Yutaka Kishimoto

The University of Tokushima, Tokusima, Tokushima, Japan

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Publications (60)99.35 Total impact

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    ABSTRACT: We have investigated the successive magnetic phase transitions of the A-site ordered perovskite LaMn3Cr4O12 by measuring 139La nuclear magnetic resonance (NMR) spectra. The successive magnetic transitions are revealed by a very small but clear increase in the full width at half maximum (FWHM) of the 139La NMR signal at T M2 = 150 K and by the disappearance of the signal due to the line broadening at temperatures below T M1 = 50 K. These two magnetic transitions are associated with independent orderings within the Cr-site sublattice at T M2 and within the Mn-site sublattice at T M1. Regarding the magnetic structure of the Cr-site sublattice at temperatures below T M2, the cancellation of internal fields at the La site, ruling out ferromagnetic and multi- q antiferromagnetic structures, is consistent with the typical antiferromagnetic structure for perovskites, such as G-, A- and C-type orders. The uncanceled internal field at the La site at temperatures below T M1 is also consistent with these typical antiferromagnetic structures in the Mn-site sublattice. The differences in the temperature dependences of the Knight shift K( T) and the bulk susceptibility χ( T) are consistently explained by the independent magnetic orderings of the Cr- and the Mn-site sublattices and the cancellation of internal field originating from the Cr-site sublattice at the La site at temperatures below T M2.
    Journal- Korean Physical Society 08/2013; 63(3):640-643. · 0.51 Impact Factor
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    ABSTRACT: We have investigated the magnetic properties of quasi-one-dimensional antiferromagnet BaCo2V2O8 by the measurements of 51V-NMR spectrum. With decreasing temperature, K ‖ shows a broad peak around 30 K signaling the 1D short-range order in the Co2+ spin chain. The estimated Weiss temperature θ = -105 K, hyperfine coupling constants A {hf/‖} = 2.1 kOe/µ B for H ‖ c and A {hf/⊥} = 2.6 kOe/µ B for H ⊥ c at 13 kOe are comparable to those at 70.6 kOe. These results suggest that static magnetic properties in the paramagnetic state are nearly independent of external magnetic field, although the ground Néel state is suppressed by the application of external magnetic field above H c = 39 kOe along c-axis. In the antiferromagnetic state below T N = 5.4 K, the double rectangle-like powder pattern was observed, indicating two V sites exposed to different size of internal fields, H A1 = 2.1 and H A2 = 3.8 kOe. The internal fields at V sites are explained by taking into account of the classical and pseudo-dipolar fields from Co2+ magnetic moments.
    Journal- Korean Physical Society 08/2013; 63(3):739-742. · 0.51 Impact Factor
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    ABSTRACT: We have investigated magnetic properties of the A-site ordered perovskite manganite LaBaMn2O6 by muon spin relaxation. In the ferromagnetic metal (FM) phase below TC ~ 330 K, no muon spin oscillation is observed in the time spectra, where the muon spin relaxation significantly deviates from a single exponential form. These results may suggest a large spatial distribution of spin dynamics, i.e. the inhomogeneous magnetic nature in the FM phase of LaBaMn2O6, which has been also observed in the FM phase of A-site disordered perovskite manganites. It is, therefore, suggested that there exists the spatial distribution of spin dynamics in the FM phase of the perovskite manganites irrespective of the order/disorder at the A-site of perovskite structure.
    Journal of Physics Conference Series 12/2012; 391(1):2096-.
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    ABSTRACT: We report on our 51V-NMR study of static and dynamical magnetic properties in the quasi-one-dimensional antiferromagnet BaCo2V2O8. Although the NMR spectrum shows well-defined antiferromagnetic (AF) order in the N\'eel ground state, the AF characteristic from the NMR spectrum is incomplete between 3.5 K and TN= 5.4 K, which could be affected by quantum spin fluctuations. The AF NMR spectrum indicates two V sites experiencing different magnetic field magnitudes, HA1=2.1 kOe and HA2=3.8 kOe. These internal fields could be explained by accounting for the classical and the pseudo-dipolar fields from Co2+ spins with a proposed magnetic structure based on the neutron diffraction measurements. In the paramagnetic state, the nuclear spin relaxation is dominated by AF spin fluctuations through the dipolar-type coupling between V and surrounding Co2+ ions. The linear relation between the nuclear spin-lattice relaxation rate 1/T1T and the magnetic susceptibility chi indicates that the Q component of magnetic susceptibility chi(Q) is roughly proportional to chi, where Q is the AF wave number. A change in slope of 1/T1T with respect to \chi around 150 K suggests a change in the AF spin fluctuation spectrum.
