arXiv subjects

Taketo Moyoshi

Publications and source records attributed to Taketo Moyoshi.

17 recordsLinked to original sources

arXiv2021

Magnetic structures and phase transitions of A-site Ordered Double-Perovskite SmBaMn2O6

Magnetic structures and the relationship between spin and charge-orbital orderings of an A-site ordered double-perovskite manganite SmBaMn2O6, an anticipated multiferroic material, were investigated by means of neutron diffraction. The spin arrangement in MnO2 planes perpendicular to the c axis is revealed to be the same as that in the A-site disordered half-doped manganites CE-type but the stacking pattern is found to be different displaying a unique twofold period. The temperature dependence of the superlattice magnetic and nuclear reflections clarifies that the antiferromagnetic spin ordering occurs at a temperature slightly lower than the temperature at which a rearrangement of the charge-orbital orderings occurs. The result evidences that the rearrangement leads the spin ordering. The intensities of the magnetic reflections are found to change across Tf = 10 K, suggesting a spin-flop by 90 [deg.] while keeping the Mn spin ordering pattern unchanged.

cond-mat.str-el↗

arXiv2018

Neutron-scattering study of yttrium iron garnet

The nuclear and magnetic structure and full magnon dispersions of yttrium iron garnet Y$_3$Fe$_5$O$_{12}$ have been studied by neutron scattering. The refined nuclear structure is distorted to a trigonal space group of $R\bar{3}$. The highest-energy dispersion extends up to 86 meV. The observed dispersions are reproduced by a simple model with three nearest-neighbor-exchange integrals between 16$a$ (octahedral) and 24$d$ (tetrahedral) sites, $J_{aa}$, $J_{ad}$, and $J_{dd}$, which are estimated to be 0.00$\pm$0.05, $-$2.90$\pm$0.07, and $-$0.35$\pm$0.08 meV, respectively. The lowest-energy dispersion below 14 meV exhibits a quadratic dispersion as expected from ferromagnetic magnons. The imaginary part of $q$-integrated dynamical spin susceptibility $χ$"($E$) exhibits a square-root energy-dependence in the low energies. The magnon density of state is estimated from the $χ$"($E$) obtained on an absolute scale. The value is consistent with a single polarization mode for the magnon branch expected theoretically.

cond-mat.mtrl-sci↗

arXiv2017

Time-of-Flight Three Dimensional Neutron Diffraction in Transmission Mode for Mapping Crystal Grain Structures

The physical properties of polycrystalline materials depend on their microstructure, which is the nano-to-centimeter-scale arrangement of phases and defects in their interior. Such microstructure depends on the shape, crystallographic phase and orientation, and interfacing of the grains constituting the material. This article presents a new non-destructive 3D technique to study bulk samples with sizes in the cm range with a resolution of hundred micrometers: time-of-flight three-dimensional neutron diffraction (ToF 3DND). Compared to existing analogous X-ray diffraction techniques, ToF 3DND enables studies of samples that can be both larger in size and made of heavier elements. Moreover, ToF 3DND facilitates the use of complicated sample environments. The basic ToF 3DND setup, utilizing an imaging detector with high spatial and temporal resolution, can easily be implemented at a time-of-flight neutron beamline. The technique was developed and tested with data collected at the Materials and Life Science Experimental Facility of the Japan Proton Accelerator Complex (J-PARC) for an iron sample. We successfully reconstructed the shape of 108 grains and developed an indexing procedure. The reconstruction algorithms have been validated by reconstructing two stacked Co-Ni-Ga single crystals and by comparison with a grain map obtained by post-mortem electron backscatter diffraction (EBSD).

physics.ins-det↗

arXiv2009

Studies of Multiferroic System of LiCu2O2 II Magnetic Structures of Two Ordered Phases with Incommensurate Modulations

Neutron diffraction and 7Li-NMR have been applied to determine the multiferroic system LiCu2O2, which has four chains (ribbon chains) of edge-sharing CuO4 square planes in a unit cell. We have confirmed that there are successive magnetic transitions at TN1=24.5 K and TN2=22.8 K. In the T region between TN1 and TN2, the quasi one-dimensional spins (S=1/2) of Cu2+ ions within a chain have a collinear and sinusoidally modulated structure with Cu-moments parallel to the c-axis and with the modulation vector along the b-axis. At T < TN2, an ellipsoidal helical spin structure with the incommensurate modulation has been found. Here, we present detailed parameters, describing the modulation amplitudes, helical axis vectors as well as the relative phases of the modulations of four ribbon chains, which can well reproduce both the NMR and neutron results in the two magnetically ordered phases. This finding of the rather precise magnetic structures enables us to discuss the relationship between the magnetic structure and the multiferroic nature of the present system in zero magnetic field, as presented in our companion paper (paper I), and open a way how to understand the reported electric polarization under the finite magnetic field.

