Kuwahara Hideki

We have grown single crystals of mixed-valence pseudobrookite titanate MgTi2O5-Ti3O5 systems (Mg1-xTi2+xO5) and have systematically investigated their physical properties as a novel canonical mixed-valence system which shows a successive change from the nonmagnetic band insulating phase (MgTi2O5) to the charge-ordered spin-singlet dimer phase (Ti3O5). In comparison with Al1-xTi2+xO5 [R. Takahama, Phys. Rev. Mater. 4, 074401 (2020)2475-995310.1103/PhysRevMaterials.4.074401], we have clarified that, in the lower x region of Mg1-xTi2+xO5, the physical properties are greatly affected by the change of electron density with a variation of x and we have found that the doped electrons enhance the optical conductivity along a Ti-Ti ladder direction. However, we have also clarified that, in the higher x region, almost all Ti3+ ions produced by an increase in x contribute not to an increase in itinerant electrons but to a development of Ti3+-Ti3+ dimer correlation. These observations indicate a crossover from the conductance along the ladder direction to a nontrivial conductance under the background of the dimer correlation.

Barium tantalate with a tetragonal tungsten bronze structure, Ba3Ta5O15, and compounds where Ba is partly substituted by various rare earths (R), Ba3-xRxTa5O15, were synthesized as single crystals, and their transport, magnetic, and magnetotransport properties were investigated. It was found that the compounds with Eu substitution show a relatively large negative magnetoresistance arising from the interaction between the conduction electrons in the Ta 5d orbital and the localized spins in the Eu 4f orbital.

Preparation conditions of titanium oxide (TiO₂) powders were examined by the hydrolysis of titanium potassium oxalate (K₂TiO(C₂O₄)₂), through the homogeneous precipitation method (80oC for 24 h) and hydrothermal treatment (160 or 170oC for 1 h). According to the Rietveld analysis, almost a single phase of anatase TiO₂ could be obtained by the hydrothermal treatment at 160oC for 1 h, followed by the heating at 900oC for 10 min in air. The molar ratio of anatase to rutile TiO₂ was found to be controlled by optimizing the hydrothermal conditions in the solution and the heating conditions in air for the photocatalytic activity.

In BaV10O15 with quasitriangular V lattices, we found unique superstructures characterized by (qq0)-type peaks in the momentum space, where q varies continuously with temperature between q = 0 and 1/3 and locks at 1/3. Such a threefold superstructure is likely caused by the dense arrangement of the V trimers associated with the orbital ordering of V. The results indicate that the superstructure of the V trimers, each of which is also a superstructure, gives rise to the ordered phases with peculiar characteristics.

The electron-doped SrTiO3 exhibits good thermoelectric properties, which makes this material a promising candidate of an n-type oxide thermoelectric device. Recent studies indicated that only a few percent co-doping of La and Mn in SrTiO3 substantially reduces the thermal conductivity, thereby greatly improving the thermoelectric figure of merit at room temperature. Our time-of-flight neutron scattering studies revealed that by doping both La and Mn into SrTiO3, the inelastic scattering spectrum shows a momentum-independent increase in the low-energy spectral weight approximately below 10 meV. The increase in the low-energy spectral weight exhibits a clear correlation with thermal conductivity. The correlation is attributed to dynamical and local structural fluctuations caused by the Jahn-Teller instability in Mn3+ ions coupled with the incipient ferroelectric nature of SrTiO3, as the origin of the low thermal conductivity.

© Published under licence by IOP Publishing Ltd. We compare the magnetic (Ni) substitution effects in Cu3Mo2O9 to the nonmagnetic (Zn) ones. Changes in the lattice constants show similar behaviours for both impurity substituted samples. In specific heat measurements, we observe the anomalous specific heat at the phase transition temperature T N. T N in the Ni substituted sample increases to 8.8 K; while T N in the Zn substituted sample decreases to 4.0 K. In magnetization measurement on the Ni substituted sample, we obtain the Van-Vleck type constant magnetization. We suggest the existence of the single-ion anisotropy of the substituted Ni2+ ions, which is a possible candidate for the origin of the increment of T N in the Ni substituted sample.

