Shintaro Yasui

Analysis for crystal structure of Bi(Fe,Sc)O3 thin films and their electrical properties

Thin films of Bi(Fe1−xScx)O3 (BFSO) system were fabricated on (111)Pt∕TiO2∕SiO2∕(100)Si substrates by chemical solution deposition to improve the electrical resistivity by substituting electrically unstable Fe3+ cations for stable Sc3+ cations. A single phase of perovskite was obtained in the range of x=0–0.30, in which selective replacement of Fe3+ and Sc3+ was confirmed in x=0–0.15 by using Raman measurement. The leakage current density of the BFSO films was reduced by increasing x. A well-saturated polarization–electric field hysteresis loop was obtained for BFSO films with x=0.15, showing remanent polarization of approximately 35μC∕cm2.

Crystal Structure Analysis of Epitaxial BiFeO3–BiCoO3 Solid Solution Films Grown by Metalorganic Chemical Vapor Deposition

Epitaxial (001)-oriented (1-x)BiFeO3?xBiCoO3 solid solution films with x = 0?0.33 were grown on (100)SrTiO3 substrates at 700 ?C by metalorganic chemical vapor deposition. The crystal structure of the films was characterized by high-resolution X-ray diffraction analysis and Raman spectroscopy. Unit cell volume and the lattice parameter were changed with increasing x. The BiFeO3 film with x = 0 has rhombohedral symmetry and those with x = 0.16 and 0.21 have a mixture of rhombohedral and tetragonal symmetries. Finally, tetragonal symmetry was observed for the film with x = 0.33 together with a small amount of the contamination phase. This result suggests that the symmetry of (1-x)BiFeO3?xBiCoO3 films changed from rhombohedral to tetragonal with increasing x similarly to Pb(Zr,Ti)O3 having a large piezo response.

Crystal Structure and Electrical Properties of {100}-Oriented Epitaxial BiCoO3–BiFeO3 Films Grown by Metalorganic Chemical Vapor Deposition

xBiCoO3–(1 - x)BiFeO3 (x = 0–0.22) films of 400 nm thickness were grown on (100)c SrRuO3∥(100) SrTiO3 substrates by metalorganic chemical vapor deposition. The changes in the crystal structure and electrical properties of the films with x were investigated. The constituent phase changed from rhombohedral to a mixture of rhombohedral and tetragonal, and to tetragonal with increasing x, but the x for this transition is different from that of 200-nm-thick films grown on (100) SrTiO3 substrates. The x of the morphotropic phase boundary that consisted of a mixture of tetragonal and rhombohedral symmetries depended on the film thickness. The remanant polarization continuously decreased with increasing x, in good agreement with the results obtained with the {100}-oriented Pb(Zr,Ti)O3 epitaxial films owing to the decrease in the crystal anisotropy of the films.

Enhancement of ferroelectric and magnetic properties in BiFeO3 films by small amount of cobalt addition

Both the ferroelectric and magnetic properties of polycrystalline BiFeO3 films fabricated by chemical solution deposition were enhanced by adding small amounts of cobalt. Addition of 3at.% cobalt to BiFeO3 films increased the remanent polarization from 49to72μC∕cm2 and decreased the electric coercive field from 0.54to0.44MV∕cm. The ferroelectricity degraded when the cobalt concentration exceeded 9at.% due to the formation of the secondary phases of Bi2Pt. The saturation magnetization was drastically enhanced by the addition of cobalt up to 12at.%. This is because the magnetic moments are not canceled locally since the differences of magnetic moment between B-sites. The saturation magnetization decreased when the cobalt content exceeded 15at.%, thereby attributing to the formation of a nonmagnetic secondary phase of Bi2Pt. It is concluded that both ferroelectric and magnetic properties were enhanced, provided only small amount of cobalt were added to the films.

Anomalous magnetoresistance in the spinel superconductor LiTi2O4.

LiTi2O4 is a unique compound in that it is the only known spinel oxide superconductor. The lack of high quality single crystals has thus far prevented systematic investigations of its transport properties. Here we report a careful study of transport and tunnelling spectroscopy in epitaxial LiTi2O4 thin films. An unusual magnetoresistance is observed which changes from nearly isotropic negative to prominently anisotropic positive as the temperature is decreased. We present evidence that shows that the negative magnetoresistance likely stems from the suppression of local spin fluctuations or spin-orbit scattering centres. The positive magnetoresistance suggests the presence of an orbital-related state, also supported by the fact that the superconducting energy gap decreases as a quadratic function of magnetic field. These observations indicate that the spin-orbital fluctuations play an important role in LiTi2O4 in a manner similar to high-temperature superconductors.

