Language：English   Publishing type：Research paper (scientific journal)  

Ferroelectric materials are widely used in modern electric devices such as memory elements, filtering devices and high-performance insulators. Ferroelectric crystals have a spontaneous electric polarization arising from the coherent arrangement of electric dipoles (specifically, a polar displacement of anions and cations). First-principles calculations and electron density analysis of ferroelectric materials have revealed that the covalent bond between the anions and cations, or the orbital hybridization of electrons on both ions, plays a key role in establishing the dipolar arrangement. However, an alternative model-electronic ferroelectricity-has been proposed in which the electric dipole depends on electron correlations, rather than the covalency. This would offer the attractive possibility of ferroelectric materials that could be controlled by the charge, spin and orbital degrees of freedom of the electron. Here we report experimental evidence for ferroelectricity arising from electron correlations in the triangular mixed valence oxide, LuFe 2O4. Using resonant X-ray scattering measurements, we determine the ordering of the Fe2+ and Fe3+ ions. They form a superstructure that supports an electric polarization consisting of distributed electrons of polar symmetry. The polar ordering arises from the repulsive property of electrons-electron correlations-acting on a frustrated geometry.

DOI： 10.1038/nature04039

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Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

DOI： 10.1007/s11664-024-11586-9

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Other Link： https://link.springer.com/article/10.1007/s11664-024-11586-9/fulltext.html

Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

DOI： 10.1103/physrevmaterials.8.064402

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevMaterials.8.064402/fulltext

Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/acs.inorgchem.3c02405

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.3390/ma16051989

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

We report an attempt valence control of the mixed valence iron triangular oxide YbFe2O4 to develop an effective technique controling the frustration of charges in strongly correlated electron systems. The electrochemical doping of Li + into YbFe2O4 was examined on a cell-type sample similar to the Li-ion secondary battery cell. Systematic changes in the lattice constant and Fe – Fe and Fe–Yb distance were observed with Li doping. Maximum value of the doping was over 300 mAh/g. An EXAFS experiment indicated that Li positioned between Yb octahedron layer (U-layer) and Fe-bipyramidal layer (W-layer). However, detailed change of iron valence state of YbFe2O4was not clearly observed because of the superimpose of the signal from iron metal nano particles in XANES observation. We discuss that the uncertainty might arise from the inhomogeneous distribution of the sample particle size, which might prevent the homogeneous doping of Li because the doping occurs on the surface of each nano-particles.

DOI： 10.1016/j.jpcs.2021.110468

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We report the magnetic field effect on the dielectric constant of the charge-ordered ferroelectric LuFe2O4. Below the Néel temperature of 240 K, we found decreases in capacitance and resistance by the magnetic field. The variation in dielectric constant with the magnetic field was investigated by inverse complex capacitance analysis with a series of equivalent electric circuits having a log-normal distribution of the relaxation time. The phenomenon can be explained by the spin- and charge-ordered domain model, in which the entropy of Fe spins affects the arrangement of Fe charges.

DOI： 10.7566/JPSJ.90.113705

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Physical Society of Japan  

Total reflection gamma-ray resonant spectra of a Cr/Fe/Cr/MgO thin film in which a Fe-57 monolayer was embedded at a depth of 6 nm below the surface were studied using a synchrotron Mossbauer source. Due to an interference between electron and nuclear resonant scatterings, the spectra showed asymmetric absorption and scattering profiles depending on beam incidence angles. The spectra were reproduced via theoretical simulations based on an anomalous scattering factor associated with nuclear resonance. The origin of the asymmetric spectral profiles was explained by a change in the phase factor of nuclear resonant scattering amplitude at the Fe-57 enriched monolayer. These results pave the way for advanced local magnetic analyses on the deeply buried interface of functional magnetic thin films and direct phase determination of gamma-rays in thin films.

DOI： 10.7566/jpsj.90.084705

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

At a metal-ferroelectric junction, it has been considered that the electric polarization of the ferroelectric material can affect the electronic structure of the neighboring metal. Here, we demonstrate that the valence state of Pd can be shifted to the unstable high value of 4+ by the electric field of electric polarization in ferroelectric BaTiO3. Study of the absorption fine structure of both hard and soft x rays revealed the existence of Pd4+ states on the surface of Pd oxide nanoparticles. The positions of Pd and oxygen atoms are shifted in opposite directions by the electric field due to the electric polarization of ferroelectric BaTiO3. The atomic displacement of Pd and O forms a zigzag structure, in which the coordination number of Pd atoms is changed from four to six, producing a quadrivalent state. This report presents experimental evidence that ferroelectric polarization can control the electronic states of neighboring metal atoms, and we suggest that using the ferroelectric support effect may produce a new type of catalyst.

DOI： 10.1063/5.0066289

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ROYAL SOC CHEMISTRY  

YbFe<INF>2</INF>O<INF>4</INF> is a charge-ordered ferroelectric that exhibits coupling between magnetization and electric polarization near room temperature and crystallizes in a rhombohedral structure (R3m). This study presents an attempt to fabricate stoichiometric and epitaxial YbFe<INF>2</INF>O<INF>4-delta</INF> films with a nearly single-domain structure using an RF magnetron sputtering method. The (0001)-oriented epitaxial films of YbFe<INF>2</INF>O<INF>4-delta</INF> on YSZ (111) substrates via reactive sputtering method exhibited clear three-fold symmetry normal to the substrate without the formation of twin domains rotated by 60 degrees. The oxygen stoichiometry of the epitaxial YbFe<INF>2</INF>O<INF>4-delta</INF> was improved by controlling an oxygen partial pressure (P<INF>O<INF>2</INF></INF>) during the deposition. The films showed a sharp ferrimagnetic transition, and the transition temperature (T<INF>N</INF>) increased linearly to approximately 245 K with decreasing P<INF>O<INF>2</INF></INF>. The magnitude of magnetization of the obtained films was comparable to that of bulk single crystals. Further, the electron diffraction pattern of the stoichiometric films confirmed the presence of three-dimensional charge order, which is consistent with the behavior of the bulk crystals as well.

DOI： 10.1039/d1ce00834j

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We report the band structure of Ba-deficient BaTiO3 as a p-type semiconductor, studied by a combination of light reflectance and photoelectron yield spectroscopy. Two acceptor levels were observed at the tail of a valence band. As the quantity of Ba vacancies increased, the density of state of the two acceptor levels also increased. The levels of the conduction band minimum and the valence band maximum shifted far away from the vacuum level, but the bandgap seems to be independent of Ba deficient concentration. For classical semiconductors such as Si and GaAs, the observation of impurity levels is restricted to low temperatures (similar to 20K) owing to their narrow bandgaps. Oxide semiconductors have now been demonstrated with wide bandgaps and acceptor levels, at normal operating temperatures, which could lead to new device designs in the future.

DOI： 10.1063/5.0033761

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We report an analysis of the dielectric glasslike state of the spin- and charge-ordered system Lu2Fe3O7, which has structural and physical properties analogous to those of the electronic ferroelectrics RFe2O4 (R: rare earth). Aging and memory effects of resistivity were found in the electric impedance response. We performed a numerical analysis with equivalent electric circuits that have a logarithmic Gaussian distribution of resistance and capacitance. The distribution parameters well explained the glassy nature of the electric and dielectric properties between 100 and 200 K and the distribution variations of charge- and magnetic-ordered domains underlying the glass nature.

DOI： 10.7566/JPSJ.90.024710

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Publishing type：Research paper (scientific journal)  

Temperature dependence of charge-ordered crystal structures and domain structures in RFe2O4 (R: Y and Lu) was investigated by energy-filtered transmission electron microscopy, combined with conventional transmission electron microscopy. The presence of three-dimensional to two-dimensional charge ordering transition were observed in both YFe2O4 and LuFe2O4 on heating. Furthermore, real-space images obtained with the energy-filtered transmission electron microscopy revealed that YFe2O4 has less anisotropic nanometer-scale charge-ordered domains than LuFe2O4. These findings in RFe2O4 indicate the importance of the interchange interactions between Fe-O bilayers in addition to those within bilayers in the structural phase transitions associated with charge ordering in this system.

DOI： 10.1080/00150193.2021.1984762

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE RESEARCH  

Skewed band structures have been empirically described in ferroelectric materials to explain the functioning of recently developed ferroelectric tunneling junction (FTJs). Nonvolatile ferroelectric random access memory (FeRAM) and the artificial neural network device based on the FTJ system are rapidly developing. However, because the actual ferroelectric band structure has not been elucidated, precise designing of devices has to be advanced through appropriate heuristics. Here, we perform angle-resolved hard X-ray photoemission spectroscopy of ferroelectric BaTiO3 thin films for the direct observation of ferroelectric band skewing structure as the depth profiles of atomic orbitals. The depth-resolved electronic band structure consists of three depth regions: a potential slope along the electric polarization in the core, the surface and interface exhibiting slight changes. We also demonstrate that the direction of the energy shift is controlled by the polarization reversal. In the ferroelectric skewed band structure, we found that the difference in energy shifts of the atomic orbitals is correlated with the atomic configuration of the soft phonon mode reflecting the Born effective charges. These findings lead to a better understanding of the origin of electric polarization.

DOI： 10.1038/s41598-020-67651-w

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE SA  

© 2019 Elsevier B.V. Dielectric and magnetocaloric measurements are carried out for the chromite TmCrO3. This oxide was reported to be multiferroic below the Néel temperature (TN) of ∼125 K, likely due to a structural transformation. The dielectric response shows large dielectric constants below 300 K. However, from the analyses of loss tangent, AC conductivity and dielectric modulus, this behavior is rooted in hopping of charge carriers rather than electric dipoles, as proposed for some other chromites. No dielectric anomaly is found at TN. The magnetocaloric effect shows that the magnetic transitions at TN as well as the spin reorientation temperature are of a second order. This result strongly suggests the absence of magnetostructural transition at TN in accord with no observation of ferroelectric transition at this temperature.

DOI： 10.1016/j.jallcom.2019.06.359

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

Interfaces in nanocarbon materials are highly important, as they determine the properties of carbon-based devices. In terms of carrier and thermal transport properties, the interfacial features are often more important than the intrinsic characteristics. Herein, we describe how 1 min Joule annealing of carbon nanotube (CNT) yarns can convert the interfacial amorphous carbon into graphene fragments. After 1 min Joule annealing, we have obtained multiwalled CNT yams with extremely high Seebeck coefficients (+/- 100 mu V/K) and high thermoelectric power factor (400 and 1000 mu W/mK(2)) at room temperature, both with or without polyethylenimine doping. Theoretical simulations and experimental measurements helped to determine the optimal annealing conditions in terms of a rapid transformation of the interfacial amorphous carbon between the bundled CNTs in the yarn into graphene fragments at similar to 2000 K. The present approach represents significant progress in energy materials science, as it provides a guiding principle for the design of interfaces in nanocarbon materials with potential applications in energy-harvesting systems.

DOI： 10.1021/acsaem.9b01736

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We report a wide-range reciprocal space observation with neutron diffraction in an iron vacancy controlled YbFe2O4 crystal, which is proposed as a prototype of electronic ferroelectrics arising from iron charge ordering. The increase of the charge ordering transition temperature and the disappearance of spin competitive transition were discovered in this stoichiometric crystal. We suppose that the lower ionic deficiency of this crystal enhances the development of spin and charge order coherence length, and consequently, the intrinsic nature of spin and charge ordering of YbFe2O4 could be detected. The experiment was performed with a two-dimensional neutron detector, and all super-lattice spots and its modulations above magnetic ordering temperature TN = 245 K are successfully explained with six domains of a monoclinic charge-order super-lattice cell. A rather extended two dimensional (2D) spin correlation above TN was identified with polarized 3He neutron spin filter and we found a clear indication of a coupling between the 2D spin correlation and charge ordering between TN and 300 K, at which a new lattice distortion phase may set in.

DOI： 10.7566/JPSJ.88.044701

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Language：Japanese   Publisher：The Physical Society of Japan  

DOI： 10.11316/jpsgaiyo.74.2.0_2103

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We report the modulation in charge ordering structure of ferroelectric YbFe2O4 by magnetic ordering from x-ray diffraction experiments and magnetic measurements. The incommensurate modulation was observed on (n n 3m+3/2) type diffraction signal around magnetic ordering temperature where n and m are integer. The modulation was also observed on charge ordering signal, indicating spin-charge coupling. The incommensurate modulation was observed only on (n/3 n/3 3m) type charge ordering signal. This selectivity can be explained by polar charge ordering models.

DOI： 10.1063/1.5026921

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Well-crystallized epitaxial YbFe2O4 films were prepared on an α-Al2O3(001) substrate using an Fe3O4 buffer layer. Fe3O4 has a relatively small lattice mismatch with both YbFe2O4 and α-Al2O3. Electron diffraction analysis combined with transmission electron microscopy revealed the epitaxial relationship to be α-Al2O3[110](001) ∥ Fe3O4[](111) ∥ YbFe2O4[110](001). Moreover, superlattice spots due the Fe2+-Fe3+ charge order state of YbFe2O4 were clarified. The Fe2+/Fe3+ ratio in YbFe2O4 was nearly stoichiometric. The film exhibited a ferrimagnetic transition at ∼220 K and a nonlinear current-voltage characteristic at room temperature. These results confirmed the good crystallinity and stoichiometry of the obtained YbFe2O4 films.

DOI： 10.7567/JJAP.57.010305

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Language：Japanese   Publishing type：Research paper (other academic)   Publisher：The Physical Society of Japan  

DOI： 10.11316/jpsgaiyo.73.2.0_530

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

We have succeeded in obtaining the crystal-site-selective spectra of the collinear antiferromagnet Fe3BO6 using a synchrotron Mossbauer diffractometer with pure nuclear Bragg scattering at SPring-8 BL11XU. Well-resolved 300, 500, and 700 reflection spectra, having asymmetric line shapes owing to the higher-order interference effect between the nuclear energy levels, were quantitatively analyzed using a formula based on the dynamical theory of diffraction. Reasonable hyperfine parameters were obtained. The intensity ratio of Fe1 to Fe2 subspectra is in accordance with the nuclear structure factor. However, when the spectrum is measured at the peak position of the rocking curve (very near the Bragg position), the value of the center shift deviates from its intrinsic value. This is also due to the dynamical effect of.-ray diffraction. To avoid this problem, it is necessary to use diffraction angles near the foot of the rocking curve, approximately 0.02 degrees apart from the peak position.

