National Institute Of Materials Physics - Romania

Nanostructured powders processed by ball milling of a mixture of Fe and Fe3O4 at room temperature are shown to undergo an incomplete redox reaction with formation of FeO during the milling process. This reaction is favored by the high energy introduced during the mechano-alloying process. Concurrent effects of milling such as grain refinement down to the nanometre scale lead at the end of the milling processes to a mixed multiphase powder of nanograins, with Fe and Fe oxide grains inter-dispersed. We show that in the as-milled Fe/Fe3O4 powder, during milling process, wustite (FeO) is formed as a consequence of the redox reaction. Moreover, with increasing temperature, the system undergoes an inverse phase transformation towards the initial Fe and Fe3O4 phases until about 450 degrees C. Above this temperature the reduction reaction Fe + Fe3O4 = 4FeO is reinitiated, resulting in sharp decrease of Fe and Fe3O4 content from about 550 degrees C and almost complete disappearance of these phases at about 900 degrees C. This transformation was investigated via an energy-dispersive in situ X-ray diffraction experiment using the synchrotron radiation. This study allows direct collection of X-ray patterns after few minutes exposure, at selected temperatures, ranging between 20 degrees C and 1000 degrees C. The structural and magnetic characterizations of the nanograin powders, as-milled and annealed at several temperatures, are studied using XRD, SEM and magnetic measurements. Such ferromagnetic-antiferromagnetic composites are extensively studied as they exhibit exchange bias effect, with a large impact in technological applications. The magnetic behaviour and intrinsic mechanisms leading to the occurrence of exchange bias effects are discussed and related to the samples microstructural features. A significant exchange bias effect, related to FeO content, is observed for as-milled sample, the effect being less pronounced upon annealing the nanograin powder. (C) 2011 Elsevier B.V. All rights reserved.

SrZrO3 powders are obtained by solid state reaction from SrCO3 and ZrO2 precursors, without involving intermediate calcination and grinding steps. The resulted powders are essentially within a single phase, with sub-micron average crystallite size. Pellets of these powders show a relatively poor sintering behavior, when fired up to 1600 degrees C. Alternatively, spark plasma sintering technique is used in order to obtain nearly 100% dense samples at the expense of excessive grain coarsening (i.e., up to 5 mu m in diameter). Crystalline structure, composition and morphology of the specimens obtained in this work are investigated by X-ray diffraction, scanning and transmission electron microscopy together with energy dispersive X-ray spectroscopy. (C) 2011 Elsevier B.V. All rights reserved.

Hydrogen adsorption in Metal-Organic Frameworks is improved by using platinum and palladium on different carbon support: activated carbon or carbon nanofibres. Although, hydrogen adsorption is directly dependent of the specific surface area of the material, it is not the only factor contributing to the hydrogen adsorption, as presented in this work. Direct synthesis of metal-organic frameworks on supported Pt and Pd catalysts, reduces the surface area significantly, but the presence of the catalysts overcomes this drawback by improving hydrogen adsorption by a factor of 2.2, in case of platinum and by a factor of 1.4 in the case of palladium, compared to the metal-organic framework without catalysts.

Superconducting Y123 thin films with (001), (100/010) and (103/110) orientations were obtained by molecular-organic- source MBE (MOMBE) for optimum growth temperatures of 600, 515 and 600 degrees C, respectively, using a Ba(DPM)(2)(phen)2 source material and ozone atmosphere. Superconductivity was obtained without post processing. According to the literature, use of a Ba(DPM)(2)(phen)(2) precursor was expected to yield a lower optimum growth temperature, but there were no significant differences between the use of this source and a Ba(DPM)(2) precursor. For our c-axis oriented Y123 film, the optimum growth temperature was higher than the reported record-low growth temperature (500 degrees C by MOCVD using Ba(DPM)2(phen)2) but it was lower than the values for other growth techniques. This can have a positive impact on fabrication of layered devices or integration through the decrease of inter-diffusion between the layers. Results also suggest that processes are complex and other details than the source type should be carefully considered.

We report on an original technique for nanoengineered pinning centers that combine three previously reported approaches: substrate decoration, quasi-multilayers and targets with secondary phase nanoinclusions. We have used a 4% BZO-doped YBCO target, and Ag nanodots, all grown by PLD. Such an approach gave interesting results in terms of pinning landscape, as proved by TEM studies. Angle-dependent measurements of critical current I-c showed that, for smaller fields, the absolute maximum in I-c occur for fields perpendicular to the a-b planes, while at larger fields the absolute maximum in I-c occurs for fields parallel to the a-b planes, in both cases with a clear second, local maxima. Measurements also showed a smooth change in the character of pinning with magnetic field, from the out-of-plane to in-plane-dominant pinning. For the out-of-plane magnetic field, the highest Ic-w (critical current per cm width) obtained so far, at 77.3 K, occurred in a 5.8 mu m Ag nanodots / BZO-doped YBCO trilayer: 782 A/cm-w in self-field, 167 A/cm-w in 1 T and 18 A/cm-w in 3 T.

