M. Hagiwara

Osaka University, Suika, Ōsaka, Japan

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Publications (259)523.85 Total impact

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    ABSTRACT: A suite of experimental tools, including high-field magnetization and electron spin resonance (ESR) studies in magnetic fields of up to 50 T and heat capacity studies up to 9 T, have revealed antiferromagnetic order in single crystals of the Heisenberg S=2 chain compound MnCl3(bpy), where bpy is 2,2′-bipyridine. The Néel temperature, which depends on the strength of the applied magnetic field and its orientation with respect to the crystalline axes that was revealed by heat capacity measurements, is near 11.5 K in zero field. The spin-flop transition is identified in the magnetization curve acquired at 1.7 K and at μoHSFc=24 T along the c axis. The transition field HSF is lower than that expected from the previous antiferromagnetic resonance (AFMR) studies on a powder sample. The identification of the long-range antiferromagnetic order resolves an earlier report by Granroth et al. [Phys. Rev. Lett. 77, 1616 (1996)] that identified MnCl3(bpy) as an S=2 Haldane system down to 40 mK. The ESR studies identify a wide range of antiferromagnetic resonance modes that provide additional microscopic information about the g values (ga*=2.09, gb=1.92, and gc=2.07), the zero-field splitting constants, D/kB=−1.5 K and E/kB=−0.17 K when the nearest-neighbor spin interaction J/kB=31.2 K, which is evaluated from fitting the susceptibility, and the anisotropy of this compound (easy axis is the c axis, the second easy-axis is the b axis, and the hard axis is the a* axis), when using a standard (two-sublattice) AFMR analysis that does not quantitatively reproduce the observed HSFc value. The observed resonance mode indicates the frequency minimum at HSFc.
    Full-text · Article · Jan 2016
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    ABSTRACT: Recently, magnetic substances with chirality, namely the handedness of the magnetic structure, have attracted considerable attention because of the anomalous phenomena which appear in magnetic fields. CrNb3S6 is one of the chiral magnets formed by exchange and Dzyaloshinsky-Moriya (DM) interactions. Electron spin resonance (ESR) measurements of CrNb3S6 in magnetic fields parallel to the c-axis (helical axis) have been performed to evaluate the exchange and the DM constants that determine the helical structure. Fitting the ESR data to a calculated mode based on a spin wave theory yields values for the ferromagnetic inter-plane exchange constant J/kB = 16.2 K, the DM constant D/kB = 1.29 K, and the single-ion anisotropy constant . From the Curie-Weiss temperature θCW ∼ 145 K, large intra-plane ferromagnetic exchange interactions are suggested.
    Preview · Article · Dec 2015 · Physics Procedia
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    ABSTRACT: We report on the results of magnetic susceptibility and high field magnetization measurements on single crystals of SrCo2V2O8, which is regarded as an S = 1/2 quasi one-dimensional Ising-like antiferromagnet and is almost isostructural to BaCo2V2O8 that has exhibited a field-induced order-to-disorder transition. The magnetization measurements have been performed at 1.4 K below the Néel temperature TN = 4.8 K in pulsed fields of up to 50 T. The temperature dependence of magnetic susceptibility parallel to the c-axis (chain axis) has a broad maximum near 40 K, which is typical of a low dimensional antiferromagnet, while the susceptibilities perpendicular to the c-axis do not show such a maximum. The magnetization at 1.9 K along the c-axis shows a steep increase at 3.8 T as in BaCo2V2O8, corresponding to the field-induced order-to-disorder transition. The magnetization curve along the c-axis up to the saturation magnetization is well fitted to the magnetization curve calculated by the exact solution based on the Bethe ansatz for an S = 1/2 XXZ model, and this analysis provides parameter values for the exchange constant J/kB = 78 K, the anisotropic exchange parameter ɛ = 0.50, and the g-value along the c-axis gc = 6.1. In contrast, the magnetization curves perpendicular to the c-axis show gradual increase up to 50 T.
    Preview · Article · Dec 2015 · Physics Procedia
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    Takanori Kida · Akira Okutani · Hiroyuki Yoshida · Masayuki Hagiwara
