M. Hagiwara

Osaka University, Suika, Ōsaka, Japan

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Publications (234)477.8 Total impact

  • Journal of the Physical Society of Japan 03/2015; 84(3):034601. DOI:10.7566/JPSJ.84.034601 · 1.48 Impact Factor
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    ABSTRACT: We have performed high-field electron spin resonance (ESR) measurements of the isolated antiferromagnetic dimer system Na2Co2(C2O4)3(H2O)2 by illuminating circularly polarized microwaves at 162 GHz. Different from a usual ESR signal by the magnetic dipole transition, the resonance signal, coming from the direct transition between the singlet and triplet states in this compound, is suggested to show absence of circular dichroism. From this curious behavior, we consider that the direct transition in Na2Co2(C2O4)3(H2O)2 is driven by oscillating electric fields of microwaves. We propose that the spin current mechanism is responsible to induce a finite probability for the electric-dipole-active transition between the singlet and the triplet states in Na2Co2(C2O4)3(H2O)2.
    Applied Magnetic Resonance 01/2015; DOI:10.1007/s00723-015-0661-8 · 1.15 Impact Factor
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    ABSTRACT: We have performed electron spin resonance (ESR) measurements in the low temperature magnetic-ordered state of polycrystalline samples of CdYb2S4. This material is one of the rare-earth spinel compounds in which Yb3+ ions form the pyrochlore lattice, and consequently, CdYb2S4 possesses anomalous properties caused by geometrical frustration and anisotropic terms. Some ESR signals have been detected, and their resonance fields are well-fitted by linear lines in the frequency vs. magnetic-field plane. One of the ESR modes appears to intersect the origin of this plane, suggesting a nearly gapless feature in the magnetically ordered state of this material. Two additional ESR modes appear above H c = 2.6 T, indicating that there is a field-induced phase transition at H c. Given the nearly gapless feature of CdYb2S4, we expect that the ground state of CdYb2S4 must be either the Palmer-Chalker state or the ψ 2 state.
    Applied Magnetic Resonance 01/2015; DOI:10.1007/s00723-015-0651-x · 1.15 Impact Factor
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    ABSTRACT: We have performed NMR experiments on the quasi one-dimensional frustrated spin-1/2 system LiCuVO$_4$ in magnetic fields $H$ applied along the c-axis up to field values near the saturation field $H_{\rm sat}$. For the field range $H_{\rm c2}<H<H_{\rm c3}$ ($\mu_0H_{\rm c2}\approx 7.5$T and $\mu_0H_{\rm c3} = [40.5 \pm 0.2]$T) the $^{51}$V NMR spectra at $T$ = 380mK exhibit a characteristic double-horn pattern, as expected for a spin-modulated phase in which the magnetic moments of Cu$^{2+}$ ions are aligned parallel to the applied field $H$ and their magnitudes change sinusoidally along the magnetic chains. For higher fields, the $^{51}$V NMR spectral shape changes from the double-horn pattern into a single Lorentzian line. For this Lorentzian line, the internal field at the $^{51}$V nuclei stays constant for $\mu_0 H > 41.4$T, indicating that the majority of magnetic moments in LiCuVO$_4$ are already saturated in this field range. This result is inconsistent with the previously observed linear field dependence of the magnetization $M(H)$ for $H_{\rm c3}<H<H_{\rm sat}$ with $\mu_0H_{\rm sat}=45$T [L. E. Svistov {\it et al}., JETP Letters {\bf 93}, 21 (2011)]. We argue that the discrepancy is due to non-magnetic defects in the samples. The results of the spin-lattice relaxation rate of $^7$Li nuclei indicate an energy gap which grows with field twice as fast as the Zeeman energy of a single spin, therefore, suggesting that the two-magnon bound state is the lowest energy excitation. The energy gap tends to close at $\mu_0H \approx 41$T. Our results suggest that the theoretically predicted spin-nematic phase, if it exists in LiCuVO$_4$, can be established only within the narrow field range $40.5 < \mu_0 H < 41.4$T .
    Physical Review B 10/2014; 90(13). DOI:10.1103/PhysRevB.90.134401 · 3.66 Impact Factor
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    ABSTRACT: We have prepared polycrystalline samples of LaSrRh$_{1-x}$Ga$_x$O$_4$ and LaSr$_{1-x}$Ca$_x$RhO$_4$,and have measured the x-ray diffraction, resistivity, Seebeck coefficient, magnetization and electron spin resonance in order to evaluate their electronic states. The energy gap evaluated from the resistivity and the Seebeck coefficient systematically changes with the Ga concentration, and suggests that the system changes from a small polaron insulator to a band insulator. We find that all the samples show Curie-Weiss-like susceptibility with a small Weiss temperature of the order of 1 K, which is seriously incompatible with the collective wisdom that a trivalent rhodium ion is nonmagnetic. We have determined the $g$ factor to be $g$=2.3 from the electron spin resonance, and the spin number to be $S$=1 from the magnetization-field curves by fitting with a modified Brillouin function. The fraction of the $S$=1 spins is 2--5%, which depends on the degree of disorder in the La/Sr/Ca-site, which implies that disorder near the apical oxygen is related to the magnetism of this system. A possible origin for the magnetic Rh$^{3+}$ ions is discussed.