The martensitic transformation characteristics in two series of cobalt substituted Ni-Fe-Ga Ferromagnetic Shape Memory alloys have been studied by differential scanning calorimetry, X-ray diffraction, electrical resistivity and thermomagnetic measurements. Co substitution for Fe or Ni promotes an increase of the Curie temperatures but the changes of the martensitic transformation temperatures are in agreement with the valence electron concentration and unit cell volume variations. A large hysteresis observed between cooling and heating curves as feature of a first order phase transition was evidenced by r(T) and thermomagnetic measurements.

Iron/iron carbide core and carbon shell nanoparticles with improved magnetic properties were successfully synthesized by laser pyrolysis. As iron and carbon precursors, iron pentacarbonyl and pure or argon-diluted acetylene/ethylene mixtures, respectively, were used. The aim of the present optimization is the improvement of the magnetic properties of the nanomaterials by the increase of the iron percent in powders simultaneously to the maintaining of the protective character of the carbon coverage of nanoparticles. The chemical content and the crystalline structure were monitored by EDX, XRD and TEM techniques. In the first study, the content of acetylene as carbon source was diminished from 75% to 0%. Consequently the percent iron increased from 10 at.% to 28 at.% while oxygen remained relatively constant (around 5 at.%). In the second step, only diluted ethylene was used (maximum 87.5 vol.% Ar). In this case, an increase of iron to 46 at.% is observed. An optimum 50% carbon source dilution was found. Above this value, the carbon content increases and below it, superficial oxidation increases through the diminishing of the carbon shell. The magnetic properties and the Fe phase composition of the Fe-C samples were analyzed by temperature dependent Mossbauer spectroscopy. (C) 2010 Elsevier B.V. All rights reserved.

Complex oxides demonstrate specific electric and magnetic properties which make them suitable for a wide variety of applications, including dilute magnetic semiconductors for spin electronics. A tin-iron oxide Sn(1-x)FexO(2) nanoparticulate material has been successfully synthesized by using the laser pyrolysis of tetramethyl tin-iron pentacarbonyl-air mixtures. Fe doping of SnO2 nanoparticles has been varied systematically in the 3-10 at% range. As determined by EDAX, the Fe/Sn ratio (in at%) in powders varied between 0.14 and 0.64. XRD studies of Sn(1-x)FexO(2) nanoscale powders, revealed only structurally modified SnO2 due to the incorporation of Fe into the lattice mainly by substitutional changes. The substitution of Fe3+ in the Sn4+ positions (Fe3+ has smaller ionic radius as compared to the ionic radius of 0.69 angstrom for Sn4+) with the formation of a mixed oxide Sn1-xFexO(2) is suggested. A lattice contraction consistent with the determined Fe/Sn atomic ratios was observed. The nanoparticle size decreases with the Fe doping (about 7 nm for the highest Fe content). Temperature dependent 57Fe Mossbauer spectroscopy data point to the additional presence of defected Fe3+-based oxide nanoclusters with blocking temperatures below 60 K. A new Fe phase presenting magnetic order at substantially higher temperatures was evidenced and assigned to a new type of magnetism relating to the dispersed Fe ions into the SnO2 matrix. (C) 2010 Elsevier B.V. All rights reserved.

Oxalates containing various 3d transitional elements and positive NH(4) or negative OH groups were newly synthesized. Each above-mentioned component has directly influenced the structure, the electronic or interaction properties, while some unexpected behaviors were revealed by various magnetic and Mossbauer measurements. The main magnetic parameters, the long-range anti-ferromagnetic couplings observed at very low temperature and, particularly the uncompensated moment are discussed in detail. The induced lower spin states for bivalent ions and especially the anti-parallel arrangement of the spins belonging to trivalent and bivalent iron inside the molecule are also emphasized. (C) 2010 Elsevier B.V. All rights reserved.

We investigated the relaxation of the irreversible magnetization in a series of 200 nm thick YBa2Cu3O7/PrBa2Cu3O7 [(YBCO)(n)/(PrBCO)(m)] superlattices, where the thickness m of the nonsuperconducting PrBCO layer (measured in unit cells) was kept to m = 4 (sufficient to decouple the superconducting YBCO layers), whereas the thickness n of the YBCO layer was varied between 2 and 20 unit cells. The analysis of standard zero-field-cooling dc magnetization relaxation data obtained in the low temperature T region with the applied magnetic field H oriented along the c axis reveals the occurrence of a crossover elastic (collective) vortex creep at low T-plastic vortex creep at high T, generated by the T dependent macroscopic currents induced in the sample during measurements. For thin superlattices (n < 20) the creep crossover temperature T-cr proportional to n, and T-cr decreases linearly with increasing ln(H) for a fixed n. This crossover represents an alternative to the elastic vortex glass behavior reported for superlattices, as well as to 'quantum vortex creep' at unexpectedly high T inferred for thin films. We also discuss the absence of an increase of the magnetically determined critical current density with decreasing YBCO thickness in our superlattices, which apparently contradicts the collective pinning theories.