Authorship：Corresponding author   Publishing type：Research paper (scientific journal)  

© 2020, The Author(s). 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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Authorship：Lead author,　Corresponding author   Language：Japanese  

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We demonstrate an efficient method of fabrication for pure lead nanoparticles with non-oxidized surfaces, supported on perovskite lead strontium titanate particles with a size distribution from tens to several hundreds of nanometers. The fabrication method is based on o-precipitation and subsequent annealing in air at 600 degrees C. Transmission electron microscopy revealed that the pure lead nanoparticles show structural fluctuations under intense electron-beam irradiation, like gold and platinum nanoparticles.

DOI： 10.1088/0957-4484/20/29/295704

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

Synchrotron radiation x-ray diffraction measurement was performed to investigate the size effect in ferroelectric nanosized (Pb0.7Sr0.3)TiO3 particles with various sizes ranging from 10 to 200 nm. The 200 and 002 Bragg reflections were separated using low energy synchrotron radiation x-rays. The peak profiles of the 002 reflections show a large broadening and asymmetry for all particle sizes compared with those of the 200 reflections. These anomalously wide and asymmetric peak profiles become marked for peaks with a large l. The aspect ratio of the lattice constants c/a and the atomic distance between the anions and cations decrease gradually in the vicinity of the surface of the particles, indicating that the asymmetrical profiles can be attributed to the formation of a gradational system.

DOI： 10.1088/0953-8984/21/2/025903

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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：Elsevier BV  

DOI： 10.1016/j.physb.2024.415687

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

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Publishing type：Research paper (scientific journal)   Publisher：Wiley  

Silica coating is performed onto silver tree‐like fractal structures, which are self‐grown in a solution, through a wet process using tetraethyl orthosilicate. Surface‐enhanced Raman scattering (SERS) of para‐aminothiophenol (p‐ATP) is measured on the silver tree‐like fractal structures with/without silica layer at the excitation wavelength of 532 nm. p‐ATP is chemically transformed into dimercaptoazobenzene (DMAB) on the non‐coated silver tree‐like fractal structures, where DMAB peaks are clearly observed, during the SERS measurements. The DMAB peaks decrease/disappear on the silica‐coated ones although the p‐ATP peaks were observed. The results indicate that the chemical transformation is inhibited on the silica‐coated ones. The sensitivity is decreased by half compared to the non‐coated silver tree‐like fractal structures, where the lower detection limit is estimated to be ∽2×10^‐5 mol/L for p‐ATP. The silica coating is advantageous for inhibiting chemical transformations of analytes, enabling identification/estimation of chemicals in unknown sample with SERS similarly to conventional Raman spectroscopy.

This article is protected by copyright. All rights reserved.

DOI： 10.1002/pssa.202300085

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

Millimeter wave absorption exceeding 70 GHz has been performed in a Co-substituted Ca–La magnetoplumbite (M-type) ferrite. A polymerized complex method was used to increase the substitution of La and Co ions in the M-type ferrite. The increase in the magnetic anisotropy field combined with the remaining significant saturation magnetization results in good magnetic resonance absorption in the millimeter range of 70–100 GHz. Above 70 GHz, good millimeter wave absorption of more than −20 dB was achieved in 0.25 mm thickness plates of Ca0.46La0.54Fe11Co0.28O19-η and Ca0.42La0.58Fe11Co0.58O19-η. Highly Co-substituted Ca–La M-type ferrites may, thus, be promising candidates for magnetic absorption materials operating in frequencies of up to 100 GHz.

DOI： 10.1063/5.0133578

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

We report on a direct measurement method for electrocaloric effects, the heating/cooling upon application/removal of an electric field in dielectric materials, based on a lock-in thermography technique. By use of sinusoidal excitation and multi-harmonic detection, the actual temperature change can be measured by a single measurement in the frequency domain even when the electrocaloric effect shows a nonlinear response to the excitation field. We demonstrated the method by measuring the temperature dependence of the electric-field-induced temperature change in two Sr-doped BaTiO3 systems with different ferroelectric-paraelectric phase transition temperatures, where we introduce the procedure for extracting the pure electrocaloric contribution free from heat losses and Joule heating due to leakage currents. This method can be used irrespective of the type of dielectric material and enables simultaneous estimation of the polarization change and power dissipation during the application of an electric field, making it a convenient imaging measurement method for the electrocaloric effect.