DOI： 10.7566/JPSJ.86.084701

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ACADEMIC PRESS INC ELSEVIER SCIENCE  

Phase relations in the pseudo-binary systems RFeO3-R2Ti2O7 (R: Lu, Ho and Dy), RGaO3-R2Ti2O7 (R: Lu and Er), LuAlO3-Lu2Ti2O7 and RAO(3)-R2Ti2O7 (R: Lu and Yb. A: Cr and Mn) at elevated temperatures in air were determined by means of a classic quenching method. There exist Lu(Fe1-xTix)O3+x/2, R(Ga1-xTix)O3+x/2 (R: Lu and Er) and Lu(Al1-xTix)O3+x/2 (2/3 &lt;= x &lt;= 3/4) having the Yb(Fe1-xTix)O3+x/2-type of crystal structure (x=0.72, space group: R3m, a(angstrom)=17.9773 and c(angstrom)=16.978 as a hexagonal setting) in these pseudo binary systems. Eighteen compounds R(A(1-x)Ti(x))O3+x/2 (R: Lu-Sm and Y, A: Fe, Ga and Al) were newly synthesized and their lattice constants as a hexagonal setting were measured by means of the X-ray powder diffraction method. The R occupies the octahedral site and both A and Ti does the trigonalbipyramidal one in these compounds. Relation between lattice constants for the rhombic R(A(1-x)Ti(x))O3+x/2 and the monoclinic In(A(1-x)Ti(x))O3+x/2 are as follows, ah approximate to 5 x b(m), ch approximate to 3 x c(m) x sin beta and a(m)=3(1/2) x b(m), where ah and ch are the lattice constants as a hexagonal setting for R(A(1-x)Ti(x))O3+x/2 and a(m), b(m), cm and beta are those of the monoclinic In(A(1-x)Ti(x))O3+x/2. Crystal structural relationships among a-InGaO3 (hexagonal, high pressure form, space group: P63/mmc), InGaO3 (rhombic, hypothetical), (RAO(3))(n)(BO)(m) and RAO(3)(ZnO)(m) (R: Lu-Ho, Y and In, A: Fe, Ga, and Al, B: divalent cation element, m, n: natural number), the orthorhombic-and monoclinic In(A(1-x)Ti(x))O3+x/2 (A: Fe, Ga, Al, Cr and Mn) and the hexagonal-and rhombic R(A(1-x)Ti(x))O3+x/2 (R: Lu-Sm and Y, A: Fe, Ga and Al) are schematically presented. We concluded that the crystal structures of both the alpha-InGaO3 (high pressure form, hexagonal, space group: P63/mmc) and the hypothetical InGaO3 (rhombic) are the key structures for constructing the crystal structures of these compounds having the cations with CN=5.

DOI： 10.1016/j.jssc.2017.04.001

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Publishing type：Research paper (international conference proceedings)  

© 2017 Taylor &amp; Francis Group, LLC. The CXD method have been shown to be applicable to the crystal characterization as well as the domain observation. The present CXD technique could evaluate the crystal coherence length up to 10 μm and the wide variety of samples were chosen in this study. We have shown how the ideal Bragg reflection deforms as the disorder and the defects increases and form the speckle patterns, the typical scattering patterns of CXD, by changing the samples. Finally we have successfully observed the μm/sub-μm domain arrangements in LuFe2O4/(1−x)Pb(Zn1/3Nb2/3)O3-xPbTiO3(x = 0.09), where it is hard to be observed by the conventional diffraction techniques.

DOI： 10.1080/00150193.2017.1350058

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Slow magnetic relaxation and the Fe ion stoichiometry were investigated in spin and charge frustrated system YbFe2O4. DC susceptibility, AC susceptibility, aging process and electron diffraction observation were carried out on nano order sized YbFe2O4 single-phase powders with the Fe/Yb=2.00, 2.02, and 2.04 ratios. The variation of the cluster glass behavior was studied in relation between the magnetic relaxation and the various chemical compositions. With the increase of the Fe/Yb ratios, the magnetic coherence length increased and the magnetic aging time goes slow down. The observed critical slowing down of the glassy fluctuation is interpreted by the development of the spin cluster size. This indicates that the spin glass like property of this material arises from the competition between various sized magnetic domains having ferrimagnetic moments. Additionally, electron diffraction experiments showed that the increase of Fe/Yb ratios from Fe/Yb=2.00 enhances the development of the charge ordering coherence in triangular lattice. This study shows that the measurement of magnetic fluctuation for nano order sized particles gives the essential information about the spin cluster fluctuation in RFe2O4.

DOI： 10.1016/j.jpcs.2016.12.014

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE SA  

Magnetic and dielectric properties have been studied for LaCrO3, PrCrO3, and their solid solution La0.5Pr0.5CrO3, which belong to a family of the ferroelectric orthochromite series RCrO3 (R: rare earths). The magnetic measurements confirm that the materials show canted antiferromagnetic ordering at 240-288 K. Neutron diffraction patterns could be fitted with the centrosymmetric Pnma, which is different from the non-centrosymmetric structure proposed for the ferroelectric phase of NdCrO3. The large dielectric constants are likely due to the hopping of charge carriers as proposed previously for other chromites. (C) 2017 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.matchemphys.2016.12.074

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

We have succeeded for the first time in obtaining a crystal-site-selective Mossbauer spectrum of Fe3O4 using the Mossbauer diffractometer at SPring-8 BL11XU. In order to extract the nuclear resonant scattering, which provides the crystal-site-selective emission spectrum, reflections having a Bragg angle near 45 degrees were used. The 666 and 10 10 0 reflection spectra reveal only B- and A-site spectra, respectively. Mossbauer diffraction spectroscopy enables us to measure the crystal-site-selective spectrum and to determine the precise hyperfine structure. This technique provides new possibilities for studying the local crystallographic and magnetic structure on iron sublattices in various multisite materials.

DOI： 10.7566/JPSJ.86.023706

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We report the observation of the electric spontaneous polarization in YbFe2O4, one candidate "electronic ferroelectric,"where ferroelectricity originates from the polar charge order. Though we have proposed that in LuFe2O4 polar charge ordering of iron ions having different valence states give rise to ferroelectricity, some reports questioned not only the electronic origin but also the existence of the ferroelectricity itself in this system. In response to this, we show a direct macroscopic evidence of the existence of the ferroelectricity by a clear P-E hysteresis loop of YbFe2O4 belonging to the same system as that of LuFe2O4. Although further investigations are required about the electronic origin, we report the direct observation of the macroscopic electric polarization.

DOI： 10.1063/1.4974994

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

We report a possible charge ordering model of triangular mixed valence material YbFe2O4. It was found that a crystal synthesized with high stoichiometry on iron vacancy shows new and simple extinction rule in the superlattice reflections. The Laue class of the diffraction signal is expressed as 2/m. With the consideration of the charge neutrality and the superlattice signal of (1/3 1/3 0), we derived 5 possible charge ordering models of this material.

DOI： 10.1080/00150193.2017.1349945

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

Size effect and lattice strain effect for charge ordered structure and magnetic properties were investigated on the nano particles of geometrically spin and charge frustrated system YbFe2O4. The lattice strain of the nano particle was controlled with the ratio of Yb and Fe (2.00 Fe/Yb 2.04) in liquid synthesizing method. It was found that the lattice strain is not affected by the magnetic transition temperature, however the anomalous behavior at a lower magnetic transition (T-LT) with an increment of the Fe / Yb ratio was detected. The development of charge ordering was two-dimensional and magnetic ordering goes down for all samples in spite of the variation of the local lattice strain. This is the first report of the coupling of charge and spin ordering to the size effect in this material. These results suggest that the discussion on the electric polarization induced from the charge ordering in this material need to care the long-range interaction like surface effect.

DOI： 10.1080/00150193.2017.1349913

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPRINGER  

A Mossbauer diffractometer has been developed by using Fe-57 nuclear resonant scattering apparatus at SPring-8 BL11XU in order to obtain a crystal-site-selective Mossbauer spectrum. A theta-2 theta goniometer was newly installed between the nuclear monochromator and a detector. From a single crystal Fe3O4 mounted on the goniometer, the 111, 222, and 220 reflected gamma-rays were used to collect the diffraction spectra at room temperature. The intensity ratio of the two subspectra, corresponding to A-and B-site Fe ions, changes notably according to the reflection index. The diffraction spectrum is composed of a major absorption spectrum and a minor emission spectrum. The former is given by the. ray due to the electron scattering and nuclear absorption, whereas the latter is given by the gamma-ray due to the nuclear resonant scattering. Interference effects between these two gamma-rays are also seen as line broadenings, asymmetric line shapes, and slope of the base lines. These features can be successfully expressed by a Fano function. We consider that the emission spectrum due to the nuclear resonant scattering represents crystal-site-selective Mossbauer spectrum.

DOI： 10.1007/s10751-016-1370-x

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Levels of the conduction band minimum and the valence band maximum in ion-deficient BaTiO3 particles were investigated with optical band gap and ionization energy measurements. Though it is known that the quantification of the band structure in an insulator is difficult, due to the poor electrical conductivity of BaTiO3, systematic variation in the band energy levels was found that correlated with the introduction of vacancies. Photoelectron yield spectroscopy provided direct observation of the occupancy level of electrons, which is altered by the presence of oxygen and barium vacancies. In addition, the conduction band deviation from the vacuum level was determined by optical reflectance spectroscopy. Our results show that: (1) Introduction of oxygen vacancies forms a donor level below the conduction band. (2) The conduction band is shifted to a lower level by a larger number of oxygen vacancies, while the valence band also shifts to a lower level, due to the reduction in the density of O 2p orbitals. (3) Introduction of barium vacancies widens the band gap. Since barium vacancies can induce a small number of oxygen vacancies with accompanying charge compensation, this behavior suppresses any large formation of donor levels in the gap states, indicating that cation vacancies can control the number of both donor and acceptor levels. Published by AIP Publishing.

DOI： 10.1063/1.4964803

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Rare-earth iron oxides (RFe2O4) have attracting attention as new electronic device materials because of their numerous functionalities, such as electronic ferroelectricity, ferrimagnetism, and high infrared absorption. In this paper, nearly monophasic YbFe2O4 films were prepared on alpha-Al2O3 (001) substrates by the spin coating method using an aqueous-based YbFe2O4 solution. The solution was composed of a stoichiometric ratio of Yb(CH3COO)(3) and Fe(NO3)(3) with excess chelating agents. After heat treatment above 800 degrees C, well-crystallized and highly (001)-oriented YbFe2O4 started to epitaxially form on the substrate under controlled oxygen partial pressure with H-2/CO2 gas mixtures. X-ray pole figure analysis confirmed the following epitaxial relationship: YbFe2O4[100](001)//alpha-Al2O3[100](001). Moreover formation of an Fe3O4 interracial layer between YbFe2O4 and alpha-Al2O3 was detected by high-resolution transmission electron microscopy. Presence of the Fe3O4 interracial layer seemed to release the lattice misfit with the substrate. The Fe2+/Fe3+ ratio in the obtained YbFe2O4 films was nearly stoichiometric and the indirect bandgap assigned to Fe2+ -&gt; Fe3+ charge transfer excitation was found to be similar to 0.4 eV by optical spectroscopy. A clear magnetic transition from the paramagnetic state to the ferrimagnetic state occurred at similar to 230 K. (C) 2016 Author(s).

DOI： 10.1063/1.4961639

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

In strongly correlated electron systems, enhanced fluctuations in the proximity of the ordered states of electronic degrees of freedom often induce anomalous electronic properties such as unconventional superconductivity. While spin fluctuations in the energy-momentum space have been studied widely using inelastic neutron scattering, other degrees of freedom, i.e., charge and orbital, have hardly been explored thus far. Here, we use resonant inelastic x-ray scattering to observe charge fluctuations proximate to the charge-order phase in transition metal oxides. In the two-leg ladder of Sr14-xCaxCu24O41, charge fluctuations are enhanced at the propagation vector of the charge order (q(CO)) when the order is melted by raising temperature or by doping holes. In contrast, charge fluctuations are observed not only at q(CO) but also at other momenta in a geometrically frustrated triangular bilayer lattice of LuFe2O4. The observed charge fluctuations have a high energy (similar to 1 eV), suggesting that the Coulomb repulsion between electrons plays an important role in the formation of the charge order.

DOI： 10.1038/srep23611

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We observed all-in type Nb tetrahedral displacement in nonmagnetic pyrochlore niobates A(2)Nb(2)O(7) (A = Nd0.5Ca0.5 and Y0.5Ca0.5) through the analysis of the neutron pair distribution function and the extended x-ray absorption function spectroscopy. The all-in type Nb tetrahedral displacement, which has the character of a charge singlet state, is driven by the formation of the bonding orbital. The diffuse scattering in the x-ray diffraction, which has the resonant component in the Nb L-3 edge, indicates that the all-in type Nb tetrahedral displacement has the periodicity with its short-range correlation.

DOI： 10.1103/PhysRevB.93.085109

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

DOI： 10.11316/jpsgaiyo.71.1.0_2691

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

DOI： 10.11316/jpsgaiyo.71.1.0_2708

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Language：Japanese   Publisher：The Physical Society of Japan  

DOI： 10.11316/jpsgaiyo.71.1.0_2709

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Language：English   Publisher：The Materials Research Society of Japan  

<p>Fermi level and valence state were investigated in RFe₂O₄ (R = Y, Dy, Ho, Er, Tm, Yb, Lu, Sc, or In), which shows magnetoelectric effect originating from charge and magnetic orders of Fe²⁺ and Fe³⁺. We succeeded in controlling the Fermi level of YbFe₂O₄ system by Ti⁴⁺ substitution for Yb³⁺. The variation in the ratio of Fe²⁺ to Fe³⁺ is suggested to the origin of the Fermi level variation. This report suggests the possibility of simple selection of conduction type in functional oxide.</p>

DOI： 10.14723/tmrsj.41.269

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We report the effect of iron vacancies on the magnetic property of spin and charge ordered system YbFe₂O₄ single crystals. The excess amount of iron compensates for the evaporation of iron during crystal growth. We concluded that samples grown with the standard method contain iron vacancies of more than 10%. It is considered that increasing the iron in the starting material recovers the iron stoichiometry, and the proper magnetic property appears. With recovery of the chemical stoichiometry, the magnetic hysteresis loop changes from a double loop to a single loop, which indicates the development of spin coherence and the appearance of a large ferrimagnetic domain. The anomaly found in the thermomagnetization curve at approximately 150 K, the so-called T_LT transition, disappears with better stoichiometry. This fact indicates that T_LT is not a kind of phase transition but an effect of iron vacancy. Our discovery demonstrates that most of the crystals used in RFe₂O₄ research might contain many iron vacancies. The confused discussions on this material about the spin order model, the charge order model, and so on might reconsider such chemical stoichiometry, including iron vacancies.