Electric-field-temperature phase diagram of Mn-doped Bi0.5(Na0.9K0.1)0.5TiO3 ceramics

An electric field–temperature (E-T) phase diagram for a lead-free 0.5 mol. % Mn-doped Bi(Na0.1K0.9)TiO3 ceramics was investigated. The x-ray diffraction, dielectric and polarization measurements revealed relaxor behavior and were used to characterize the stability regions of the non-ergodic relaxor, ergodic relaxor and electric field induced ferroelectric states. As indicated by the polarization–current density profiles, transformation between two electric fields, induced ferroelectric states with opposite polarization direction arise via a two-step process through an intermediate relaxor state. Interplay between the ferroelectric state conversion and intermediate relaxor state is governed by the dynamics of polarization relaxation. The presented E-T phase diagram revealed the effects of the applied electric field and temperature on stability regions. This is of special interest since the Bi0.5(Na0.1K0.9)0.5TiO3 ceramics were proposed as a potential piezoceramic material.

Epitaxial LiCoO2 Films as a Model System for Fundamental Electrochemical Studies of Positive Electrodes

Epitaxial LiCoO2 (LCO) thin films of different orientations were fabricated by pulsed laser deposition (PLD) in order to model single-crystal behavior during electrochemical reaction. This paper demonstrates that deposition of conductive SrRuO3 between a SrTiO3 (STO) substrate and an LCO film allows (1) epitaxial growth of LCO with orientation determined by STO and (2) electrochemical measurements, such as cyclic voltammetry and impedance spectroscopy. Scanning transmission electron microscopy (S/TEM and SEM) has demonstrated an orientation relationship between LCO and STO of three orientations, (111), (110) and (100), and identified a LCO/electrolyte surface as consisting of two crystallographic facets of LCO, (001) and {104}. The difference in the orientation of LCO accounts for the difference in the exposed area of {104} planes to the electrolyte, where lithium ions have easy access to fast diffusion planes. The resistance for lithium ion transfer measured by electrochemical impedance spectroscopy had inverse correlation with exposed area of {104} plane measured by TEM. Chemical diffusivity of lithium ions in LCO was measured by fitting electrochemical impedance spectroscopy data to a modified Randles equivalent circuit and allowed us to determine its dependence on film orientation.

Epitaxial growth of metastable multiferroic AlFeO3 film on SrTiO3 (111) substrate

GaFeO3-type AlFeO3 is consisted of oxygen octahedra and tetrahedra containing Al and Fe ions and is known to have a non-centrosymmetric polar structure with space group Pna21. We tried to grow epitaxial GaFeO3-type AlFeO3 films on SrTiO3 (111) substrates by pulsed laser deposition technique. Both the atomic arrangement of close-packed and the atomic distance of the substrate surface played important roles in stabilizing GaFeO3-type AlFeO3 on the substrate. Piezoresponse force microscopy measurements clearly showed that GaFeO3-type AlFeO3 films have ferroelectricity at room temperature. In addition, AlFeO3 film also showed pinched-like hysteresis loop with TN ∼ 317 K.

Annealing Temperature Dependences of Ferroelectric and Magnetic Properties in Polycrystalline Co-Substituted BiFeO3 Films

Multiferroic Co-substituted BiFeO3 films were fabricated by chemical solution deposition method followed by post deposition annealing at various temperatures. The substitution of cobalt of B-sites for iron in BiFeO3 was promoted at relatively high temperatures. The B-site substitution by cobalt promoted increases in saturation magnetization and spontaneous magnetization. By substitution, leakage current density was suppressed in a high-electric-field region, and ferroelectric hysteresis (P–E) loops became measurable even at room temperature. The optimal annealing temperature for the coexistence of a high remanent polarization and a high remanent magnetization was 923 K having a high B-site substitution ratio of cobalt.

Ultrafast switching of ferroelastic nanodomains in bilayered ferroelectric thin films

The dynamic switching of ferroelastic nanodomains in ferroelectric PbZr0.3Ti0.7O3/PbZr0.7Ti0.3O3 bilayers was investigated. Synchrotron microdiffraction using a high-speed pulse generator reveals that electric field pulses as short as 200 ns can switch the ferroelastic domain. Multiples of random distribution analysis of the field-induced changes in diffraction peak intensities finds a dynamic strain change from 0.2 to 1% with increasing the pulse width. Raman spectroscopy shows considerable decreases in A1(1TO) soft mode intensity after applications of short pulses, confirming the ferroelastic switching. The results demonstrate that ferroelastic domains can indeed move at time scales of the order of hundreds of nanoseconds.