Takanari Kashiwagi

University of Tsukuba, Tsukuba, Ibaraki, Japan

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Publications (37)64.56 Total impact

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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). · 3.66 Impact Factor
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    ABSTRACT: We have performed ESR measurements on the triangular lattice antiferromagnet CuFeO2 to study the detailed frequency and field dependences of its magnon modes, which were recently revealed to be excited by oscillating electric fields. The results of the analysis for the magnon modes based the spin wave theory, and the polarization dependence measurements for the ESR signals are reported.
    Journal of Physics Conference Series 12/2012; 400(3):2039-.
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    ABSTRACT: We have performed high-field multifrequency electron spin resonance (ESR) and high-field magnetization measurements in magnetic fields H of up to 53 T on single crystals of the kagome-lattice antiferromagnet KFe3(OH)6(SO4)2. We have analyzed the magnetization curve and the ESR excitation modes for H∥c by using two kinds of anisotropy origins, the Dzyaloshinsky-Moriya (DM) interactions and the single-ion anisotropy, the former of which is inevitable in a kagome-lattice antiferromagnet. We obtained good agreement between experiment and calculation for the case of the DM interactions. In addition, we have clarified the origin of a field-induced metamagnetic transition observed in the magnetization curve and determined the intraplane and interplane exchanges and the DM interaction parameters.
    Physical review. B, Condensed matter 03/2012; 85(9). · 3.77 Impact Factor
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    ABSTRACT: In this paper, we report results of high field ESR measurements of the triangular lattice antiferromagnet CuFeO2. From polarization measurements for the ESR signals, we show an appearance of the electromagnon excitation in the field-induced 1/5 plateau phase of this compound.
    Journal of Low Temperature Physics 03/2012; 170(5-6). · 1.18 Impact Factor
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    ABSTRACT: We present the results of the magnetization and dielectric constant measurements on untwinned single-crystal samples of the frustrated S=1/2 chain cuprate LiCu2O2. Magnetic phase transitions were observed. A spin-flop transition of the spiral spin plane was observed for the field orientations H∥a, b. The second magnetic transition was observed at H≈15 T for all three principal field directions. This high-field magnetic phase is discussed as a collinear spin-modulated phase, which is expected for an S=1/2 nearest-neighbor ferromagnetic and next-nearest-neighbor antiferromagnetic chain system.
    Physical review. B, Condensed matter 02/2012; 85(5). · 3.77 Impact Factor
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    ABSTRACT: We have performed multifrequency electron spin resonance (ESR) measurements of the triangular lattice antiferromagnet CuFeO2, which shows peculiar magnetic ordering and field-induced phase transitions, in high magnetic fields of up to 53 T. Our analysis of the ESR modes and energy dispersion explains the overall behaviors of magnetic excitations in the collinear 4-sublattice phase of this material. We show changes of the exchange interactions due to the lattice distortion of CuFeO2, suggesting strong spin-lattice coupling in this material. In addition, we evaluate the precise value of the magnetic anisotropy in CuFeO2 from the ESR and magnetization measurements. The presence of relatively large second- and third-nearest-neighbor intralayer exchange interactions is also indicated. We discuss that the strong spin-lattice coupling, the large farther neighbor exchange interactions, and the finite axial anisotropy are important to understand the magnetic phase transitions of CuFeO2.
    Physical review. B, Condensed matter 09/2011; 84(10). · 3.77 Impact Factor
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    ABSTRACT: A novel topologically frustrated pentanuclear cluster helicate [{Cu(II)(μ-L)(3)}(2)Cu(II) (3)(μ(3)-OH)](3+) (L(-)=3,5-bis(2-pyridyl)pyrazolate) has been synthesized and characterized. This cluster has a helical arrangement of ligands around the central metal core. Dzyaloshinsky-Moriya interactions are essential components to observe a gradual magnetization and forbidden transitions of high-field/multi-frequency (HF/MF)-ESR. The origin of the magnetic anisotropy of this compound is influenced by its helical spin structure, and consequently, the Cu(5)-cluster helicate introduces a unique magnetic anisotropy. This observation is a direct evidence of the topological part of the new spin phase in a magnetic system.
    Chemistry - A European Journal 09/2010; 16(36):11139-44. · 5.93 Impact Factor
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    ABSTRACT: We have carried out electron spin resonance (ESR) measurements on powder samples of sodalite loaded with Na at several frequencies between 9.7 and 35GHz and at temperatures between 1.5 and 60K. The ESR absorption spectrum below a Néel temperature TN turns into an asymmetric spectrum with a long tail at low fields from a symmetric one above TN. The line shape of the spectra below TN is analyzed by a powder pattern simulation of the antiferromagnetic resonance spectra with easy-plane anisotropy. The calculated line shape reproduces the experimental one considerably well by assuming a Gaussian distribution of the zero-field energy gap. We have evaluated a small anisotropy field of about 2×10−4T by using the exchange coupling constant calculated from the Weiss and the Néel temperatures. This result indicates that the sodalite loaded with Na is quite an ideal Heisenberg antiferromagnet as expected from the s-electron character of Na clusters and the cubic arrangement of nano-spaces in the sodalite.
