National Institute Of Materials Physics - Romania

Publications

5914 articles found

4231. Heusler bulk materials as targets for pulsed laser deposition: growth and characterisation

Authors: Manea, AS; Monnereau, O; Notonier, R; Guinneton, F; Logofatu, C; Tortet, L; Garnier, A; Mitrea, M; Negrila, C; Branford, W; Grigorescu, CEA

Published: FEB 15 2005, JOURNAL OF CRYSTAL GROWTH, 275, E1792, DOI: 10.1016/j.jcrysgro.2004.11.213

This work concerns the preparation and characterisation of homogeneous bulk Co2Mn (Si, Ge, Ga, Sn Sb0.8Sn0.2) to be used as targets for pulsed laser deposition of magnetic contacts for spintronic devices. The samples have been prepared by direct synthesis from elemental powders followed by vertical gradient freeze. Characterisation of their structural properties and compositional homogeneity was performed by X-ray diffraction and energy-dispersive X-ray spectroscopy combined with scanning electron microscopy. The results show one-phase for the Heusler alloys. The Curie temperatures of the samples have been measured by both thermogravimetric analysis and differential scanning calorimetry and are in a good agreement with the previously published values. The saturation magnetisation values are close to the values for single crystals. (C) 2004 Elsevier B.V. All rights reserved.

4232. Interstitial void structure in Cu-Sn liquid alloy as revealed from reverse Monte Carlo modelling

Authors: Hoyer, W; Kleinhempel, R; Lorinczi, A; Pohlers, A; Popescu, M; Sava, F

Published: FEB 9 2005, JOURNAL OF PHYSICS-CONDENSED MATTER, 17, S36, DOI: 10.1088/0953-8984/17/5/004

A model for the structure of copper-tin liquid alloy has been developed using the standard reverse Monte Carlo method. The interstitial void structure (size distribution) was analysed. The effects of various kinds of voids (small size and large size) on the interference function and radial distribution function were investigated. Predictions related to the formation of some ternary alloys by filling the interstices of the basic alloy were advanced.

4233. Temperature dependence of the electron paramagnetic resonance spectra of Mn2+, impurity ions in PbWO4 single crystals

Authors: Stefan, M; Nistor, SV; Goovaerts, E; Nikl, M; Bohacek, P

Published: FEB 2 2005, JOURNAL OF PHYSICS-CONDENSED MATTER, 17, 728, DOI: 10.1088/0953-8984/17/4/014

The temperature variation of the fine and hyperfine parameters of Mn2+ in single crystals of PbWO4 in the low temperature range reveals the presence of a resonant mode of frequency omega = 8.8 x 10(12) rad s(-1). Moreover, above 60 K, where the temperature induced broadening becomes dominant, a T-2 variation of the relaxation time was inferred from the analysis of the temperature dependence of the Mn2+ linewidth. This variation is attributed to a Raman relaxation process due to the coupling with the same local resonant mode.

4234. The influence of Cu on the morphological and chemical properties of nanostructured TiO2 films

Authors: Gartner, M; Ghita, A; Anastasescu, M; Osiceanu, P; Dobrescu, G; Zaharescu, M; Macovei, D; Modreanu, M; Trapalis, C; Kordas, G

Published: FEB 2005, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 7, 405, DOI:

TiO2 multilayer films were deposited by the sol-gel method on glass and SiO2/glass substrates. 10% of Cu was added to the coating solution, to study its effect on the amorphous to anatase phase transition, with a view to improving the photocatalytic activity. Densification of the films was obtained by thermal treatment at 500 degrees C for 1 hour in oxidative and reductive conditions. In order to check the correlation between the influence of copper, substrate and annealing temperatures on the structural, morphological and optical properties, the samples were characterized by FTIR, XPS, SEM and fractal analysis. Transformation of CuO from as-prepared samples into Cu2O and metallic Cu, depending on the treatment conditions, was shown. The composition and stoichiometry of the oxide films in the first atomic layers were quantitatively determined as chemical state relative concentrations.

4235. High frequency investigation of graded gap injectors for GaAs Gunn diodes

Authors: Montanari, S; Forster, A; Lepsa, MI; Luth, H

Published: FEB 2005, SOLID-STATE ELECTRONICS, 49, 250, DOI: 10.1016/j.sse.2004.08.014

In this work, we present a detailed high frequency characterization of conventional and optimized Gunn-diodes containing a graded gap injector. It is shown that the Al-concentration in the hot electron injector influences the occupation of the Gamma- and L-valleys and hence the electron drift velocity in the active Gunn layer. Quantitative values for the occupation of the L-valley can be extracted from the S-parameter measurements. (C) 2004 Elsevier Ltd. All rights reserved.

