Publications

The leakage current in all oxide epitaxial (La,Sr)MnO3-ferroelectric-(La,Sr)MnO3 structures, where the ferroelectric layer is either BaTiO3 or Pb(Zr0.2Ti0.8)O-3, was analyzed on a broad range of temperatures and for different thicknesses of the ferroelectric layer. It was found that, although the structures are nominally symmetric, the current-voltage (I-V) characteristics are asymmetric. The leakage current depends strongly on the thicknesses of the ferroelectric layer, on temperature and on the polarity of the applied voltage. Simple conduction mechanisms such as space charge limited currents or thermionic emission cannot explain in the same time the voltage, temperature, and thickness dependence of the experimentally measured leakage currents. A combination between interface limited charge injection and bulk controlled drift-diffusion (through hopping in the case of BTO and through band mobility in the case of PZT) is qualitatively explaining the experimental I-V characteristics. (C) 2013 AIP Publishing LLC.

Metal-ferroelectric-metal structures based on epitaxial Pb(Zr0.2Ti0.8)O-3 thin films are prepared by pulsed laser deposition on single crystal SrTiO3 substrates ((001) orientation) with buffer SrRuO3 layer as bottom electrode. Pt, Cu, and SrRuO3 are used as top contacts. The current-voltage (I-V) measurements reveal a strong influence of the top electrode interface on the magnitude of the leakage current and the shape of the I-V characteristics. The lowest current values are obtained for top Cu and the highest for top Pt. Diode-like behavior is obtained for top Cu and Pt, but the forward and reverse biases are opposite in sign. Contrary to the case of BiFeO3 layers deposited on the same type of substrates, it was found that the diode-like behavior is not switchable with the polarization reversal although the polarization values are comparable. It is also shown that the metal-ferroelectric-metal (MFM) structure based on Pb(Zr,Ti)O-3 (PZT) can be simulated and modeled as a back-to-back connection of two Schottky diodes. The diode-like behavior of the MFM structure can be induced by a slight asymmetry of the potential barriers at the electrode interfaces behaving as Schottky contacts. The study ends with a critical discussion of the MFM structures based on PZT and BiFeO3 (BFO) layers. It is shown that the switchable diode-like behavior is not uniquely determined by the polarization reversal and is not a general characteristic for MFM structures. Such behavior may be present only if the polarization induced band-bending at the interface is generating an accumulation layer at the interface. This could be possible in BiFeO3 based MFM structures due to the lower band gap compared to Pb(Zr0.2Ti0.8)O-3 thin films. (C) 2013 AIP Publishing LLC.

Hexagonal ferrites BaxSr1-xFe12O19 (x = 0.05, 0.15, 0.25, 0.35) were prepared by sol-gel and conventional solid state reaction techniques. Their magnetic properties, investigated by magnetometry and Mossbauer spectroscopy, were discussed in correlation to structural aspects. Whereas the hexaferrites obtained by sol-gel are almost single phase, the ones prepared by the conventional solid state reaction technique present a low amount of BaFe2O4 as secondary phase. All samples show a relative high coercive field and remanent magnetization, as specific features for permanent magnet behavior. The five Fe3+ sites of the specific elemental cell have been revealed and their relative occupancy has been derived from the Mossbauer spectra. The analyzed samples show a sensitivity of the magnetic properties (magnetic moment and anisotropy) with respect to the occupancy of the different Fe sites. (C) 2013 Elsevier B. V. All rights reserved.

Pure EuFeO3 and ceria were prepared via a sol-gel citrate route and were calcined at 700 degrees C. Separately, a 20 wt% EuFeO3/CeO2 was prepared via incipient wetness impregnation of calcined ceria with citrate precursors and calcined in the same conditions. SBET, XRD, toluene chemisorption, toluene-TPD and in situ Eu-151 and Fe-57 Mossbauer investigations were performed. All characterization data indicate the formation of a well-dispersed EuFeO3 at the surface of CeO2. During the oxidation of toluene Eu-151 and Fe-57 Mossbauer spectroscopy suggested a cooperative effect of the cations. The catalysts were tested in total oxidation of toluene. In terms of intrinsic activity for toluene combustion, supported catalysts were more active than bulk EuFeO3. All structural changes during operation time were reversible. (C) 2012 Elsevier B.V. All rights reserved.

We report on the deposition of Al doped ZnO (AZO) thin films on unheated polyethylene terephthalate (PET) substrates by pulsed laser deposition technique using a UV excimer laser and Al2O3:ZnO ceramic targets (1.5 and 2 wt% Al2O3). The deposited AZO films have been investigated by atomic force microscopy, scanning electron microscopy, X-ray diffraction, and optical spectrophotometry. Films present excellent optical and electrical properties (transmission in the visible range T > 85%; resistivity at room temperature rho = 1.3 x 10(-3) Omega cm) as electrodes for plastic solar cells. A good correlation was found between deposition conditions (laser fluence) and structural, morphological, optical and electrical properties. (C) 2013 Elsevier B.V. All rights reserved.