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    ABSTRACT: We have investigated the magnetotransport properties of polycrystalline samples of Ag2CrO2, which is a metallic two-dimensional triangular antiferromagnetic compound (TN ∼24K). The electrical resistivity of this compound displayed metallic behavior and an abrupt decrease at TN. Below TN, almost linear positive magnetoresistance was observed. On the other hand, just above TN, a large negative magnetoresistance of -38% at 8 T was observed. The absolute value of the negative magnetoresistance decreases with increasing temperature. The Hall resistance showed a nonlinear behavior as a function of the magnetic field below TN. These results suggest that the transport properties of Ag2CrO2 are closely related to its unusual antiferromagnetic spin-structure.
    Preview · Article · Dec 2015 · Physics Procedia
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    ABSTRACT: Recent theories have revealed topological differences between one-dimensional (1D) Heisenberg antiferromagnets (HAFs) with odd and even integer spins (S). With this renewed interest, we have investigated the low-field magnetic susceptibility, specific heat, and high-field magnetization of single-crystal samples of MnCl3(bpy), bpy=2,2t-bipyridine, which is regarded as an S = 2 quasi-1D HAF. With the magnetic field oriented along the a*, b and c directions, the magnetic susceptibilities possess broad maxima around 100K. When the magnetic field is parallel to the b-axis, a sharp peak at 11K is observed in the magnetic susceptibility, while a smaller feature is detected in the specific heat, indicating a transition to long-range antiferromagnetic order. At 1.7K, the high-field magnetization curve along the c-axis shows a spin-flop transition, while the responses in the other directions show monotonic increases. These experimental findings confirm that MnCl3(bpy) is not an ideal model system to study the S=2 Haldane phase.
    Preview · Article · Dec 2015 · Physics Procedia
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    ABSTRACT: We report on high-field electron spin resonance (ESR) studies of magnetic excitations in the spin-1/2 triangular-lattice antiferromagnet Cs2CuBr4. Frequency-field diagrams of ESR excitations are measured for different orientations of magnetic fields up to 25 T. We show that the substantial zero-field energy gap, K, observed in the low-temperature excitation spectrum of Cs2CuBr4, (Zvyagin et al 2014 Phys. Rev. Lett.112 077206) is present well above TN. Noticeably, the transition into the long-range magnetically ordered phase does not significantly affect the size of the gap, suggesting that even below TN the high-energy spin dynamics in Cs2CuBr4 is determined by short-range-order spin correlations. The experimental data are compared with results of model spin-wave-theory calculations for spin-1/2 triangular-lattice antiferromagnet.
    No preview · Article · Nov 2015 · New Journal of Physics
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    Full-text · Dataset · Nov 2015
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    ABSTRACT: We have studied orbital dynamics in the spin liquid candidate Ba3CuSb2O9 using multifrequency electron-spin resonance. We prepared two high-quality single crystals. The crystal with a slight copper deficiency shows a structural phase transition at around 200 K due to the cooperative Jahn-Teller effect, accompanied with orbital ordering. In contrast, the crystal with almost perfect stoichiometry shows no orbital ordering down to the lowest temperature of 1.5 K. Dramatic change in the g factor anisotropy as a function of frequency and temperature demonstrates orbital quantum fluctuations at a nearly constant time scale of ∼100ps below 20 K, evidencing the emergence of an orbital liquid state in this quantum spin liquid compound.
    Full-text · Article · Nov 2015 · Physical Review B
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    ABSTRACT: High field electron spin resonance (ESR) and magnetization measurements reveal the crucial role of the strong spin-lattice coupling to generate the peculiar phase transitions in the chromium spinel oxide CdCr2O4, which possesses a spin-driven Jahn-Teller transition and a field-induced 1/2-magnetization plateau state. From our analysis of the ESR modes and the spin wave dispersion, which was observed from the previous neutron scattering studies, these magnetic properties are shown to originate from the modifications of the exchange interactions due to the lattice distortions. The evaluated exchange constants are examined by the magnetoelastic theory proposed by Penc etal.
