桑原 英樹

We have investigated anisotropic properties of magnetism and dielectricity in CaBaCo4O7 single crystal, which has layered structure consisting of CoO4 tetrahedra. The crystallographic symmetry of CaBaCo4O7 is Pbn21 at room temperature, which breaks the inversion symmetry. The magnetic moments along the a- and b-axes rise at 64 K, suggesting a weak ferromagnetism. We have also observed an increase of electric polarization and a peak of dielectric constant along the b- and c-axes at the magnetic transition temperature. In addition, the large magnetocapacitance effect was observed near above the transition temperature: ΔεC(HB)/εC(0) ≡ [ C(HB) - εC(0)]/εC(0) reaches over 50 % at 68 K.

We have investigated the magnetic and dielectric properties of orthorhombic Eu1-yYyMnO3 (0 ≤ y ≤ 0.6) single crystals without the presence of the 4f magnetic moments of the rare-earth ions. In y ≥ 0.2, the magnetic-structure driven ferroelectricity is observed. The ferroelectric transition temperature is steeply reducing with increasing y. In y ≥ 0.52, two ferroelectric phases (P ∥ a and P ∥ c) are coexistent at low temperatures. In these phases, ferroelectricity has different origin, which is evidenced by the distinctive poling-electric-field dependence of electric polarization. Namely, the electric polarization along the c-axis (Pc) is easily saturated by a poling electric field, therefore Pc is caused by the bc-spiral antiferromagnetic order. On the other hand, the electric polarization along the a-axis (Pa) is probably attributed to the collinear E-type antiferromagnetic order, because Pa is unsaturated even in a poling field of 106 V/m. © 2011 The Physical Society of Japan.

We have investigated electron- and hole-doping effects on A-site ordered perovskite manganite NdBaMn2O6, which has the A-type (layered) antiferromagnetic (AFM) ground state. Electrons (holes) are introduced by partial substitution of Ba2+ (Nd3+) with Nd3+ (Ba2+). Electron-doping generates ferromagnetic (FM) clusters in the A-type AFM matrix. With increasing the electron- doping level, the volume fraction of the FM phase or the number of the FM clusters is abruptly increasing. In contrast, the A-type AFM phase is robust against the hole-doping, and no FM correlation is observed in the hole-doped NdBaMn2O6.

The conditions for the preparation of regular-shaped Eu 2+-doped calcium silicon nitride ((Ca 0.99Eu 0.01) 2Si 5N 8) powder were examined by the carbothermal reduction and nitridation (CRN) of oxide/nitride powder prepared by ultrasonic spray pyrolysis. Two kinds of silicon sources, i.e., (i) colloidal silica (CS) and (ii) silicon nitride (Si 3N 4 SN), were used for the spray pyrolysis. Firstly, the solution containing 0.057 mol·dm -3 Ca(NO 3) 2, 0.140 mol·dm -3CS and 5.71 × 10 -4 mo·dm -3Eu(NO 3) 3was spray-pyrolyzed at 600°C, and the resulting powder was calcined at 800°C for 10 min in air. The single phase but agglomerated particles of (Ca 0.99Eu 0.01) 2Si 5N 8 were obtained when the CRN (the relative carbon amount to the stoichiometric amount (m)\\ 1.5) of spray-pyrolyzed/calcined oxide powder was conducted at 1300°C for 2 h in nitrogen (N 2) atmosphere. Secondly, the solution containing 0.0285 mol·dm -3Ca(NO 3) 2, 0.070 mol·dm -3SN and 2.855 × 10 -4 mol·dm -3Eu(NO 3) 3 (one-half of the solution concentration compared to the case of CS utilization) was spray-pyrolyzed, and the CRN of spray-pyrolyzed oxide/nitride powder was conducted at 1500°C for 2 h to obtain (Ca 0.99Eu 0.01Si 5N 8 in N 2 atmosphere. The microscopic observation showed that this powder was composed of regular-shaped particles with sizes of approximately 2-3 μm. The excitation and emission peaks of this powder appeared at around 370 and 590 nm, respectively, showing the emission of orange light.