DOI： 10.1063/5.0137686

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Language：Japanese   Publishing type：Research paper (conference, symposium, etc.)  

DOI： 10.11517/jsaikbs.127.0_08

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Publishing type：Research paper (scientific journal)   Publisher：MDPI AG  

Single-walled carbon nanotube (SWCNT)/TiO2 hybrids were synthesized using 1,10-bis(decyloxy)decane-core PAMAM dendrimer as a molecular glue. Upon photoirradiation of a water dispersion of SWCNT/TiO2 hybrids with visible light (λ > 422 nm), the hydrogen evolution reaction proceeded at a rate of 0.95 mmol/h·g in the presence of a sacrificial agent (1-benzyl-1,4-dihydronicotinamide, BNAH). External quantum yields (EQYs) of the hydrogen production reaction photosensitized by (6,5), (7,5), and (8,3) tubes were estimated to be 5.5%, 3.6%, and 2.2%, respectively, using monochromatic lights corresponding to their E22 absorptions (570 nm, 650 nm, and 680 nm). This order of EQYs (i.e., (6,5) > (7,5) > (8,3)SWCNTs) exhibited the dependence on the C2 energy level of SWCNT for EQY and proved the hot electron extraction pathway.

DOI： 10.3390/nano12213826

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

Silver tree-like fractal structures are self-grown, where needle growths and branching are iterated, via rapid reduction of silver ions in a solution. Many self-similar frameworks exist at different scales in the whole structure, and geometry is characterized by a fractal dimension. Here, we perform a control of the fractal geometry by facile growth engineering related to the viscosity of the solvent, with acetone and ethanol, on the silver tree-like structure. The geometry changed from needle leaf-like to broadleaf-like structure as the viscosity increased, which resulted in the difference in the fractal dimensions ranging from 1.743 to 1.812, at 303 K. The silver fractal structures exhibited optical responses over a wide wavelength range from 400 to 900 nm, which was also visualized by the dark-field observations with an optical microscope. We demonstrated surface-enhanced Raman scattering (SERS) measurements of para-aminothiophenol at 532, 633, and 785 nm on the silver tree-like fractal structures with three types of different fractal geometries. Compared with the common SERS substrates, the merit of using dendrite structures is the availability of multiple laser wavelengths for SERS excitation and detection on the same sample, with keeping significant enhancement effect. This feature also allows us direct observation of plasmon-mediated chemical transformations of molecules through multi-color excitation SERS measurements.

DOI： 10.1007/s00339-022-05976-1

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Other Link： https://link.springer.com/article/10.1007/s00339-022-05976-1/fulltext.html

Language：Japanese   Publishing type：Research paper (conference, symposium, etc.)  

DOI： 10.11517/jsaikbs.125.0_07

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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：PHYSICAL SOC JAPAN  

A BaTiO3-delta thin film with oxygen vacancies was prepared on a Pt/Al2O3 (0001) substrate by RF magnetron sputtering. The prepared thin film was preferentially grown in the [111] direction. The BaTiO3-delta thin film with a reduced interplanar distance (d(111)) exhibited the Ti3+ valence state of 4.4%, which is closely related to concentration of oxygen vacancies. The activation energy estimated from the Arrhenius plot of the electrical conductivity was approximately 61 meV. The electrical conductivity of similar to 10(-5) S/cm at room temperature did not depend on the oxygen gas partial pressure. The photoemission spectra exhibited the O-H bond peak and donor state near the Fermi level. These results indicate that the BaTiO3-delta/Pt/Al2O3 (0001) thin film has surface electron -proton mixed conduction at room temperature.