    Physical Review B 09/2014; 90(14). DOI:10.1103/PhysRevB.90.144402 · 3.66 Impact Factor
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    ABSTRACT: From high field electron spin resonance measurements in illuminating polarized light, we have revealed the existence of electromagnon, i.e., magnon excitation by oscillatory electric fields of light, in the field-induced 1/5-plateau phase of the triangular lattice antiferromagnet CuFeO2. We indicate that peculiar magnon modes, which generate uniform fluctuation of the vector spin chirality at wave vector k = 0, appear in the magnetic ordered phase with a collinear spin structure on triangular lattice. Our experimental results demonstrate that such magnon modes couple with an electric component of light, leading to the emergence of the electromagnon. Moreover, the measurements in circularly polarized light exhibit an anomalous behavior that circular dichroism, which is usually found in magnetic resonance, is absent in the resonance signal of the electromagnon. The microscopic mechanism of the electromagnon in CuFeO2 is also discussed.
    Physical Review B 08/2014; 90(6). DOI:10.1103/PhysRevB.90.060413 · 3.66 Impact Factor
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    ABSTRACT: We succeed in growing single crystals of SmCd11 by the Cd self-flux method, and clarified the magnetic properties. SmCd11 is an antiferromagnet with a Néel temperature TN1 = 4.7 K, with four successive magnetic transitions at TN2 = 3.5, TN3 = 3.4, TN4 = 2.8, and TN5 = 2.3 K. The magnetization at 1.3 K for \(H\parallel \langle 100 \rangle \) is characteristic, indicating a spin-flip transition at 90 kOe and a canting process of magnetization in magnetic fields up to 300 kOe. From the magnetization, an ordered moment of Sm3+ with the CEF-doublet ground state is estimated to be 0.1 μB/Sm.
    Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES2013); 06/2014
  • Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES2013); 06/2014
  • Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES2013); 06/2014
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    ABSTRACT: Unconventional high temperature superconductivity as well as three-dimensional bulk Dirac cone quantum states arising from the unique d-orbital topology has been a recent priority research area in physics. In iron pnictide compounds, although transport phenomena arisen from this multiple band Fermi surface are intriguing and scientifically important, they still do not give an adequate matching to neither experimental observations on the band picture nor theoretical calculations and a debate continues. Here we describe a new analytical approach of mobility spectrum, in which the carrier number is conveniently described as a function of mobility without any hypothesis about the number of carriers, on both longitudinal and transverse transport of high quality single crystal Ba(FeAs)$_2$ in a wide range of magnetic field. We show that the major numbers of carriers reside in large parabolic hole and electron pockets with very different topology as well as remarkably different mobility spectra, while the minor number of Dirac carriers resides in both hole- and electron- Dirac quantum states with the largest mobility as high as 70,000 cm$^2$(Vs)$^{-1}$.
    New Journal of Physics 03/2014; 16(9). DOI:10.1088/1367-2630/16/9/093062 · 3.67 Impact Factor
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    ABSTRACT: We present a comprehensive structural study on perovskite-type 6H-Ba3CuSb2O9, which exhibits a spin-orbital short-range ordering on a honeycomb-based lattice. By combining synchrotron x-ray diffraction, electron spin resonance, ultrasound measurement and Raman spectroscopy, we found that the static Jahn-Teller distortion is absent down to the lowest temperature in the present material, indicating orbital ordering is strongly suppressed. We discuss such an unusual state is realized with the help of spin degree of freedom, leading to a spin-orbital entangled liquid state.
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    ABSTRACT: Spin-1/2 Heisenberg antiferromagnets Cs2CuCl4 and Cs2CuBr4 with distorted triangular-lattice structures are studied by means of electron spin resonance spectroscopy in magnetic fields up to the saturation field and above. In the magnetically saturated phase, quantum fluctuations are fully suppressed, and the spin dynamics is defined by ordinary magnons. This allows us to accurately describe the magnetic excitation spectra in both materials and, using the harmonic spin-wave theory, to determine their exchange parameters. The viability of the proposed method was proven by applying it to Cs2CuCl4, yielding J/kB=4.7(2) K, J'/kB=1.42(7) K, [J'/J≃0.30] and revealing good agreement with inelastic neutron-scattering results. For the isostructural Cs2CuBr4, we obtain J/kB=14.9(7) K, J'/kB=6.1(3) K, [J'/J≃0.41], providing exact and conclusive information on the exchange couplings in this frustrated spin system.
