National Institute Of Materials Physics - Romania

A study of the magnetic properties of an assembly of non-interacting iron oxide-based nano-particles is presented. The measured average particle size is found to approach the expected thickness of the spin disordered layer. In this regime, the magnetization is characterized by two parameters: T-b above which the particle acts as a single domain, and T-E below which intra-particle coupling between pinned and unpinned spins become important. (C) 2004 Elsevier B.V. All rights reserved.

Tin-doped hematite nanoparticles (1-x)alpha-Fe2O3-xSnO(2) were synthesized using the hydrothermal method. The samples were characterized using Mossbauer spectroscopy as a function of the substitution level x. The recoilless fraction was determined using the dual absorber method introduced by us [M. Sorescu, Mater. Lett. 54 (2002) 256]. It has been observed that the recoilless fraction of the hematite samples first decreases with increasing the amount x of tin substitution, then increases as iron enters the SnO2 lattice and, finally, it slightly decreases again as the particle size of the nanoparticles decreases. (C) 2004 Elsevier B.V. All rights reserved.

The crystallographic and magnetic properties of the Y3Ni13-xCoxB2 series of compounds, with the Nd3Ni13B2-type structure and xless than or equal to5, were investigated by powder x-ray diffraction and magnetic measurements. The susceptibility and magnetization data show that Y3Ni13B2 is a weak itinerant antiferromagnet with a Neel temperature T-N=68 K. A weak ferromagnetic component appears at T-N, suggesting uncompensated antiferromagnetic structure. The compounds with x=0.5 and 1, also order antiferromagnetically, with T-N=94 and 106 K, respectively. The intrinsic weak ferromagnetic component increases with the cobalt content and a field-induced transition, from the antiferromagnetic state to a ferromagnetic state, is observed at the critical transition fields 5.6 and 1.6 T, at 5 K for x=0.5 and 1, respectively. A heterogeneous ferromagnetic state and in-plane magnetic anisotropy are determined for the compounds in the 2less than or equal toxless than or equal to5 composition range. The Curie temperature and spontaneous magnetization increase linearly with the cobalt content. Based on the experimental data, the T-x phase diagram is constructed for xless than or equal to5. The effect of spin fluctuations is highlighted in the nickel-rich compounds in the 3:13:2 and related systems.

The photostimulated luminescence (PSL) from Eu-doped fluorobromo- and fluorochlorozirconate glass-ceramics is reviewed. The PSL is attributed to the characteristic emission of Eu2+ present in BaX2 (X = Br, Cl) crystallites which form in the glass upon thermal processing. Both the photoluminescence intensity and the PSL efficiency increase with the size of the crystallites in a way that is consistent with a PSL-inert surface layer surrounding the crystallites. The increasing crystallite sizes also result in decreased optical transparency at the stimulation wavelength due to scattering. (C) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Using Raman scattering and Fourier transform infrared (FTIR) spectroscopy, we show that the vibrational properties of single-walled carbon nanotubes (SWNTs) functionalized with conducting polymers (CPs), such as polyaniline (PANI) and poly (3, 4-ethylenedioxythiophene) (PEDOT), are different depending on the synthesis method. In the case of SWNTs electrochemically functionalized with PANT, the increase in intensity of the Raman band at 178 cm(-1), associated with radial breathing modes of bundled SWNTs, indicates that an additional roping of nanotubes with PANT takes place. Besides, the functionalization of SWNTs with PANT induces strong steric hindrance effects observed in FTIR spectra by the enhancement of bands at ca. 740-750 and 772 cm(-1) associated to the deformation vibrations of the benzoid and quinoid ring, respectively. Raman spectra recorded after the electropolymerization of 3, 4-ethylenedioxythiophene (EDOT) on the SWNTs film reveal that a side-wall functionalization takes place. (c) 2005 Elsevier B.V. All rights reserved.