We have established detailed magnetoelectric phase diagrams of (Eu0.595Y0.405)(1-x)TbxMnO3 (0 <= x <= 1) and (Eu,Y)(1-x)GdxMnO3 (0 <= x <= 0.69), whose average ionic radii of R-site (R: rare earth) cations are equal to that of Tb3+, in order to reveal the effect of rare earth 4f magnetic moments on the magnetoelectric properties. In spite of the same R-site ionic radii, the magnetoelectric properties of the two systems are remarkably different from each other. A small amount of Tb substitution on R sites (x similar to 0.2) totally destroys ferroelectric polarization along the a-axis (P-a), and an increase in Tb concentration stabilizes the P-c phase. On the other hand, Gd substitution (x similar to 0.2) extinguishes the P-c phase, and slightly suppresses the P-a phase. These results demonstrate that the magnetoelectric properties of RMnO3 strongly depend on the characteristics of the rare earth 4f moments.

We developed a high-speed polarizing microscope imaging system combined with a 37 T pulse magnet. With utilizing this system, we can visualize some kinds of structural transitions induced by high magnetic fields. The efficiency of this system was tested through observation of the field-induced collapse of charge-orbital ordering (COO) in mixed-valent manganites. In perovskite-type Nd(1/2)Sr(1/2)MnO(3), quantitative analyses of the obtained polarizing microscope images clearly show the discontinuous change in birefringence accompanied with hysteresis, which are characteristic of the first order transitions. This observation of field-induced melting of the COO was also successful in a layered manganite La(1/2)Sr(3/2)MnO(4) in which changes in magnetization and magnetoresistance at the transition are less clear. The present success in observation of the melting of the COO demonstrates the potential applicability of this system to visualize other kind of structural transitions even in a tiny piece of crystal.

A high-speed polarizing microscope system combined with a 37 T pulse magnet has been developed. This system was applied to successfully visualize the field-induced collapse of charge-orbital ordering in a layered manganite La1/2Sr3/2MnO4. Quantitative analyses of the obtained polarizing microscope images provided clear evidence of this transition in contrast to rather moderate changes in magnetization and magnetoresistance. The ability of this system to carry out quantitative analysis was further tested through the observation of Faraday rotation in a Tb3Ga5O12 crystal. The Verdet constant determined from the polarizing images is in reasonable agreement with that in literature. Local intensity analyses of the images indicate that we can investigate magneto-optical signals within an accuracy of 0.85% in an area of 9.6x9.6 mu m(2).

We have investigated the magnetoelectric properties of (Ca 1-xSrx)2CoSi2O7 (0 ≤ x ≤ 1) crystals with a quasi-two-dimensional structure. In Ca 2CoSi2O7 (x =0), a canted antiferromagnetic transition occurs at 5.6 K. The transition temperature TN is increasing with increasing Sr concentration, and the rises of the magnetization and dielectric constant become larger. Since the dielectric constant shows large change at TN and the magnetocapacitance effect is observed below TN, a coupling between the magnetism and dielectricity is strong in (Ca1-xSrx)2CoSi2O7. The positive magnetocapacitance is reduced by Sr substitution, and is not observed in x ≥ 0.5. Namely, the compound of x ≥ 0.5 does not show the magnetic-field-induced electric polarization. On the other hand, the negative magnetocapacitance is enhanced by Sr substitution. © 2010 IOP Publishing Ltd.

We have investigated the R-site randomness effect of R/Ba-ordered RBaMn2O6 (R = rare earth) by using Y1-yLa yBaMn2O6 (0 ≤ y ≤ 1) in which R (Y,La) and Ba are regularly arranged while Y and La randomly occupy the R-site. YBaMn2O6 (y = 0) undergoes charge/orbital ordering (CO) transition at TCO 500 K while LaBaMn2O6 (y = 1) shows ferromagnetic metallic (FM) behavior below 350 K. In 0 ≤ y ≤ 0.6, TCO decreases with an increase in y. In 0.6 ≤ y ≤ 0.8, R-site randomness causes strong phase separation tendencies among the FM, antiferromagnetic (AFM), and CO insulating (COI) states. The large magnetoresistance is observed in the phase separated region. © 2010 IOP Publishing Ltd.