We studied the physical properties of Ba1-xSrxV13O18 single crystals, which exhibit three different phases caused by the orbital and charge degrees of freedom in V t2g states. We found from the magnetic susceptibility and the NMR measurement that the low-temperature phase of this series of compounds is characterized by the spin-singlet state. We also found from the electrical resistivity and optical conductivity spectra that these compounds become heavily anisotropic in the low-temperature phase with V trimerization, indicating that the orbital ordering of V t2g states causes the anisotropy of the electronic structure of this series of compounds.

We have investigated the magnetic properties of EuTi1-xAlxO3 with 0≤x≤1 and established a detailed magnetic phase diagram. Substitution of Ti4+ with Al3+, which oxidizes magnetic Eu2+ (J=7/2) into nonmagnetic Eu3+ (J=0), turns the antiferromagnetic insulating ground state (x=0) into the ferromagnetic (FM) insulating one (0.10≤x≤0.50). The Eu2+/Eu3+ mixed valence is indispensable for the FM behavior. In 0.10≤x≤0.33, the Curie temperature TC has a plateau at ∼4 K, and the FM correlation is most enhanced around x=0.25. In 0.33≤x≤1,Al3+ substitution suppresses the FM correlation due to the dilution of magnetic Eu2+ ions. Our present results would provide useful information for magnetoelectric phase control of EuTiO3-based compounds.

We found large changes in the low-temperature specific heat (low-T C) in the lightly Mn-substituted electron-doped perovskites Sr0.95La0.05Ti1−yMnzO3 with y = 0.02 and 0.04 by applying magnetic fields up to 9 T. The changes in the low-T C are qualitatively well explained by the Schottky specific heat (CSch) of localized spins of the Mn 3d electrons in weak internal magnetic fields via itinerant electrons. However, the actual numbers of localized spins estimated from CSch are about 30% smaller than the expected values. Part of the localized spins of the Mn 3d electrons may disappear due to Kondo coupling with the itinerant electrons.

NMR study has been performed on the quasi-one dimensional antiferromagnet BaCo2Si2O7, in which three inequivalent CoO4 tetrahedra, slightly tilted from one another, are linked at vertices to form a chain along the easy-axis c. In its magnetically ordered state at 3.6 K, 59Co-NMR spectra were measured in the field region up to 12 T. From the observed three distinct sets of quadrupolar-split peak groups, three different hyperfine fields at Co sites were determined. They are slightly tilted from c-axis and are all different both in magnitude and direction, which is in agreement with the non-collinear magnetic structure.

The two-legged spin ladder Cu(CO3)0.5(ClO4)(H2O)0.5(NH3)2.5 consists of a rung formed by two Cu(II)’s and of a spacing molecule CO32− between each two rungs. The non-centrosymmetric shape of CO32− molecule brings a slight bond alternation along the leg, and hence the system can be considered as an alternating spin chain, which is confirmed so far by the temperature dependence of magnetic susceptibility. In order to investigate its spin state at low temperatures, we have performed experiments of 1H-NMR, magnetization and specific heat under wide range of magnetic field, and have found the critical diverging of longitudinal relaxation rate 1/T1, the spectral broadening and the lambda-type anomaly in specific heat at TN≃ 3.4 K, indicating the existence of long range magnetic order. In paramagnetic state well above TN, 1/T1 showed a power-law temperature dependence, suggesting the realization of Tomonaga Luttinger liquid state.

We studied how Mn substitution affects the thermoelectric properties and thermal excitations of the electron-doped perovskite Sr1-xLaxTiO3 by measuring its electrical and thermal transport properties, magnetization, specific heat, and inelastic neutron scattering. Slight Mn substitution with the lattice defects enhanced the Seebeck coefficient, perhaps because of coupling between itinerant electrons and localized spins or between itinerant electrons and local lattice distortion around Mn3+ ions, while it enhanced anharmonic lattice vibrations, which effectively suppressed thermal conductivity in a state of high electrical conductivity. Consequently, slight Mn substitution increased the dimensionless thermoelectric figure of merit for Sr1-xLaxTiO3 near room temperature.

Strain, magnetization, dielectric relaxation, and unpolarized and polarized neutron diffraction measurements were performed to study the magnetic and structural properties of spinel Co1-xV2+xO4. The strain measurement indicates that, upon cooling, Delta L/L in the order of similar to 10(-4) starts increasing below T-C, becomes maximum at T-max, and then decreases and changes its sign at T*. Neutron measurements indicate that a collinear ferrimagnetic order develops below T-C and upon further cooling noncollinear ferrimagnetic ordering occurs below T-max. At low temperatures, the dielectric constant exhibits a frequency dependence, indicating slow dynamics. These results indicate the existence of an orbital glassy state at low temperatures in this nearly metallic frustrated magnet.