    Physical review. B, Condensed matter 09/2012; 86(9). · 3.66 Impact Factor
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    ABSTRACT: We report NMR studies of TbCoGa5, which has the tetragonal HoCoGa5 structure and exhibits two antiferromagnetic (AF) transitions at TN1=36.2 K and TN2=5.4 K. From a symmetry analysis of internal magnetic fields at orthorhombic Ga sites, we have successfully determined the magnetic structures in the AF-I (TN2<T<TN1) and AF-II (T<TN2) phases. The AF-I phase is a collinear AF order with a propagation vector q=[1/2,0,1/2] and ordered moments parallel to the [001] direction. In the AF-II phase, on the other hand, we found a noncollinear AF structure described by double propagation vectors q1=[1/2,0,1/2] and q2=[0,1/2,1/2], where the moments tilt away from the [001] direction toward [100], keeping a constant value along the [001] direction. In the context of these results, we discuss the possible presence of magnetic frustration in this system.
    Physical Review B 12/2011; 84(21). · 3.66 Impact Factor
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    ABSTRACT: Magnetic excitation in the two-dimensional triangular lattice LiVO2 and its itinerant analogue LiVS2 are investigated by 51V- and 7Li-NMR experiments. The nuclear spin-lattice relaxation rate 1/T1 on 51V and 7Li nuclei shows thermally activated behavior with different size of spin gap, Δ/kB ~ 3400 K for LiVO2 and ~ 1900 K for LiVS2, below the transition temperature Tc from magnetic to non-magnetic ground state. In 51(1/T1(T))/51(1/T1(Tc)) vs. T/Tc plot, the normalized relaxation rate for both materials seems to fall on the same curve. This scaling behavior indicates that the ground states of LiVO2 and LiVS2 are the same, i.e., V3+ (3d2 S = 1) spins form spin singlet with the formation of triangular vanadium clusters, although the replacement of oxygen by sulfur may broaden the band width and largely suppresses Tc from ~ 500 K in LiVO2 to ~ 310 K in LiVS2. However, the magnetic properties in the high temperature phase are different between LiVO2 and LiVS2. The value of 7(1/T1T) in LiVS2 becomes smaller with decreasing temperature, which suggests the development of pseudogap above Tc, in qualitatively consistent with the results from the magnetic susceptibility and the small change in entropy at Tc. This is a contrast to the case in LiVO2 where 51(1/T1T) is proportional to 1/T and it is explained by a localized spin model well above Tc.
    Journal of Physics Conference Series 09/2011; 320(1):2028-.
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    ABSTRACT: We report a neutron diffraction and muon spin relaxation muSR study of static and dynamical magnetic properties of BaCo2V2O8, a quasi-one-dimensional spin-chain system. A proposed model for the antiferromagnetic structure includes: a propagation vector k_{AF} = (0, 0, 1), independent of external magnetic fields for fields below a critical value H_c(T). The ordered moments, of 2.18 \mu_B per Co ion, are aligned along the crystallographic c-axis. Within the screw chains, along the c axis, the moments are arranged antiferromagnetically. In the basal planes the spins are arranged ferromagnetically (forming zig-zags paths) along one of the axis and antiferromagnetically along the other. The temperature dependence of the sub-lattice magnetization is consistent with the expectations of the 3D Ising model. A similar behavior is observed for the internal static fields at different muon stopping sites. Muon spectra measured at weak longitudinal fields and temperatures much higher than T_N can be well described using a single muon site with an exponential muon spin relaxation that gradually changes into an stretched exponential on approaching T_N. The temperature-induced changes of the relaxation suggest that the Co fluctuations dramatically slow down and the system becomes less homogeneous as it approaches the antiferromagnetic state. Comment: 7 pages, 8 figures
    Physical review. B, Condensed matter 10/2010; · 3.66 Impact Factor
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    ABSTRACT: 11B NMR measurements have been performed on rare earth ternary borides, YRuB2, LuRuB2 and (Tm0.4Y0.6)RuB2 to investigate their electronic states. From the measurements of temperature dependence of spin-lattice relaxation rate 1/T1, YRuB2 and LuRuB2 are found to be Bardeen-Cooper-Schrieffer (BCS) superconductors with the energy gap 2Delta(0)=3.52 kBTc and the superconducting transition temperatures Tc=7.6 K (YRuB2) and 10.2 K (LuRuB2). A very broad and temperature dependent spectrum is observed in (Tm0.4Y0.6)RuB2. The magnitude of 1/T1T is considerably larger in (Tm0.4Y0.6)RuB2 than in YRuB2 and LuRuB2. These facts suggest the contribution of 4f electrons to the hyperfine field at 11B nuclei in (Tm0.4Y0.6)RuB2.