DOI： 10.14723/tmrsj.41.139

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The far-infrared reflectance spectra of layered iron oxide LuFe₂O₄ have been investigated at external pressure up to 8 GPa. The observed phonon modes continuously shift to the higher energy side with increasing pressure below 6 GPa. However, it is found that the spectral profile of the phonon modes drastically changes around 6 GPa. From the analysis of the phonon modes using a Lorentz oscillator model, it is shown that the spectral changes correspond to a structural phase transition that occurs at 6 GPa.

DOI： 10.14723/tmrsj.41.109

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Physical Society of Japan  

The Morin transition of a high-quality flux-grown single crystal of α-Fe2O3 was studied by 57Fe synchrotron radiation Mössbauer diffraction. The measured rocking curves and Mössbauer spectra gave direct evidence that the coexistence of magnetic phases during the Morin transition occurred over a wide temperature range. The complex magnetic structure at the phase coexistence induced a large magnetostrictive distortion in the α-Fe2O3 crystal surface. In contrast, however, when the antiferromagnetic phase was dominant at low temperatures, the distortion disappeared, and the initial high crystal perfection was recovered. The spectral line shapes were discussed in terms of the interference between electronic and nuclear scatterings.

DOI： 10.7566/JPSJ.85.054705

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DOI： 10.11316/jpsgaiyo.71.1.0_1273

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DOI： 10.11316/jpsgaiyo.71.2.0_2549

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We report the effects of elemental substitutions in multiferroic {\it R}Fe$_{2}$O$_{4}$ ($R$: Y, Ho-Lu, In). The substitution by nonmagnetic Ga$^{3+}$ at the Fe site, which has not been reported so far, shows a drastic decrease in the ferrimagnetic transition temperature ($T_{\rm N}$). This is plausibly owing to the suppression of magnetic interactions between Fe ions, based on our previous results of other substituted systems such as {\it R}FeCoO$_{4}$. Dielectric constants near room temperature were comparable to those of $R$Fe$_{2}$O$_{4}$. The suppression of dielectric loss in this system is suitable to application. The {\it R}-site substitution was also attempted by the large R$^{3+}$ ions such as Dy$^{3+}$. The solubility limit was up to about 10\% of {\it R} ions, as confirmed by XRD and EXAFS measurements. $T_{\rm N}$ was raised by 5-10 K for Dy$^{3+}$ substitution.

DOI： 10.1002/pssc.201400250

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We give a brief review of the experimental research on a triangular mixed valence iron oxide RFe2O4 (R = Y, Dy, Ho, Er, Tm, Yb, Lu, Sc or In). Interest in this material has been increasing every year because of the fascinating but complicated interaction between spin, charge and the orbital state of iron ions in frustrated geometry. Reports collected in this review cover experimental research on crystallography, chemical analysis, bulk and thin film preparation, magnetic, dielectric, diffraction with neutrons, x-ray and electron, optical and x-ray absorption, Mossbauer spectroscopy and other methods that incorporate the use of modern scientific technology and knowledge. The report mainly focuses on experimental facts since 1990 on which an early review by Siratori has been published (Kimizuka et al 1990 Handbook on the Physics and Chemistry of Rare Earths vol 13, ed K A Gschneidner Jr and L Eyring

DOI： 10.1088/0953-8984/27/5/053201

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Soft x-ray magnetic circular dichroism (XMCD) have been measured on the single crystal of the charge-ordered type multiferroelectric material LuFe2O4. By comparing the XMCD results with the bulk magnetizations, the valence-specific magnetizations (VSMs) of Fe2+ and Fe3+ ions are obtained. Both Fe2+ and Fe3+ ions show positive and negative remanent VSMs below the spin ordering temperature TSO∼250 K, showing the ferrimagnetic long-range ordering directly. Both of the remanent VSMs disappear above the TSO and the field dependencies of the VSMs behave in paramagnetic fashions with convex and concave curvatures. However, the signs of the VSMs of Fe2+ and Fe3+ remain opposite, nevertheless the system is in a paramagnetic phase. This is direct evidence of the robust antiferromagnetic interaction between Fe2+ and Fe3+ ions even above the TSO. As temperature increases, furthermore, the opposite VSM of Fe3+ is reversed around the three-dimensional charge ordering temperature TCO∼330 K showing that both net magnetic moments of Fe2+ and Fe3+ become parallel to the applied magnetic field. Possible correlation between these unusual magnetic behaviors and the ferroelectric properties of LuFe2O4 is discussed.

DOI： 10.1103/PhysRevB.91.014410

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Soft x-ray magnetic circular dichroism (XMCD) have been measured on the single crystal of the charge-ordered type multiferroelectric material LuFe2O4. By comparing the XMCD results with the bulk magnetizations, the valence-specific magnetizations (VSMs) of Fe2+ and Fe3+ ions are obtained. Both Fe2+ and Fe3+ ions show positive and negative remanent VSMs below the spin ordering temperature T-SO similar to 250 K, showing the ferrimagnetic long-range ordering directly. Both of the remanent VSMs disappear above the TSO and the field dependencies of the VSMs behave in paramagnetic fashions with convex and concave curvatures. However, the signs of the VSMs of Fe2+ and Fe3+ remain opposite, nevertheless the system is in a paramagnetic phase. This is direct evidence of the robust antiferromagnetic interaction between Fe2+ and Fe3+ ions even above the T-SO. As temperature increases, furthermore, the opposite VSM of Fe3+ is reversed around the three-dimensional charge ordering temperature T-CO similar to 330 K showing that both net magnetic moments of Fe2+ and Fe3+ become parallel to the applied magnetic field. Possible correlation between these unusual magnetic behaviors and the ferroelectric properties of LuFe2O4 is discussed.

DOI： 10.1103/PhysRevB.91.014410

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DOI： 10.1007/978-3-319-13242-6_61

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：WILEY-V C H VERLAG GMBH  

We report the effects of elemental substitutions in multiferroic RFe2O4 (R: Y, Ho-Lu, In). The substitution by nonmagnetic Ga3+ at the Fe site, which has not been reported so far, shows a drastic decrease in the ferrimagnetic transition temperature (TN). This is plausibly owing to the suppression of magnetic interactions between Fe ions, based on our previous results of other substituted systems such as RFeCoO4. Dielectric constants near room temperature were comparable to those of RFe2O4. The suppression of dielectric loss in this system is suitable to application. The R-site substitution was also attempted by the large R3+ ions such as Dy3+. The solubility limit was up to similar to 10% of R ions. TN was raised by 5-10 K for Dy3+ substitution. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

DOI： 10.1002/pssc.201400250

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DOI： 10.11316/jpsgaiyo.70.2.0_2405

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DOI： 10.11316/jpsgaiyo.70.2.0_865

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DOI： 10.11316/jpsgaiyo.70.2.0_2489

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DOI： 10.11316/jpsgaiyo.70.2.0_2505

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

The charge ordering of Fe2+ and Fe3+ ions in LuFe2O4 induces ferroelectricity below 330 K. In this study, the X-ray absorption spectra (XAS) at the Fe L-2,L-3-edge are investigated to study charge ordered- and disordered-states. The polarization dependence of XAS revealed that the energy level of (d(xy), d(x2-y2)) is lower than that of (d(yz), d(zx)). In the charge-ordered state, the Fe L-3-XAS shows a double-peak structure, which is a characteristic of Fe2+ and Fe3+ ions. In the high-temperature region, the double-peak structure becomes a broad single peak, because of overlap with the thermally excited state. (C) 2014 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.elspec.2014.08.008

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We report the change in the magnetic properties of multiferroic YbFe2O4 caused by cooling in the presence of an electrical current. The magnetization measured after the cooling is similar to 10-20% smaller than that after cooling without a current. The current also affects the magnetic anisotropy below similar to 100 K; the magnetic coercivity is increased by similar to 10-20%. These phenomena may arise from the coexistence of ferromagnetic and antiferromagnetic domains or a disturbance in the formation of charge-ordered domains. From the viewpoint of application, the enhancement of anisotropy by an electrical current provides a method for stabilizing magnetization other than the well-known method of exchange bias.

DOI： 10.7566/JPSJ.83.063708

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Various types of iron oxides loaded on ferroelectric BaTiO3 and nonferroelectric ZrO2 supports have been studied by Fe-57 Mossbauer spectroscopy to investigate the valence instability of iron oxides affected by ferroelectric polarization fluctuation. Although the iron oxide loaded on ZrO2 shows a typical reduction process at high vacuum and high temperature, the iron oxide loaded on BaTiO3 shows an opposite process, i.e., iron oxide oxidization. We propose the dynamics of band structures of ferroelectrics in the paraelectric phase to understand this phenomenon. The oscillation of band bending causes electron transfer from an iron oxide to a ferroelectric support. This result suggests that the surface charges of ferroelectrics are useful in developing new functional materials with metal compounds. (C) 2014 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.53.05FB24

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Precise measurements of specific heat were performed on high quality single crystals of multiferroic LuFe2O4 by using an ac calorimeter. The thermal anomalies corresponding to four known phase transitions were observed at 220K (T-g), 260K (T-N), 380K (T-3CO) and 590K (T-2CO). In addition to them, a new clear anomaly was detected at 305K. No other distinct anomaly could be observed below 220K down to 100K. It is clarified that the temperature dependence of the order parameter associated with this new phase transition at 305K is quite similar to those of spontaneous polarization reported previously by Ikeda etal.

DOI： 10.1080/00150193.2014.890505

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Journal of the Japan Society of Powder and Powder Metallurgy  

Rare-earth iron oxides (RFe2O4) have attracting attention as new electronic device materials because of their abundant functionalities such as ferroelectricity, ferrimagnetism, and high infrared absorption. These properties are strongly related to the mixed iron valance states between Fe2+ and Fe3+. We tried to prepare RFe2O4 films by using soft chemical method. The films were prepared by spin-coating of viscous aqueous solutions of Yb(CH3COO)3 and Fe(NO3)3 with citric acid. The molar ratio between Yb:Fe was 1:2 and the citric acid: metal ratio was set to 3.75:1. After the coating, the films dried at 100�C for 10 min in air were, heat-treated at 900�C for 0.2 h in H2/CO2 gases to control oxygen partial pressure, and quenched. The obtained films were characterized by using XRD, SEM, EDS, M�ssbauer, and optical-absorption spectra. The XRD pattern clearly indicated the well-crystallized and nearly monophasic YbFe2O4 formed on the substrates above 900�C to control the oxygen partial pressure. Equal amount of Fe2+ and Fe3+ ions was present in the films. The films showed small band-gap and large optical absorbance in infrared region.

DOI： 10.2497/jjspm.61.S327

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPRINGER  

Fe-57 Mossbauer spectroscopy was performed on two types of Fe oxide nanoparticles supported on a typical ferroelectric, BaTiO3. It was found that the valence state of FeO nanoparticles changed to a mixed 2+/3+ state at high temperature where BaTiO3 shows paraelectric behaviour. We attribute this phenomenon to the fluctuation of electric dipoles which realizes carrier injection into the Fe oxides. This is the first report which discusses a dynamical valence state of transition metal oxides supported on ferroelectrics.

DOI： 10.1007/s10751-012-0687-3

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The magnetocapacitance effect was investigated using impedance spectroscopy on single crystals of LuFe2O4. The intrinsic impedance response could be separated from the interfacial response and showed a clear hysteresis loop below T-Ferri similar to 240 K under the magnetic field. The neutron diffraction experiment under the magnetic field proves the origin of the dielectric property related to the motion of the nanosized ferromagnetic domain boundary. These results imply that the modification of the microscopic domain structure is responsible for the magnetoelectric effect in LuFe2O4. DOI: 10.1103/PhysRevLett.110.117602

DOI： 10.1103/PhysRevLett.110.117602

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We have investigated the magnetic properties of a single crystal of multiferroic YbFe2O4. Ferrimagnetic ordering of Fe spins is observed at similar to 250 K. The magnetization measured during field cooling becomes negative below similar to 10 K owing to the antiparallel coupling between the Fe and Yb sublattices. The magnetization below similar to 10 K is flipped by changing the applied magnetic field in a positive direction, a property which may be used to the development of magnetic devices. Magnetocaloric measurements show a broad peak of the entropy change, likely owing to a multiple magnetic transition.

DOI： 10.1088/1742-6596/428/1/012032

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We report nonlinear conductivity associated with a phase transition in polar charge ordered material YbFe2O4. To investigate the existence of nonlinear conductivity, we measured temperature dependence of conductivity as a function of applied electric current and estimated the sample heating precisely. Then we found the nonlinear conductivity below three dimensional charge ordering temperature 340 K. The result is consistent with diffraction observation.

DOI： 10.1080/00150193.2013.773874

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We have investigated the magnetic properties of R2Fe 3O7 (R=Yb and Lu), which belongs to the homologous family of electronic ferroelectric RFe2O4. Magnetization measurements show ferrimagnetic ordering at the Néel temperature (T N) of ∼270 K, which is slightly higher than that of RFe 2O4 (TN∼230-250 K). Observation of spin glass and exchange bias shows a coexistence of ferromagnetic and antiferromagnetic interactions, similarly to RFe2O4. This situation gives rise to a complex magnetic change and a broad peak of magnetic entropy change. Some characteristics, such as refrigeration temperatures higher than in RFe2O4, seem to offer a possibility of this system to applications. © 2013 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ssc.2013.08.030

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An exchange bias (EB) behavior has been observed for multiferroic RFe2O4 (R Y, Er, Tm, Yb, Lu, and In). The materials with small R3+ ions (R = Tm, Yb, Lu, and In) exhibit large EB fields (greater than or similar to 1 kOe) below similar to 100-150 K. This property is rooted in a magnetically glassy state, arising from the competition between ferromagnetic and antiferromagnetic domain interactions. In addition, the EB field tends to be more enhanced for smaller R3+ ions. Hence, the EB is controlled by the substitution at the R-site in this series.