© 2015 The Authors. Published by Elsevier B.V. We report the substitution effects of W6+ ions on multiferroic material Cu3Mo2O9 having a dis- torted tetrahedral quasi-one-dimensional spin structure. In Cu3Mo2O9, the weak ferromagnetic component of the spin moments and the ferroelectricity coexist below TN = 7.9K. In case of Cu3(Mo,W)2O9, the nonmagnetic Mo6+ ions are replaced with the nonmagnetic W6+ ions. We study the specific heat, magnetization, and dielectric constants, and show the magnetic-field- temperature phase diagrams in the single crystals with various W concentrations. We compare the phase diagram of Cu3(Mo,W)2O9 to that of (Cu,Zn)3Mo2O9 and discuss the similarity and the difference between the direct and the indirect substitution effects. The efficiency of the W substitution is much weaker than that of the Zn substitution.

© 2015 The Authors. Published by Elsevier B.V. It has been demonstrated that the muon spin rotation measurements under electric field give helpful information about the electrically induced magnetism, e.g., the cross correlation effects in multiferroic materials. We have developed an electric-field application system up to 500V for the Dolly spectrometer at the Paul Scherrer Institute. We report the electric-field effects on the μSR spectrum in the multiferroic material Cu3Mo2O9, where a slightly canted antiferromagnetic long-range order appears together with the ferroelectricity below 8K. In the muon-spin rotation spectrum at 1.5K, two kinds of the internal magnetic fields are clearly observed as a beating oscillation. The muon-spin spectrum depends on the electric fields along the c axis of the crystal along which the spontaneous electric polarization appears. From the fitting of the spectra in time and frequency domains, it is shown that the observation of the electric-field dependence on the muon-spin spectra clearly indicates a change of the internal magnetic fields induced by the application of the external electric fields. We propose a model with one muon-stopping site which explains the observed spectra qualitatively. This model is based on the magnetic excitations in Cu3Mo2O9 obtained from the inelastic neutron-scattering experiments.

Numerous attempts have been made to realize crossed coupling between ferroelectricity and magnetism in multiferroic materials at room temperature. BiFeO3 is the most extensively studied multiferroic material that shows multiferroicity at temperatures significantly above room temperature. Here we present high-field experiments on high-quality mono-domain BiFeO3 crystals reveal substantial electric polarization orthogonal to the widely recognized one along the trigonal c axis. This novel polarization appears to couple with the domains of the cycloidal spin order and, hence, can be controlled using magnetic fields. The transverse polarization shows the non-volatile memory effect at least up to 300 K.

We have investigated effects of impurity substitution of Fe for Co sites on magnetic and dielectric properties in CaBa(Co1-xFex)(4)O-7 crystals in magnetic fields. The non-doped compound, CaBaCo4O7 with the polar space group Pbn21, shows a large change of electric polarization AP along the c axis below a ferrimagnetic transition temperature. With increasing x, the transition temperatures for the ferrimagnetic and ferroelectric-like phases are shifted towards lower and these phases are completely collapsed above x=1.56%. We have also investigated the magnetic field effects for CaBa(Co1-xFex)407 crystal with a doping level x=0.5 where the ferrimagnetic and ferroelectric-like phases sustain. Then a relatively large magnetocapacitance, Delta epsilon(H)/epsilon(0) [epsilon(H) - epsilon(0)]/epsilon(0), of 45% at 58 K was observed, which is not significantly reduced from the parent compound.

The temperature dependence of the complex dielectric constant of RbCoBr3 along the hexagonal c-axis was measured in the frequency range from 1 kHz to 1MHz under magnetic fields of up to 12 T applied parallel to the c-axis. On the basis of the observed dielectric behavior, a probable interplay between lattice and spin systems, and appropriate crystal structures for the three phases below the upper Néel point were deduced. A broad peak of the real part ε{lunate}′c(T) at approximately 30K was found to exhibit frequency dispersion with two kinds of relaxation time. These relaxation times seem to correspond closely to the magnetic relaxation times ascribed to the domain-boundary motion in Ising spin chains. In the presence of magnetic fields above 8 T, this peak split into two subpeaks. This magnetic field dependence of ε{lunate}′c(T) reflects the suppression of antiferromagnetic ordering under the magnetic field. These findings imply the correlation between the ferroelectric and antiferromagnetic phase transitions in RbCoBr3.

