Materials Science Forum Vols. 587-588

Abstract: Absorption and fluorescence spectra in the range from 350 to 1750 nm of several Tm3+- doped optical performs (rods) for optical fiber drawing were measured. Silica-based matrices of Al2O3-GeO2-P2O5-Sb2O3-SiO2 composition doped with several thousands ppm of Tm3+ were characterized. The preforms were fabricated by the Modified Chemical Vapor Deposition and by the solution doping methods. A new method, Genetic Algorithm SPEctra Decomposition was adopted for processing of the measured absorption spectra. This decomposition made it possible to calculate the oscillator strengths of Tm3+ absorption levels. Fluorescence bands of Tm3+ at 800 nm or 1640 nm were found in fluorescence spectra measured on the preform samples when excited at 1064 nm only.

Abstract: In this work we studied the structural properties of SiGe alloys with different Ge molar compositions co-implanted with manganese and arsenic ions. The ions were implanted at room temperature to fluences of 1×1015, 5×1015 and 1×1016 cm–2 and energies of 170 keV (Mn) and 200 keV (As) in order to achieve the overlap of the implanted profiles. The alloys were studied with Rutherford Backscattering/Channeling spectrometry (RBS/C) and X-ray Diffraction (XRD) techniques. After implantation the implanted region (150 nm) turns into amorphous according with RBS/C. The evolution of the lattice parameter was studied using XRD. The annealing at 550°C induces the recrystallization of the amorphous layer for the sample implanted with the lower fluence and the full recovery is complete after annealing at 700°C. The samples implanted with higher fluences did not reveal any noticeable recovery. The Mn and As profiles do not exhibit significant changes during the annealing at 550oC.

Abstract: Thin film nanocomposites of cobalt ferrite (CoFe2O4) dispersed in barium titanate (BaTiO3) matrix, have been deposited with different cobalt ferrite concentrations (from 20% to 70% CoFe2O4), as well as pure barium titanate and cobalt ferrite thin films (end members). The films were prepared by pulsed laser ablation on platinum covered Si(001) substrates. The films structure was studied by X-ray diffraction and their surface was examined by scanning electron microscopy (SEM). The magnetic properties were measured in a SQUID magnetometer. The results show that the deposited films are polycrystalline with a slight (111) barium titanate phase orientation and (311) CoFe2O4 phase orientation. The grain sizes measured from the X-ray diffraction peak widths, for both phases, are in the range 40nm to 100nm. However, as the concentration of the cobalt ferrite increases, the grain size of the BaTiO3 phase decreases, from 100nm to 30nm, up to 40% CoFe2O4 concentration beyond which the BaTiO3 grain size has an approximately constant value near 30nm. On the other hand the cobalt ferrite grain size does not show a clear trend with increasing cobalt ferrite concentration, fluctuating in the range 20nm to 30nm. The magnetic measurements show an increase of the magnetic moment from the low concentration region where the magnetic grains are more isolated and their magnetic interaction is small, towards the bulk value at higher CoFe2O4 concentrations. Also, a strong reduction of the magnetization with increasing temperature was observed, due to the corresponding decrease of the magnetocristalline anisotropy of the cobalt ferrite.

Abstract: In the present work, the spectroscopic ellipsometry (1.5 - 5.5 eV) was used to investigate the effects of current density induced microstructural variations and their influence on the electronic states of as-prepared and a-Si:H coated porous silicon (PS). The pseudodielectric responses of the low and high current densities (5 and 40 mA/cm2) were analyzed using a multilayer model within the effective medium approximation. The FTIR investigation reveals the enhancement of surface oxide (Si-Ox) layer with current density and the improvement of the Si-Hx band after a-Si:H coating.

Abstract: In this work, a study of the sensitivity enhancement of spin valve sensors, when located in close proximity to magnetic flux guides, is presented. The magnetoresistance (MR) of spin-valve sensors, lithographically patterned into stripes with lateral dimensions, (length) l = 500 µm, (width) wsensor = 1, 2, 6 µm and placed near one/two Co93.5Zr2.8Nb3.7 (CZN) magnetic flux guide, is characterized at room temperature. CZN has a high permeability that together with a defined microstructured shape, is able to concentrate the magnetic flux in a small area, leading to an increase in sensor's sensitivity. The magnetic field amplification is estimated by comparison of sensor sensitivity with/without magnetic flux guides, in the linear operation range, and studied as a function of different parameters. Besides an enhancement in sensitivity, sensors also exhibit an important increase in the hard axis coercivity and a shift from MR(H=0) = 0.5, both attributed to the magnetic flux guides. Amplification factors of the order of 20 are observed..