DOI： 10.7566/JPSJ.90.014707

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

We report here the formation of carbon-carbon bonds via carbon-hydrogen bond activation catalysed by multi-walled carbon nanotubes (mwcnts), the catalytic activity of which is influenced by nanocarbon morphology and structure. Control of nanocarbon defects and edges allows the realisation of a high-performance carbon-based catalyst that can replace its metal-based counterparts.[GRAPHICS].

DOI： 10.1007/s10562-019-02951-z

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Publishing type：Research paper (scientific journal)   Publisher：The Royal Society  

This paper reports the structural and optical investigations of the structural colour of the weevil
<italic>Lamprocyphus</italic>
<italic>augustus</italic>
. The photonic crystal structure within the weevil's scales was investigated using sequential focused ion-beam milling and scanning electron microscopy imaging. We carefully analysed the reconstructed three-dimensional structure to determine the unit cell of the photonic crystal. It was found that the cuticle network of the cubic unit cell perfectly matches the previously reported diamond-based network. However, different results were obtained for the crystal orientations of the small crystal domains that comprise the entire photonic crystal structure in the scales:
<monospace>&lt;111&gt;</monospace>
directions are highly preferred along the surface normal of the scale. This finding explains the fact that the scale is almost uniformly coloured despite the multi-domain structure. It is confirmed experimentally and theoretically that the wavelength range of the reflection band corresponds to the gap of the photonic band.

DOI： 10.1098/rsif.2018.0360

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Other Link： https://royalsocietypublishing.org/doi/full-xml/10.1098/rsif.2018.0360

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier B.V.  

We decorated nano-crystalline LiNbO3 (LN) particles, which played the role of artificial solid electrolyte interfaces (SEIs) on an active material, namely LiCoO2 (LC). Conventional one-step synthesis using metal organic decomposition (MOD) was used initially, but resulted in the presence of an undesired Li-rich compound, Li3NbO4 (L3N), along with a counterpart phase, Co3O4. Hence, we incorporated crystalline LN into the LC matrix via a two-step synthesis method. Treatment at 500–700 °C resulted in single-phase LN-decorated LCs with improved high rate capabilities. The improvement in the rate capability is related to the temperature dependencies of the two most important parameters: 1) the degree of LN crystallinity, which corresponds to the improvement in the polarization, and 2) the density of the triple phase junctions, which act as active Li ion pathways. The optimized capacity, 78 mAh/g at 20 C, of the specimen annealed at 600 °C was 20 C, which is approximately 1.7-fold larger than that of amorphous LN-decorated LC and is 2.6 times higher than that of bare LC. This implied the introduction of a dielectric polarization architecture had a greater impact on the improvement to the rate capability than Li transportation through the amorphous LN.

DOI： 10.1016/j.ssi.2017.11.020

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

Well-crystallized epitaxial YbFe2O4 films were prepared on an alpha-Al2O3(001) substrate using an Fe3O4 buffer layer. Fe3O4 has a relatively small lattice mismatch with both YbFe2O4 and alpha-Al2O3. Electron diffraction analysis combined with transmission electron microscopy revealed the epitaxial relationship to be alpha-Al2O3[110](001) parallel to Fe3O4[211](111) parallel to 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 similar to 220K and a nonlinear current-voltage characteristic at room temperature. These results confirmed the good crystallinity and stoichiometry of the obtained YbFe2O4 films. (C) 2018 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.57.010305

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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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Authorship：Corresponding author   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   Publisher：The Materials Research Society of Japan  

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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Language：English   Publisher：IOP PUBLISHING LTD  

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

Photoinduced growth of the antiferromagnetic state in the ferrimagnetic phase were demonstrated in double layered Fe oxide LuFe2O4 by 12 fs infrared pulse under inter-layer excitation. Inter-layer charge imbalance successively induces the changes of charge/magnetic structures interacting with local Fe-O stretching and inter-layer sliding phonons through the exchange interaction.