DOI： 10.1143/JPSJ.81.033704

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The coexistence of ferroelectricity and magnetic order in epitaxial hexagonal YbFeO3 (h-YbFeO3) thin films deposited on yttria stabilized zirconia substrates is examined using an impedance analyzer, a second harmonic generation (SHG) microscope, an X-ray diffractometer, a transmission electron microscope and a superconducting quantum interference device magnetometer. The results show that h-YbFeO3 films exhibit ferroelectricity below 350K with threefold translational periodicity along the [110] direction similar to rare earth manganese oxides, and ferrimagnetism with a four-up and two-down spin configuration of Fe+3 and Yb+3 ions along the c-axis at low temperatures. In addition, a possibility of new ferroelectric phases with an electronic origin induced by the magnetic ordering is discussed based upon SHG and D-E hysteresis measurements.

DOI： 10.1143/JPSJ.81.024719

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Magnetic and dielectric properties have been investigated for Bi2CuO4, which has the same chemical formula as that of the parent materials of cuprate superconductors R2CuO4 (R: rare earths). Magnetization measurements show the antiferromagnetic transition of the Cu2+ spins at similar to 42 K, as reported previously. Dielectric measurements for the frequencies of 1 kHz to 1 MHz show that the dielectric constants are 100-500 at room temperature. The dielectric dispersion reveals that the dielectric response lacks spatial coherence, a property which indicates the possible existence of phase separation as suggested for La2CuO4. The imaginary part of dielectric response gives the activation energy of 0.22 eV, suggesting that the dielectric response is governed by the electron hopping between the Cu ions. (C) 2011 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2011.05.049

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In order to clarify the stability of charge ordered structure (CO) in charge- and spinfrustrated ferrite YbFe$_{2}$O$_{4}$, we have investigated changes of the CO structure by partial substitution of Mn$^{2+}$ for Fe$^{2+}$ in YbFe$_{2}$O$_{4}$ by a transmission electron microscope (TEM), incombination with conventional dielectric measurement. It is revealed that subtle substitution of Mn$^{2+}$ for Fe$^{2+}$ in YbFe$_{2}$O$_{4}$ destroyed drastically the CO structure with the wave vector of q=$<$1/3 1/3 1/2$>$ and, instead, induced polar clustering structure characterized by honeycomb shaped diffuse scatterings in the reciprocal space. The formation of polar clustering structure should be responsible for the characteristic dielectric dispersion.

DOI： 10.1088/1742-6596/320/1/012085

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The electronic properties of Eu2.75C60 are studied using magnetic susceptibility and electron spin resonance (ESR) from 2 to 300 K. Both the magnetic susceptibility and the ESR parameters clearly show an anomaly around the valence transition temperature, T-V = 70 K. The magnetic susceptibility shows weak temperature dependence above T-V, while it changes drastically to Curie-Weiss behavior below T-V. The low-temperature susceptibility can be reproduced by assuming the moment of free Eu2+ ions. This result reveals that Eu2.75C60 changes from the intermediate valence state to the divalent state below T-V. Although ESR signals above T-V should be attributed to conduction electrons, the ESR intensity below T-V follows the Curie-Weiss law with a distinct increase in the g-factor. This should be associated with a strong localization of pi electrons. We also found that, below similar to 17 K, the isothermal magnetization exhibits a weak hysteresis and thermoremanent magnetization appears. These results suggest that valence-ordered Eu2.75C60 undergoes antiferromagnetic ordering with a weak ferromagnetic component at the Neel temperature, T-N = 17 K.

DOI： 10.1103/PhysRevB.83.245103

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

The frequency dependence of the complex dielectric constant of LuFe2O4 single crystal is measured by terahertz time-domain spectroscopy. We observed a broad infrared active mode in the real and imaginary parts of the complex dielectric constants at around 25 cm(-1).

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The variation of AC dielectric constant in the function of the DC magnetic fields was found in low oxygen vacant LuFe2O4 single crystal in which the electric polarization arise from the polar charge ordering. These magneto-electric coupling should be explained by a novel mechanism where the boundary motion of the polar charge ordered domain is affected by the magnetic (spin) domain boundary motion.

DOI： 10.1080/00150193.2011.577371

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Changes of charge ordered (CO) structure by partial substitution of Mn for Fe in YbFe2O4 were investigated by a transmission electron microscope (TEM), in combination with conventional dielectric measurement. It is revealed that partial substitution of Mn2+ for Fe2+ in YbFe2O4 destroyed drastically the CO structure with the wave vector of q=&lt;1/3 1/3 1/2&gt;. Consequently polar clustering structure giving rise to honeycomb-shaped diffuse streaks was found in YbFeMnO4. The random distribution of polar clustering structure gives rise to characteristic broad dielectric dispersion.

DOI： 10.1088/1757-899X/18/9/092047

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The low-temperature superstructure of YFe2 O4-δ was carefully investigated by transmission electron microscopy. We found that the unique superstructure at about 100 K is characterized by 1̄ /142/71/14 -type superlattice reflection spots, suggesting the presence of charge-reordering process. The low-temperature high-resolution images clearly show superlattice modulations in Y-O layers as well as Fe-O layers. The modification of intrabilayer charge interactions due to lattice distortions in Y-O layers is discussed to play a crucial role in the stabilization of long-periodic superstructures at low temperatures. © 2010 The American Physical Society.

DOI： 10.1103/PhysRevB.82.184119

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Magnetic and dielectric properties of perovskite manganites R0.5Sr0.5MnO3(R = Gd, Tb and Dy) have been investigated. DC and AC magnetic measurements showed short-range glassy magnetic ordering at T-g similar to 40 K. Such ordering was observed by neutron diffraction and is ascribable to the size mismatch of R3+ and Sr3+ settled randomly at the same crystallographic site. Dielectric constants for each material were similar to 1000-10,000 between similar to 50 and similar to 300 K and showed broad maximums above T-g. Dielectric dispersion showed poor coherency of the motion of polar regions, plausibly because of the size-mismatch effect; both the magnetic and dielectric properties of this system are governed by the randomness at the R/Sr site. The tan delta and EXAFS data suggest that the dielectric response is rooted in a transfer of the Mn-3d electrons. (C) 2010 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.materresbull.2010.07.024

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O-2 gas sensing behaviors are studied in picene thin film field-effect transistors (FETs) with hydrophobic polymer (Cytop(TM) or polystyrene) coated SiO2 gate dielectrics. Picene thin film FETs show a rapid reduction of hysteresis in transfer curves recorded in forward and reverse measurement modes, compared to a picene FET with hexamethydisilazane-coated SiO2. The picene FETs show very sensitive O-2 gas sensing effects down to similar to 10 ppm. A quantitative analysis is presented of the gate voltage (V-G) dependent mobility induced by O-2 exposure, i.e., the suppression of drain current at high V-G. (C) 2010 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.orgel.2010.06.003

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Here we report the structural study of NbSe3 topological crystals via synchrotron x-ray measurements on individual microcrystals. Topological crystals are characterized by the shape of a ring or a twisted loop, thereby representing a translational and orientational order confined in topologically nontrivial, curved geometries. The crystal lattice of topological crystals is under considerable amount of strain. The volume-averaged strain asymptotically increases with thickness and curvature of a crystal. We present a structural model in which a ring crystal is composed of well-ordered domains in the shape of concentric cylinders. The internal structure reflects periodic accumulation and release of strain during the growth.

DOI： 10.1103/PhysRevB.82.014105

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Simultaneous synchrotron radiation and transport measurements for the organic thyristor family compound theta-(BEDT-TTF)(2)RbZn(SCN)(4) have revealed the presence of microscopic competing domains between the twofold and 3 x 4 charge orderings in a triangular lattice. In contrast to conventional correlated electron systems, charge ordering is a unique critical parameter of phase competition. With external current, only the twofold ordering melts, which is responsible for the emergence of a nonlinear response.

DOI： 10.1143/JPSJ.79.044606

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A new oxide, Bi(Mg1/2Ti1/2)O3 was prepared using a high-pressure high-temperature technique at 8 GPa and 1000°C. As-sintered Bi(Mg1/2Ti1/2)O3 has the BiFeO 3-type perovskite structure with the rhombohedral R3c. After ground, crystal structure changed to tetragonal P4mm. Structure parameters of tetragonal Bi(Mg1/2Ti1/2)O3 were refined from laboratory X-ray powder diffraction data (a = 4.033(10) Å, and c = 3.938(10) Å). Bi(Mg1/2Ti1/2)O3 does not decompress at ambient pressure, but the structure changed from rhombohedral to tetragonal on heating above 800° C. This phase transition was nonreversible. Bi(Mg 1/2Ti1/2)O3 is expected to be one of the end members of lead-free ferroelectric materials. © (2010) Trans Tech Publications.

DOI： 10.4028/www.scientific.net/KEM.421-422.30

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Doping effects on the charge ordered (CO) structure in RFe 2O4 (R=Lu, Yb) were investigated by an energy-filtered transmission electron microscope (EF-TEM), in combination with conventional magnetic measurement. The repalcement of Cu2+ and Co2+ for Fe2+ in RFe2O4 (R=Lu, Yb) destroyed drastically the CO structure. In LuFeCuO4, the CO structure with q=1/3 1/3 1/2 disappear and short-ranged domains consisting of cationic ordering of Fe3+ and Cu2+ ions are formed, which gives rise to characteristic zigzag-shaped diffuse streaks with diffuse spots at the h/3-δ h/3-δ 0-type positions (h: integer, δ=0.06) in reciprocal space. In addition, the magnetic measurement revealed that LuFeCuO4 exhibits a magnetic transition around 50K, which is much lower than the Neel temperature of 250K in LuFe2O4. In YbFeCoO4, clusters consisting of Fe3+ and Co2+ in the triangular lattice are formed and give rise to honeycomb-shaped diffuse streaks. © 2010 IOP Publishing Ltd.

DOI： 10.1088/1742-6596/200/1/012128

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Conditions for epitaxial growth of hexagonal YbFeO3(h-YbFO) using the PLD method are examined. Ferroelectricity of h-YbFO is confirmed at room temperature by dielectric, dilatometric, SHG and TEM techniques which show anomalous temperature behaviors around 350K. SQUID reveals ferrimagnetic spin configuration along the c-axis in low temperature. © 2010 IEEE.

DOI： 10.1109/ISAF.2010.5712256

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We report the observation of a modulated structure and a ferromagnetic insulating state in a high quality single crystal of a nine-layer BaRuO3. Using x-ray scattering, the modulated satellites were observed to double the unit cell along the c-axis at low temperature. The ferromagnetic insulating state is confirmed by magnetic and resistivity measurements. Analyzing the peak profiles from the modulation and host structure respectively, showed a lattice distortion at T similar to 55 K. These findings elucidate the intimate relationship between ferromagnetism and lattice distortion in a nine-layer BaRuO3.

DOI： 10.1088/0953-8984/22/3/036003

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A magneto-electric (ME) effect was observed in a charge-ordered ferroelectric LuFe2O4. The dielectric response in a three-dimensional charge-ordered phase is strongly influenced by magnetic field, especially around the fern-magnetic ordering temperature. We also show the relation between the oxygen stoichiometry and the ME signal character above the magnetic transition temperature.

DOI： 10.1088/1742-6596/200/1/012077

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DOI： 10.14723/tmrsj.35.111

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Language：English   Publisher：The Japan Society of Applied Physics  

We investigated the average crystal structure and microstructures of LuFe0.56Ti0.44O3 by transmission electron microscopy (TEM) in combination with dielectric measurements. In this work, high-quality polycrystalline samples of LuFeM0.56Ti 0.44O3 were successfully synthesized in air by the conventional solid-state reaction. The average crystal structure at room temperature is characterized by a hexagonal structure (space group: P6 3cm). Dielectric measurements revealed that LuFe 0.56Ti0.44O3 shows broad dielectric peaks at approximately 450 and 580K, one of which is considered to originate from the formation of polar domains at the nanometer scale. © 2009 The Japan Society of Applied Physics.

DOI： 10.1143/JJAP.48.09KB04

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The effects of oxygen deficiency on charge ordering were investigated by transmission electron microscopy at various deficiencies and temperatures of YFe2O4-delta. We found that the detailed nature of the charge ordering structures is associated with changes in the superlattice reflections and diffuse scattering. They go from (1/3 1/3 1/2)-type superlattice reflection to (1/3 1/3 l)-type straight-type diffuse scattering, then to zigzag-type diffuse scattering. With the oxygen deficiencies increased, the interchange correlation of Fe-O bilayers obviously decreases, yet the triple superperiodicity along [110](*) remains unchanged. These findings on frustrated ferrites indicate the stability of in-plane threefold charge ordering and the importance of the interchange interaction between bilayers in the structural phase transitions.

DOI： 10.1103/PhysRevB.80.092104

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The partial oxidation of [Ni-2(II/II)(RCS2)(4)] (R = Et (1), n-Pr (2), and n-Bu (3)) with iodine affords the MMX chain compounds [Ni-2(II/III)(RCS2)(4)I](infinity) (R = Et (4), n-Pr (5), and n-Bu (6)), respectively. The crystal structures of 4-6 consist of neutral one-dimensional (1-D) chains with a repeating -Ni-Ni-I- unit. The room-temperature (RT) structure of 4 indicates a charge-polarization (CP) state {-Ni(2.5-delta)+-Ni(2.5+delta)+I--Ni(2.5-delta)+ -Ni(2.5+delta)+-I- (delta &lt;&lt; 0.5)} close to an averaged valence state judged by the Ni-I distances. In contrast, 5 and 6 exhibit a Vold periodicity of a -Ni-Ni-l- unit due to the disorder of the dithiocarboxylato ligands. Compounds 4-6 show typical semiconducting behavior and exhibit an intense sharp absorption band centered at 5400 cm(-1), which is attributed to a Mott-Hubbard gap due to a relatively large on-site Coulomb repulsion energy U of the nickel atoms. The high-temperature magnetic susceptibilities of 4-6 can be described by a 1-D Heisenberg antiferromagnetic chain model with vertical bar J vertical bar/k(B) ranging from 898(2) to 939(3) K. Compounds 4 and 5 undergo a spin-Peierls (SP) transition at relatively high T-sp = 47 and 36 K, respectively, which are accompanied by superlattice reflections corresponding to a 2-fold -Ni-Ni-l- period below T-sp. By determining the superstructure of 4 at 26 K, we conclude that the valence-ordered state changes from the CP in the RT phase to the altemate charge-polarization (ACP) state of -Ni(2.5-delta)+-Ni(2.5+delta)+-I--Ni(2.5-delta)+-I-Ni(2.5+delta)+-Ni(2.5-delta)+-I- -in the SP phase. Such a spin-Peierls transition could not be observed for 6.