The surface of a ferroelectric BaTiO3(001) single crystal was studied using synchrotron radiation induced x-ray photoelectron diffraction (XPD), x-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and low-energy electron diffraction (LEED). AFM, XPS, and LEED show that the surface is BaO terminated with a (1 x 1) reconstruction. The Ba 4d, Ti 2p, and O 1s XPD results were compared with multiple scattering simulations for out-of- (P+, P) and in-plane (P-in) polarizations using a genetic algorithm to determine atomic rumpling and interlayer relaxation. Linear combinations of the XPD simulations of the surface structure of each polarization state allow determination of the domain ordering. The best agreement with experiment is found for 55% P+, 38% P-, and 7% P-in. The rumpling is smaller at the surface than in the bulk, suggesting that both domain ordering and surface structural changes contribute to screening of the polarization.

The influence of milling type on the structural and magnetic behaviour of mechanically milled SmCo5/Fe65Co35 has been studied. By adjusting the mill speed parameters, (Omega/omega), three types of milling have been performed: shock mode process (SMP), friction mode process (FMP) and a combined shock followed by a friction mode process. It was found that in SMP conditions the remanent magnetization and composite coercivity increase with increasing milling time. The best exchange coupling, with a coercivity of 1.05 T, was obtained for samples milled for 8 h in SMP and annealed for 2 h at 550 degrees C. On the other hand, the low energy of FMP does not succeed in producing a microstructure adapted to favour the interphase exchange interactions between the soft and hard magnetic phases. A combined milling process does not change significantly the microstructure obtained previously after 8 h of SMP. This indicates that the microstructure of the composite is formed primarily during SMP while FMP, less energetic, does not drastically affect the crystallite size and crystallite shape of SMP samples. X-ray diffraction patterns were used to follow the evolution of the structure and microstructure. The diffusion of Fe and Co from Fe65Co35 to SmCo5 phase leads to the formation of Sm-2(Co,Fe)(17)/Fe-Co composite. The magnetic behaviour was checked from hysteresis curves and dM/dH vs. H plots. (C) 2013 Elsevier B.V. All rights reserved.

The growth of gold layers on Pb(Zr,Ti)O-3 (PZT) deposited on SrTiO3 is investigated by X-ray photoelectron spectroscopy in the Au thickness range 2-100 angstrom. Two phases are identified, with compositions close to nominal PZT. The 'standard' phase is represented by all binding energies (Pb 4f, Ti 2p, Zr 3d, O 1s) sensibly equal to the nominal values for PZT, whereas the 'charged' phase exhibits all core levels are shifted by similar to 1 eV toward higher binding energies. By taking into account also scanning probe microscopy images together with recent photoemission results, the 'charged' phase belongs to P(+) regions of PZT, whereas the 'normal' phase corresponds to regions with no net ferroelectric polarization perpendicular to the surface. Au deposition proceeds in a band bending of Phi(PZT) - Phi(Au) similar to 0.4-0.5 eV for both phases, identified as similar shifts toward higher binding energies of all Pb, Ti, Zr, O core levels with Au deposition. The Au 4f core level exhibits also an unusually low binding energy component 1 eV below the 'nominal' Au 4f binding energy position (metal Au). This implies the existence of negatively charged gold, or electron transfer from PZT to Au, although the 'normal' PZT phase have a higher work function, as it is derived from the band bending. Most probably this charge transfer occurs toward Au nanoparticles, which have even higher ionization energies. High resolution transmission electron microscopy evidenced the formation of such isolated nanoparticles. (C) 2013 Elsevier B.V. All rights reserved.

We investigated the influence of rare earth (RE) additions on the structural, dielectric, magnetic and magnetostrictive properties of cobalt ferrite bulk materials. Three lanthanide ions with different ionic radii and magnetic properties were used as dopants: Dy, Gd, La. The structural properties of the sintered samples were investigated using X-ray diffraction, Raman spectroscopy and scanning electron microscopy. The room temperature hysteresis loops were obtained using a vibrating sample magnetometer. The frequency dependence of dielectric permittivity and tangent loss was investigated in the 20 Hz-20 kHz range. The variation of the magnetoelectric response with applied dc magnetic field was analyzed. The insertion of rare earth elements influenced the structural characteristics of the samples (density, cell edge, porosity) and the magnetic and electric properties. An increase in coercive field and a decrease in saturation magnetization and magnetostriction coefficient were observed. These results can be partially explained by the weaker nature of the RE3+-Fe3+ interaction compared to Fe3+-Fe3+ interaction and to the antiferromagnetic alignment of the REFeO3 second phase observed in the diffraction patterns after sintering process. (C) 2012 Elsevier BY. All rights reserved.

Within the framework of density functional theory, the electronic structure and magnetic properties have been studied for the Ti2FeSn full-Heusler compound. The ferromagnetic state is found to be energetically more favorable than paramagnetic and antiferromagnetic states. The spin-polarized results show that Ti2FeSn compound has half-metallic ferromagnetic character with a total spin moment of 2 mu(B) and a band gap in the minority spin channel of 0.489 eV, at the equilibrium lattice constant a = 6.342 angstrom. (C) 2013 Elsevier Ltd. All rights reserved.