DOI： 10.1007/978-3-319-13242-6_61

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

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

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

We synthesized well-crystallized ilmenite-type FeGeO3 to investigate its structural and magnetic properties. Ilmenite-type FeGeO3 were synthesized by high-pressure synthesis technique using a Kawai-type multi-anvil apparatus. Their structural and magnetic properties were examined by XRD analysis with Rietveld refinement, transmission electron microscopy, Mossbauer spectroscopy and SQUID-magnetization measurements. The subsequent annealing after the high-pressure synthesis was effective to improve the crystallinity of the ilmenite-type FeGeO3. The primary particle size of ilmenitetype FeGeO3 was similar to 15 nm. Some particles seemed to be covered with non- or poorly crystalline phase even after the annealing, and others showed a lamellar microstructure parallel to the (0 0 1) plane. Theilmenite-type FeGeO3 showed typical antiferromagnetic behavior with the Neel temperature of 79 K. (c) 2012 Elsevier Inc. All rights reserved.

DOI： 10.1016/j.jssc.2012.11.020

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

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

The change of electrical and lattice dynamical properties of ferroelectric Ca0.6Sr0.4Bi4Ti4O15 (CSBT) ceramics were studied by doping with 0.1 wt.% of CeO2 and MnO2. The loss tangent and the AC conductivity decreased markedly due to the reduction of charge carrier. The activation energy of doped-CSBT increased owing to the reduction of intrinsic defects in the crystal lattice. For doped and pure CSBT, the appearance of thermal hysteresis and the satisfaction of the Cure-Weiss law by the dielectric susceptibility data assigned the first order nature of a ferroelectric phase transition. Especially in ferroelectric phases, the soft optic phonon mode showed the significant softening towards T-1 (at T-1, the square of soft mode frequency omega(2)(s) -&gt; 0) and omega(2)(s) was approximately proportional to T-1-T, where T-1 is similar to 70A degrees C above T-C = 672A degrees C, demonstrated a first order displacive nature of a ferroelectric phase transition. The invariance of lattice dynamical feature indicated that the crystal lattice did not influence markedly by the doping ions.

DOI： 10.1007/s10832-012-9684-9

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

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

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

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

A characteristic nonlinear electric responses were found in a charge ordered type ferroelectrics LuFe2O4 with less oxygen deficiency. Cole - Cole circular in the dielectric response varies in the function of the amplitude of the AC electric field. With a synchrotron radiation diffraction experiment under the current revealed the variation of the coherence length of the polar charge ordered W-layer along c-axis. The nonlinear response with the increase of the electronic current may relates to the electron hopping in the charge stripe structure.

DOI： 10.1080/00150193.2011.577294

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

Dynamical properties of protonic conductor K3H(SeO4)(2) were examined by using various techniques, Brillouin scattering, quasi elastic, and inelastic neutron scattering. No significant anomaly was observed in longitudinal and transverse acoustic modes. In each experiment, a Lorentzian shaped spectrum was observed at extremely low energy, (less than 0.1 meV). The spectra in Brillouin and inelastic neutron scattering at the zone boundary originate from rotational modes of SeO4 tetrahedra at Gamma- and L-point, respectively. The peak positions, shapes, and widths of the spectra are consistent with each other, while the integrated intensities show complementary temperature dependence. The lifetimes of those modes, about 10(-10) s, agree with the stationary time of conduction protons obtained from Q dependence of width in quasi elastic neutron scattering.

DOI： 10.1080/00150193.2011.577715

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The dynamical properties of protonic conductor K3 H (SeO4) 2 were examined by Brillouin scattering. Both longitudinal-acoustic modes (c33 and c11) and a transversal mode (c44) monotonously depend on temperature, and no soft mode was observed. A central mode with a width about 6 GHz was observed above phase-transition temperature, TC. The central mode has a symmetry Eg corresponding to the rotational mode of SeO4 tetrahedra. The Eg mode is compatible with a zone-boundary soft mode, L3, which accounts for the symmetrical change from R 3̄ m to C2/c. The intensity of the central mode declines in the vicinity of TC. This shows that chaotic rotational motion changes into a zone-boundary soft mode with decreasing temperature. © 2010 The American Physical Society.