4236. Interface trap density in amorphous La2Hf2O7/SiO2 high-kappa gate stacks on Si

Authors: Mereu, B; Dimoulas, A; Vellianitis, G; Apostolopoulos, G; Scholz, R; Alexe, M

Published: FEB 2005, APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING, 80, 257, DOI: 10.1007/s00339-004-2910-9

The present paper investigates the interface trap density of a new high-kappa gate dielectric stack, La2Hf2O7/SiO2 on Silicon. Amorphous La2Hf2O7 thin films are deposited by metal evaporation in the presence of atomic oxygen beams on an ultra-thin SiO2 layer (1.5 nm) grown by rapid thermal oxidation on a p-type Si substrate. A combination of electrical (C-V) and cross sectional TEM measurements indicates a value of the dielectric constant kappa of about 19 +/- 2.2. The interface states density (D-it) was measured using the conductance method for different La2Hf2O7 thicknesses ranging from 3 nm to 14 nm. Dit ranges from 3.4 x 10(10) eV(-1) cm(-2) to 4.8 x 10(12) eV(-1) cm(-2) and shows a quasi-linear dependence on the La2Hf2O7 layer thickness. The interface state density increase with film thickness could have different explanations, such as oxygen de-passivation and/or defect generation at the Si-SiO2 interface, formation of a new Si-SiO2 interface by Si oxidation or alternatively, the SiO2 interfacial layer reduction by oxygen vacancies.

4237. Photoluminescence of manganese- and copper-doped CdS nanowires

Authors: Ghiordanescu, V; Sima, M; Enculescu, I; Grecu, MN; Mihut, L; Secu, M; Neumann, R

Published: FEB 2005, PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE, 202, 454, DOI: 10.1002/pssa.200406927

Arrays of US: Mn2+ : Cu+ micro- and nanowires grown in polycarbonate ion-track templates exhibit photoluminescence in the spectral domain ranging from 500 to 800 mn at room temperature. A comparison with similar US and CdS:Mn2+ wire arrays is presented. The individual contributions to the emission spectra of Cu+ and Mn2+ ions in the US matrix are explained using their energy level schemes. Also SEM, EDX and EPR data are given or these wires. (c) 2005 WILEY-VCH Verlag GmbH A Co. KGaA, Weinheim.

4238. Effect of UV radiation on the semi-interpenetrating polymer networks based on polyurethane and epoxy maleate of bisphenol A

Authors: Rosu, L; Cascaval, CN; Ciobanu, C; Rosu, D; Ion, ED; Morosanu, C; Enachescu, M

Published: JAN 15 2005, JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY A-CHEMISTRY, 169, 185, DOI: 10.1016/j.jphotochem.2004.06.008

The behavior of two semi-interpenetrating polymer networks (S-IPNs) based on polyurethane (PU) and epoxy maleate of bisphenol A (EMBA) under exposure to UV radiation for long period of times (up to 200 h) was investigated. The networks obtained by the specific mixture under certain thermal treatment included in their structures crosslinked EMBA and linear PU. Advanced photooxidative degradation took place for long time of UV irradiation (over 160 h, especially), with almost a total destruction of the raw structures. The important modification of both the surface and of the morphology of the tested samples with the time of irradiation was also analyzed. Simultaneously, as compared to results obtained for short time of UV radiations (up to 10 h), where the mechanical properties of the tested samples improved, long time of UV radiations led to deterioration of many of the mechanical properties. (C) 2004 Elsevier B.V. All rights reserved.

4239. Local interactions and electronic phenomena in substituted LaFeO3 perovskites

Authors: Russo, U; Nodari, L; Faticanti, M; Kuncser, V; Filoti, G

Published: JAN 14 2005, SOLID STATE IONICS, 176, 102, DOI: 10.1016/j.ssi.2004.06.001

Effects of electronic charge disproportionation were observed by Fe-57 Mossbauer spectroscopy at low temperatures in substituted LaFeO3 perovskites as (La1-xCax)FeO3 and La(Fe1-xMgx)O-3 (xless than or equal to0.5). The presence of both Fe3+ and Fe5+ species was evidenced in both series of compounds. Direct evidence for disproportionation was supported by correlating the Mossbauer results with the chemical determination of tetravalent iron at RT. The evolution of hyperfine parameters vs. substitution degree was comparatively analysed for Fe3+ and Fe5+ ions in relation to their electronic microscopic origin. Electron delocalisation processes are discussed in terms of ion distribution and valence state. (C) 2004 Elsevier B.V. All rights reserved.

4240. Structural phase transition induced in Fe50Rh50 alloys by high pressure

Authors: Kuncser, V; Nicula, R; Ponkratz, U; Jianu, A; Stir, M; Burkel, E; Filoti, G

Published: JAN 11 2005, JOURNAL OF ALLOYS AND COMPOUNDS, 386, 11, DOI: 10.1016/j.jallcom.2004.04.139

The equiatomic composition of the FeRh alloy was studied by in situ high pressure energy dispersive X-ray experiments. Pressures as high as 19 GPa were generated using a diamond anvil cell. An incomplete bcc --> fcc phase transition was evidenced, both bcc and fcc phases coexisting above 10.0 GPa. The transition is accompanied by a decrease of the lattice parameter of the remaining bcc phase by approx. 1.5%. A substantial low lattice parameter was observed for the newly formed fee phase, reflecting a rather compressed structure. (C) 2004 Elsevier B.V. All rights reserved.