    Journal of Physics and Chemistry of Solids 04/2010; 71(4):544-547. · 1.53 Impact Factor
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    ABSTRACT: We report the results of multi-frequency electron spin resonance (ESR) measurements on single crystals of Ni(C5H14N2)2N3(PF6) which is regarded as the one-dimensional Heisenberg antiferromagnet with spin one, namely the Haldane magnet, at very low temperatures down to about 100 mK. We observed the lowest resonance branch below about 500 mK for the field along the chain direction (H||c), which was observed previously only in an inelastic neutron scattering experiment at 30 mK. We compare the resonance branch with that calculated by a phenomenological field theory, and discuss the field dependence and the temperature sensitivity of this ESR branch.
    Journal of Physics Conference Series 02/2010; 200(2):022012.
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    ABSTRACT: We have performed multi-frequency ESR measurements on the geometry frustrated triangular lattice antiferromagnet CuFeO2. The observed ESR modes are analyzed based on a mean field theory, taking into account a distortion of the triangular lattice, which was found by recent synchrotron X-ray diffraction measurements.
    Journal of Low Temperature Physics 01/2010; 159(1):80-83. · 1.18 Impact Factor
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    ABSTRACT: Sodium clusters arrayed in aluminosilicate sodalite are known to show antiferromagnetism below the Néel temperature TN of 48 K. We have performed electron spin resonance (ESR) measurements on a powder specimen of this material at an X-band microwave frequency (9.7 GHz) with temperature varying between 4 and 300 K. The ESR spectra show asymmetric broadening below TN, which can be regarded as a powder pattern of the antiferromagnetic resonance (AFMR) signal. From the analysis of the AFMR fields at low temperatures using the exchange-coupling constants evaluated from the magnetic susceptibility, the anisotropy field is estimated to be significantly small at about 1-2 Oe. These results indicate that this material is an ideal Heisenberg antiferromagnet resulting from the character of s-electrons in zeolite cages with a cubic arrangement.
    Journal of the Physical Society of Japan 08/2009; 78(8):4723-. · 2.09 Impact Factor
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    ABSTRACT: Sensitivity to detect diamagnetic anisotropy ΔχDIA of inorganic oxides was improved by increasing intensity of horizontal field from 1.6 T to 5.0 Tesla. The field induced a rotational oscillation of a magnetically stable axis of a sample, which was suspended with a thin fiber. Accurate Δχ values are obtained when restoration torque of the fiber is negligibly small compared to torque due to magnetic anisotropy. Accordingly, Δχ value at a level of 5×10-10 emu/g can be acquired, even if the mass of the sample is relatively small (0.318 g). Necessity of realizing Δχ measurement at low temperature is discussed, which realizes separation of Δχ DIA from a paramagnetic anisotropy caused by isolated magnetic ions.
    Journal of Physics Conference Series 04/2009; 156(1):012023.
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    ABSTRACT: Technical basis to detect small diamagnetic anisotropy ΔχDIA is established in a method using microgravity μG. Sensitivity of Δχ can be improved by increasing length of measurable period of rotational oscillation of magnetically stable axis with respect to field direction. In order to achieve the above condition, position of the diamagnetic sample should be stabilized in static field area, which was realized by introducing a magnetic circuit with high field homogeneity. Function of a mobile sample stage was effective in minimizing translational motion of sample. Accordingly, magnitude of measurable anisotropy may be decreased to a level of 10-10 emu/g. ΔχDIA of materials with high crystal symmetry, namely hexagonal ice, ZnO, SiC and BaTiO3, can be obtained by the above improvement. Magnetically active property due to magnetic anisotropy may be recognized for almost all the solid materials when sensitivity reaches the above level.
    Journal of Physics Conference Series 04/2009; 156(1):012024.
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    ABSTRACT: An ultra-low temperature multi-frequency electron spin resonance (ESR) apparatus has been developed for use at a temperature down to about 200 mK by utilizing a 3He-4He compact dilution refrigerator and a vector network analyzer. The microwave frequency can be varied between 30 GHz and 700 GHz almost continuously. Either of the two kinds of copper blocks with a sample holder for typical two configurations, namely Faraday and Voigt configurations, is attached to a stainless steel block neighboring to a mixing chamber which provides ultra-low temperarures down to 160 mK. A magnetic field up to 16 T is produced with a superconducting magnet (14 T at 4.2 K and 16 T at 2.2 K). Thus, we have a potential to perform ESR measurements in a very wide frequency-field window at ultra-low temperatures.
    Journal of Physics Conference Series 03/2009; 150(1):012015.