DOI： 10.1103/PhysRevB.81.012301

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

The dynamical properties of protonic conductor K3H(SeO4)(2) were examined by Brillouin scattering. Both longitudinal-acoustic modes (c(33) and c(11)) and a transversal mode (c(44)) monotonously depend on temperature, and no soft mode was observed. A central mode with a width about 6 GHz was observed above phase-transition temperature, T-C. The central mode has a symmetry E-g corresponding to the rotational mode of SeO4 tetrahedra. The E-g mode is compatible with a zone-boundary soft mode, L-3, which accounts for the symmetrical change from R (3) over barm m to C2/c. The intensity of the central mode declines in the vicinity of T-C. This shows that chaotic rotational motion changes into a zone-boundary soft mode with decreasing temperature.

DOI： 10.1103/PhysRevB.81.012301

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

The size effects on the successive phase transition of lanthanum doped ferroelectric bismuth titanate Bi3.5La0.5Ti3O12 (BLT) nano-particles have been investigated. A dielectric anomaly appears around 520 degrees C for all the size of nano-particles, corresponds to the low temperature monoclinic to orthorhombic phase transition that remains unaffected by the decrease of particle size. The optical soft phonons show a considerable broadening due to the reduction of particle size and completely overdamped below 34 nm. These results demonstrate a size induced ferroelectric orthorhombic to paraelectric tetragonal phase transition. It is confirmed by the temperature-dependence of the soft optical phonon that the first order displacive nature of the orthorhombic to tetragonal phase transition is hold even in nano-system.

DOI： 10.1080/00150191003697062

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

Ferroelectric lead-based perovskite particles have been fabricated by co-precipitation method which a Ca or Ba modified Pb site. Co-precipitates were first synthesized at ambient temperature and then annealed in air at various temperatures to produce lead-base perovskite particles. We demonstrate that the morphology, crystal structure and soft mode behavior are influenced by the particle size.

DOI： 10.1080/00150191003697203

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

The effects of MnO2 doping on the microstructure and phase transition of ferroelectric CaBi4Ti4O15 (CBT) ceramics with x mol % of MnO2 (CBT-xM; x = 0, 1, 3) have been studied. The single-phase crystal structure and the plate-like grain morphology are clearly observed for all compositions, while extended grains are observed with x = 3. The dielectric permittivity and dielectric loss show that the Mn2+ and Mn3+ ions are preferentially incorporated into the A- and B-sites, respectively, up to a limiting extent of approximately x = 2. The ac conductivity decreases and the activation energy increases with increasing Mn content owing to the decrease in the number of conducting carriers and increase in covalence, respectively. The Curie temperature T-C is independent of MnO2 doping within the experimental accuracy, and the dielectric constant satisfies the Curie-Weiss law above T-C. The temperature dependence of the soft mode with x = 0 and 1 shows a significant softening towards T-1 similar to T-C + 113 degrees C (at T-1, the square of the soft mode frequency omega(2)(s) -&gt; 0). omega(2)(s) is approximately proportional to T - T-1. These findings indicate that the displacive nature of the phase transition is not affected by MnO2 doping, at least below x = 3. (C) 2009 The Japan Society of Applied Physics

DOI： 10.1143/JJAP.48.09KC10

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

An efficient fabrication method for pure lead nanoparticles supported on lead-based perovskite particles has been developed using co-precipitation. The annealing of the co-precipitates in air at 600 degrees C produced lead nanoparticles with non-oxidized and non-contaminated surfaces. High-resolution transmission electron microscopy revealed that the pure lead nanoparticles with an average size of similar to 4 nm showed structural fluctuations under an intense electron beam irradiation, similar to gold and platinum nanoparticles. This result implied that the lead nanoparticles prepared by this co-precipitation process are expected to be used as catalysts.