    No preview · Article · Oct 2015 · Physical Review B
  • A. Okutani · S. Kimura · T. Takeuchi · M. Hagiwara
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    ABSTRACT: We have performed high-field multi-frequency electron spin resonance (ESR) measurements of a single crystal of the S = 1/2 quasi one-dimensional (1D) Ising-like antiferromagnet BaCo2V2O8 at 1.5 K in magnetic fields of up to 14 T applied along [110] and [100] directions. Two main ESR branches, which are ascending and descending with increasing magnetic fields, are observed from 400 GHz at zero magnetic field in both cases. The low ESR mode shows a softening near 10 T for H || [100], while no softening was observed below 14 T for H || [110]. This difference in the magnetic excitations may explain the distinct phase diagrams for these two directions.
    No preview · Article · Sep 2015 · Applied Magnetic Resonance
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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.
    No preview · Article · Sep 2015 · Applied Magnetic Resonance
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    ABSTRACT: We explore orbital dynamics in the spin liquid candidate Ba3CuSb2O9 using multi-frequency electron spin resonance. We prepared two high quality single crystals. The crystal with a slight copper deficiency shows a structural phase transition at around 200 K due to the cooperative Jahn-Teller effect, accompanied with orbital ordering. In contrast, the crystal with almost perfect stoichiometry shows no orbital ordering down to the lowest temperature of 1.5 K. Dramatic change in the g-factor anisotropy as a function of frequency and temperature demonstrates orbital quantum fluctuations at a nearly constant time scale of ~ 100 ps below 20 K, evidencing the emergence of an orbital liquid state in this quantum spin liquid compound.
    Full-text · Article · Aug 2015
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    ABSTRACT: Crossover phase diagrams in the magnetic field versus temperature (H-T) plane of the nonmagnetic heavy-fermion metamagnets UT2Zn20 (T:Ir, Co) are studied by measuring the magnetic and electronic transport properties. The crossover phase diagrams of UIr2Zn20 and UCo2Zn20 are composed of a low-magnetic-field region (LFR) and a high-magnetic-field region (HFR), which are characterized by magnetic properties and the Hall effect, respectively. The LFR is found to form a closed area in the H-T plane, which is a quite different feature from the conventional uranium heavy-fermion compounds and the first observation in uranium compounds. From the drastic anomaly in the Hall effect at a metamagnetic field of UIr2Zn20, it is found that the metamagnetic behavior in UIr2Zn20 corresponds to a crossover from the heavy-fermion state to the field-induced ferromagnetic or polarized paramagnetic state accompanied by the reconstruction or topology change of Fermi surfaces. In UCo2Zn20, on the other hand, no sign of abrupt change in the electronic state at the metamagnetic field is observed. These contrastive crossover phase diagrams and the electronic state changes at the metamagnetic field are due to the different hybridization strengths between the 5 f electrons of U atoms and the d electrons of Ir and Co atoms, leading to the differences in magnetic correlation and crystalline electric field ground state or the degree of itinerancy of 5 f electrons.
    No preview · Article · Jul 2015 · Journal of the Physical Society of Japan
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    ABSTRACT: Inelastic neutron scattering is used to study the finite-temperature scaling behavior of the local dynamic structure factor in the quasi-one-dimensional quantum antiferromagnet NTENP ($\text{Ni(N,N'-bis(3-aminopropyl)propane-1,3-diamine)(}\mu\text{-NO}_2\text{)ClO}_4$), at its field-induced Ising quantum critical point. The validity and the limitations of the theoretically predicted scaling relations are tested.
    Full-text · Article · Jul 2015 · Physical Review B
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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.
    No preview · Article · Mar 2015 · Applied Magnetic Resonance