We demonstrate local crystal structure analysis based on annular dark-field (ADF) imaging in scanning transmission electron microscopy (STEM). Using a stabilized STEM instrument and customized software, we first realize high accuracy of elemental discrimination and atom-position determination with a 10-pm-order accuracy, which can reveal major cation displacements associated with a variety of material properties, e.g. ferroelectricity and colossal magnetoresistivity. A-site ordered/disordered perovskite manganites Tb(0.5)Ba(0.5)MnO(3) are analysed; A-site ordering and a Mn-site displacement of 12 pm are detected in each specific atomic column. This method can be applied to practical and advanced materials, e.g. strongly correlated electron materials.

We have investigated the magnetic and dielectric properties of Ca2CoSi2O7 crystal. The dielectricity and magnetism of Ca2CoSi2O7 are strongly coupled below a canted antiferromagnetic transition temperature (T-N). Magnetic fields induce electric polarization below T-N. Interestingly, the magnetic-field-induced electric polarization is detected even without poling electric fields. Below T-N, a canted antiferromagnetic-paramagnetic transition is induced by magnetic fields. The large magnetocapacitance is observed around T-N. The origin of the large magnetocapacitance is due to the magnetic-field-induced the canted antiferromagnetic-paramagnetic transition.

We have investigated the magnetic and dielectric properties of A(2)CoSi(2)O(7) (A=Ca, Sr, and Ba) crystals with a two-dimensional network of CoO(4) and SiO(4) tetrahedra connected with each other through the corners. In Ca(2)CoSi(2)O(7), a weak ferromagnetic transition occurs at 5.7 K, where the dielectric constant parallel to the c axis shows a concomitant anomaly. The large magnetocapacitance effect is observed below 5.7 K; Delta epsilon(H)/epsilon(0) equivalent to [epsilon(H)-e(0)]/epsilon(0) reaches 13 % at 5.1 K. These results indicate a strong coupling between the magnetism and dielectricity in Ca(2)CoSi(2)O(7). Sr(2)CoSi(2)O(7) shows a similar magnetoelectric behavior to that of Ca(2)CoSi(2)O(7). In contrast, in Ba(2)CoSi(2)O(7), which has the different arrangement of SiO(4) and CoO(4) tetrahedra from that of Ca(2)CoSi(2)O(7), the magnetocapacitance is hardly observed. The key for the magnetocapacitance effect of A(2)CoSi(2)O(7) lies in the quasi-two-dimensional crystal structure.

We have investigated temperature and magnetic-field dependence of dielectric properties in the orthorhombic GdMnO3 single crystal which is located near the phase boundary between the ferroelectric/spiral-antiferromagnetic phase and the paraelectric/A-type-antiferromagnetic one. In this compound, strong phase competition between these two phases results in a unique phase diagram with large temperature and magnetic-field hystereses. Based on the phase diagram, we have successfully demonstrated the persistent and reversible phase switching between them by application of magnetic fields.

We have studied the effects of band filling and bandwidth-control on the chemical potential in perovskite manganites R(1-x)A(x)MnO(3) (R: rare earth, A: alkaline earth) by measurements of core-level photoemission spectra. A suppression of the doping-dependent chemical potential shift was observed in and around the CE-type charge-ordered composition range, indicating that there is charge self-organization such as stripe formation or its fluctuations. As a function of bandwidth, we observed a downward chemical potential shift with increasing bandwidth due to the reduction of the orthorhombic distortion. After subtracting the latter contribution, we found an upward chemical potential shift in the ferromagnetic metallic region 0.3 < x < 0.5, which we attribute to the enhancement of double-exchange interaction involving the Jahn-Teller-split e(g) band.