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

Dynamical properties of the polar nanoregions (PNRs) of a relaxor ferroelectric Pb(Ni1/3Nb2/3)O-3-0.29PbTiO(3) single crystal are studied by using Brillouin scattering to elucidate the mechanism of the diffuse phase transition. The temperature dependence of Brillouin scattering shows anomalies at three temperatures: at T-d related to the appearance of the PNRs, at T* associated with the growth of the PNRs, and at T-f related to the macroscopic structural phase transition; all these temperatures are closely related to the dynamical behavior of the PNRs. The thermal hysteresis around T-f in the longitudinal acoustic properties can be interpreted by considering the growth process of the PNRs. From the anomalous acoustic properties, the relaxation time of the polarization fluctuation is evaluated. It is found that the relaxational process responsible for the central peak originates from the polarization fluctuation that is coupled with the longitudinal acoustic mode.

DOI： 10.1143/JPSJ.77.033707

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

Bi4 - xLaxTi3O12 (BLT) nano-particles have been synthesised by the chemical co-precipitation method utilizing a polymeric precursor. The particle size decreases from 100 to 40 nm with the increase of La content (x = 0 1) owing to the increase of a reaction temperature. The axial ratio a/b of BLT decreases with the increase of La content reflects a relaxation of structural distortion from its ideal tetragonal structure. A monoclinic to orthorhombic transition occurs at La 0.75. The existence of the soft mode in Raman spectra above x = 0.75 demonstrates a displacive nature of orthorhombic to tetragonal phase transition. No noticeable size effect on physical properties is observed.

DOI： 10.1080/10584580802540942

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

Raman scattering of lead nickel niobate Pb(Ni1/3Nb2/3)O3 (PNN) single crystals grown by the flux method has been performed to investigate the temperature dependence of the optical phonons of relaxors. Anomaly of the temperature dependence of the Raman intensity at 780 cm-1 was observed. Polar microregions promote the growth of volume gradually above the Curie temperature ( 170 K), and then freeze around 200 K. From the extrapolation of the temperature dependence of the depolarization ratio, we tried to estimate Burns temperature.

DOI： 10.1080/00150190802365962

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

Structural phase transitions in relaxor ferroelectric materials remain to be one of the most puzzling issues in materials science. In the present work, we have investigated relaxational and vibrational properties of a relaxor ferroelectric 0.71Pb(Ni1/3Nb2/3)O-3-0.29PbTiO(3) single crystal by means of inelastic light scattering measurements. Analysis based on the spectra has shown anomalous behaviors at Burns' temperature T-d, freezing temperature T-f those are related to a development of polar nanoregions (PNRs). The PNR must play a key role in the relaxor nature, and obtained results provide new insights into the relaxational dynamics in the PNRs and into how they develop below Tf.

DOI： 10.1109/TUFFC.2007.590

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

The paraelectric to ferroelectric phase transitions were studied by Raman scattering in Ca-doped lead titanate ceramics. The soft optic modes responsible for the ferroelectric phase transitions were clearly observed. The A(1) (1TO) mode, which softens toward T, contains four subpeaks. The frequencies of these subpeaks depend to a great extent on the Ca content and the temperature. These behaviors are discussed on the basis of the anharmonicity of lattice vibrations.

DOI： 10.1109/TUFFC.2007.585

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

Dielectric and light scattering studies on a Pb(Ni1/3Nb2/3)O-3-0.29PbTiO(3) single crystal have been conducted in a wide temperature range to clarify the relaxational properties of polar nanoregions (PNRs). The temperature dependences of the relaxation time observed in light scattering spectra show two characteristic temperatures: associated with enhancement of the correlation length in the PNRs T* and related to a temperature-induced macroscopic structural phase transition Tf. These results are closely related to the behavior of the PNRs. We suggest that two relaxation processes are attributed to the central peak and that the origins of the observed relaxation processes are two types of switching processes of dipole moments in the PNRs. In addition, the activation energy at 0 K has been evaluated as H-0B = 2.11 x 10(3) K.

DOI： 10.1143/JJAP.46.7151

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：INST PURE APPLIED PHYSICS  

Nanosized perovskite (Pb0.7Sr0.3)TiO3 powders have been studied by Raman scattering to characterize the dielectric property derived from the particle size effect. All the optical phonons were assigned while referring to previously reported Raman spectra of PbTiO3. The frequencies of the soft E(1TO) and A, (1TO) modes show a slight hardening with a decrease in particle size. The size dependences of the clamped dielectric constants along the a and c axes characterized by the effect of the phonon confinement are evaluated by the Lyddane-Sachs-Teller relation using the observed frequencies of polar modes with E(x, y) and A(1)(z) symmetries, respectively.