DOI： 10.1021/ic900630a

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ACADEMIC PRESS INC ELSEVIER SCIENCE  

We have investigated the magnetic and dielectric properties of YbFe2-xMnxO4 (0 &lt;= x &lt;= 1), which is all Fe-site-substituted system of new multiferroic oxides RFe2O4 (R = Y, Ho-LU). X-ray diffraction measurements show that a solid solution is formed between YbFe2O4 (x = 0) and YbFeMnO4 (X = 1) for (0 &lt;= x &lt;= 1), whereas only compounds with x = 1 (i.e., RM1M2O4: M-1 and M-2 = trivalent and divalent cations, respectively) have been known for the Fe-site substitution in RFe2O4. The valence of the Mn ion is determined to be similar to 2+, consistently with the Suppression of low-temperature magnetization by the Mn substitution. The magnetic transition temperature (T-N) and the dielectric constant (epsilon&apos;) decrease monotonically with increasing x. This result plausibly confirms that the magnetic and dielectric properties in oxides isostructural with RFe2O4 are governed by the electron transfer between Fe-site ions, Unlike ordinary ferroelectrics and dielectrics, in which the ionic displacement plays a key role. The possibility for application is briefly discussed as well. (C) 2009 Elsevier Inc. All rights reserved.

DOI： 10.1016/j.jssc.2009.04.008

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

C-60 and picene thin film field-effect transistors (FETs) in bottom contact structure have been fabricated with poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PED-OT:PSS) electrodes for a realization of mechanical flexible organic FETs. The C-60 thin film FETs showed n-channel enhancement-type characteristics with the field-effect mobility mu value of 0.41 cm(2\) V-1 s(-1), while the picene thin film FET showed p-channel enhancement-type characteristics with the p of 0.61 cm(2) V-1 s(-1). The p values recorded for C-60 and picene thin film FETs are comparable to those for C-60 and picene thin film FETs with Au electrodes. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.orgel.2009.01.006

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Language：Japanese   Publisher：The Crystallographic Society of Japan  

The rare-earth iron oxides RFe₂O₄ (R = Y, Ho-Lu) have a rhombohedral crystal structure, consisting of an alternating stacking of triangular lattices of R, Fe and O ions. Recently, we have proposed that this system shows a new mechanism of ferroelectricity ; the ferroelectric state originates from a polar charge-ordered structure of Fe²⁺ and Fe³⁺ on a triangular lattice. In this article, a review is given on the details of this ferroelectricity, which were revealed by measurements of synchrotron X-ray diffraction and dielectric properties of one of the RFe₂O₄ oxides, LuFe₂O₄.

DOI： 10.5940/jcrsj.51.162

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We report a synchrotron X-ray study of charge density waves (CDWs) in an o-TaS3 crystal. CDW of o-TaS3 has been known to undergo a commensurate-incommensurate transition at 100 K, below which the wavevector locks in with the pristine lattice. However, the details of the commensurate-incommensurate transition remain open problems, the example being whether the discommensuration exists or not in the neighborhood of the transition. We exploited the beamline BL02B1 of SPring-8. A two-dimensional detector was placed at a distance of 1300mm from the sample, providing an angle resolution of 0.0076 degrees. Temperature dependence of the Bragg peak (0 0 2) and satellite peak (1 - 12) + q was measured from 7 to 180 K. We found that a new phase in the temperature range of 130-50 K, where two independent CDWs coexist. These waves are incommensurate and commensurate CDWs with longitudinal wave vectors q(c) = 0.252c* and 0.250c*, respectively. By lowering the temperature, intensity of the incommensurate CDW was decreased, while that of the commensurate CDW was increased. Below 50K, the incommensurate CDW was completely diminished. Based on the concept of discommensuration, we suggested the dislocation configuration from the intensity of the two CDWs. (C) 2008 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physb.2008.11.017

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

We have measured and analyzed the resistivity and voltage-current characteristics of the charge-ordered organic salt theta-(BEDT-TTF)(2)CsM(SCN)(4) (M = Zn, Co, and Co(0.7)Z(0.3)). At 4.3 K, all the samples show giant nonlinear conduction, in which the c-axis resistivity markedly decreases with external current by a factor of 10 against a small electric field of 1 V/cm. The temperature dependence of the resistivity is well described by an activation law, the activation energy of which is evaluated to be 2 meV in the low-current limit, and linearly decreases with external current. The activation energy and x-ray intensity near q(2) = (0, k, 1/2) are found to show the same dependence on current density. This coincidence strongly suggests that the energy gap of the charge order indexed by q(2) is an order parameter of this phenomenon.

DOI： 10.1143/JPSJ.78.024714

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

Diluted effect on the charge ordered (CO) structure in dielectric materials LuFeMO4 (M = Cu, Co) were investigated by an energy-filtered transmission electron microscopy (TEM), in combination with a conventional TEM. It is revealed that partial substitution of Cu2+ and Co2+ for Fe2+ in LuFe2O4 destabilized the CO structure. In LuFeCuO4, there exist characteristic zigzag-shaped diffuse streaks and diffuse spots at the h/3-delta h/3-delta 0-type positions (h: integer, delta = 0.06) in reciprocal space, which originate from static cationic ordering of Fe3+ and Cu2+ ions in the triangular lattice. Moreover, real-space images by the energy-filtered TEM revealed that nanometer-sized domain structures with the 5 similar to 10 nm size are formed. In contrast, honeycomb-shaped diffuse streaks were found in LuFeCoO4, in which no domain structures are observed in real-space images.

DOI： 10.1080/00150190902873204

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The magnetic and dielectric properties of perovskite manganites R0.5Ca0.5MnO3 (R = Eu-Lu), which show charge ordering of the Mn-3d electrons below room temperature, have been investigated. Their dielectric constants were similar to 1000-10000 at around room temperature. Inflections or peaks of temperature dependences of the dielectric constants were observed at around the charge-ordering temperatures. Analyses of tan delta for R = Eu-Lu and EXAFS spectra for R = Dy suggested that the dielectric properties of the present system are rooted in a special arrangement and transfer of the Mn-3d electrons. The two-peak structure of tan d indicates that dielectric measurement can be used to observe homogeneity or inhomogeneity of sample, as mentioned in our previous paper on the ruthenium perovskite oxides.

DOI： 10.1080/00150190902852208

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

We report dielectric and magnetic properties of a single crystal of LuFe2O4 with different degrees of oxygen deficiency. The oxygen stoichiometry was monitored with the gas mixture ratio of CO2 and CO during the crystal growing process. A sample having high resistivity of about 10 k cm sample was found at room temperature. The relation between the control of the charge frustration and the completeness of the charge ordering is discussed.

DOI： 10.1080/00150190902859187

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Language：Japanese   Publisher：The Materials Research Society of Japan  

Recently, there has been considerable interest in the dielectric properties of the cubic perovskite CaCu₃Ti₄O₁₂. The material has shown to have relatively high temperature independent dielectric permittivity over a wide temperature range from 100 K to 600 K. Below ～ 100 K the permittivity drops by almost three orders of magnitude, but is not accompanied by a structural phase transition. We studied CaCu₃Ti₄O₁₂ hoping to find signatures of local structural distortions that may explain the observe anomalies in the dielectric properties. Experiments were carried out using synchrotron radiation at several temperatures between room temperature and 10 K. Reconsideration is necessary for interpreting a huge permittivity by the local disorder model.

DOI： 10.14723/tmrsj.34.15

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DOI： 10.1088/1742-6596/150/4/042068

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We have investigated the physical properties and local atomic structure of LuFe₂O₄, a material belonging to the multiferroic system RFe₂O₄ (R: rare earths) which shows the new type of ferroelectricity owing to the charge ordering of Fe ions. Magnetic measurements showed the ferromagnetic ordering of Fe spins at ～230 K, as reported previously. From dielectric measurements, the dielectric constant (ε) of ～10000 was obtained at around room temperature. In addition, a peak of ε was observed near the charge-ordering temperature (～330 K). The peak temperature was insensitive to the frequency of AC electric field; this behavior is different from that found in a previous study. The pair-distribution function analysis on a high-energy X-ray diffraction pattern revealed that LuFe₂O₄ has the systematic modulation of the local atomic displacement.

DOI： 10.14723/tmrsj.34.51

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC APPLIED PHYSICS  

We investigated the effect of oxygen annealing on dielectric properties of LuFeCuO4 mainly by dielectric measurements and transmission electron microscopy experiments. It was found that the leakage current density decreases with high-pressure oxygen annealing and the size of nanodomains with orientational polarization increases, which is characterized by the short-range ordering of Fe3+ and Cu2+ ions on a triangular lattice. In addition, the absolute value of relative permittivity epsilon &apos; decreases with oxygen annealing and epsilon &apos; exhibits a broad peak in the temperature window between room temperature and 550 K, which is similar to that observed in relaxor ferroelectric materials. The present experimental results suggest that the number of oxygen vacancics has a crucial effect on dielectric properties of LuFeCuO4. [DOI: 10.1143/JJAP.47.8464]

DOI： 10.1143/JJAP.47.8464

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Magnetic and dielectric correlation in triangular mixed valence iron oxide RFe(2)O(4) is discussed. The frustrated interaction for antiferromagnetic spins as well as charges realizes the auto-organized superlattice cell of iron ions. The characteristic charge and spin superlattice cell drives the magnetoelectric cross-correlation. The charge supercell detected with a resonant x-ray scattering measurement has an electric polarization that forms a class of new ferroelectrics.

DOI： 10.1088/0953-8984/20/43/434218

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：EPL ASSOCIATION, EUROPEAN PHYSICAL SOCIETY  

We report X-ray diffraction measurements of the charge-ordered organic conductor theta-(BEDT-TTF)(2)CsZn(SCN)(4) at various temperatures with external currents. The two kinds of diffuse scattering observed at q(1) = (2/3 k 1/3) and q(2) = (0 k 1/2) exhibit remarkable difference in the interplane correlation along the b-axis, the temperature dependence, and the electric-current dependence. These observations suggest that the two diffuse scatterings come from spatially different charge-ordered domains. The external current melts the q(2)-type domains, which can control the competition of the two orders. Copyright (C) EPLA, 2008

DOI： 10.1209/0295-5075/84/26002

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

We report a synchrotron X-ray Study of charge density waves (CDWs) in an o-TaS3 crystal. We found that two independent CDWs coexist ill the temperature range of 130-50K. These waves are incommensurate and commensurate CDWs with longitudinal wave vectors q(c) = 0.252c* and 0.250c*, respectively. The temperature and electric current dependences of the intensity of the two CDW satellites were measured. We found that the commensurate CDW was converted to the incommensurate CDW at 80 K by inducing current flow. Our observation was interpreted in terms of the dynamics of topological defects. We determined the edge dislocation configuration from the electric Current dependence of the intensity of the two CDWs. The result implies Cor the first time that discommensurations are induced ill the commensurate CDW by applying an electric field.

DOI： 10.1143/JPSJ.77.093708

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC APPLIED PHYSICS  

The effect of oxygen vacancies on the charge ordered structure at room temperature (RT) in YFe2O4-delta was investigated by transmission electron microscopy (TEM) and it was found that the charge ordered structure is sensitive to the amount of oxygen vacanceis. Real-space images of the three-dimensional charge ordered structure revealed that the average sixe of charge ordered domains can be estimated to be 20 nm. These nanosized charge ordered domains are responsible for the dielectric properties of YFe2O4-delta was investigated by transmission electron microscopy (TEM) and it was found that the charge ordered structure is sensitive to the amount of oxygen vacancies. Real-space images of the three-dimensional charge ordered domains are responsible for the dielectric properties of YFe2O4-delta. The charge ordering processes were also examined by in-situ TEM.

DOI： 10.1143/JJAP.47.7595

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC APPLIED PHYSICS  

Rare-earth iron oxides of the form RFe2O4 (R = Y, Ho-Lu) show a new mechanism of ferroelectricity owing ot the charge ordering of Fe ions below similar to 330 K. To determine the physical propertie sof the isystem, the magnetic and dielectric properties of HoFe2O4 and R1-xRx&apos;Fe2O4 were investigated (R, R&apos;: rare earths). All the materials showed dielectric constants of similar to 100-10000, as reported for some other RFe2O4 oxides. For HoFe2O4, magnetic ordering occurred at a temperature lower than tose of RFe2O4 reported thus for. On the other hand, the dielectric measurements of this material suggested the development of a correlation between polar regions at higher temperatures, which was also observed for the sc-substituted system Lu1-xScxFe2O4. These results encourage detailed investigations of these materials form the viewpoints of both fundamental and applied sciences and suggest the existence of other new properties of RFe2O4.

DOI： 10.1143/JJAP.47.7599

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

The magnetic and dielectric properties of InFe2O4, InFeCuO4, and InGaCuO4 have been investigated. All these materials are isostructural with RFe2O4 (R = Y, Ho-Lu), which shows ferroelectricity due to iron-valence ordering. InFe2O4 exhibits ferrimagnetic ordering at T-C similar to 242 K and a dielectric constant (epsilon) of similar to 10000 at around room temperature. These properties resemble those of RFe2O4; the origins of the magnetic and dielectric phenomena are likely common in InFe2O4 and RFe2O4. From measurements of the other two materials, we found that both T-C and epsilon are decreased in the order of InFe2O4, InFeCuO4, and InGaCuO4. This result strongly supports the previously reported explanation based on an electron transfer between the Fe-site ions for the corresponding rare-earth systems. Therefore, we propose that the dielectric properties of the oxides isostructural with RFe2O4 are plausibly governed by electron transfer; this situation is different from that of ordinary ferroelectrics and dielectrics, in which the displacement of cations and anions is important. In addition, InFeCuO4 and InGaCuO4 exhibit large epsilon values (epsilon &gt; similar to 1500). In consideration of this property, we discuss the possible applications of these oxides.