We have investigated the magnetic and dielectric properties of BaCo2Si2O7 single crystal with a quasi-one-dimensional structure to explore novel magnetoelectric functionality. The Weiss temperatures estimated in paramagnetic region are -20K (H parallel to c) and -74K (H perpendicular to c), indicating the large magnetic anisotropy due to the highly anisotropic structure. The weak ferromagnetic magnetization rises up at 21 K, where the dielectric constant perpendicular to the c-axis decreases concomitantly. In addition, we have observed the magnetocapacitance effect below 21 K. Delta epsilon(H)/epsilon(0) reaches 0.2% at 5.5 K. These results suggest that there exists a substantial coupling between magnetism and dielectricity. (C)2007 Elsevier B.V. All rights reserved.

We have investigated heavily doped Nd1-xSrxMnO3 with 0.55 <= x <= 0.75 in detail. In 0.60 <= x <= 0.72, ferromagnetic correlation accompanying magnetoresistance (MR) is observed at low temperatures. The ferromagnetic correlation and the concomitant MR are most enhanced at x = 0.625 where the 3z(2) - r(2) and x(2) - y(2) orbital orders compete with each other in a bicritical manner. This strongly suggests that the orbital fluctuation arising from the competition gives rise to the ferromagnetic fluctuation closely linked to the MR. (C) 2007 Elsevier B.V. All rights reserved.

We have studied the chemical potential shift as a function of temperature in Nd1-xSrxMnO3 by measurements of core-level photoemission spectra. For ferromagnetic samples (x = 0.4 and 0.45), we observed an unusually large upward chemical potential shift with decreasing temperature in the low-temperature region of the ferromagnetic metallic phase. This can be explained by the double-exchange (DE) mechanism if the e(g) band is split by the dynamical and/or local Jahn-Teller effect. The shift was suppressed near the Curie temperature (TC), which we attribute to the crossover from the DE to lattice-polaron regimes.

We investigate the time-dependent reflectivity of the layered manganite La0.5Sr1.5MnO4 at low temperature. We find that the reflectivity oscillates with an anomalously long period, e.g., several tens of hours. The period varies with wavelength and the character of the oscillation depends on the illumination condition. The oscillation is clearly observed below T similar to 20 K and almost disappears above this temperature. Furthermore, in the temperature-cycle measurements, the reflectivity oscillation shows a particularly peculiar behavior.

We have studied the chemical potential shift and changes in the electronic density of states near the Fermi level (E (F)) as functions of temperature and carrier concentration in Pr1-x Ca (x) MnO3 (PCMO) and Nd1-x Sr (x) MnO3 (NSMO) by measurements of photoemission spectra. Suppression of the chemical potential shift as a function of carrier concentration has been observed in PCMO and NSMO near and in the composition range where the CE-type antiferromagnetic charge-ordered (CO) phase appears at low temperatures. This result indicates that there is charge self-organization on a microscopic scale such as stripe formation in this composition range. In the ferromagnetic metallic phase of NSMO, we found a large temperature-dependent chemical potential shift at low temperatures and attributed this to double-exchange mechanism. Suppression of the temperature-dependent chemical potential shift near Curie temperature was observed, possibly associated with the formation of correlated polarons. In the valence band near the E (F) of PCMO, spectral weight was transferred with hole doping, leading to a finite intensity at E (F) even in the paramagnetic insulating phase above the CO phase for x greater than or similar to 0.3, and as the temperature was lowered, a clear gap opening was observed in the CO phase.

A neutron diffraction study was performed on a single crystal of 4f-moment-free perovskite manganite Eu0.595Y0.405MnO3 to elucidate the relation between the spontaneous ferroelectric polarization and the magnetic structure. Eu0.595Y0.405MnO3 shows an incommensurate magnetic ordering below T-N = 47 K. The ferroelectric polarization P along the c axis appears below 25 K, and it is replaced by P along the a axis below 23 K. We have observed the change of magnetic structure at temperatures where the ferroelectric polarization appears and changes its polarization direction. (c) 2007 Elsevier Ltd. All rights reserved.

