National Institute Of Materials Physics - Romania

Vortex excitations and creep regimes in YBa2Cu3O7 (YBCO) films with nonsuperconducting nanorods (columnar defects) and/or nanoparticles in a low external magnetic field H oriented perpendicular to the film surface have been identified using standard DC magnetization relaxation measurements. It was found that by increasing the pinning energy dispersion through nanoparticle addition the relevant changes in the temperature T variation of the normalized magnetization relaxation rate S appear at relatively high temperatures (T similar to 65K for H = 2 kOe), owing to the inhibition of double vortex- kink formation. This indicates that the often observed S(T) maximum at low T (around 30 K) is not related to the excitation of double vortex kinks and super-kinks in specimens with columnar defects, as previously believed. The low-T S(T) maximum is present in the case of strongly pinned specimens without nanorods, as well, and depends on the film thickness, which definitely shows that it is caused by thermo-magnetic instabilities, as recently suggested. (C) 2014 Elsevier B.V.All rights reserved.

xLi(2)O-(1 x)alpha-Fe2O3 (x=0.1, 0.3, 0.5, and 0.7) nanoparticle systems were successfully synthesized by mechanochemical activation of Li2O and alpha-Fe2O3 mixtures for 0-12 h of ball milling time. The study aims at exploring the formation of magnetic oxide semiconductors at the nanoscale, which is of crucial importance for catalysis, sensing and electrochemical applications. X-ray powder diffraction (XRD), Mossbauer spectroscopy and magnetic measurements were used to study the phase evolution of xLi(2)O-(1 x)alpha-Fe2O3 nanoparticle systems under the mechanochemical activation process. Rietveld refinement of the XRD patterns yielded the values of the particle size as function of composition and milling times and indicated the presence of Li-substituted hematite and tetra lithium iron oxide LiFeO2, along with the formation of multiple phases for large x values and long milling times. The Mossbauer studies showed that the spectrum of the mechanochemically activated composites evolved from a sextet for hematite to sextets and a doublet upon duration of the milling process with lithium oxide. Magnetic measurements recorded at 5 K to room temperature (RT) in an applied magnetic field of 50,000 Oe showed that the magnetization of the milled samples is larger at low temperatures than at RT and increases with decreasing particle size. Zero field cooling measurements made possible the determination of the blocking temperatures of the specimens as function of ball milling time and evidenced the occurrence of superparamagnetism in the studied samples. This result correlates well with the observed presence of a quadrupole-split doublet in the Mossbauer spectra. (C) 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Sintered bulks require new approaches for the control and improvement of their functional characteristics. Nanostructuring, synergy effects at interfaces in a composite, and technology specific features are essential in this regard. In this work, for a better understanding of challenges, complex relationships between materials and technology and different practical concepts such as "transferability", "composite within a composite", and "multilevel design", and "multifunctionality", a comparative analysis is proposed for very different nano structured materials such as La0.9Sr0.1Ga0.8Mg0.2O3-x x solid electrolyte, hard B4C-based materials, and doped MgB2 superconductors. Samples were obtained by reactive spark plasma sintering (SPS). SPS is reconfirmed as a powerful processing technique that generates high-density unique bulk materials impossible to fabricate by other methods. However, it is shown that SPS is not universal and its suitability should be carefully considered depending on materials and targeted applications. (C) 2014 The Japan Society of Applied Physics

A mixture of B2O3 and amorphous B with the mole ratio of 1:14 is shown to react and form B6O ceramic under in-situ spark plasma sintering (SPS) conditions. The optimum SPS temperature and time to obtain phase B6O are 1250 C and 30 min, respectively, and Rietveld refinement of the XRD patterns indicates that the oxygen occupancy of B6Ox is reasonably high at x = 0.89(99). However, to reach high density above 98%, SPS temperatures of 1700-1800 degrees C are necessary and a one-step twotemperature in-situ reactive SPS was designed and applied. As-prepared B6O dense ceramic has Vickers hardness (36.7 +/- 1.2 GPa) and fracture toughness (K-1c = 4.2 +/- 0.15 MPa. m(1/2)) comparable with the highest values reported in literature for the bulks obtained by processing routes of already reacted B6O powders (ex-situ routes).

Dense boron carbide (above 95%) was achieved through high pressure (300 MPa) and low temperature (1600 degrees C) Spark Plasma Sintering (SPS). This approach resulted in improvement of fracture toughness and of dynamic toughness when compared to corresponding toughness values of the sample sintered by conventional SPS (2100 degrees C, 50 MPa). Dynamic toughness was extracted from Split Hopkinson Pressure Bar measurements. Results are understood based on microstructure and on very different behaviour of the samples in respect to residual B2O3 and carbon available in the raw B4C powder. (C) 2014 The Ceramic Society of Japan. All rights reserved.

