National Institute Of Materials Physics - Romania

Phase transition and field driven hysteresis evolution of a two-dimensional Ising grid consisting of ferroelectric-antiferroelectric multilayers that take into account the long range dipolar interactions were simulated by a Monte-Carlo method. Simulations were carried out for a 1 + 1 bilayer and a 5 + 5 superlattice. Phase stabilities of components comprising the structures with an electrostatic-like coupling term were also studied. An electrostatic-like coupling, in the absence of an applied field, can drive the ferroelectric layers toward 180A degrees domains with very flat domain interfaces mainly due to the competition between this term and the dipole-dipole interaction. The antiferroelectric layers do not undergo an antiferroelectric-to-ferroelectric transition under the influence of an electrostatic-like coupling between layers as the ferroelectric layer splits into periodic domains at the expense of the domain wall energy. The long-range interactions become significant near the interfaces. For high periodicity structures with several interfaces, the interlayer long-range interactions substantially impact the configuration of the ferroelectric layers while the antiferroelectric layers remain quite stable unless these layers are near the Neel temperature. In systems investigated with several interfaces, the hysteresis loops do not exhibit a clear presence of antiferroelectricity that could be expected in the presence of anti-parallel dipoles, i.e., the switching takes place abruptly. Some recent experimental observations in ferroelectric-antiferroelectric multilayers are discussed where we conclude that the different electrical properties of bilayers and superlattices are not only due to strain effects alone but also due to long-range interactions. The latter manifests itself particularly in superlattices where layers are periodically exposed to each other at the interfaces.

We investigated the vortex dynamics in the noncentrosymmetric superconductor Li2Pt3B in the temperature range 0.1-2.8 K. Two different logarithmic creep regimes in the decay of the remanent magnetization from the Bean critical state have been observed. In the first regime, the creep rate is extraordinarily small, indicating the existence of an unconventional very effective pinning mechanism. At a certain time, a vortex avalanche occurs that increases the logarithmic creep rate by a factor of about 5 to 10 depending on the temperature. This may indicate that certain barriers against flux motion are present and they can be opened under increased pressure exerted by the vortices.

Lead free ceramics in the system (1-x)KNN x(Bi2O3,CaO,CuO) with 0.0 <= x <= 0.1 were synthesized by the usual solid state reaction and their structural and piezoelectric properties were investigated Dense samples with densities of about 4 68 g/cm(3) were prepared by sintering at an optimum temperature of 1150 degrees C for 6 h Only perovskite phase was detected by X-ray investigation in all samples A typical MPB where orthorombic-tetragonal phases coexist was detected for a doping level of 0.06 where the piezoelectric parameters showed maximum values Planar coupling factor, as much as 0.46, piezoelectric charge constant of 270 pC/N and mechanical quality factor Q(m) of 75 were recorded for the samples situated within the MPB.

Lead titanate (PT) ceramics co-doped with 8 to 13 mol% Ni-Nb, by substituting titanium ions, were prepared by conventional ceramic method and the influence of Ni-Nb doping level on their mopho-structural, electromechanical and dielectric properties was investigated. Dense tetragonal PT ceramics were obtained for 8 to 12 mol% Ni-Nb co-doping. Lattice anisotropy decreased from 1.053 to 1.046 with increasing doping level. Large thickness coupling factors of 0.4 were obtained for 8, 11, and 12 mol% Ni-Nb. Large electromechanical anisotropy of 10 was obtained for 8 mol% Ni-Nb. It was found that Ni-Nb co-doping has both softening and hardening effects as a function of donor or acceptor behavior of different amounts of Ni-Nb. For 13 mol% Ni-Nb, segregation of a small amount of a secondary phase occurs, lattice constants and tetragonality significantly decrease and the resulting ceramic becomes very hard.

Thin films of Ba(Ti1-xSnx)O-3 with x = 0.13 have been deposited by rf-magnetron sputtering and their growth morphology and structure vs annealing temperature, in oxygen atmosphere, have been investigated The films deposited on silicon-platinum substrates, at room temperature, in Ar atmosphere were amorphous The as-obtained films had a mean surface roughness (RMS) around 6.529 nm and a grain size of about 150 nm By annealing the film up to 1150 degrees C in oxygen, the grains size increases up to 1-1.5 mu m in diameter Films containing BaTi0.87Sn0.13 as crystalline phase with the cubic structure of BaTiO3 and, bundle-like morphology were obtained by heating the film at 1250 degrees C for 3h in oxygen The as-obtained film showed the preferential (110) and (100) orientations