DOI： 10.1021/ic702365k

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

DOI： 10.1143/JPSJ.77.065004

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

We report our recent discovery of novel ferroelectricity arising from the polar ordering of Fe3+ and Fe2+ in a mixed valence triangular lattice oxide LuFe2O4, where the electric polarization is not a result of ionic displacement. The polar ordering of Fe3+ and Fe2+ was confirmed with a resonant x-ray scattering study in SPring-8. The origin of such ordering is the competitive interaction between Fe3+ and Fe2+ in the triangular lattice, i.e., the charge frustration. The polar superlattice of Fe3+ and Fe2+ develops below 350 K, where the electric polarization appears. The ferro electricity arising from the polar charge ordering or the polar electron distribution may have great potential for the future application of ferroelectrics.

DOI： 10.1109/TUFFC.2008.753

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Language：Japanese   Publisher：The Crystallographic Society of Japan  

Recent works revealed that a ferroelectric polarization in the charge-frustrated system LuFe₂O₄ originates from an emergence of the charge ordering, which is characterized by a regular arrangement of Fe²⁺ and Fe³⁺ ions on the frustrated triangular lattice. In this work, we have investi-gated both the charge ordered (CO) structure and charge ordering processes mainly by means of the transmission electron microscopy (TEM) . Both the CO structure and charge ordering processes depend strongly on the amount of the oxygen deficiency (δ) involved in LuFe₂O_4-δ. In the case of δ-0, the three-dimensional CO structure with the modulation vector of q= [1/3 1/3 1/2] was formed at room temperature. On the other hand, as δ was increased, the three-dimensional CO structure changed into the two-dimensional CO one and the coherent length becomes shorter along the [001] direction. For proper understanding of the dielectric and magnetic properties in LuFe₂O₄ and related materials, it should be essential to prepare LuFe₂O_4-δ samples with properly controlled oxygen-concentration and elucidate the physical properties in LuFe₂O_4-δ with δ-0.

DOI： 10.5940/jcrsj.50.150

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：MATERIALS RESEARCH SOCIETY JAPAN-MRS-J  

We investigated magnetic/dielectric properties and related microstructures in LuFeCuO(4), which was obtained by substituting Cu(2+) for Fe(2+) in LuFe(2)O(4). It was found that LuFeCuO(4) shows a characteristic low-frequency dielectric dispersion around 300K. We analyzed it using the Debye model and found that LuFeCuO(4) is a dielectric compound with an orientational polarization. Electron diffraction experiments revealed that there exists characteristic zigzag-shaped diffuse scattering in LuFeCuO(4). In addition, we found the presence of nano-sized domains, which consist of a short-range ordering of Fe(3+) and Cu(2+) on the triangular lattice. The present results suggest that the short-range ordering gives rise to some anomalous dielectric properties in LuFeCuO(4).

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DOI： 10.1107/S0108767308083657

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The magnetic and dielectric properties of a perovskite manganite Tb0.5Ca0.5MnO3 were investigated. Magnetic measurements showed three characteristic temperatures, originating from charge, ordering (similar to 360K), antiferromagnetic ordering (similar to 120K), and a transition to a glassy state (similar to 50 K). The coexistence of the charge and antiferromagnetic orderings was confirmed by powder neutron diffraction. AC dielectric measurements showed large dielectric constants larger than similar to 1000 at approximately room temperature. The temperature dependence of the dielectric constant changed just below the charge-ordering temperature. From the analysis of tan delta, the activation energy for switching the directions of polar regions was calculated to be similar to 0.09 eV, which is close to the activation energy of the Mn 3d electron transfer. These results strongly suggest that the dielectric properties of this material are governed by the ordering and transfer of the Mn 3d charges.

DOI： 10.1143/JJAP.46.7171

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：KOREAN PHYSICAL SOC  

RMn2O5 (R = Y and rare earth) shows successive magnetic and ferroelectric phase transitions at about 45 K, 40 K, 39 K, 20 K and 10 K and is called a multiferroic compound. We have investigated the lattice modulation vectors (qL) of YMn2O5 and TbMn2O5 by using single crystal Synchrotron Radiation X-ray diffraction and compared them with the magnetic propagation vectors (qM) studied by neutron diffraction. The relation of the propagation vectors between magnetic materials and dielectrics is found to be qL = 2qM. A preliminary structure analysis of the atomic displacement pattern in the commensurate ferroelectric phase of TbMn2O5 is discussed.

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The magnetic and dielectric properties have been investigated for the isostructural oxides of RFe2O4, RFeMO4, and RGaCuO4 (R=Yb and Lu, M=Co and Cu). The magnetization measurements for RFe2O4 showed ferrimagnetic ordering at similar to 250 K. This system also exhibited large dielectric constants of similar to 10 000-30 000 at around room temperature, which is attributable to the charge-ordering-induced ferroelectricity, as was proposed in our recent paper. The magnetic transition temperatures are lowered to similar to 45-90 K for RFeMO4. Magnetic ordering is not found for RGaCuO4. ac magnetic susceptibility measurements indicate that magnetic ordering becomes short-ranged by the substitution at the Fe site. The overall characteristic behavior of the magnetic properties is explained in terms of the change of a spin value as well as the dilution of magnetic interactions. Although the ac dielectric measurements show the existence of polar regions in each material, the dielectric constants below 300 K become smaller in the order of RFe2O4, RFeCoO4, RFeCuO4, and RGaCuO4. From the analysis of the dielectric dispersion, the distribution of the fluctuation time of polar regions is wider in RFeCoO4, RFeCuO4, and RGaCuO4 than in RFe2O4; a coherent motion of polar regions is suppressed in the substituted systems. By comparison to the results from the magnetic measurements, the dielectric properties are discussed in connection with a charge transfer between the transition-metal 3d orbitals, consistently with the proposed mechanism of the ferroelectricity in RFe2O4.

DOI： 10.1103/PhysRevB.76.024423

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The magnetic and dielectric properties are investigated for TmFe2O4 and TmFe2CuO4. The properties of TmFe,04, which shows the ferroelectricity from iron-valence ordering, are essentially the same as those of the other RFe2O4 oxides, i.e., ferrimagnetic ordering at around 250 K and dielectric constants larger than 3000 at around room temperature. On the other hand, the Cu-substituted oxide TmFeCuO4 shows no magnetic transition and considerably smaller dielectric constants. The change of the properties is likely attributable to a reduction of a charge transfer between the transition-metal ions. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jmmm.2006.10.327

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We have performed X-ray powder diffraction measurements in a mixed valence compound of Fe2BO4 using synchrotron radiation. The diffraction patterns obtained at several temperatures between 85 and 386K were analyzed by the Rietveld method. The monoclinic-orthorhombic transition, ascribed to charge-ordering of Fe2+ and Fe3+ ions, was confirmed by the temperature dependence of the monoclinic angle beta. No superlattice reflection due to the charge-ordering was detected. The temperature dependences of the lattice parameters show anomalies around 270K as well as the monoclinic-orthorhombic transition temperature of 327-347K. The structural change around 270K is attributable to the onset of the partial charge-delocalization. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jmmm.2006.10.184

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DOI： 10.1103/PhysRevB.76.094113

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We report a novel ferroelectricity arise from the polar ordering of Fe3+ and Fe2+ in a mixed valence triangular lattice oxide LuFe2O4, where the electric polarization do not arise from the ionic displacement. The polar ordering of Fe3+ and Fe2+ was confirmed with a resonant X-ray scattering study in SPring-8. The origin of such ordering is the competitive C, interaction between Fe3+ and Fe2+ in the triangular lattice, i.e. the charge frustration. The polar superlattice of Fe3+ and Fe2+ develops below 350K where the electric polarization appears. The ferroelectricity arising from the polar charge ordering or the polar electron distribution may have a great potential for the future application of ferroelectrics.

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We report a polar ordering of electrons in RFe2O4, which consistently explains the ferroelectric property of this material. Pyroelectric current measurement, low frequency AC dielectric constants, resonant X-ray scattering diffraction experiment and the consideration for the charge frustration, consistently lead the presence of ferroelectricity originated from the ordering of Fe3+ and Fe2+. The ferroelectric material originated from the polar electron distribution and free from the ionic displacement may potentially have great possibilities in future applications and fundamental researches.

DOI： 10.1080/00150190701196112

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The 2D crystallites of polar organic dye molecules on an aqueous subphase exhibit a thermochromic phase transition. The visible absorption band of the crystallites depends on the temperature and the composition of the subphase. In-situ observations of the phase transition were carried out by a grazing incidence X-ray diffraction method. A shift and a split of the diffraction peaks were observed between the different phases, indicating a change in the molecular arrangement of the crystallites in their chromic transition.

DOI： 10.1080/00150190601182477

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Change of the charge ordered (CO) structure by substituting Cu2+ for Fe2+ in LuFe2O4 was investigated by means of the transmission electron microscopy. The CO structure in LuFe 2O4 is characterized by the modulated structure with the wave vector of q=1/3[1-13/2] and the average size of the CO domains can be estimated to be about 10-20nm. On the contrary, the Cu substitution in LuFe 2O4 destroyed the CO structure drastically and induced characteristic local lattice distortion, which gives rise to characteristic diffuse scattering in the reciprocal space. High-resolution lattice images revealed that there exist nano-scale clusters, which are characterized as the short-range ordering of the Fe3+ and Cu2+ ions on the triangular lattice. In addition, the magnetic measurement revealed that LuFeCuO4 exhibits an antiferromagnetic transition around 50K, which is lower than the Neel temperature of 250K in LuFe2O4. © 2007 Materials Research Society.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

RMn2O5 (R = Y and rare earth) shows successive magnetic and ferroelectric phase transitions at about 45, 40, 39, 20 and 10 K. We have reinvestigated the magnetic structure of YMn2O5 at the commensurate phase (T = 25 K) using a single crystal four-circle diffractometer in order to discuss the mechanism of magnetoelectric interaction and the origin of ferroelectricity. We also observed the lattice modulation vectors (q(L)) to compare the magnetic propagation vectors (q(M)) by synchrotron X-ray diffraction. Improved magnetic structure data are compared with the theory recently proposed. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physb.2006.05.179

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-V C H VERLAG GMBH  

Magnetic and dielectric properties of the ruthenium double perovskites La2MRuO6 (M = Mg, Co, Ni, and Zn) were investigated. The magnetization measurements for M = Co and Ni showed the existence of magnetic order at 20-30 K. Though the oxides with M = Zn and Mg exhibit a deviation from the Curie-Weiss law, magnetic order was not clearly observed. The result of La2ZnRuO6 was different from that previously reported, in which a ferromagnetic transition was found at around 165 K. The AC dielectric measurements for M = Co and Ni showed large dielectric constants (typically larger than 1000) at around room temperature, suggesting both the formation of short-ranged polar regions and the magnetic origin of large dielectric constant. In addition, two peaks were found for the temperature dependence of the tan delta component for La2NiRO6. The behavior suggests the existence of two different polar regions. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

DOI： 10.1002/pssa.200669549

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

Resonant X-ray scattering (RXS) in Y1-xCaTiO3 has been investigated near the Y K-edge. The resonant signal has been clearly found at a forbidden reflection, (500), and the intensity shows an azimuthal angle dependence with a twofold symmetry. The RXS intensities have almost no Ca concentration dependence, although the RXS intensities at the Ti K-edge markedly decrease with increasing Ca concentration. The origin of the RXS at the Y K-edge is clarified to be the deviation from 180 degrees of the Ti-O-Ti bond angle, namely, the tilts of the octahedra around the Y ion. We also discuss the RXS mechanism at the main edge of the Ti K-edge.

DOI： 10.1143/JPSJ.75.094706

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DOI： 10.1103/PhysRevLett.96.169901

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

A powerful method to determine the hidden structural parameters in functional molecules has been developed. Local valence arrangements that dominate the material properties are sometimes not three-dimensionally ordered. This method that comprises diffuse X-ray scattering and resonant X-ray scattering is suitable in such cases. Using this method, we present clear evidence of the low-dimensional valence arrangement in two halogen-bridged one-dimensional metal complexes, so- called MMX chains. This family allows us to control many physical and structural parameters by chemical substitution of bridging halogen, counterions, or metal ions, and one of our samples carries an unusual metallic phase. It is demonstrated with this complex that the present method makes it possible to have microscopic insight to low-dimensionally ordered systems.

DOI： 10.1021/ja060111j

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

The magnetic and dielectric properties of TmFe2O4 were investigated. Magnetization measurements showed a ferrimagnetic transition around 240 K. A modulation of this ordering with a slow spin relaxation was found below similar to 70-100 K. A characteristic low-frequency dielectric dispersion was also obtained, which is analogous behavior to that for a few other RFe2O4 oxides reported previously. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physb.2006.01.155

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We prepared high-purity oxygen-free and single-phase Ni nanocrystals using the gas condensation and deposition method in order to examine the random anisotropy model (RAM). The grain size of an as-prepared sample was estimated to be 8-10 nm. Thermal annealing increased grain sizes (D) up to 22 nm. Clear D-6 dependence of the coercive force on a grain size smaller than 13 nm was obtained for samples prepared on both plastic and KBr single-crystal substrates. For larger grain sizes, H-c decreased with D-1 and these results clearly conform to the RAM in single-element nanocrystal systems. A local anisotropy constant was estimated from the slope of the D-6 relation and the peak of H-c at D=13 nm. (c) 2006 American Institute of Physics.

DOI： 10.1063/1.2188597

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We investigated the charge-ordered (CO) state in spinel AlV2O4 by electron diffraction, synchrotron x-ray diffraction, and magnetic measurements. It was found that the CO structure appearing below 700 K is characterized by the formation of V clusters (heptamers), each of which is consisting of 7 vanadium atoms and is in a spin-singlet state as a total. Theoretical consideration indicates that this unique molecularlike V heptamer is stabilized by a strong bonding of vanadium t(2g) orbitals.

DOI： 10.1103/PhysRevLett.96.086406

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

A new type of pulse magnet, x-ray and neutron transparent pulse magnet, is suggested for use in high-field diffraction experiments. The magnet is made of aluminum-based material, so we expect that the scattered beam penetrates the magnet body and hence the total number of reflection points available substantially increases. This feature possibly allows full determination of the lattice and magnetic structure in strong magnetic fields. To realize this idea, we constructed a prototype transparent magnet consisting of aluminum wire and duralumin reinforcement. With a portable capacitor bank, the maximum field of 25 T was successfully generated in a 1.6 mm bore, even though the aluminum wire is less strong and less conducting than copper wire. Since the total energy is small (E-bank similar to 100 J), high repetition rate, which is necessary for sufficient signal-to-noise ratio, is realized. Technical details, coil performance, and future prospects will be described.