The magnetic and dielectric properties of åkermanite Ca2CoSi2O7 single crystals were investigated in pulsed high magnetic fields. In magnetic fields along the c-axis, Ca2CoSi2O7 shows a magnetization plateau in a wide field range below saturation. Magnetization processes for fields along the a- and b-axes show multiple anomalies and different traces, indicating the breaking of four-fold symmetry. Measurements of the magnetoelectric effect exhibit changes in electric polarization depending on changes in the spin system. The experimentally determined quadratic magnetoelectric tensor is consistent with that expected in the crystal symmetry of orthorhombic P21212.

We studied single crystals of BaV13O18, in which V ions with an average valence of + 2.62 (3d(2.38)) form a quasi-fcc lattice. We found that the ground state of the stoichiometric crystal is insulating, whereas that of the off-stoichiometric crystal is metallic with a large effective mass. This metallic state can be explained by the Kondo coupling between the magnetic moment existing at the lone V ion and itinerant carriers in the V tetramers.

We have investigated the Mn-site substitution effect in Nd0.45Sr0.55MnO3 single crystal, which has an A-type layered antiferromagnetic (A-AFM) phase with the 3d(x2-y2)-type orbital-order. Substitution of Fe or Ga for Mn-site suppresses both the A-AFM order and competing ferromagnetic (FM) correlation, whereas Cr substitution suppresses only the A-AFM order but reactivates the underlying FM correlation via double-exchange mechanism along the AFM coupled c-direction. In Nd0.45Sr0.55Mn0.95Cr0.05O3, the A-AFM state with the orbital-order is changed into the orbital-disordered three-dimensional FM metallic state by applying magnetic field of mu H-0 = 12 T, which is much smaller than that of the parent compound Nd0.45Sr0.55MnO3. (C) 2014 Author(s).

The luminescence properties of Eu2+-doped barium calcium silicon oxynitrides (general formula: Ba4-xCaxSi 6ON10: Eu2+ (x = 1.8 ∼ 2.2)) were examined, together with the preparation conditions for the solid state reaction. Firstly, the stoichiometric amounts of Ba, Ca3N2, SiO2 and Si3N4 were mixed with Eu2O3 (activator the molar ratios of Eu to (Ca + Ba + Eu) (= m 0.05 to 1.0)) for the preparation of Ba1.8Ca22Si6ON10: Eu2+, Ba2.0Ca2.0Si6ON10: Eu2+ and Ba2.2Ca1.8Si6ON10: Eu2+, and then the mixtures were heated at a temperature between 1300 and 1550°C for 10 h in N2 atmosphere. The single phase compounds, which were prepared by the heating at and above 1350°C, emitted the red color light whose wavelength could be changed from 638 to 653 nm with increasing m value from 0.05 to 1.00, due to the excitation at the wavelength of 528 nm. © (2014) Trans Tech Publications, Switzerland.

We have tried to improve The n-type thermoelectric properties of The electron- doped Perovskite Sr1-xLaxTiO3 by a Mn substitution. The 1 ∼ 2 % Mn substitution enhances The Seebeck coefficient (S) and reduces The thermal conductivity (κ) by about 50 % at room temperature (RT) without largely increasing The resistivity for The 5 % electron-doped SrTiO3. Consequently, The power factor at RT keeps a large value comparable to that of Bi2Te3 and The dimensionless figure-of-merits at RT increases twofold by The slight Mn substitution. Such a large reduction of κ at RT is perhaps due to The effect of Jahn-Teller active Mn3+ ions, around which dynamical local lattice distortion may occur.