    Journal of Physics Conference Series 06/2009;
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    ABSTRACT: To clarify the electronic state in a two-dimensional triangular lattice LiVS2, we have performed 51V- and 7Li-NMR measurements. Below the phase transition temperature Tc of about 310 K from the paramagnetic state to a nonmagnetic state, the Knight shift of both 51V and 7Li does not depend on temperature. The 51V and 7Li spin-lattice relaxation rates 1/T1 show the exponential temperature dependence below Tc, which indicates a gap structure of the electronic state. These results are in agreement with a trimer singlet of V3+ spins below Tc. The increase in 7Li 1/T1 above Tc indicates the existence of the pseudo gap and agrees with the increase in paramagnetic susceptibility with increasing temperature and with the small entropy change at Tc compared with that in LiVO2.
    Journal of the Physical Society of Japan 01/2009; 78. · 2.09 Impact Factor
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    ABSTRACT: In order to investigate the Li+ ionic diffusion and the electronic states in a mixed conductor beta-LiGa with high Li+ ionic diffusibility and electron/hole conductivity, 7Li NMR linewidth and spin-lattice relaxation measurements have been performed in 44.0, 47.0, and 50.0 at. % Li beta-LiGa samples at 10.03 MHz in the temperature range between 10 and 320 K. The onset temperature TMN=70 K of the motional narrowing in 50.0 at. % sample has been determined from the temperature dependence of the linewidth. The Li+ ionic diffusion is found to contribute to the spin-lattice relaxation rate 1/T1 down to ˜0.5 TMN even below TMN where the motional narrowing does not occur. The high diffusibility of Li+ ions has been proved from a microscopic point of view. At low temperatures, the relations 1/T1T=3.5× 10-4, 3.8× 10-4, and 5.1× 10-4 s-1 K-1 are observed in 44.0, 47.0, and 50.0 at. % Li samples, respectively. The density of states of conduction electrons at the Fermi level in these compounds becomes higher with increasing Li content, which is consistent with the predictions by band calculations.
    Journal of the Physical Society of Japan 01/2009; 78. · 2.09 Impact Factor
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    ABSTRACT: 55Mn NMR spectra have been measured for the first time in the A-site ordered YBaMn2O6 whose ground state is of antiferromagnetic insulator, and LaBaMn2O6 whose ground state is of ferromagnetic metal. These spectra consist of several much narrower lines compared with those observed in the A-site disordered R1-xAxMnO3.
    Physica B Condensed Matter 04/2005; 359:1291-1293. · 1.28 Impact Factor
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    ABSTRACT: 139La-NQR measurement has been made in a superconductor La3Ir2Ge2. The spin-lattice relaxation rate 1/T1 measured in the normal state is proportional to the temperature T, indicating that La3Ir2Ge2 is a simple metal. In the superconducting state, a small coherence peak in 1/T1 appears just below the superconducting transition temperature Tc=4.7K and 1/T1 decreases exponentially well below Tc with decreasing temperatures. It is concluded that La3Ir2Ge2 is a BCS superconductor with an energy gap 2Δ(0)/kBTc=3.52.
    Physica B Condensed Matter 01/2005; 359:503-505. · 1.28 Impact Factor
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    ABSTRACT: LaRu3Si2 reveals superconductivity below the superconducting transition temperature Tc∼6.5K, which is important concerning with a valence fluctuating material CeRu3Si2 with Tc∼1K, in which 4f spins are considered to depress its superconducting transition. The magnetic susceptibility of LaRu3Si2 is very dependent on temperature, which indicates a narrow band of conduction electrons. To investigate the relation between the superconducting state and 4f spin fluctuations, 139La NQR relaxation rate 1/T1 has been measured in LaRu3Si2 without 4f spins. In the normal state, 1/T1 is proportional to temperature. The superconductivity in LaRu3Si2 is found to be BCS type because 1/T1 shows a coherence peak just below Tc and an Arrhenius type dependence exp(−2Δ/kBT) well below Tc.
    Journal of Magnetism and Magnetic Materials 05/2004; 272:507-508. · 2.00 Impact Factor
  • Yutaka Kishimoto, Yu Kawasaki, Takashi Ohno
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    ABSTRACT: In order to study the magnetism in RRh3B2 (R = La, Ce, Nd, Sm, Eu and Gd), 11B NMR has been measured systematically. LaRh3B2 was shown to be simply paramagnetic in the normal state, and GdRh3B2 and NdRh3B2 ferromagnetic with the well localized moments. In CeRh3B2, the K-chi plot and 1/T1 can be interpreted in a localized moment model with a large reduction in the effective moment, and a small magnetic moment is considered to be localized at the Ce site below Tc. For SmRh3B2, 1/T1 cannot be explained by the localized moment model, and it was found that the antiferromagnetic spin fluctuation exists in the paramagnetic state. The temperature dependence of 1/T1T indicates that EuRh3B2 is a weak antiferromagnet.