cond-mat.mtrl-sci↗

arXiv2008

Preparation of the oxypnictides and studies on their superconductivity

LaFe1-yCoyAsO1-xFx (x=0.11) with various y values were prepared and their electrical resistivities, superconducting diamagnetisms and Hall coefficients have been measured. 75As- and 135La-NMR studies have also been carried out In spite of the successful Co-doping, we have not found any meaningful correlation of Tc with y, which indicates that the Tc-suppression by Co-doping is considered not to be so significant as expected for superconductors with nodes. Even for superconductors without nodes, it may not be easy to expect this small effect on Tc, if there are two different (disconnected) Fermi surfaces whose order parameters have opposite signs. The data of the NMR Knight shift indicate that the Cooper pairs are in the singlet state and the spin susceptibility is almost fully suppressed at low temperature.

cond-mat.supr-con↗

arXiv2008

On the Superconductivity of LaFe1-yCoyAsO1-xFx

We have prepared the superconducting system LaFe1-yCoyAsO1-xFx (x=0.11) and carried out measurements of their electrical resistivities r and superconducting diamagnetisms. 75As- and 135La-NMR studies have also been carried out. The Knight shift observed for 75As has been found to be suppressed by the superconductivity, while for 135La, the shift is almost insensitive to the superconductivity. This result presents rather strong experimental evidence for the singlet pairing. The Co-doping effect on the superconducting transition temperature Tc is not so significant as expected for superconductors with nodes, suggesting that the potential scattering does not seem to primarily suppress the superconductivity. Even for superconductors without nodes, it may not be so trivial to expect this small effect, if there are two different (disconnected) Fermi surfaces whose order parameters have opposite signs. As a possible explanation of this small Tc suppression seems to be related with the loss of the itinerant nature of the electrons.

cond-mat.supr-con↗

arXiv2008

Magnetic Excitations of Spin-Gap System Na3Cu2SbO6 with Distorted Honeycomb Structure

Magnetic excitation spectra of quantum spins of Na3Cu2SbO6 with the distorted honeycomb structure have been measured by neutron inelastic scattering. The intensity distribution and the dispersion curves of the excitations indicate that the spin system can be well understood by considering one-dimensional spin chains formed of Cu2+ ions on a lattice of the alternation of the shorter- and longer-spacings along the b axis. The exchange interactions between the neighboring spins with the shorter- and longer-spacings correspond to ferromagnetic and antiferromagnetic ones, respectively. These findings are consistent with those we reported previously on the basis of macroscopic measurements. The observed spin gap is 8.9 meV and the antiferromagnetic- and ferromagnetic-interactions have been found to be 13.9 and -12.5 meV, respectively.

cond-mat.str-el↗

arXiv2008

NMR studies of Successive Phase Transitions in Na0.5CoO2 and K0.5CoO2

59Co- and 23Na-NMR measurements have been carried out on polycrystalline and c-axis aligned samples of Na0.5CoO2, which exhibits successive transitions at temperatures T = 87 K (= Tc1) and T = 53 K (= Tc2). 59Co-NMR has also been carried out on c-axis aligned crystallites of K0.5CoO2 with similar successive transitions at Tc1 ~ 60 K and Tc2 ~ 20 K. For Na0.5CoO2, two sets of three NMR lines of 23Na nuclei explained by considering the quadrupolar frequencies nuQ ~1.32 and 1.40 MHz have been observed above Tc1, as is expected from the crystalline structure. Rather complicated but characteristic variation of the 23Na-NMR spectra has been observed with varying T through the transition temperatures, and the internal fields at two crystallographically distinct Na sites are discussed on the basis of the magnetic structures reported previously. The internal fields at two distinct Co sites observed below Tc1 and the 591/T1-T curves of Na0.5CoO2 and K0.5CoO2 are also discussed in a comparative way.

cond-mat.str-el↗

arXiv2008

Co-NMR Knight Shift of NaxCoO2 \dot yH2O Studied in Both Superconducting Regions of the Tc-nuQ3 Phase Diagram Divided by the Nonsuperconducting Phase

In the temperature (T)-nuQ3 phase diagram of NaxCoO2 \dot yH2O, there exist two superconducting regions of nuQ3 separated by the nonsuperconducting region, where nuQ3 is usually estimated from the peak position of the 59Co-NQR spectra of the 5/2-7/2 transition and well-approximated here as nuQ3~3nuQ,nuQ being the interaction energy between the nuclear quadrupole moment and the electric field gradient. We have carried out measurements of the 59Co-NMR Knight shift (K) for a single crystal in the higher-nuQ3 superconducting phase and found that K begins to decrease with decreasing T at Tc for both magnetic field directions parallel and perpendicular to CoO2-planes. The result indicates together with the previous ones that the superconducting pairs are in the spin-singlet state in both phases, excluding the possibility of the spin-triplet superconductivity in this phase diagram. The superconductivity of this system spreads over the wide nuQ3 regions, but is suppressed in the narrow region located at the middle point of the region possibly due to charge instability.