Boron carbide ceramic was prepared by reactive Spark Plasma Sintering under N-2-atmosphere and for different heating times and maximum pressure regimes. Split-Hopkinson Pressure Bar (SHPB), indentation, XRD and microscopy measurements were performed for samples characterization. It is shown that SHPB toughness control depending on SPS regime is possible and the main reason is introduction of nitrogen into B4C ceramic. Complex relationships between processing conditions, sintering mechanism, material's specifics, static and dynamic mechanical properties are discussed. Improvement of dynamic toughness is through mechanisms resembling those working for static load conditions such as cracks deflection and pull out, but there are also significant differences. (c) 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Novel metal nanoporous transition metal oxides M (x) O (y) (Co3O4, CuO) have been synthesized by thermal decomposition of inorganic salts precursors (acetates, nitrates) impregnated into hexagonal mesoporous silica (OMS, ordered mesoporous silica) of SBA-15 type (prepared in-house) at different precursor loadings, the mesocomposites thus obtained being monitored after each impregnation-calcination step by small and wide angle powder XRD. The pore size for the ordered silica host range from 5.08 to 7.06 nm. Retention of the hexagonal silica framework has been observed in spite of the temperatures up to 500 A degrees C. Mesoporous Co3O4 has been obtained by leaching the silica through overnight HF dissolution, which partially preserved the small-range ordering found in the parent Co3O4@OMS composite prior to leaching. Both Co3O4 (meso) and Co3O4@SBA-15 have been tested in methane oxidation and were found to be superior to the bulk Co3O4 performance, with mesoporous Co3O4 being able to fully oxidize methane to CO2 and H2O at 350 A degrees C, while Co3O4@OMS exhibits a lower activity with 20 % conversion at 350 A degrees C. CuO@OMS shows the lowest activity, with only similar to 13 % conversion at 500 A degrees C.

Magnetoresistive Fe-Co based thin film structures were produced using thermionic vacuum arc method. The purpose of this work was to obtain significant magnetic response on different granular combinatorial structures. The Giant Magnetoresistive (GMR) and combination between GMR and Tunneling Magnetoresistive (TMR) properties of the thin film structures were the aim of the work. The proposed method in order to obtain the desired granular structures is based on electron beam emitted by an externally heated cathode, accelerated by a high anodic potential. The work consisted in preparing two sets of samples, first being a combination between Fe-Co as magnetic materials embedded in a Cu matrix, with a total thickness of 200 nm. The second structure was a combination between Fe-Co (50%-50%) alloy, embedded in a matrix of Cu combined with MgO with a thickness of 200 nm. Both sets of samples were obtained by three simultaneous types of discharges. Because of the substrate positioning in respect with the three anode-cathode systems, different material concentrations were obtained, confirmed by Energy Dispersive Spectroscopy (EDS) measurement results. Structural and morphological properties were investigated using Scanning Electron Microscopy; Atomic Force Microscopy and EDS. Electrical properties of the obtained samples were studied using the 4 point measurement method. The magnetic properties were first studied using a non-destructive optical method called MOKE (Magneto-Optical Kerr Effect). Electrical resistance behavior of the granular type structures was studied for different values of the magnetic field, up to 03 T, at different values of the sample temperature. The magnetoresistive effect obtained for the two sets of samples varied from 1.5% to 81% in respect with the substrate positioning and sample temperature for a constant magnetic field. (C) 2014 The Authors. Published by Elsevier B.V. All rights reserved.

In order to study specific phenomena at ferromagnetic/semiconducting interfaces, of potentially high interest in spintronics and information technology, structural aspects and magnetic properties of Fe thin films grown on Si(001) substrates by RF sputtering have been investigated using Fe-57 conversion electron Mossbauer spectroscopy (CEMS) and magneto-optic Keer effect (MOKE). Films of different thicknesses have been deposited either directly on crystalline Si substrates or on Cu buffer layers. An inherent Fe oxide layer is observed in all as prepared films, with a relative thickness decreasing drastically with the deposition time. The Cu buffer layer does not diminish either the interfacial diffusion or the oxidation process. An efficient method to prepare sharper oxygen- and silicon-free interfaces for an improved spin injection, via thermal treatment in hydrogen atmosphere, is proposed. Accordingly, the hydrogenation treatments are very efficient in the modification of the ferromagnetic film structure, phase composition, magnetic properties and interfacial mixing. (C) 2013 Elsevier B.V. All rights reserved.

The FePt system has important perspectives as high-temperature corrosion-resistant magnets. In the form of rapidly solidified melt-spun ribbons, FePt-based magnets may exhibit in certain cases a two-phase hard-soft magnetic behaviour. The present paper deals with a microstructural and magnetic study of FePtAgB alloys with increasing Ag content. The aim is to identify and confirm the effect of Ag addition in decreasing the temperature of the FePt disorder-order structural phase transformation. A detailed high-resolution transmission electron microscopy study is employed, and the alternative disposal of hard and soft regions within the two-phase microstructure is observed and interpreted with respect to the X-ray diffraction results. In the as-cast Ag-containing samples, it is shown that there is an optimum of the Ag content for which best magnetic properties are obtained. Ag addition creates a nonlinear behaviour of the coercive field and the ordering parameter, similar to the RKKY interaction-induced interlayer exchange coupling (IEC) observed in magnetic layers separated by non-magnetic spacer layers. Direct formation of the L1(0) phase from the as-cast state in the FePtAgB alloys is reported with magnetic parameters compatible to other exchange spring permanent nanomagnets. These findings open novel perspectives into utilization of such alloys in applications requiring magnets operating in high-temperature industrial environments.