Abstract: The all oxide magnetic multilayer system [LaNiO3/SmFeO3]n (for n = 1 and 2), grown on single crystal SrTiO3(100) substrates, has been produced using the laser MBE (Molecular Beam Epitaxy) technique. We have made a systematic study of the electrical transport properties in the temperature range from 15–300K. As part of this work, we have made a detailed study of the metallic properties of the LaNiO3 layer as a function of the oxygen partial pressure (pO2) and substrate temperature (TS). We have measured magnetic layers of SmFeO3 with LaNiO3 electrodes as a function of the magnetic layer thickness (10 – 470 nm). A non-metallic behaviour is observed with evidence of a “hopping” mechanism at low temperatures. For the n = 2 multilayers, we have measured the temperature dependence of resistance for the sample series with varying LaNiO3 interlayer thickness (t = 2 – 30 nm). We observe an appreciable increase of the low temperature resistance for the interlayer thicknesses between 3.75 – 7.5 nm. This could indicate a change in coupling from ferromagnetic to antiferromagnetic between the magnetic layers.

Abstract: Cu(In,Ga)Se2 (CIGS) thin film semiconductors are among the most attractive materials for thin film solar cell applications. Conversion efficiency exceeding 19% has been achieved for CIGS absorber layers deposited by three-stage co-evaporation technique. From a technological point of view the sputtering deposition process is more attractive than thermal co-evaporation, however, solar cell parameters obtained so far are worse. The highest efficiency value reported for co-sputtered CIS thin films is less than 8% and there is no data found for CIGS layers produced by a similar technique. We have developed a hybrid RF-magnetron sputtering/evaporation method for the deposition of the CIGS absorber layer. In this method Cu and In are sequentially sputtered from metallic targets in the presence of Se vapour. Ga depth profiling leads to a band gap grading which is known to play an important role in cell performance. Here, we report the results of our work on three different ways of Ga incorporation into the CIGS thin films. They consisted of sputtering from In-GaSe, Cu-GaSe composite targets and Ga evaporation. The Ga content and distribution across the layer thickness was investigated by AES measurements. The CIGS formation kinetics, structural and compositional studies were performed by SEM, XRD and AES measurements.

Abstract: The thermally stimulated discharge current (TSDC) method is a very sensitive and a very selective technique to analyze dipole disorientation and the movement of de-trapped space charge (SC). We have proposed a variant of the TSDC method, namely the final thermally stimulated discharge current (FTSDC) technique. The experimental conditions can be selected so that the FTSDC is mainly determined by the SC de-trapping. The temperatures of the maximum intensity of the fractional polarization peaks obtained at low temperature, in the range of the local (secondary) relaxation, are in general about 10 to 20 K above the poling temperature. Measurements of the FTSDC in a wide temperature range demonstrate the existence of an apparent peak at a temperature Tma shifted with about 10 to 30 K above the charging temperature Tc. The shift of Tma with respect to Tc depends on the experimental conditions. The peak width at the half maximum intensity decreases as Tc increases and the thermal apparent activation energy increases. The variations are not monotonous revealing the temperature range where the molecular motion is stronger and consequently the charge trapping and de-trapping processes are affected. Our results demonstrate that there is a strong similarity between the elementary peaks obtained by the two methods, and the current is mainly determined by SC de-trapping. Even the best elementary peaks are not fitted very well by the analytical equation, indicating that the hypothesis behind this equation have to be reconsidered.

Abstract: In this work, we present a study of the spin disorder resistivity ( ρm∞) and the electronic specific heat coefficient ( γ) in Gd4(Co1-xCux)3 compounds, with x = 0, 0.05, 0.10, 0.20, 0.30. The experimental results show a strongly non-linear dependence of ρm∞ on the de Gennes factor which, in similar intermetallic compounds, is usually attributed to the existence of spin fluctuations on the Co 3d bands and its amplification by the thermal disorder of the Gd magnetic moments through the Gd-Co exchange coupling. Using a novel combined analysis of ρm∞ and γ, we show, however, that only electron band structure changes are involved in the anomalous behaviour of ρm∞ and that a linear dependence of ρm∞ on the de Gennes factor is obtained when the variation of the effective mass is properly taken into account.

Abstract: Manganites of general formula ABMnO3 (where A is a trivalent rare-earth ion and B is a divalent dopant) have interesting properties, namely colossal magnetoresistance and their applicability as materials for active magnetic regenerators. La0.70Sr0.30MnO3 (LSMO) is a ferromagnet presenting considerable magnetocaloric effect, with operating temperature TC ~ 370 K and magnetic entropy variation comparable to pure Gadolinium. The high value of TC makes LSMO unsuitable for room-temperature magnetic refrigeration applications, but by substituting La with the high-magnetic moment ion Er, TC is lowered and total magnetic entropy increases. We have found a limit of solid solubility of Er ions in LSMO, in samples prepared by either solid state or sol-gel techniques in previous works [1], in accordance with other authors [2]. We now present a more detailed study of this limit of solubility, with more samples prepared with Er substitution close to the solubility limit and SEM microscopy clearly showing the changes in microstructure caused by the formation of a secondary ErMnO3 phase, in accordance with x-ray diffraction data and TC variation along the series. The magnetocaloric properties of the series are also presented, showing the increase of Relative Cooling Power along the series, in applied magnetic fields up to 1 T.