cond-mat.supr-con↗

arXiv2007

Relationship between Magnetic Structure and Ferroelectricity of LiVCuO4

Neutron scattering studies and measurements of the dielectric susceptibility and ferroelectric polarization P have been carried out in various magnetic fields H for single-crystal samples of the multiferroic system LiVCuO4 with quasi one-dimensional spin 1/2 Cu2+ chains formed of edge-sharing CuO4 square planes, and the relationship between the magnetic structure and ferroelectricity has been studied. The ferroelectric polarization is significantly suppressed by the magnetic field H above 2 T applied along a and b axes. The helical magnetic structure with the helical axis parallel to c has been confirmed in H=0, and for H//a, the spin flop transition takes place at H=2 T with increasing H, where the helical axis changes to the direction parallel to H. The ferroelectric polarization along a at H=0 is found to be proportional to the neutron magnetic scattering intensity, indicating that the magnetic order is closely related to the appearance of the ferroelectricity. The relationship between the magnetic structure and ferroelectricity of LiVCuO4 is discussed by considering the existing theories.

cond-mat.str-el↗

arXiv2006

Successive Magnetic Transitions of the Kagome Staircase Compound Co3V2O8 Studied in Various Magnetic Fields

For the spin-3/2 kagome staircase system Co3V2O8, magnetic field (H)-temperature (T) phase diagrams have been constructed for the fields along three principal directions up to 5 T, using results of various macroscopic measurements on single crystal samples and also using neutron diffraction data taken on both powder and single crystal samples under H along c. In zero magnetic field, the system exhibits three transitions at temperatures Tc1~11.2 K, Tc2~8.8 K and Tc3~ (6.0-7.0) K. The single crystal data present clear evidence for the noncollinear nature of the magnetic structures in all magnetically ordered phases below Tc1. The sinusoidal nature of the incommensurate modulation of the ordered moment reported in the former work has been confirmed between Tc1 and Tc2, that is, no higher harmonics of the modulation have been detected even for the present large single crystal. Even in the phase of commensurate modulation between Tc2 and Tc3, we have not detected any higher harmonics of the modulation. The phase diagrams show that the magnetically ordered phases sensitively change to other phases with H, indicating that the geometrical frustration inherent in this system is important for the determination of the phase diagram. No evidence for ferroelectric transitions has been observed in the measurements of the dielectric constant applying the electric fields along three crystallographic axes, a, b and c. Only small dielectric anomalies closely connected with the magnetic phase transitions have been found.

cond-mat.str-el↗

arXiv2006

Studies of the Anomalous Hall Effect and Magnetic Structure of Nd2Mo2O7 -Test of the Chirality Mechanism-

Neutron scattering studies have been carried out under the magnetic fields H//[0_11] and H//[001] on a single crystal of Nd2Mo2O7, whose Hall resistivity(rhoH) exhibits quite unusual H- and temperature(T)-dependences. Material parameters such as the single ion anisotropies of the Mo- and Nd- moments and exchange coupling constants among the Mo-Mo, Mo-Nd and Nd-Nd moments, have been determined to reproduce various kinds of experimental data taken as a function of H and T. For example, the neutron Bragg intensities, magnetization curves and the magnetic specific heats have been reproduced by the common parameters. By using the magnetic structure reproduced by these parameters, the spin chirality (chi) of Mo spins or the fictitious magnetic flux Phi proportional to chi has been calculated as a function of H and T by using equation chi= , where the bracket means the statistical average. (Note that we do not use the equation chi= x , because the local nature of the chirality should be correctly considered.) Comparing the calculated results with the observed rhoH, we can conclude that the unusual behavior of rH cannot be understood consistently only by the chirality mechanism.

cond-mat.str-el↗

arXiv2006

Magnetic properties of superconducting cobalt oxides NaxCoO2.yH2O

Studies of the NMR Knight shift K of Na0.3CoO2.yH2O have been carried out in detail. The suppression of K by the occurrence of the superconductivity reported previously by the present authors in both magnetic field directions perpendicular and parallel to the c axis has been confirmed, indicating that the Cooper pairs are in the singlet state. The anisotropy of the suppression amplitudes is consistent with the anisotropy of the hyperfine coupling constant Aspin estimated by the K-kai plot. It has also been found that even samples, which do not exhibit a significant amount of the Curie-Weiss-like increase of the uniform magnetic susceptibility kai with decreasing temperature T, exhibit the superconducting transition, which indicates that the superconducting Na0.3CoO2.1.3H2O is not necessarily be in the proximity region of the ferromagnetic phase. It has also been confirmed that the superconducting transition temperature Tc of the samples prepared by mixing Na0.7CoO2, H2O (or D2O) and bromine for 4 h, and separating the resultant powder from the solution by filtration, depends on the elapsed time t after filtration. For these samples, the Curie constant of kai estimated at low temperatures increases with t, supporting the idea that the number of the lattice imperfection possibly due to the oxygen-vacancy-formation increases with t. Even for such samples, the superconductivity appears, which seems to exclude the possibility of the anisotropic superconducting order parameter. These results are favorable for the full-gapped superconductivity.