DOI： 10.1088/174-6596/51/1/113

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Electrical conduction properties of YFe2O4, which is expected to be charge ordering type ferroelectrics, were investigated. YFe 2O4 was synthesized in reduced atmosphere at 1200°C Oxygen partial pressure of the reduced atmosphere was controlled by the equilibrium state of CO and CO2. YFe2O4 is paramagnetic at room temperature and has Néel temperature around 250K. The Néel temperature was decreased with increasing the amount of oxygen deficiency. Moreover, YFe2O4 showed ohmic conduction from 260 to 100 K. From the complex impedance measurements, the equivalent circuits of YFe2O4 with different oxygen deficiency were determined at various temperatures. © 2007 Materials Research Society.

DOI： 10.1557/proc-0966-t07-27

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：EDP SCIENCES S A  

High resolution X-ray structure analyses and electronic structure calculation revealed the condition for the charge ordering(CO) observed in the (TMTTF)(2)X (Fabre) salts. The phase diagram of the electronic states including 2k(F) and 4k(F) CO has been proposed associated with magnitudes Of Molecular dimerization and tetramerization.

DOI： 10.1051/jp4:2005131008

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ACADEMIC PRESS INC ELSEVIER SCIENCE  

It was found that the manganese perovskite oxides Ln(0.5)Ca(0.5)MnO(3) (Ln = Ho, Er, Tm, Yb and Lu) have an orthorhombic structure (space group Pnma). The Mn-O-Mn angles were calculated to be similar to 148-150 degrees, revealing an existence of a large crystallographic distortion in these oxides. Electrical resistivity measurements indicated both an insulating nature and a small magnetoresistance effect, both of which are owing to narrow bandwidths of the Mn-3d electrons arising from the crystallographic distortion. DC magnetization measurements showed the three characteristic temperatures, which could be assigned to charge-order, antiferromagnetism or Mn moments, and possible glassy states. All of these temperatures were decreased for the heavier Ln ions, which is explained in connection with both a difference of ionic radii of Ln(3+) and Ca2+, and a lowering of electron transfer. The charge-ordering transition was not clearly observed only for Lu0.5Ca0.5MnO3 containing the smallest lanthanide ion, plausibly due to a large randomness of magnetic interactions arising from the ionic radii difference of Lu3+ and Ca2+. In addition, preliminary measurements of AC dielectric response suggested that these manganites belong to a so-called multiferroic system. (C) 2005 Elsevier Inc. All rights reserved.

DOI： 10.1016/j.jssc.2005.08.033

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：OXFORD UNIV PRESS  

We examined microstructures related to the charge ordering in YFe2O4-delta, by transmission electron microscopy. It is found that two types of characteristic diffuse scatterings appear at room temperature. One is characteristic diffuse streaks elongated along the [0001] direction through the (1/3 1/3 (2) over bar /3 0)-type reciprocal positions and the other is diffuse spots at (1/3 - eta 1/3 - eta (2) over bar /3 + 2 eta 0)-type incommensurate positions (eta approximate to 0.066). Real-space images revealed that the former diffuse scattering is related to nano-domains due to the charge ordering and the latter one is related to those due to vacancy ordering. The presence of the nano-sized domains should be strongly related to the dielectric anomalies found in YFe2O4-delta.

DOI： 10.1093/jmicro/dfi009

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMERICAN PHYSICAL SOC  

We have succeeded in detecting ferro-type orbital states in Ca2-xSrxRuO4, which is the first outcome in a 4d Mott transition system by the resonant x-ray scattering interference technique. For x=0 (Mott insulator), the resonant signal for d(xy) orbital ordering is observed even at room temperature, in which the Jahn-Teller distortion is negligible. The signal disappears near the metal-insulator transition. On the other hand, in a metallic phase for x=0.5, orbital polarization with d(yz/zx) character dominates. With lowering temperature, the magnitude of the resonant signal slightly decreases owing to the additional influence of the gamma band with d(xy) character.

DOI： 10.1103/PhysRevLett.95.026401

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The effects of hydrostatic pressure on a structure and the corresponding resonant x-ray scattering (RXS) spectrum have been investigated for the hole-orbital-ordered compound (C2H5NH3)(2)CuCl4. We have found a structural phase transition at P-c similar to 4 GPa, as suggested by the Raman scattering measurement, by observing a peak splitting resulting from an orthorhombic-to-monoclinic symmetry breaking. The reduction of the Jahn-Teller distortion (JTD) toward P-c is also ascertained by structural analyses. The gradual change in the color of the crystal is also observed in connection with the structural change near P-c. The red-colored transparency of the crystal indicates that the system is still insulative above P-c. The observed RXS and fluorescence spectrum at ambient pressure were precisely analyzed. We experimentally confirmed that the RXS intensity comes from the polarization of the density of states of p(x) and p(y) symmetries, which is mainly dominated not by the on-site 3d-4p Coulomb interaction but by the JTD of the CuCl6 octahedron. charged Cl ions neighboring the Cu site. It is also expected that the RXS intensity is proportional to JTD; however, our RXS study under a high pressure shows no striking change as JTD is suppressed by the application of pressure. On the other hand, the RXS intensity becomes zero above P-c. The results indicate that the local environment of the Cu2+ ion markedly changes. Two possible structures of EA(2)CuCl(4) above P-c are proposed.

DOI： 10.1103/PhysRevB.72.014123

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The molecular alignment of a merocyanine (MC) J-aggregate monolayer at the air-water interface was determined by a grazing incidence x-ray diffraction method. The obtained molecular arrangement apparently shows that the conventional formula, which accounts only for the transition dipole interaction, is not sufficient to figure out the exciton band wavelength, suggesting the importance of the electric dipole (ED) interaction. We derived a simple formula for the ED interaction energy under an extended dipole approximation and clarified the ED contribution in the MC J aggregate.

DOI： 10.1103/PhysRevLett.94.136404

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：KOREAN PHYSICAL SOC  

We examined structural changes related to charge ordering (CO) in the temperature window between 100 K and room temperature (BT) in YFe2O4-delta by transmission electron microscopy. Characteristic diffuse scattering elongated along the [001] direction through the (1/3 1/3 0)-type reciprocal positions was found at RT, which is related to the microdomains due to the CO with a size of 10 - 20 run. In addition, it was also found that the superlattice structures characterized by the (1/3 - eta1/3 + eta0)-type modulation vector, which should be due to the vacancy ordering, coexist with the microdomains due to the CO. On the other hand, in stoichimetric YFe2O4, We found temperature variations of the superlattice structures on warming from 100 K, which should be strongly related to the dielectric anomalies in YFe2O4.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

In a series of oxides RFe2O4 (R = rare earth between Dy and Lu, or Y), equal numbers of Fe2+ and Fe3+ are located on a layer composed of two triangular nets stacked together. These oxides have several phase transitions where the arrangement of Fe2+ and Fe3+ ions transforms from one type to another. Unique dielectric properties of these oxides have been explained in connection with the charge ordering.
Recent developments on the structure of the ordered phases are reviewed with comments on their correlation with the observed dielectric dispersion.

DOI： 10.1080/00150190590926085

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

The recently found chromic phase transition of merocyanine J-aggregates monolayer on the aqueous subphase containing Cd2+ and Mg2+ was investigated by the grazing incidence X-ray diffraction method. The evidence of the structural difference between the phases of J-band at 620 nm and 590 nm was obtained by the investigations on the monolayers prepared at the different molar ratios of Cd2+ and Mg2+. The change in a Bragg's peak was observed at the ratio of the ions, where the J-band wavelength also exhibited the discrete shift. The relationship between the obtained results and the phase diagram determined by the shift of the J-band wavelength was also discussed from the viewpoint of the formation mechanism of J-aggregate crystallites.

DOI： 10.1080/00150190590926319

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Language：English   Publisher：INT UNION CRYSTALLOGRAPHY  

DOI： 10.1107/S0108767305086083

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The effects of hydrostatic pressure on a structure and the corresponding resonant x-ray scattering (RXS) spectrum have been investigated for the hole-orbital-ordered compound (C2H5NH3)2CuCl4. We have found a structural phase transition at Pc∼4GPa, as suggested by the Raman scattering measurement, by observing a peak splitting resulting from an orthorhombic-to-monoclinic symmetry breaking. The reduction of the Jahn-Teller distortion (JTD) toward Pc is also ascertained by structural analyses. The gradual change in the color of the crystal is also observed in connection with the structural change near Pc. The red-colored transparency of the crystal indicates that the system is still insulative above Pc. The observed RXS and fluorescence spectrum at ambient pressure were precisely analyzed. We experimentally confirmed that the RXS intensity comes from the polarization of the density of states of px and py symmetries, which is mainly dominated not by the on-site 3d-4p Coulomb interaction but by the JTD of the CuCl6 octahedron. charged Cl ions neighboring the Cu site. It is also expected that the RXS intensity is proportional to JTD
however, our RXS study under a high pressure shows no striking change as JTD is suppressed by the application of pressure. On the other hand, the RXS intensity becomes zero above Pc. The results indicate that the local environment of the Cu2+ ion markedly changes. Two possible structures of EA2CuCl4 above Pc are proposed. © 2005 The American Physical Society.

DOI： 10.1103/PhysRevB.72.014123

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We examined structural changes related to the charge ordering (CO) in the temperature window between 100K and room temperature (RT) in YFe2O4-δby transmission electron microscopy. Characteristic diffuse scattering elongated along the [001] direction through the (1/3 1/3 0)-type reciprocal positions was found at RT, which is related to nano-sized domains due to the CO. In addition, we found the presence of the superlattice structures characterized by the (1/4 1/4 0)-type modulation vector at 150K and by the (0 1/2 0)-type modulation vector at 200 K. The changes in the superlattice structures as a function of the temperature should be strongly related to the dielectric anomalies in YFe2O4-δ.

DOI： 10.1080/10584580490898876

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Language：English   Publisher：The Physical Society of Japan (JPS)  

Structural feature of Eu0.6Sr0.4MnO3 through the magnetic-field-induced insulator-to-metal (IM) transition has been investigated by X-ray powder diffraction in a magnetic field using synchrotron radiation. No superlattice reflections due to the charge and orbital ordering are detected in the insulator phase down to 20 K. Both of the insulator phase and the induced metal phase are proved to be in orthorhombic structure with Pnma space group, and thus a change of the crystal symmetry is not detected on the IM transition. However, the lattice constants abruptly increase and the reflection intensities abruptly change through the transition. These suggest a rotation of MnO6 octahedron in a manner to increase Mn-O-Mn angles. This structural change is consistent with a gain in electron transfer energy in the double exchange mechanism that stabilizes the metallic and ferromagnetic state. © 2004 The Physical Society of Japan.

DOI： 10.1143/JPSJ.73.3059

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Below T-N=289 K, the antiferromagnet CoO exhibits magnetic long-range order described by a propagation vector along the trigonal axis, whereas the lattice is distorted monoclinicly along both the tetragonal and the trigonal axes. In order to elucidate this inconsistency, we performed neutron and synchrotron x-ray diffraction experiments on single crystals of CoO, and found a series of magnetic reflections represented by a propagation vector along the tetragonal axis below T-N. These results suggest an altered magnetic structure of CoO, which includes both the trigonal and the tetragonal propagation vectors, has a monoclinic symmetry, and which is in accordance with the distorted lattice symmetry.

DOI： 10.1103/PhysRevB.70.184411

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Other Link： http://orcid.org/0000-0003-2764-0908

We report a resonant X-ray diffraction and a transmission electron microscope observation on triangular layered oxide RFe2O4 (R = Lu and Y). The observed super structure is concluded to be originated from the charge ordering on triangular plane. The origin of the charge ordering is interpreted as the competing interaction of the charges between nearest and next nearest neighbor charges on triangular lattice. © 2004 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jmmm.2003.12.155

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Structural feature of Eu0.6Sr0.4MnO3 through the magnetic field induced insulator-to-metal transition has been investigated by using synchrotron X-ray diffraction in the magnetic field. While keeping the same Pnma symmetry, the lattice constant b jumps at the transition field, which is responsible for magnetoelastic effect. We suggest that the structural change through the transition is a rotation of MnO 6 octahedron in a manner that the Mn-O-Mn angle increases toward 180°. © 2003 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jmmm.2003.11.139

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Magnetic properties were investigated for the rare-earth 3D-transition metal oxides with the perovskite structure. Intriguing magnetic phenomena were reviewed for a few systems: magnetization peak effect in the titanates, magnetization reversal in the chromites and metallic ferromagnetism in the cobaltites. The results suggest an important role of the rare-earth ions for the magnetic properties of such complex oxides.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society  

DOI： 10.1103/PhysRevB.70.134512

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We report the low temperature charge ordered structure and magnetic relaxation in nearly stoichiometric YFe 2 O 4 in which the charge frustration effect is considered to be dominant. The superlattice reflections have revealed the presence of four different phases with a triple, double, quadruple and septuplex charge-superstructure in the temperature range from room temperature to 130 K. The observed successive transitions have been proposed by a previous X-ray powder diffraction study. A slow relaxation of the superstructure was found at 130 K. Time dependence of the magnetic moment was observed at about 240 K to suggest a similar relaxation. © 2003 Taylor &amp; Francis.

DOI： 10.1080/00150190390206356

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

It has been theoretically expected that critical phenomena of helimagnetism is drastically different from those of collinear antiferromagnetism due to the existence of chiral degree of freedom. In order to verify this prediction, we carried out X-ray magnetic scattering measurements on a simplest helimagnet beta-MnO2 using a synchrotron radiation. The critical exponent beta was estimated at 0.25+/-0.05 from the temperature dependence of the scattering intensity. This value is consistent with the predicted value 0.253 for chiral system rather than with 0.346 for collinear system. (C) 2003 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4526(02)02501-2

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Lattice distortions associated with charge stripe order in (formula presented) hole-doped (formula presented) are studied using synchrotron x-ray diffraction for (formula presented) and (formula presented) The propagation wave-vector and charge order correlation lengths are determined with a high accuracy, revealing that the oblique charge stripes in orthorhombic (formula presented) crystal are more disordered than the aligned stripes in tetragonal (formula presented) crystal. The twofold periodicity of lattice modulations along the c axis is explained by long-range Coulomb interactions between holes on neighboring (formula presented) planes. © 2003 The American Physical Society.

