Tokura
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    ABSTRACT: The magnetic skyrmion is a topologically stable spin texture in which the constituent spins point to all the directions wrapping a sphere. Generation and control of nanometric magnetic skyrmions have large potential, for example, reduced power consumption, in spintronics device applications. Here we show the real-space observation of a biskyrmion, as defined by a molecular form of two bound skyrmions with the total topological charge of 2, realized under magnetic field applied normal to a thin plate of a bilayered manganite with centrosymmetric structure. In terms of a Lorentz transmission electron microscopy (TEM), we have observed a distorted-triangle lattice of biskyrmion crystal, each composed of two bound skyrmions with oppositely swirling spins (magnetic helicities). Furthermore, we demonstrate that these biskyrmions can be electrically driven with orders of magnitude lower current density (<10(8) A m(-2)) than that for the conventional ferromagnetic domain walls.
    No preview · Article · Jan 2014 · Nature Communications
  • Y Tokunaga · Y Taguchi · T Arima · Y Tokura
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    ABSTRACT: In a multiferroic orthoferrite Dy_{0.7}Tb_{0.3}FeO_{3}, which shows electric-field-(E-)driven magnetization (M) reversal due to a tight clamping between polarization (P) and M, a gigantic effect of magnetic-field (H) biasing on P-E hysteresis loops is observed in the case of rapid E sweeping. The magnitude of the bias E field can be controlled by varying the magnitude of H, and its sign can be reversed by changing the sign of H or the relative clamping direction between P and M. The origin of this unconventional biasing effect is ascribed to the difference in the Zeeman energy between the +P and -P states coupled with the M states with opposite sign.
    No preview · Article · Jan 2014 · Physical Review Letters
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    ABSTRACT: We report magnetotransport properties for epitaxial thin films of SrFeO3 and SrFe0.99Co0.01O3 with possible skyrmion-related spin textures. Resonant soft x-ray diffraction measurements revealed formation of helical spin structures for both samples (Q//<111>). From magnetotransport measurements we found several distinct helimagnetic phases with multiple/single Q vectors. A steep suppression of Hall resistivity is observed above the critical field to reach the high-field conical state, indicating the presence of skyrmionlike topological spin textures at lower fields responsible for the topological Hall effect.
    No preview · Article · Dec 2013 · Physical Review B
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    ABSTRACT: The electron mobility is one of the key parameters that characterize the charge-carrier transport properties of materials, as exemplified by the quantum Hall effect as well as high-efficiency thermoelectric and solar energy conversions. For thermoelectric applications, introduction of chemical disorder is an important strategy for reducing the phonon-mediated thermal conduction, but is usually accompanied by mobility degradation. Here, we show a multilayered semimetal β-CuAgSe overcoming such a trade-off between disorder and mobility. The polycrystalline ingot shows a giant positive magnetoresistance and Shubnikov de Haas oscillations, indicative of a high-mobility small electron pocket derived from the Ag s-electron band. Ni doping, which introduces chemical and lattice disorder, further enhances the electron mobility up to 90,000 cm(2) V(-1) s(-1) at 10 K, leading not only to a larger magnetoresistance but also a better thermoelectric figure of merit. This Ag-based layered semimetal with a glassy lattice is a new type of promising thermoelectric material suitable for chemical engineering.
    Full-text · Article · Apr 2013 · Nature Materials
  • K Ueda · J Fujioka · Y Takahashi · T Suzuki · S Ishiwata · Y Taguchi · Y Tokura
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    ABSTRACT: We have spectroscopically investigated the thermally and doping-induced metal-insulator transitions for pyrochlore-type Nd_{2}Ir_{2}O_{7} as well as its Rh-doped analogs Nd_{2}(Ir_{1-x}Rh_{x})_{2}O_{7}, where the spin-orbit interaction as well as the electron correlation is effectively tuned by the doping level (x). The charge dynamics dramatically changes on an energy scale of 1 eV in the course of thermally and doping-induced metal-insulator transitions, while the insulating ground state shows a small but well-defined charge gap of 45 meV. Anomalous doping variation of the low-energy (<0.3  eV) optical-conductivity spectra at the ground state can be interpreted in terms of the phase changes among the narrow-gap Mott insulator, Weyl semimetal, and correlated metal.
    No preview · Article · Sep 2012 · Physical Review Letters
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    ABSTRACT: Infrared optical and inelastic x-ray scattering spectra have been systematically investigated in combination with first-principles calculations for paraelectric and antiferromagnetic perovskite Sr1−xBaxMnO3 (x=0–0.3) single crystals, which are close to a ferroelectric transition arising from off-center displacement of magnetic Mn4+ ions. All the phonon dispersions measured for the parent compound of x=0 agree well with the results of the first-principles calculation. As the Ba concentration increases, one optical phonon rapidly softens toward zero frequency at room temperature, while the other phonons are almost unchanged. This soft-mode behavior is also reproduced by the first-principles calculations, from which we have predicted the vibration mode of all the optical phonons. The results of the infrared measurements at various temperatures indicate that only the soft phonon mode shows marked temperature variation relevant to the antiferromagnetic transition for all x, whereas other optical modes are almost independent of temperature. The conventional evolution of a soft phonon with decreasing temperature is prevented by the onset of the magnetic order. Below the antiferromagnetic-transition temperature, the soft mode hardens with decreasing temperature and then resoftens toward the lowest temperature. A similar temperature dependence was observed in the nonzero-momentum region by means of the inelastic x-ray scattering measurements, although its magnitude decreases as the momentum is increased from zero. Such a nonmonotonic temperature profile of the soft-mode energy is well explained on the basis of a phenomenological spin-phonon coupling model, which suggests the largest coupling constant yet attained.
    Full-text · Article · Sep 2012 · Physical review. B, Condensed matter

Publication Stats

5k Citations
695.77 Total Impact Points


  • 2009-2015
    • RIKEN
      • Strong Correlation Interface Research Group
      Вако, Saitama, Japan
  • 1978-2009
    • Tohoku University
      • • Institute for Materials Research
      • • Department of Medical Genetics
      • • Division of Surgery
      Sendai-shi, Miyagi, Japan
  • 1993-2007
    • The University of Tokyo
      • • Department of Applied Physics
      • • Department of Physics
      Tokyo, Tokyo-to, Japan
  • 1996
    • Hokkaido University
      • Division of Physics
      Sapporo, Hokkaidō, Japan