    Journal of the Physical Society of Japan 01/2004; 73(7):1970-1981. · 2.09 Impact Factor
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    ABSTRACT: The magnetic susceptibility χ(T) of a mixed valence (MV) system CeRu3Si2 has been measured in the temperature T range between 2 and 300K. χ(T) is independent of T below 100K, and slightly increases with increasing T. We estimated the spin fluctuation temperature Tsf and found a good scaling of χ(T)T/C vs T/Tsf among MV compounds CeRu3Si2, CeSn3, CeRu2, CeOs3B2 and CeRu3B2. Tsf is compared and found to rule the electronic properties in these compounds.
    Journal of Magnetism and Magnetic Materials 01/2004; 272:509-510. · 2.00 Impact Factor
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    ABSTRACT: To investigate the electronic state of LaRu3Si2, we have carried out 139La-NQR measurement. In the normal state, the spin-lattice relaxation rate 1/T1 is proportional to the temperature T. 1/T1 shows a coherence peak just below the superconducting transition temperature Tc=6.5 K and decreases exponentially well below Tc. LaRu3Si2 is found to be a strong coupling BCS superconductor with a large energy gap 2Delta(0)=5.50kBTc. A relatively high Tc in LaRu3Si2 is presumably due to a high density of states of conduction electrons at Fermi surface and strong electron-phonon interaction.
    Journal of the Physical Society of Japan 01/2004; 73(1):190-193. · 2.09 Impact Factor
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    ABSTRACT: We report the results of 209Bi-NQR measurements on the heavy-fermion compound CeRhBi with the large electronic specific heat coefficient gamma ˜ 500 mJ/mol\cdotK2 at 2 K. From the Bi-NQR spectra, it is proved that no magnetic ordering occurs down to 0.15 K. The temperature dependence of nuclear spin-lattice relaxation rate 1/T1 clearly demonstrates a crossover from a localized regime at high temperature to an itinerant ground state with the Kondo temperature TK of about 8 K. It is, therefore, revealed that CeRhBi is a typical heavy-fermion compound with a non-magnetic ground state, closely located at a quantum critical point. In the context of CeRhPn (Pn = Bi, Sb, As) series, where all compounds crystallize in the same orthorhombic \varepsilon-TiNiSi-type structure, no trace of gap formation is observed in 1/T1 down to 0.2 K in CeRhBi, although the values of 1/T1 in the Kondo semiconductors CeRhSb and CeRhAs markedly decrease at low temperatures due to the presence of excitation gap.
    Journal of the Physical Society of Japan 01/2004; 73(3):694-697. · 2.09 Impact Factor
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    ABSTRACT: To investigate the electronic state of La3Ir2Ge2, we have carried out 139La-NQR measurement. In the normal state, the spin-lattice relaxation rate 1/T1 is proportional to the temperature T. 1/T1 shows a small coherence peak just below the superconducting transition temperature Tc=4.7 K and decreases exponentially well below Tc. La3Ir2Ge2 is found to be a BCS superconductor with an energy gap 2Delta (0)=3.52kBTc.
    Journal of the Physical Society of Japan 01/2004; 73(12):3389-3393. · 2.09 Impact Factor
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    ABSTRACT: To investigate the mixed-valence (MV) state in CeRu3Si2, we measured the magnetic susceptibility χ(T) in the temperature range 8–300 K. The magnetic susceptibility is constant below 100 K, and slightly increases with increasing temperature. We estimated the spin fluctuation temperature Tsf by plotting χ(T)T/C against T, where C is the Curie constant of a Ce3+ ion. CeRu3Si2 is found to be a typical MV system, indicating that χ(T)T/C is a function of T/Tsf. This scaling relation is applied to the other MV compounds such as CeSn3.
    Physica B Condensed Matter 08/2003; 336(s 3–4):398–401. · 1.28 Impact Factor
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    ABSTRACT: To investigate the relation between the non-Fermi-liquid state and the disordered state, a crystallographically disordered amorphous UPd2Al3 has been prepared. At low temperatures, the measured electronic specific heat coefficient γ has a typical temperature dependence of non-Fermi-liquid state, γ∝−logT.
    Physica B Condensed Matter 01/2003; 329:561-563. · 1.28 Impact Factor