We have investigated the thermoelectric (TE) properties for single crystals of the perovskites Sr1-x Ca (x) Ti1-y Nb (y) O-3 for 0 a parts per thousand currency sign x a parts per thousand currency sign 0.4 and 0 a parts per thousand currency sign y a parts per thousand currency sign 0.03 at temperatures below room temperature (RT). We found that SrTi0.99Nb0.01O3 showed a large power factor at low temperatures (PF = 50 A mu W/K(2)cm at 100 K similar to 90 A mu W/K(2)cm at 50 K) and the largest dimensionless TE figure-ofmerit at temperatures below 40 K (ZT similar to 0.07) among the reported materials. Such a large low-temperature TE response around a carrier concentration of 10(20) cm(-3) is due to a distinct phonon drag effect. We also found that the Ca2+ substitution for Sr2+ increased the ZT at 300 K for Sr1-x Ca (x) Ti0.97Nb0.03O3 from 0.08 to 0.105. The enhancement of the ZT around RT originates both from a large reduction of a thermal conductivity due to a randomness introduced into the crystal structure and from an unexpected enhancement of a Seebeck coefficient.

We have systematically investigated the magnetic and the dielectric properties of Eu-0.595(Y1-x Ho (x) )(0.405)MnO3 (0 a parts per thousand currency sign x a parts per thousand currency sign 1) single crystals with fine tunning of 4f magnetic moments while keeping the average size of the rare-earth (R) site the same. In the x = 0.3 crystal, the ferroelectric polarization along the c axis (P (c) ) emerges at 25 K; then, P (c) is suppressed, and P (a) appears simultaneously with decreasing temperature. On further decreasing temperature, P (a) disappears and P (c) appears again. The observed reentrant behavior of the P (c) phase indicates that the P (a) and the P (c) phases are strongly competing with each other.

We study the effects of Zn substitution on single crystals of the low-dimensional multiferroic material Cu3Mo2O9. We report the magnetic field-temperature phase diagram obtained from the temperature and the magnetic field dependences of the specific heat, the dielectric constant, the differential magnetization, and the electric polarization-electric field curve in Cu2.85Zn0.15Mo2O9. We obtain three phases: a paraelectric and a ferroelectric phase, and a ferroelectric phase with a large coercive electric field. The origin of the last one is discussed based on the charge redistribution effect on a half-filled Mott insulator. In this phase, the Zn substitution induces a robust localized ferroelectric polarization that increases the coercive electric field.

We have investigated the magnetic and the dielectric properties of Ayenkermanite Sr2CoSi2O7 and Sr2MnSi2O7 single crystals. Sr2CoSi2O7 shows a canted antiferromagnetism with a large magnetic anisotropy at temperatures below 7 K. By applying a magnetic field along the [110] direction, an electric polarization emerges along the c axis at temperatures below the magnetic transition temperature. In high magnetic fields, the saturation magnetization becomes enhanced only when the electric polarization appears. In contrast, the magnetization of Sr2MnSi2O7 is isotropic at all temperatures, and its magnetic-field-induced electric polarization is very small. These results suggest that the electronic configuration of the transition-metal ion is of crucial importance for the appearance of electric polarization.

We have investigated magnetic and dielectric properties of åkermanite Sr2CoSi2O7 single crystal in which the orbital hybridization is controllable by external magnetic fields. Sr 2CoSi2O7 shows the electric polarization that is originated from asymmetric orbital hybridization induced by magnetic field. This induced electric polarization was observed even at temperatures considerably higher than the magnetic transition temperature. The polarization data obtained for the paramagnetic phase are scaled with the square of the induced magnetization. By the discussion of the crystal symmetry, the space group of a polar state of åkermanite materials is probably Cmm2. © 2012 Springer Science+Business Media New York.

The optimum conditions for the fabrication of transparent glass body in the ZnO-B2O3 (ZB) and ZnO-B2O3-Bi 2O3 (ZBB) systems were examined by a pressureless firing and subsequent oxygen-supplied hot isostatic pressing (HIP). The pressurelessly-fired opaque ZB glass body possessed the maximum relative density (99.1%) when fired at 620°C for 15 min in air, and was hot isostatically pressed at 630°C for 12 h under the pressure of 150MPa (80%Ar-20%O 2 atmosphere) to eliminate the residual pores the light transmittance of this glass body was in the range of 35 to 51% (380 to 800 nm). In order to fabricate the transparent glass body at lower temperature, the fabrication conditions of ZBB glass body were examined. The pressurelessly-fired ZBB glass body possessed a maximum relative density (98.8%) when fired at 500°C for 2 h in O2 atmosphere and hot isostatically pressed at 490°C for 12 h the transmittance of the transparent ZBB glass body was in the range of 48 to 58% (500 to 800 nm). © Published under licence by IOP Publishing Ltd.