cond-mat.supr-con↗

arXiv2006

Search for Magnetic Excitation Spectra of NaxCoO2.yD2O -Neutron Scattering-

Neutron inelastic scattering studies on aligned crystals of NaxCoO2.yD2O have been carried out. Significant scattering has been observed for phonons originating from the vibrations of the short-range ordered D2O molecules. No firm evidence for the magnetic fluctuations has been found for the NaxCoO2.yD2O, while the very weak intensity modulation observed in the j scans along (1/2, 1/2, j) suggests the existence of the antiferromagnetic fluctuations in the non-deuterated parts of the sample, the volume fraction of which is estimated to be about 26 %. The present results indicate that magnetic fluctuations, especially ferromagnetic ones are rather weak, in the deuterated system, which is consistent with the results of the NMR Knight shift reported by the present authors' group. We discuss the short-range ordered state of D2O, too by analyzing the observed diffuse scattering.

cond-mat.supr-con↗

arXiv2006

Magnetic Structure of YBaCo4O7 with Kagome- and Triangular-Lattices

Neutron diffraction studies have been carried out in the temperature(T) range between 10 K and 130 K on a single crystal of YBaCo4O7, which has the stacking of kagome and triangular lattices of CoO4 tetrahedra along the c-axis. Structural transitions have been found at two temperatures Tc1~70 K and Tc2~105 K. With decreasing T, magnetic order appears along with the transition at Tc2, but it does not grow to an ideal long-range order even at the lowest T studied (~10 K). Two groups of magnetic diffuse reflections, which originate from the Co-moments of the kagome and triangular lattices, are observed separately in the reciprocal space. With further decreasing T, the growth rates of the intensities of these two are enhanced by another transition at Tc1. These magnetic behaviors seem to be related to the release of the geometrical frustration. At 10 K, the patterns of the magnetic short-range order have been determined for both sites of the kagome and triangular lattices.

cond-mat.str-el↗

arXiv2005

Magnetic Correlation of NaxCoO2 and Successive Phase Transitions of Na0.5CoO2 -NMR and Neutron Diffraction Studies-

Data taken by various kinds of methods including NMR/NQR and neutron scattering are presented for NaxCoO2, in particular, for Na0.5CoO2. Attentions have also been paid to the x dependence of the electronic nature. The pseudo-gap-like behavior observed in the region of x<xc~0.6 is emphasized. For samples with x<0.6, signals from two kinds of Co sites with different quadrupole frequencies nQ, ~4.1 MHz and ~2.8 MHz were observed, and the former value of nQ, which is nearly equal to that of the superconducting system Na0.3CoO2.yH2O, becomes dominant with decreasing x to 0.3. For Na0.5CoO2, the Co sites with the larger nQ value have the larger magnetic moments. They align antiferromagnetically at Tc1~87 K with their direction within the plane, while the Co sites with the smaller nQ value have the smaller moments. They align along the direction parallel to the c axis. The ordered moments at the two distinct sites exhibit the same T dependences, indicating that the existence of these sites is not due to a macroscopic phase separation. Although we have observed the splitting of the zero-field NMR signals from the Co sites with the smaller nQ value at the second transition temperature Tc2~53 K, any other anomaly of the ordered magnetization has not been observed there in the T dependences of the neutron and the NMR data. As x decreases, the two-dimensional nature of the electrons is enhanced and the antiferromagnetic correlation increases. Based on these results, the superconducting pair state in Na0.3CoO2.yH2O is discussed.

cond-mat.str-el↗

arXiv2005

59Co-NMR Knight Shift of Aligned Crystals and Polycrystalline Samples of Superconducting Na0.3CoO2.1.3H2O

Temperature (T) dependence of 59Co-NMR Knight shifts K of Na0.3CoO2.1.3H2O has been studied, where samples of randomly oriented powder and aligned crystals have been used for the applied magnetic fields H // ab plane and H //c-axis, respectively. For both directions of H, the shift K decreases below the superconducting transition temperature Tc(H) with decreasing T, indicating that the superconducting electron pairs are in the singlet state. The upper critical fields Hc2(T) determined from the K(H)-T curves are found to be consistent with the values reported by the resistivity measurements for both directions of H.

cond-mat.supr-con↗