Bio-glass films were deposited by radio-frequency magnetron sputtering technique onto medical grade Ti6Al7Nb alloy substrates from prepared silica based bio-glass target. A low deposition temperature was used (150 degrees C) and three different working pressures, followed by annealing in air at 550 and 750 degrees C. A quasi-stoichiometric target to substrate atomic transfer was found for Si, Ca and P, along with strong enrichment in Na and depletion in K and Mg, as evidenced by the energy dispersive microanalysis. The best results, taking into account stoichiometry and surface roughness, were obtained for the BG layers deposited at 0.3 Pa argon working pressure. The infrared spectroscopy of the as-sputtered and of the annealed films evidenced the characteristic molecular vibrations of silicate, phosphate and carbonate functional groups. The as-deposited films are amorphous and became partly crystalline after annealing at 750 degrees C, as evidenced by X-ray diffraction. The pull-out measurements, performed with a certified pull-test machine, gave very strong film-substrate adhesion strength values. For the non-crystalline layers, the pull-out strength is higher than 85 MPa, and decreases after annealing at 750 degrees C to 72.9 +/- 7.1 MPa. The main objective of this work was to establish the influence of the working pressure upon the composition and morphology of the as-deposited films, and of the annealing temperature upon structure and film substrate adhesion. (C) 2009 Elsevier B. V. All rights reserved.

The aim of this paper was to study the influence of crystallization heat treatments up to 750 degrees C, upon the integrity and structure of hydroxylapatite thin films. Two different types of hydroxylapatite coatings were investigated: abrupt structures prepared by magnetron sputtering and respectively functional graded structures deposited by a co-sputtering technique. A clear dependence was noticed between adherence values and annealing temperature and was correlated with the SEM, XRD and FTIR results. X-ray diffraction and FTIR measurements showed that the hydroxylapatite coatings annealed at 400 degrees C were less crystalline, similar to biological apatites. Over 550 degrees C, the apatite coatings obtained were well crystallized and preferentially oriented on (002) HA crystal plane. The adherence of HA coating increased with annealing the temperature up to 550 degrees C due to diffusion phenomena. At 550 degrees C the adhesion test results indicated better adhesion values in case of functional graded structures (> 85 MPa) in comparison with abrupt ones (similar to 70 MPa). At 750 degrees C an adherence decreasing occurred for both types of structures. This is mainly due to the effect of a significant thermal stress owed to the mismatch of the thermal expansion coefficients between the crystallized HA and Ti substrate. The presence of rutile lines in XRD spectrum suggests an increasing of the diffusion phenomena and a strong substrate oxidation at the HAM interface with annealing temperature.

Epitaxial ferroelectric PbZr0.2Ti0.8O3 thin films were grown by pulsed laser deposition. PbZr0.2Ti0.8O3 was doped with Cr acting as acceptor ion. Microstructural characterization was performed by (high resolution) transmission electron microscopy. The voltage dependence of polarization, dielectric constant, and leakage current were measured with respect to the Cr content. To derive the electronic properties, PZT was considered as a wide-gap semiconductor which allows treating the metal-PZT interface as a Schottky contact. The Cr was found to facilitate the elastic relaxation of the film. Furthermore, the leakage current was increased through a reduction of the Schottky barrier. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3141733]

The leakage current in epitaxial BiFeO3 capacitors with bottom SrRuO3 and top Pt electrodes, grown by pulsed laser deposition on SrTiO3 (100), SrTiO3 (110), and SrTiO3 (111) substrates, is investigated by current-voltage (I-V) measurements in the 100-300 K temperature range. It is found that the leakage current is interface-limited and strongly dependent on the orientation of the substrate. The potential barriers at the electrode interfaces are estimated to about 0.6, 0.77, and 0.93 eV for the (100), (110), and (111) orientations, respectively.

The structural peculiarities of titanium oxide films formed by pulsed laser deposition have been investigated. A pure titanium target was irradiated by a Nd:YAG laser (355 and 532 nm). The work studied the influence of the beam wavelength, and oxygen pressure (20, 40, 80, and 160 mTorr) upon the layer structure. The films deposited at room temperature were partly crystalline. The crystalline fraction was a mixture of titanium sub-oxides. The 532 nm wavelength seems to favour the oxidation of titanium leading to TiO(2) formation even at room temperature. After annealing, crystalline TiO(2-) anatase formed at both irradiation wavelengths, but only at low oxygen pressures (20 and 40 mTorr). The best crystallization occurs in the layers deposited at the lowest oxygen pressure (20 mTorr), at both laser beam wavelengths; most anatase formed in the sample obtained at 20 mTorr, 532 nm. At 20 mTorr the films were compact, for both wavelengths, but also tilted or randomly distributed columnar grains were observed at higher pressure. The films deposited with 355 nm, were thinner than those with 532 nm; at 355 nm the thickness decreased at higher pressure, while at 532 nm the dependence was opposite.