Two-dimensional mapping of triaxial strain fields in a multiferroic BiFeO3 thin film using scanning x-ray microdiffraction

The dramatically enhanced polarizations and saturation magnetizations observed in the epitaxially constrained BiFeO3 (BFO) thin films with their pronounced grain-orientation dependence have attracted much attention and are attributed largely to the constrained in-plane strain. Thus, it is highly desirable to directly obtain information on the two-dimensional (2D) distribution of the in-plane strain and its correlation with the grain orientation of each corresponding microregion. Here the authors report a 2D quantitative mapping of the grain orientation and the local triaxial strain field in a 250 nm thick multiferroic BFO film using a synchrotron x-ray microdiffraction technique. This direct scanning measurement demonstrates that the deviatoric component of the in-plane strain tensor is between 5x10(-3) and 6x10(-3) and that the local triaxial strain is fairly well correlated with the grain orientation in that particular region. (c) 2007 American Institute of Physics.X1145Nsciescopu

Surface Modification of TiO2 Nanoparticles with Phenyltrimethoxysilane in Dye-sensitized Solar Cells

Phenyltrimethoxysilane (PTMS) was anchored onto the sensitized TiO2 nanoparticles. This insulating molecular layer effectively inhibited the charge recombination at the interface of TiO2/electrolyte in the dye sensitized solar cells (DSCs) without sacrificing the dye-loading capacity of the nanocrystalline TiO2. DSCs using PTMS-modified TiO2 exhibited a short-circuit current (J(SC)) of 15.9 mA/cm(2), an open-circuit voltage (V-OC) of 789 mV, and a fill factor (FE) of 68.2%, yielding an overall conversion efficiency (eta) of 8.55% under 100 mW/cm(2) illumination. The resulting cell efficiency was improved by similar to 10% as compared with the reference cell.X1133Ysciescopu

Enhanced thermodynamic stability of tetragonal-phase field in epitaxial Pb(Zr,Ti)O-3 thin films under a two-dimensional compressive stress

A two-dimensional thermodynamic model was developed to account for the observed difficulty in the fabrication of epitaxial Pb(Zr,Ti)O-3 (PZT) thin films in which tetragonal and rhombohedral phases coexist. The thermodynamic formalism based on the Landau-Devonshire's phenomenological theory predicts the enhanced thermodynamic stability of the tetragonal-phase field under a two-dimensional compressive stress. We have experimentally proved this prediction by fabricating an epitaxially oriented tetragonal PZT thin film on MgO substrate with the target composition corresponding to the bulk morphotropic phase boundary (MPB). (C) 1998 American Institute of Physics.open1160sciescopu

Recovery process of degraded ferroelectric properties in the forming-gas-annealed Pt/Bi4-xLaxTi3O12/Pt capacitor

The recovery of ferroelectric properties in the forming-gas-annealed Pt/Bi4-xLaxTi3O12/Pt (Pt/BLT/Pt) capacitor was studied by examining changes in ferroelectric responses, phase evolution, and spatial distributions of relevant species during the recovery annealing. The degraded ferroelectric properties were practically restored to their original values after the recovery annealing at 600 degreesC for 10 min in an O-2 atmosphere. The following recovery process has been delineated from the present study: (i) the removal of impregnated protons from the degraded capacitor due to the chemical potential difference of protons between the forming-gas-annealed capacitor and the contacting atmosphere, and (ii) the restoration of perovskite BLT phase with the help of replenishment of the Bi and oxygen losses via diffusion from the neighboring intact region to the Bi-depleted columnar region located beneath the top Pt electrode. (C) 2003 American Institute of Physics.open1156sciescopu

Modulated spin structure responsible for the magnetic-field-induced polarization switching in multiferroic TbMn2O5

Orthorhombic TbMn2O5 (o-TMO) is a well-known multiferroic manganite with the remarkable property of polarization switching at 3 K under a bias magnetic (H) field along the a axis of Pb2(1)m. To theoretically account for this outstanding observation, we have proposed a modulated spin structure under the saturated bias H field by considering the relative strength of the three relevant exchange parameters in o-TMO. The proposed modulated structure based on density-functional theory (DFT) calculations is described in terms of the spin angle phi between the neighboring Mn4+-Mn3+ spin moments on the a-b plane. We have shown that the computed DFT polarization plotted as a function of f satisfactorily accounts for the observed H-field-induced polarization switching. We have further theoretically shown that the square of the critical field strength (H-c) needed for the polarization switching is inversely proportional to the degree of the extrinsic magnetoelectric coupling. The computed partial charge density demonstrates that the H-field-induced polarization switching also accompanies with the switching in the sign of the excess valence-electron density.open1134sciescopu