We investigated the crystal structures of single-crystal Nd0.2Sr0.8MnO3 by means of synchrotron x-ray diffraction measurement and transmission electron microscopy. We observed the coexistence of distinct crystal-structural phases, i.e., tetragonal (orbital ordered) and cubic (orbital disordered) phases, over a significantly wide range of temperatures. The ratio of these phases varied continuously with temperature. These indicate that the crystal exhibits a diffuse phase transition of the 3z(2) - r(2)-type orbital ordering. Moreover, we observed the anisotropic evolution of the orbital-ordered nanodomains in the cubic phase. This specific nature of the phase transition probably originates from the competition between the orbital order and disorder.

We have investigated orthorhombic RMnO3 ( R = ( Gd1-yTby)) crystals near the phase boundary between the paraelectric A- type- antiferromagnetic ( AF) phase ( PA) and the ferroelectric transverse- spiral- AF one ( FS). The spiral AF structure breaks inversion symmetry and induces the ferroelectric polarization through the inverse Dzyaloshinskii - Moriya ( DM) interaction. We have found that the PA - FS phase boundary is located at 0.15 < y < 0.2. In the y = 0.15 compound, PA and FS phases appear in the cooling scan, while FS is not observed in the warming scan. This result suggests that FS observed in y = 0.15 is a metastable state arising from the competition between PA and FS. Furthermore, we demonstrate the phase control between these competing phases by the application of the external magnetic field and external quasihydrostatic pressure.

The electronic states of Nd0.45Sr0.55MnO3 in the two-dimensional metal phase have been revealed by the high-resolution Mn 2p-3d resonant photoemission. The vanishingly weak intensity at E-F indicates an anomalous metal due to the two-dimensionality of the electronic states. (c) 2006 Elsevier B.V. All rights reserved.

We have investigated the role of the 4f moment on the magnetoelectric (ME) effect of orthorhombic RMnO3 (R = rare earth ions). In order to clarify the role of the 4f moment, we prepared three samples: (Eu, Y)MnO3 without the 4f moment, TbMnO3 with the anisotropic 4f moment, and (Gd, Y)MnO3 with the isotropic 4f moment. The ferroelectric behaviors of these samples are different from each other in a zero magnetic field. (Eu, Y)MnO3 and (Gd, Y)MnO3 show the ferroelectric polarization along the a axis in the ground state, while TbMnO3 shows it along the c axis. Such difference may arise from the influence of the anisotropic Tb3+ 4f moment. The direction of the ferroelectric polarization of RMnO3 is determined by the internal magnetic field arising from the 4f moment. (c) 2006 Published by Elsevier B.V.

The magnon dispersion of Nd0.60Sr0.4MnO3 has been measured along the [qqq], [qq0], and [00q] propagation directions using inelastic neutron scattering. Modes attributed to both Mn-Mn interactions and Nd-Mn interactions were detected. At low temperatures, and for all three directions, the low q Mn-Mn dispersion closely followed a Heisenberg nearest-neighbor model calculation, before showing marked softening at higher values of q. For the [qq0] and [00q] directions, this softening appears to correspond with intersection with 20 and 40 meV phonon branches, respectively. At low temperatures, and for all three directions, the Nd-Mn dispersion followed the calculated Heisenberg dispersion for all measured points. Additionally, the temperature dependence of the magnon stiffness parameter D was measured up to T-c = 273 K, and showed conventional temperature renormalization behavior. (c) 2005 Elsevier B.V. All rights reserved.