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    ABSTRACT: The S=1 quasi-one dimensional Heisenberg antiferromagnet Ni(C5H14N2)2N3(PF6), abbreviated as NDMAP, has been studied by electron spin resonance (ESR) in a magnetic field exceeding the critical field (Hc) above which this compound shows a long range order (LRO) at sufficiently low temperatures. Our recently developed ESR apparatus with a dilution refrigerator is utilized to study the spin excitations of NDMAP above Hc for H∥c, because this compound exhibits the LRO below about 1 K. The spin excitation modes above Hc observed below 1 K do not show any difference from those at 1.5 K and the lowest excitation mode observed at 30 mK in the inelastic neutron experiments above Hc was not observed at about 200 mK in this study. Accordingly, the result suggests that the lowest excitation mode is very sensitive to temperature.
    Journal of Physics Conference Series 03/2009; 150(4):042083.
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    ABSTRACT: We have developed high-field and multifrequency (HFMF) electron spin resonance (ESR) apparatus for the magnetic fields up to 65T at frequencies up to about 6 THz. In addition to this pulsed field ESR apparatus, we are making a multifrequency ESR apparatus with very high sensitivity in a static field. We report the results of ESR studies on BaCoV2O8 and NiGa2S4, followed by recent developments and future plans of our HFMF ESR apparatus.
    Applied Magnetic Resonance 01/2009; 36(2):269-274. · 0.83 Impact Factor
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    ABSTRACT: In order to understand physical properties of the field-induced phase of a spin-gap system, we performed high-field and multifrequency electron-spin-resonance (ESR) measurements on single crystals of the S=1 quasi-one-dimensional Heisenberg antiferromagnet, namely, the Haldane magnet, Ni(C5H14N2)2N3(PF6) , abbreviated as NDMAP. This compound has an energy gap (Haldane gap) at zero field and one of the excited triplet branches goes down on applying magnetic fields, resulting in the gap closing at a critical field Hc around 5 T that is slightly different depending on the field direction. First, we studied the angular dependence of spin excitations below 14 T. Two sets of resonance modes caused by two types of Ni2+ chains in NDMAP are observed. These data are analyzed by comparing with a phenomenological field theory (PFT). The experimental results between Hc and about 12 T are well fitted with the calculated ones by the PFT, but the fitting above 12 T is not satisfactory. Therefore, we studied spin excitations at much higher magnetic fields up to about 55 T. Several ESR signals are observed above Hc for each crystallographic axis, and one or two of them survive in the high-field region above about 15 T. One mode approaches a paramagnetic resonance line at high fields and the other mode broadly changes with magnetic fields. These modes fit well with the conventional antiferromagnetic resonance modes with biaxial anisotropy. This result suggests that the quantum fluctuations are suppressed by strong magnetic field and the spin excitations change from a quantum nature to a classical one in high magnetic fields.
    Physical review. B, Condensed matter 01/2009; 79(2). · 3.77 Impact Factor
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    ABSTRACT: The S = 1 quasi-one-dimensional Heisenberg antiferromagnet [Ni(C5H14N2)2N3](PF6), abbreviated as NDMAP, has been studied by electron spin resonance in a magnetic field above the critical field (H c). We studied angular and frequency dependences of spin excitations. The angular dependence of the spin excitations in the vicinity of H c is explained well by a phenomenological field theory, but the agreement between the experiment and the calculation is not satisfactory above 10 T. In high magnetic fields above 15 T, we obtained some characteristic spin excitations which are well explained by conventional antiferromagnetic resonance modes. These results suggest that the spin excitations change from a quantum state to a classical one due to the suppression of quantum fluctuations by high magnetic fields.
    Applied Magnetic Resonance 01/2009; 36(2):309-316. · 0.83 Impact Factor
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    ABSTRACT: High field magnetization and high-frequency electron spin resonance measurements on the perovskite Co oxide Sr1−xYxCoO3−δ (x=0.22 and 0.25) have been performed in magnetic fields up to 53 T. We have observed the spin-state transition, induced by the external magnetic fields, for the sample with x=0.25. A distinctive feature of this field-induced spin-state transition is that it is assisted by an internal field, originating from ferromagnetic exchange interaction between Co3+ ions.
    Physical review. B, Condensed matter 11/2008; 78(18). · 3.77 Impact Factor
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    ABSTRACT: Spin resonance of the triplet excitations is studied experimentally in the Haldane magnet PbNi2V2O8 . The absorption spectrum has features of spin S=1 resonance in a crystal field, with all three components, corresponding to transitions |±1>&lrarr2;|0> and |-1>&lrarr2;|1> , being observable. The resonance field is temperature dependent, indicating the renormalization of excitation spectrum in the interaction between the triplets. Magnetic resonance frequencies and critical fields of the magnetization are consistent with a boson version of the macroscopic field theory [J. Affleck, Phys. Rev. B 46, 9002 (1992); A. M. Farutin and V. I. Marchenko, Zh. Eksp. Teor. Fiz. 131, 860 (2007)] [JETP 104, 751 (2007)] while they contradict the fermion version and the previously used approach of noninteracting spin chains.
    Physical review. B, Condensed matter 01/2008; 77(10). · 3.77 Impact Factor