DOI： 10.1143/JJAP.46.7148

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Raman spectroscopy of lead-free piezoelectric (1-x)(Na0.5K0.5)NbO3-xSrTiO(3) ceramics and (Na1-yKy) NbO3 ones sintered by the Spark-Plasma-Sintering method have been performed to investigate the dynamical property in the intermediate phase of x = 0.05. From the comparison of Raman spectra between (1-x)(Na0.5K0.5)NbO3-xSrTiO(3) and (Na1-yKy)NbO3 at room temperature, the overdamped mode which became soften was found in the intermediate phase at x = 0.05. Although it is difficult to assign the mode because of using ceramics samples, this result has a possibility that the overdamped mode includes the soft mode observed in the orthorhombic phase.

DOI： 10.1080/00150190601187021

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

The MnO2 doping effect on ferroelectric phase transitions of CaBi4Ti4O15 ceramics has been studied by Raman scattering, dielectric measurements and scanning electron microscope. The Curie temperatures show monotonic decrease as the content of MnO2 increases. Softeing of low frequency optical modes has been observed for all the compositions, while the Raman scattering decreases markedly as the content of MnO2 increases.

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

We have studied the aging effect of bismuth sodium titanate (BNT) ceramics with point defects. With aging BNT ceramics, we have observed slight decrease in BNT permittivity. Permittivity vs. aging time is well reproduced by assumed Debye-relaxation. It is found that there are two relaxation processes in this aging effect. The diffuseness of phase transition is determined from the temperature dependence of permittivity. The diffuseness vs. composition shows the correlation clearly.

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

We have studied structural and vibrational properties of Ca-doped lead titanate ceramics. The vibrational soft modes associated with the ferroelectric phase transition have been observed. A(1)(1TO) mode, which is one of the soft modes, has four subpeaks due to anharmonicity. Composition and temperature dependences of subpeak frequencies of the A(1)(1TO) mode behaves as expected from theoretical calculation. Some subpeaks remain above the Curie temperature. This is attributed to the point detects and the internal strain.

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

Structural phase transitions in relaxor ferroelectric materials remain to be one of the most puzzling issues in solid-state physics. In the present work, we have investigated relaxational and vibrational properties of a relaxor ferroelectric 0.71Pb(Ni1/3Nb2/3)O-3-0.29PbTiO(3) single crystal by means of inelastic light scattering measurements. Analysis based on the spectra has showed anomalous behaviors at Bum's temperature T-d, freezing temperature T-f, those are related to a development of polar nanoregions (PNRs). PNR must play a key role in relaxor nature, and obtained results provide insight into the relaxational processes in the PNR and into how they develop below T-f.

DOI： 10.1109/ISAF.2007.4393242

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The lattice dynamical properties of (Pb(1-x)C(a)x)TiO3 ceramics were investigated by Raman scattering. The low frequency spectra show the strong composition and temperature dependences. The composition dependence of Raman spectra at room temperature is different from the previous work [S.-Y Kuo, C.-T Li and W.-E Hsieh, Phys. Rev. B 69, 184104 (2004).]. This difference is probably attributed to the aging effect of MnO doping. Temperature dependence of Raman spectra of (Pb0.75Ca0.25)TiO3 behaves as expected by the selection rule and the lowest frequency peak softened and broadened as approaching T-c.

DOI： 10.1080/00150190701515931

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Brillouin scattering in the relaxor ferroelectric 0.8 Pb(Ni1/3Nb2/3) O-3-0.2 PbTiO3 has been studied in the temperature range from -150 degrees C to 500 degrees C. The anomalies of the frequency shift and full width at half maximum of Brillouin component at the structural phase transition from the paraelectric to ferroelectric phases have been successively detected. The origins of the anomalies of them are discussed from the phenomenological point of view. The central peak caused by the fluctuation of polarization has been also observed in the VV and VH scattering geometries with the large free spectral range of 150 GHz. It is found the difference of central peak intensity obtained in VV and VH scattering geometries.