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    ABSTRACT: Using X-ray diffraction techniques, we have studied the crystal structure of a copper polynuclear coordination polymer Cu2(pymca)3(ClO4) (pymca = pyrimidine-2-carboxylate), which is found to crystallize as a trigonal crystal system, space group P31m, with the lattice constants a = 9.5904(18) Å and c = 5.9000(11) Å, at temperature T = 150 K. Each pymca ligand connects to two Cu2+ ions, forming a honeycomb network in the ab plane. The T dependence of the magnetic susceptibility of Cu2(pymca)3(ClO4) shows a broad maximum near T = 26K, indicating low-dimensional antiferromagnetic interactions. From the crystal structure and magnetic properties, we conclude that Cu2(pymca)3(ClO4) is a good realization of a spin-1/2 honeycomb lattice antiferromagnet.
    No preview · Article · Mar 2015 · Journal of the Physical Society of Japan
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    ABSTRACT: A magnetic phthalocyanine based carbon material has been successfully synthesized by using a low energy process that employs highly chlorinated iron-phthalocyanine as building blocks and an alkali metal as a coupling reagent. The X-ray diffraction patterns and XPS spectrum for the reaction products suggest that they consist of amorphouscarbon material that contains uniformly dispersed iron ions. The iron phthalocyanine based carbon material exhibits ferromagnetic properties at room temperature and the ferromagnetic phase transition occurs at TC=490 K.
    No preview · Article · Jan 2015
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    ABSTRACT: We report on the results of X-ray diffraction (XRD), magnetic susceptibility, and high-field magnetization measurements of oxygen molecules, which are unique magnetic homonuclear diatomic molecules with spin-1, encapsulated in single-walled carbon nanotubes (SWCNTs) with diameters of about 0.8 nm. Antiferromagnetic interactions between neighboring oxygen molecules are expected in SWCNTs, resulting in the formation of a spin-1 one-dimensional Heisenberg antiferromagnet, known as a Haldane magnet. The XRD pattern can be predicted accurately by considering the expected oxygen molecule alignment. The temperature evolution of the magnetic susceptibility and the high-field magnetization curve are typical of those for a Haldane magnet with spin-1. The results indicate that the Haldane state has been realized in a nanospaced material for the first time. This provides an alternative to the conventional condensed matter approach to forming quantum spin systems.
    Full-text · Article · Nov 2014 · Journal of the Physical Society of Japan
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    ABSTRACT: The chromium spinel oxide HgCr2O4 is known to show a peculiar 1/2-plateau in its magnetization curve. The appearance of the magnetization plateau has been interpreted by classical theories, which take into account the magnetoelastic interaction. In this study, we suggest that not only the magnetization curve but also the change in the exchange interaction, which occurs due to lattice distortions during the magnetization process, can be calculated based on the magnetoelastic theory, proposed by Penc et al. We show that the field dependence of the magnetization and the exchange constants, obtained from our previous experiments, agree well with those calculated by the magnetoelastic theory, except for the behaviors prior to the fully polarized phase. This result indicates that an unexpected phase exists just before the saturation of the magnetization in this compound.
    No preview · Article · Nov 2014 · Journal of the Physical Society of Japan
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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 .
    Full-text · Article · Oct 2014 · Physical Review B

Publication Stats

2k Citations
523.85 Total Impact Points

Institutions

  • 1998-2016
    • Osaka University
      • Center for Quantum Science and Technology under Extreme Conditions
      Suika, Ōsaka, Japan
  • 2015
    • Hiroshima University
      Hirosima, Hiroshima, Japan
  • 2011
    • Niigata University
      • Department of Physics
      Niahi-niigata, Niigata, Japan
    • Waseda University
      • Department of Applied Physics
      Edo, Tōkyō, Japan
  • 2009
    • Japan Science and Technology Agency (JST)
      Edo, Tōkyō, Japan
    • Charles University in Prague
      Praha, Praha, Czech Republic
  • 1999-2006
    • RIKEN
      Вако, Saitama, Japan
  • 2003-2004
    • Yokohama City University
      Yokohama, Kanagawa, Japan
  • 2002
    • Toyota Physical and Chemical Institute
      Seto, Aichi, Japan
  • 2001
    • Chiba University
      • Department of Physics
      Chiba-shi, Chiba-ken, Japan
  • 1992
    • Université Paris-Sud 11
      Orsay, Île-de-France, France