Magnetoelectric coupling of [00l]-oriented Pb(Zr0.4Ti0.6)O-3-Ni0.8Zn0.2Fe2O4 multilayered thin films

Multilayered thin films consisting of alternatively stacking Pb(Zr0.4Ti0.6)O-3 (PZT) and Ni0.8Zn0.2Fe2O4 (NZFO) layers were fabricated to exploit a strain-mediated coupling of piezoelectricity and magnetostriction. The 450-nm-thick PZT/NZFO multilayer fabricated by pulsed laser deposition showed magnetodielectric effects upon applying a static magnetic field. The magnetoelectric (ME) susceptibility values estimated using these magnetodielectric responses were in the range of 15-30 mV/cm Oe at a zero magnetic-field strength and were comparable to those obtained using a more commonly employed "dynamic" ME method. (c) 2007 American Institute of Physics.open116567sciescopu

Mode coupling between nonpolar and polar phonons as the origin of improper ferroelectricity in hexagonal LuMnO3

Currently, the most puzzling problem associated with the hexagonal LuMnO3 (h-LMO) is a large temperature-gap between the structural phase transition to the polar P6(3)cm phase at similar to 1290 K and the emergence of the spontaneous polarization at a substantially reduced temperature, similar to 750 K. Interestingly, this large temperature-gap is not limited to h-LMO but is a universal phenomenon valid for other rare-earth manganites. We have examined this important issue by exploiting first-principles calculations. It is shown that the structural phase transition to the polar P6(3)cm phase from the nonpolar P6(3)/mmc phase of h-LMO is mediated by the freezing-in of the zone-boundary K-3 phonon. However, the ferroelectric polarization remains at a negligibly small value until the amplitude of the K-3 phonon reaches a certain critical value above which the coupling of the polar Gamma(2) mode with the nonpolar K-3 mode is practically turned on. This coupling-induced polarization explains the observed temperature-gap in h-LMO as well as other rare-earth manganites.open1144sciescopu

Giant dielectric permittivity observed in Pb-based perovskite ferroelectrics

Giant dielectric permittivity was observed in La-modified PbTiO3 (PLT) with A-site vacancy. The observed values of PLT with A-site vacancy are 1 order of magnitude larger than those of relaxor ferroelectrics. The giant relative dielectric permittivity, coupled with a low dielectric loss (tan delta approximate to 0.03) of the PLT, potentially makes it one of the most promising materials for numerous modem technological applications.open11123sciescopu

Thermodynamic theory of stress distribution in epitaxial Pb(Zr, Ti)O-3 thin films

A phenomenological thermodynamic model has been developed to account for the effects of the film thickness on various properties of ferroelectric thin films. To this end, we have suitably incorporated a position-dependent stress distribution function into the elastic Gibbs function. Various physical properties can be predicted as a function of the film thickness using this modified thermodynamic formalism. A comparison of the theoretical predictions with experimental values of the average strain and the para-ferro transition temperature indicates that the tensile stress caused by the cubic-tetragonal displacive phase transition dominates over the compressive thermal stress in the epitaxially oriented tetragonal Pb(Zr, Ti)O-3 thin films. (C) 1999 American Institute of Physics. [S0003-6951(99)05546-1].open1158sciescopu

Dynamic magnetoelectric coupling in "electronic ferroelectric" LuFe2O4

Magnetoelectric (ME) coupling characteristics of LuFe2O4 were examined by monitoring the electrical voltage induced by an oscillating magnetic field under a static bias field (H-0). Interestingly, the room-temperature dynamic ME output exhibited a constant plateau behavior up to a certain static-field strength but showed a sudden drop above this critical value. In addition, two evidences of the intrinsic ME coupling were obtained by monitoring the pyroelectric response near the ferrimagnetic ordering temperature (similar to 250 K) and by examining the temperature-dependent magnetization near the ferroelectric transition point (similar to 345 K) between the two-dimensional charge-density-wave (CDW) state and the three-dimensional CDW state. (C) 2007 American Institute of Physics.open114951sciescopu