DOI： 10.1080/00150190601180182

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Lead-free piezoelectric (1-x)(Na0.5K0.5)NbO3-xSrTiO3 ((1-x)NKN- xST) solid solution has been fabricated around a morphotropic phase boundary (MPB) by Spark-Plasma-Sintering method. The phase diagram around MPB was studied by the dielectric measurement. The piezoelectric constant d33 and electromechanical constant kp measurements shows that MPB is about at x = 0.05. Inverse dielectric constants are well fitted by the extended Curie-Weiss law above the Curie temperature and its fitting result shows that the fitting parameters, C and n values show the maxima for 0.95NKN-0.05ST. We suggest the existence of another phase boundary around x = 0.05. To confirm the another phase boundary, the Raman spectra of 0.975NKN-0.025ST and 0.9NKN-0.1ST samples at the tetragonal phases have been measured and comparison has been done.

DOI： 10.1080/00150190701196237

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

Brillouin and Raman scatterings of a 0.71Pb(Ni1/3Nb2/3)O-3-0.29PbTiO(3) single crystal have been measured to investigate broadband inelastic spectra. The two different central peaks related to fast and slow relaxation processes have been observed separately. These two processes are attributed to the thermally activated switching of polarization in polar nanoregions. By the analysis of modified superparaelectric model, the activation energies of fast and slow relaxation processes are determined to be 3.66x10(3) and 4.31x10(2) K, respectively. The fast process with the lower activation energy probably originated from 180 degrees switching, whereas the slow one with the higher energy from non-180 degrees switching. (c) 2006 American Institute of Physics.

DOI： 10.1063/1.2393159

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

DOI： 10.1080/00150190600678699

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER INST PHYSICS  

Brillouin scattering of water-ethanol binary mixtures have been measured in the temperature range from room temperature to 343 K as a function of ethanol concentration. A maximum appears at 12 ethanol mol% in the sound velocity and at about 50 mol% in the absorption coefficient all at room temperature. Raman scattering of heavy water-ethanol mixtures was also measured at room temperature as a function of concentration. The concentration range from 0100 mol% is divided into three ranges and the behavior of the acoustic properties was discussed with three cluster models; the circular clusters are dominant in the concentration range of 0-12 mol%, layered clusters in the range of 1250 mol%, pure ethanol chain clusters in the range of 50-100 mol% at room temperature. These results are consistent with the concentration variation of OH stretching band of Raman spectra and the results in literatures.

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Brillouin and Raman spectra of a 0.71Pb(Ni1/3Nb 2/3)O3-0.29PbTiO3 single crystal were measured in two light scattering geometries to elucidate the origins of central peaks that is related to the dynamics of polar nanoregions. The results show that two Lorentzian-type peaks exist in the Brillouin spectra. The shape and the full width at half maximum of the broader peak in Brillouin spectra are almost same as that observed in Raman spectra. This indicates that two relaxations occur in the polar nanoregions. These two processes might be originated from the different switching directions of polarization. © 2007 Materials Research Society.

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

DOI： 10.1080/00150190490508675

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DOI： 10.1080/00150190390205898

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DOI： 10.1080/00150190390204240

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DOI： 10.1080/713716272

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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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DOI： 10.2497/jjspm.61.S327

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

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Language：Japanese   Publisher：The Institute of Electronics, Information and Communication Engineers  

Acoustic properties of a relaxor ferroelectric Pb(Ni_<1/3>Nb_<2/3>)O_3-0.29PbTiO_3 single crystal have been studied by Miro-Brillouin scattering to manifest the mechanism of the diffuse phase transition. These temperature dependences show anomalies at three characteristic temperatures : at Burns temperature T_d related to the formation of the PNRs, at T^* associated with the growth of the PNRs, and at freezing temperature T_f related to the temperature-induced macroscopic structural phase transition, which are closely related to the dynamical behavior of the PNRs.

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Applicant：国立大学法人 岡山大学

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