The effect of Fe and Ni doping on piezoelectric properties of a soft type piezoelectric material was investigated. The materials composition was as follows: Pb0.95Bi0.03Nb0.02Zr0.51Ti0.49-xMxO3, where M stands for the transitional metals Fe or Ni and x = 0, 0.02, 0.04, 0.06, 0.08 and 0.10. The materials were prepared by the conventional ceramic technique. X-ray diffractograms showed that compounds were completely formed and they were situated in the nanometric range with an average crystallite size of about 95 urn. The optimum amounts of doping for both types of transitional elements were situated somewhere around x = 0.06 with better results for nickel doped samples. The maximum density for 0.06 nickel doped material was 7.87 and 7.80 g/cm(3) for iron doped one. The electromechanical coupling factor k(p) for 0.06 nickel doped samples was 0.665 while for the correspondingly iron doped ones it was 0.638. The relative dielectric constant was about 4050 for nickel doped samples and 3400 for iron doped ones. The corresponding values for the charge constant d(33) were 625 and 530 pm/V, respectively. These results were discussed in terms of the positions occupied by Ni and Fe into the lattice, the type of vacancies created by this and the shift of the morphotropic phase boundary. (c) 2005 Published by Elsevier Ltd.

The magnetic properties of FINEMET-type nanocrystalline alloys and isolated ferromagnetic AgCo nanoparticles are investigated both experimentally and numerically. Theoretical models of spins that simulate ideal nanocrystalline alloys and isolated nanoparticles are considered while their magnetic properties are derived from Monte Carlo simulation of low-temperature spin ordering. Interesting features such as magnetic polarization of the matrix due to penetrating fields arising from nanograins and the role played by the crystalline fraction in the overall L magnetic behaviour, in the case of nanocrystalline alloys are investigated. For isolated nanoparticles it is shown that the competition between surface and bulk anisotropy gives rise to surface spin disorder that, together with finite-size effects, is responsible for the experimentally observed lack of saturation of the magnetization in high applied fields. These simulation results are confirmed by experimental data obtained on FINEMET nanocrystalline alloys and isolated ferromagnetic AgCo colloidal nanoparticles.

The influence of preliminary treatment in hydrogen plasma on elimination of radiation defects and formation of thermal donors has been studied in detector structures made of standard and oxygenated float zone silicon has been studied. A new type of thermal donors has been found in as-treated diodes. These thermal donors are unstable and can be eliminated by heat-treatment at 200-250 degrees C. After irradiation with 3.5 MeV electrons the detectors had been annealed at temperatures of 50-350 degrees C. It has been found that preliminary hydrogenation at 300 degrees C leads to disappearance of main vacancy-type radiation defects at lower annealing temperatures, The annealing of hydrogenated and irradiated crystals is accompanied by hydrogen redistribution and formation of hydrogen-related donors. Preliminary irradiation influences on both these processes.

Hardening laser surface offers new possibilities for the enhancement of the mechanical resistance of superficial layers. Essential for the laser treatment processes is the efficiency at which the incident laser power is coupled into the work piece. The aim of the work was to study the effect of some factors such as spot shape, beam angle of incidence and surface coating on the steel surface absorptivity. Samples made of carbon steel were laser processed using a CW CO2 - 1,2 kW laser unit. Different process parameters (beam power, spot dimension and traverse speed), were used. The microstructure of laser hardened layers which have been investigated by optical microscopy. The mechanical characterization of the layers has been done by hardness measurements. Correlation has been established between the structure of the laser-processed layers and the process parameters.

Effect of Bi additives on the piezoelectric properties of PT ceramics having the formula (Pb1-3x/2Bix(Ti0.98Mn0.02)O3, with x = 0,04, 0.06, 0.08 was investigated. The samples were prepared by conventional ceramic technique, using p.a. purity raw materials. The Mixed powders were sintered at temperatures between 1050-1300 degrees C. Poling was done in fields of about 70 kV/cm. Density and coupling factors of the samples were determined as a function of sintering temperatures, and doping level. Temperature dependence of the main piezoelectric characteristics was also investigated for temperatures as high as 500 degrees C. A Curie point of 450 degrees C, a low dielectric constant (< 140) and a large electromechanical anisotropy), (the radial mode is nearly inexistent, at room temperature and the thickness coupling factor is about 0.4) were found. These enhanced properties make such ceramics very attractive for high temperature and high frequency transducers applications.