Thermal diffusivity measurements on Nd1-xSrxMnO3 (0 <= x <= 0.5) single crystals have been performed by means of a photopyroelectric technique in the temperature range 78-320K. The critical exponents of the antiferromagnetic-paramagnetic transition in the undoped sample, as well as those of the ferromagnetic-paramagnetic transition in Nd0.6Sr0.4MnO3, have been obtained through the analysis of the inverse of the thermal diffusivity. For x = 0, the results agree with the Heisenberg model (0 alpha(Heisenberg) = -0.115), while for x = 0.4, they point to the Ising model (alpha(Ising) = +0. 110). (c) 2006 Elsevier B.V. All rights reserved.

The electronic states of the Nd0.5Sr0.5MnO3, which shows various electronic phases such as paramagnetic insulator, ferromagnetic metal, and the charge-ordered insulator depending on the temperature. have been revealed by high-resolution Mn 2p-3d resonant photoemission and X-ray absorption spectroscopy. These results indicate the importance of the relationship between electronic states and the Jahn-Teller effect. (c) 2006 Elsevier B.V. All rights reserved.

We have investigated the dielectric and magnetic properties of Eu0.595Y0.405MnO3 without the presence of the 4f magnetic moments of the rare-earth ions and have found two ferroelectric phases with polarization along the a and c axes in a zero magnetic field. A magnetic-field-induced switching from one to the other ferroelectric phase took place in which the direction of ferroelectric polarization changed from the a axis to the c axis by the application of magnetic fields parallel to the a axis. In contrast to the case of TbMnO3, in which the 4f moments of Tb3+ ions play an important role in such a ferroelectric phase switching, the magnetic-field-induced switching between ferroelectric phases in Eu0.595Y0.405MnO3 does not originate from the magnetic transition of the rare-earth 4f moments but from that of the Mn 3d spins. (C) 2006 American Institute of Physics.

The Hall resistivity and magnetization have been investigated in the ferromagnetic state of the bilayered manganite La2-2xSr1+2xMn2O7 (x=0.36). The Hall resistivity shows an increase in both the ordinary and anomalous Hall coefficients at low temperatures below 50 K, a region in which experimental evidence for the spin glass state has been found in a low magnetic field of I mT. The origin of the anomalous behavior of the Hall resistivity relevant to magnetic states may lie in the intrinsic microscopic inhomogeneity in a quasi-two-dimensional electron system. (c) 2005 Elsevier Ltd. All rights reserved.

We have investigated the Hall effect of the layered (A-type) antiferromagnetic (AF) phase of the Nd1-xSrxMnO3 (x=0.55) single crystal, in which d(x2-y2)-type orbitals lie in the ferromagnetic (FM) ab plane without charge-ordering. In the AF metallic phase of the x=0.55 sample, we have observed a negative linear dependence of the Hall resistivity (rho(xy)) as a function of magnetic field (H), in contrast to a positive sign of carriers in the FM metal (e.g., x=0.40). We have detected a remarkable change in the temperature dependence of d rho(xy)/dH at T-N, which shows a close correspondence with the change in the longitudinal resistivity (rho(xx)).

We have investigated the dielectric properties of Gd1-xTbxMnO3 crystals in magnetic fields to clarify the crossover between two distinct ferroelectric phases, GdMnO3 -type (first-order displacive) and TbMnO3 -type (second-order order-disorder). We have found that the compositional phase boundary between the phases exists around 0. 3 < x < 0.4. For the x = 0. 1 sample exposed to magnetic fields above 3 T, we have discovered successive ferroelectric phase transitions that occur at two different temperatures. We have determined the electric and magnetic phase diagram for x = 0, 0.1, and 0.3 in the plane of temperature and field in order to demonstrate the change in the ferroelectric phases.

We have performed the bulk sensitive Mn 2p-3d resonant photoemission for Nd1-xSrxMnO3 (x = 0.40, 0.47, 0.50, 0.63) in order to reveal the Mn 3d electronic states. We will report the temperature and doping dependence of the Mn 3d spectral functions. The sudden spectral change for x = 0.50 across the FM-COI transition shows the strong influence of the charge-ordering on the Mn 3d electronic states. (c) 2005 Elsevier B.V. All rights reserved.