    Physical Review Letters 02/2014; 112(7):077206. DOI:10.1103/PhysRevLett.112.077206 · 7.73 Impact Factor
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    ABSTRACT: We report on a simple method to fabricate a magnetic carbon material that contains nitrogen-coordinated transition metals and has a large magnetic moment. Highly chlorinated iron phthalocyanine was used as building blocks and potassium as a coupling reagent to uniformly disperse nitrogen-coordinated iron atoms on the phthalocyanine based carbon material. The iron phthalocyanine based carbon material exhibits ferromagnetic properties at room temperature and the ferromagnetic phase transition occurs at Tc = 490 ± 10 K. Transmission electron microscopy observation, X-ray diffraction analysis, and the temperature dependence of magnetization suggest that the phthalocyanine molecules form three-dimensional random networks in the iron phthalocyanine based carbon material.
    Journal of Applied Physics 02/2014; 115(5):054306-054306-4. DOI:10.1063/1.4863539 · 2.19 Impact Factor
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    ABSTRACT: Spin-1/2 Heisenberg antiferromagnets Cs$_2$CuCl$_4$ and Cs$_2$CuBr$_4$ with distorted triangular-lattice structures are studied by means of electron spin resonance spectroscopy in magnetic fields up to the saturation field and above. In the magnetically saturated phase, quantum fluctuations are fully suppressed, and the spin dynamics is defined by ordinary magnons. This allows us to accurately describe the magnetic excitation spectra in both materials and, using the harmonic spin-wave theory, to determine their exchange parameters. The viability of the proposed method was proven by applying it to Cs$_2$CuCl$_4$, yielding $J/k_B=4.7(2)$ K, $J'/k_B=1.42(7)$ K [$J'/J\simeq 0.30$] and revealing good agreement with inelastic neutron-scattering results. For the isostructural Cs$_2$CuBr$_4$, we obtain $J/k_B=14.9(7)$ K, $J'/k_B=6.1(3)$ K, [$J'/J\simeq 0.41$], providing exact and conclusive information on the exchange couplings in this frustrated spin system.
  • Journal of the Physical Society of Japan 12/2013; 82(12):124708. DOI:10.7566/JPSJ.82.124708 · 1.48 Impact Factor
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    ABSTRACT: We have performed electron spin resonance (ESR) studies of K_{4}^{3+} and (K_{3}Rb)^{3+} nanoclusters incorporated in powder specimens of aluminosilicate sodalite at several microwave frequencies between 9 and 34 GHz. The K_{4}^{3+} and (K_{3}Rb)^{3+} clusters are arrayed in a bcc structure and are known to show antiferromagnetic ordering below the Ne[over ́]el temperatures of T_{N} ≃72 and ≃80 K, respectively, due to the exchange coupling between s electrons confined in the clusters. We have found sudden broadenings of ESR spectra in both samples below T_{N}. The line shape of the spectra below T_{N} is analyzed by powder pattern simulations of antiferromagnetic resonance (AFMR) spectra. The calculated line shapes well reproduce the experimental ones at all the frequencies by assuming a biaxial magnetic anisotropy. We have evaluated extremely small anisotropy fields of approximately 1 Oe indicating that these materials are ideal Heisenberg antiferromagnets. We have also found that the magnetic anisotropy changes from easy-plane type to uniaxial type by changing into a heavier alkali-metal cluster and that the g value shifts to a large value beyond two below T_{N} for K_{4}^{3+} and (K_{3}Rb)^{3+} nanoclusters. These novel features of K_{4}^{3+} and (K_{3}Rb)^{3+} nanoclusters incorporated in sodalite are discussed.
    Physical Review B 11/2013; 88(17). DOI:10.1103/PhysRevB.88.174401 · 3.66 Impact Factor
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    ABSTRACT: We grew a high-quality single crystal EuGa4 with the tetragonal structure by the Ga self-flux method, and measured the electrical resistivity, magnetic susceptibility, high-field magnetization, specific heat, thermoelectric power and de Haas-van Alphen (dHvA) effect, together with the electrical resistivity and thermoelectric power under pressure. EuGa4 is found to be a Eu-divalent compound without anisotropy of the magnetic susceptibility in the paramagnetic state and to reveal the same magnetization curve between H vertical bar vertical bar [100] and [001] in the antiferromagnetic state, where the antiferromagnetic easy-axis is oriented along the [100] direction below a Neel temperature T-N = 16: 5 K. The magnetization curve is discussed on the basis of a simple two-sublattice model. The Fermi surface in the paramagnetic state was clarified from the results of a dHvA experiment for EuGa4 and an energy band calculation for a non-4f reference compound SrGa4, which consists of a small ellipsoidal hole-Fermi surface and a compensated cube-like electron-Fermi surface with vacant space in center. We observed an anomaly in the temperature dependence of the electrical resistivity and thermoelectric power at T-CDW = 150K under 2 GPa. This might correspond to an emergence of the charge density wave (CDW). The similar phenomenon was also observed in EuAl4 at ambient pressure. We discussed the CDW phenomenon on the basis of the present peculiar Fermi surfaces.