DOI： 10.1103/PhysRevB.67.140503

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The grazing incidence X-ray diffraction from the 2D polar crystallites of organic dye molecules on an aqueous subphase was measured. The structure of the 2D crystallites shows a dependence on the subphase composition, and this dependence corresponds well to the results obtained from Langmuir-Blodgett films, which consist of the multilayer of the 2D crystallites deposited on the solid planar supports. © 2003 Taylor & Francis.

DOI： 10.1080/00150190390206374

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DOI： 10.1080/00150190390206347

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The oribitally ordered phase of DyB2C2 has been studied by nonresonant x-ray diffraction with high-brilliance synchrotron radiation. From the condition of diffraction, the symmetry property of the charge distribution around dysprosium has been concluded at the quadrupolar level. The quantitative inspection, furthermore, indicates that the observed signals cannot be interpreted as arising only from the 4f electrons of dysprosium responsible for the ordering; instead, the experiment can be described rather well by considering a distortion of the metaloid network concomitant with the ordering.

DOI： 10.1103/PhysRevLett.89.206401

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It has been believed for a long time that the metal-to-insulator (MI) transition of BaVS3 is not accompanied by any spatial order of the spin and lattice. We have carried out x-ray-diffraction measurements of BaVS3 single crystals using a laboratory x-ray source as well as synchrotron radiation, and found that superlattice reflections which double the lattice constant c exist below the transition temperature. The most probable space group at the low-temperature insulator phase contains two inequivalent vanadium sites, and thus a charge disproportionation of the vanadium ions is considered the main cause of the MI transition.

DOI： 10.1103/PhysRevB.66.073108

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WORLD SCIENTIFIC PUBL CO PTE LTD  

We have performed Co 2p X-ray absorption spectroscopy (XAS) on a mixed valence hexagonal layered system NaCo2O4, which shows large thermopower and metallic conductivity. We also measured XAS for SrCoO3 and LaCoO3 as examples of typical Co ionic states. The Co ions of the last two materials are octahedrally surrounded by oxygen atoms. A Co atom in NaCo2O4 also has octahedral coordination with oxygen atoms but it is distorted slightly. The Co ions in SrCoO3 and LaCoO3 are in the d(5) and d(6) configuration respectively. The spectrum of NaCo2O4 resembles that of LaCoO3 rather than that of SrCoO3; the Co ions in NaCo2O4 can be regarded as the d(6) configuration. Also, we have tried to calculate the XAS spectra by using the charge-transfer multiplet model (CT-M). Although we could not find out the best-fitting parameters, the results suggested the itinerancy of the Co 3d states.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Taylor and Francis Inc.  

YFe2O4-δ ( δ &lt
0.005), which is a member of charge-frustrated system RFe 2 O 4 was synthesized and investigated by the low-temperature synchrotron powder X-ray diffraction. The structure between 225K and 250K is the same with the three-dimensional charge ordered state of LuFe 2 O 4 . Below the charge ordering temperature, at least five different phase transitions were detected. Since the balance of competing interaction between ionic charge essentially dominates the presence of multi phases in frustrated system, the low oxygen deficiency in YFe2O 4-δ is considered as the origin of the multi phases in low temperature. © 2002 Taylor &amp
Francis.

DOI： 10.1080/713716255

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DOI： 10.1103/PhysRevLett.87.086402

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Language：English   Publisher：The Physical Society of Japan (JPS)  

We report the dielectric anomaries at the phase transitions ofLu(Fe2+ Fe3+)O4, which is considered as a charge andspin frustrated system.Dielectric constant shows a hump, just above the transition from thehigh temperature two-dimensional to the low temperaturethree-dimensional charge ordering. A few glassy behaviors wereobserved at this transition. The presence of spontaneous polarizationin the low temperature phase was confirmed by the measurement ofpyroelectricity. A step-wise increase of the dielectric constant withincreasing temperature was observed between 150 K and 250 K as inother RFe2O4.These phenomena are explained by the presence of the polar domainswith the ordering of Fe2+ and Fe3+ and the motions of theantiphase boundaries between them.

DOI： 10.1143/JPSJ.69.1526

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X-ray scattering measurements on (Formula presented) have revealed the existence of diffuse scattering above the charge and orbital ordering transition temperature (Formula presented) The diffuse scattering has local intensity maxima at incommensurate positions (Formula presented) With decreasing temperature, (Formula presented) increases from about 0.4, and it discontinuously changes to 0.5 at (Formula presented) Characteristic patterns of diffuse intensity distributions can be qualitatively interpreted by calculations based on distortion fields associated with the Jahn-Teller effect. The observed patterns are similar to those observed above the ferromagnetic insulator-metal transition temperature in (Formula presented) reported previously. © 2000 The American Physical Society.

DOI： 10.1103/PhysRevB.62.3875

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We report a synchrotron orbital radiation study of a charge ordering in a triangular mixed valence system LuFe2O4. Using the X-ray anomalous dispersion method near the iron K-shell absorption edge, we confirmed a superstructure of Fe2+ and Fe3+ on the triangular lattice with a configuration of a √3 × √3 × 2 structure. This charge structure shows a spontaneous polarization. The existence of two dimensional charge ordering above 330K was deduced from the temperature dependence of the energy dispersion for the superlattice reflection.

DOI： 10.1080/00150199908014820

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We observed a superlattice reflection for triangular layered oxide LuFe2O4 by using anomalous X-ray dispersion with synchrotron radiation source. We found that superlattice spot (1/3, 1/3, 4.5) shows a large intensity variation upon a slight change of incident X-ray energy (wave length) from 7.114 keV (1.7437 Å) to 7.1448 keV (1.7353 Å) across the Fe absorption edge. This phenomenon is explained by superlattice structure of Fe2+ and Fe3+ on triangular lattice. The existence of the superlattice structure due to the charge ordering of iron ions on triangular lattice indicates that LuFe2O4 possesses local spontaneous polarization.

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The anomalous dielectric behavior of mixed valence system LuFe2O4 is investigated. It has been shown that LuFe2O4 undergoes successive phase transitions associated with ordering of charge configuration of Fe ion following the sequence of disorder → 2D-CDW (charge density wave) → 3D-CDW state. The stable 3D-CDW phase is characterized by a long period incommensurate antiferroelectric phase. The anomalously high dielectric constant at lower temperature as well as its frequency dependence is discussed in connection with the motion of the antiphase domain boundaries.

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A layer-structure oxide LuFe2O4 shows dielectric dispersions in two temperature regions below and above the transition between two- and three-dimensional charge orderings proposed by a diffraction study. The origin of the dispersions are considered to be related with electron hopping between Fe2+ and Fe3+ ions ordered or disordered on triangular lattices characteristic of this oxide.

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The charge ordering process of a mixed valence LuFe2O4 system has been studied, focusing attention on the frustration effect associated with the charge configuration, ft is revealed that the system undergoes successive phase transitions following the sequence of disorder→2D CDW→3D CDW. In the 2D-CDW state, the charge density wave defined by the wave vector (1/3,1/3), is formed in the hexagonal layer. On the nodal lines of the CDW, the charge order is still undetermined due to the charge frustration. From a dielectric viewpoint, this phase is viewed as random stacking of 2D ferroelectric layers. In the 3D-CDW state, antiphase stacking of the 2D ordered layer develops. At the same time, the in-plane charge order exhibits a doubly modulated long-period structure. From a dielectric viewpoint, this phase is characterized by an incommensurate antiferroelectric phase. Relevance of the charge ordering mechanism to the observed anomalously large dielectric constant in LuFe2O4 is discussed. © 1997, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.

DOI： 10.1143/JPSJ.66.3733

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Publishing type：Research paper (scientific journal)  

We have found charge ordering in perovskite-type manganese oxides at low-carrier-doping region in neutron diffraction experiments. In La1-xSrxMnO3 (x = 0.1 and 0.15), we observed superlattice reflections at commensurate positions and the onset temperature Tp of the superlattice structure coincides with the low-temperature metal-toinsulator transition point. Pr1-xCaxMnO3 compounds are also studied at the stoichiometric concentration of x = 0.25. We observed diffuse satellite peaks indicating the existence of new charge-ordered state at this concentration. © 1998 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4526(97)00611-X

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Publishing type：Research paper (scientific journal)  

Charge ordering of a mixed valence system LuFe2O4 has been studied by neutron scattering technique, focusing attention on the charge frustration effect on triangular lattice. It is revealed that the system undergoes successive phase transitions following the sequence of disorder → two-dimensional charge density wave state (2D-CDW) → three-dimensional charge density wave state (3D-CDW). In 2D-CDW state, the charge density wave defined by the wave vector (1/3 1/3) is formed in the hexagonal layer, © 1998 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4526(97)00728-X

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We report low frequency dielectric dispersions of ErFe2O4 in which equal numbers of Fe2+ and Fe3+ ions occupy the sites on a triangular lattice. The first dispersion observed between 200 K and 300 K is explained qualitatively on a model that spontaneous polarization is present in the crystal below the Verwey transition. The second dispersion at higher temperatures is characterized by a few glass-like features. It was interpreted by successive freezing of the local dipole moments accompanied by short-range ordering of Fe2+ and Fe3+ ions.

DOI： 10.1080/00150199708014115

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Dielectric dispersion was observed in a single crystal with the composition of ErFe2O3.95 below magnetic transition temperature. It is characterized by a large dielectric constant of the order of 104, and a Debye-type frequency dependence like in the poly crystalline samples of the same substance which we reported in a previous paper. Large anisotropy exists in the strength εs − ε∞ and the characteristic relaxation frequency of the dispersion. The strength is higher within the c-plane than along the c-axis, while the characteristic frequency is much larger along the c-axis than within the c-plane. The dielectric dispersion in the polycrystalline sample cannot be explained by a simple average of the single crystal results. The observed dispersion is qualitatively discussed with a model of domain wall motion on an assumption that ErFe2O4 is a polar crystal. © 1995, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.

DOI： 10.1143/JPSJ.64.1371

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We observed a few magnetic relaxation phenomena in a LuFeMgO4single crystal, that is, the difference in the magnetization between field-cooled and zero-field-cooled specimens, the appearance of thermoremanent magnetization, and the frequency dependence of the cusp temperature of AC susceptibility. The observed relaxation was interpreted in terms of thermal fluctuations and freezing of the magnetic clusters which are supposed to exist in this substance. © 1995, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.

DOI： 10.1143/JPSJ.64.4371

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We report the magnetoelectric effect of a polycrystalline sample of a triangular Ising antiferromagnet ErFe2O4-8, in which the existence of short range charge ordering was inferred from the neutron and dielectric studies. The sign of the magnetoelectric signal was reversed at 77 K, when the electric or magnetic field was applied in the opposite directions during cooling the specimen down to 77 K. The sign of the signal was independent of the direction of the static bias magnetic field in the range above 150 K. The signal was maximum at 183 K. We suggest a relation between the origin of the magnetoelectric effect and the dielectric relaxation due to an electron hopping between Fe3+ and Fe2+ sites. © 1994, Taylor & Francis Group, LLC. All rights reserved.

DOI： 10.1080/00150199408213359

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We report the frequency and the temperature dependence of the dielectric constant of polycrystalline ErFe2O4-δ with different degrees of oxygen deficiency S. a large dielectric relaxation, with the order of magnitude of 104 and nearly of the Debye type, is observed in the magnetically ordered states within the temperature and frequency ranges of 150 k to 290 K and 1 kHz to 3 MHz, respectively. The dispersion is larger in the sample with more oxygen deficiency. From the temperature dependence of the characteristic frequency, we conclude that the elementary process of the dispersion is related to electron hopping between Fe2+ and Fe3+ ions. a large value of the dielectric constant is consistent with the existence of spontaneous polarization in the magnetic phases of this oxide. We propose that the motion of the polarization domain boundaries is a determining factor of low-frequency dielectric constant. © 1994, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.

DOI： 10.1143/JPSJ.63.4556

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We report the change of the superconducting transition temperature Tc with the substitution of rare earth with cadmium in the RE1-xCdxBa2Cu3Oy system, where RE is Nd, Eu, Gd, Dy and Y. The decrease of Tc with the amount of Cd was found to be closely related to the radius of the RE3+ ion. © 1993, Taylor & Francis Group, LLC. All rights reserved.

DOI： 10.1080/01411599308207781

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Publishing type：Research paper (scientific journal)   Publisher：Informa UK Limited  

DOI： 10.1080/01411599308203485

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Language：English   Publishing type：Research paper (scientific journal)  

Short-range antiferromagnetic (speromagnetic) behavior was observed for amorphous BiFeO3 by static magnetic measurements, ac susceptibility (χac) measurement, and 57Fe Mössbauer spectra measurements. The magnetic behavior is classified into three temperature ranges: (i) paramagnet for T≳220 K
(ii) local clustering of spins for 20 K&lt
T&lt
220 K
and (iii) speromagnet in which all spins are frozen into random directions for T&lt
20 K. In the spin freezing state the magnetization shows irreversibility and the χac-T curve has a cusp. The variation of the spin freezing temperature against measuring magnetic field and measuring frequency indicate that the speromagnetic order is very similar to that of spin glass.

DOI： 10.1063/1.354179

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/0921-4534(91)91628-h

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

An improvement of a Hartshorn type alternating current magnetometer is reported. The use of a dual channel oscillator, a two-phase lock-in amplifier and a personal computer enabled us to obtain a simple feedback system for automatic balancing of the bridge. The resolution of this system is about $3\times 10^{-7}$ emu of magnetic moment or higher at the field of 0.8 Oe in amplitude and 159 Hz in frequency.

DOI： 10.1143/jjap.29.208

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DOI： 10.1016/0038-1098(88)90243-8

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DOI： 10.1088/0305-4608/18/8/004

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Language：Japanese   Publisher：The Physical Society of Japan  

&lt;p&gt;自発分極は試料内部の静電ポテンシャルを傾斜させる。強誘電体の原子軌道エネルギーはこの傾斜に応じて系統的に変化すると考えられる。この現象はバンドベンディングと呼ばれ40年以上に渡り予想されてきた。今回，BaTiO_3_においてバンドベンディング現象を直接的に観測したので報告する。本研究において得られた実験結果は，エピタキシャル製膜された強誘電体薄膜試料と深さ分解光電子分光実験を組み合わせることで初めて達成できた。&lt;/p&gt;

DOI： 10.11316/jpsgaiyo.72.1.0_2725

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Language：Japanese   Publisher：The Physical Society of Japan  

DOI： 10.11316/jpsgaiyo.71.2.0_2527

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