    Journal of the Physical Society of Japan 10/2013; 82(10):104703. DOI:10.7566/JPSJ.82.104703 · 1.48 Impact Factor
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    ABSTRACT: EuNi2P2 is known as a heavy fermion compound with an electronic specific heat coefficient γ=100 mJ/(K2\cdotmol). We grew single crystals and studied their electronic and magnetic properties by measuring the electrical resistivity, magnetic susceptibility, high-field magnetization, specific heat, and thermal expansion. The present heavy fermion state is clarified to be based on the Kondo effect as in CeRu2Si2, revealing an intensive shrinkage of the volume below about 100 K in the temperature dependence of thermal expansion. The temperature dependences of the 4f-electron contribution to the volume thermal expansion (Δ V/V)4f and the average Eu valence are found to show good scaling in EuNi2P2.
    Journal of the Physical Society of Japan 08/2013; 82(8):3708-. DOI:10.7566/JPSJ.82.083708 · 1.48 Impact Factor
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    ABSTRACT: We measured the magnetic susceptibility, high-field magnetization, magnetoresistance, specific heat, and Hall coefficient, together with the electrical resistivity under high pressures and magnetic fields, for the heavy-fermion compound YbRh2Zn20 with the cubic CeCr2Al20-type structure. The metamagnetic behavior was observed at Hm = 65 kOe for the magnetic field along the < 100> direction below Tχ_{max} = 5.3 K, at which the temperature of the magnetic susceptibility indicates a broad maximum. The coefficient A of the T2 dependence of electrical resistivity ρ=ρ0 + AT2 and the electronic specific heat coefficient C/T possess a broad maximum at Hm. The metamagnetic field Hm is found to decrease with increasing pressure and to become zero at a critical pressure Pc ≃ 5.2 GPa. Correspondingly, the A value increases drastically in magnitude, and the Fermi liquid relation is no longer satisfied at 5.2 GPa in zero magnetic field, implying a non-Fermi liquid state. On the other hand, at magnetic fields of H > 20 kOe, the low-temperature resistivity exhibits a T2 dependence even at 5.2 GPa. The A coefficient decreases rapidly with increasing magnetic field. These results indicate that an electronic state at 5.2 GPa corresponds to the quantum critical point.
    Journal of the Physical Society of Japan 08/2013; 82(8):4705-. DOI:10.7566/JPSJ.82.084705 · 1.48 Impact Factor
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    ABSTRACT: Magnetic excitations in Cs$_2$CuBr$_4$, a spin-1/2 antiferromagnet with a distorted triangular lattice, are probed by means of high-field electron spin resonance (ESR) spectroscopy. We show that the high-energy excitation spectrum of this material is not appreciably affected by the 3D ordering, indicating the presence of strong quantum fluctuations competing with the magnetic order down to well below $T_N$. Such a behavior is consistent with the quantum spin-liquid scenario with the spin dynamics determined by short-range-order correlations (presumably of 1D nature, albeit being strongly influenced by frustrated interchain interactions). The observation of three ESR branches is in a prefect agreement with a general phenomenological macroscopic theory, predicting the presence of three Goldstone modes (two gapped and one gapless) as a consequence of the complete breaking of the rotational SO(3) symmetry in anisotropic spin systems with a noncollinear ground state.

Publication Stats

2k Citations
477.80 Total Impact Points

Institutions

  • 1998–2014
    • Osaka University
      • Center for Quantum Science and Technology under Extreme Conditions
      Suika, Ōsaka, Japan
  • 2011
    • Waseda University
      • Department of Applied Physics
      Edo, Tōkyō, Japan
  • 2009
    • Japan Science and Technology Agency (JST)
      Edo, Tōkyō, Japan
  • 2007
    • Hiroshima University
      • Department of Quantum Matter
      Hiroshima-shi, Hiroshima-ken, Japan
  • 2006
    • Charles University in Prague
      • Faculty of Mathematics and Physics
      Praha, Praha, Czech Republic
  • 1999–2006
    • RIKEN
      Вако, Saitama, Japan
  • 2002
    • Toyota Physical and Chemical Institute
      Seto, Aichi, Japan
  • 1995
    • Chiba University
      • Department of Physics
      Tiba, Chiba, Japan
  • 1992
    • Université Paris-Sud 11
      Orsay, Île-de-France, France