National Institute Of Materials Physics - Romania

We measured the critical current density J(c) of optimally doped YBa(2)Cu(3)O(7) (- delta) thin films obtained by dc sputtering on (100) oriented SrTiO(3) substrates and by pulsed laser deposition (PLD) on (100) oriented MgO substrates with SrTiO(3) buffers. J(c) at various temperatures T was determined from the magnetization curves registered with a vibrating sample magnetometer for an external magnetic field H (oriented along the c axis) up to 90 kOe. It was found that J(c) of the films obtained by PLD usually overcomes the J(c) values of the sputtered films in the whole investigated (H, T) domain.

Simple Fe-Co ferromagnetic films and Fe-Mn antiferromagnetic films of different compositions were grown on epitaxial Si substrates with Ta buffer layers via thermo-ionic vacuum arc method. Fe-Co/Fe-Mn bilayer structures were obtained by following similar growing conditions as for the simple films. The magnetic behavior concerning easy axis distribution, anisotropy energy and coercive field of the Fe-Co films exchange coupled to the antiferromagnetic layers were discussed with respect to the magnetic behavior of the simple, uncoupled, ferromagnetic films. The composition of the antiferromagnetic films has a sensible influence on the magnetic parameters of the exchange coupled ferromagnetic films. The largest coupling and the narrowest easy axis distribution are induced in the bilayer system with equatomic composition of the Fe-Mn antiferromagnetic layer. (C) 2010 Elsevier B.V. All rights reserved.

Two series of cobalt substituted Ni-Fe-Ga ferromagnetic shape memory alloys with lower gallium content (< 27 at. %) were studied by differential scanning calorimetry, x-ray diffraction, and thermomagnetic measurements. Co substitution for Fe or Ni promotes an increase in the Curie temperatures and a variation in the martensitic transformation (MT) temperatures, in accord with the alloy valence electron concentration change. For alloys with MTs below room temperature, a field dependent thermomagnetic hysteresis was evidenced and discussed in connection with the effect of cobalt substitution, on the magnetic hardness of the martensite phase. A direct interpretation of the evolution of the thermomagnetic hysteresis versus the applied field was provided. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3429231]

Exchange bias AF/Fe and spin valve AF/Fe/Cu/Fe (AF=Fe50Mn50 and Ir50Mn50) multilayer systems have been prepared by molecular beam epitaxy. Thin tracer layers enriched in the Fe-57 isotope were artificially grown at the AF/Fe interface and the phase composition of the ferromagnetic layer, as well as the interfacial atomic diffusion were observed via Fe-57 conversion electron Mossbauer spectroscopy. The dependence of the magnetization reversal process on training and temperature associated effects was studied by low temperature vibrating sample magnetometry, whereas the interlayer magnetic coupling was analyzed via longitudinal magneto-optic Kerr effect.

Lithium borohydride (LiBH4) is a promising material for hydrogen storage, featuring a high gravimetric storage density (above 13 wt. % in the first decomposition step). However, studies of re-hydrogenation of the decomposition products have proven less successful, leading us to the requirement of modified-LiBH4 materials with better absorption-desorption behavior. A Mo-doped mesoporous material was obtained starting from commercial MSU-H and ammonium molybdate. This mesoporous material (MSU-H-type functionalized with molybdenum salt) was used as support in order to impregnate LiBH4. Even though the thermodynamics of LiBH4 hydrogen absorption-desorption is not modified, the desorption kinetics of supported LiBH4 is improved compared to that of pure LiBH4. Moreover, after re-hydrogenation at 450 degrees C only 3.2 wt. % H-2 of storage capacity is recovered for LiBH4, while for supported LiBH4 we achieved almost 5.2 wt. % H-2.

A simple and novel method for production of metal clusters as building blocks for nanoscale devices is presented. With this procedure, called the gas / cluster aggregation method, a wide range of clusters may be synthesised and, more important, these clusters may be subsequently modified and functionalized in-situ by adding atoms/molecules of different nature, on the surface of readily formed clusters. The cluster size is extremely well controlled by the vapour pressure of the picked-up species. Moreover, the method is versatile, since it allows multiple pick-up processes within the same rare gas cluster for producing, for example, core-shell nanoparticles with metal core and non-metallic shell or vice-versa, nano-onions, with different species successively attached to the surface of the initial picked-up cluster, and so on. Initial formation of Fe gas-stabilised clusters and core-shell nanoparticles with Fe core and Fe oxide shell, as well as their structure and morphology, are presented and discussed. The core-shell nanoparticles show incipient self-organization into hexagonal cluster superlattice. Structural, magnetic and Mossbauer spectroscopy investigations have been performed on the Fe cluster samples. The magnetic properties of supported Fe clusters show marked differences compared to the bulk. A small hysteresis is observed in the parallel applied field while in the perpendicular case, lack of saturation at the highest applied field is noticed. Such behaviour has been also observed in FeRh [1] and AgCo [2] bimetallic nanoparticles. This behaviour marks the occurrence of a strong planar magnetic anisotropy in the sample and may also be a consequence of increased surface spin disorder and finite size effects, which are typical for nanoparticles in the reported size range.

The FePtNbB alloys are of interest as a new class of exchange spring magnets. In strictly defined compositional range and after proper conditions of annealing, they develop a microstructure made of small nanograins of hard and soft magnetic phases alternatively disposed and coupled through exchange interactions. Taking advantage of the high uniaxial aniosotropy of the tetragonal hard magnetic FePt phase and high saturation magnetization of the cubic soft magnetic FePt phases, an exchange coupled nanocomposite magnet can be derived. A compositional study of FePtNbB alloys is provided in order to understand the role of each additional element. The possibility of casting alloys where two or more magnetic phases can be obtained is discussed in terms of the eutectic point of the FePt phase diagram. Also, the formation and evolution of the magnetic phases is studied by differential scanning calorimetry (DSC) and detailed analysis of these DSC scans is provided. Activation energy determined from DSC scans provides essential informations about crystallization and disorder-order phase transformation in these alloys.

Au3Zr intermetallic phase was obtained as pure phase by melt spinning technique. The sample with the same composition obtained by powder metallurgy exhibits some Au4Zr impurity. Rietveld refinements of the X-Ray data were done to obtain the unit cell parameters and atom positions inside the unit cell. Geometry optimizations for the atom positions inside the unit cell were performed by using ab-initio band structure calculations. The calculated values agree well with the ones obtained by Rietveld refinements of the diffraction data. The electronic mechanisms are explained on the basis of the projected densities of states to atom sites.

Using Pulsed Laser Deposition we have fabricated thick quasi-multilayers composed of incomplete layers of PrBa2Cu3Ox (PrBCO) nano-dots and layers of YBa2Cu3O7-delta (YBCO). The number of such sequences was between 2 and 6, with the thickness of individual YBCO layers between 565 and 885 rim, and total thickness between 1.13 and 5.31 mu m. For the thinner quasi-multilayer, DC magnetization studies showed an increase in the critical current density J(c) at all fields in comparison with a pure YBCO reference sample, while the thicker samples showed an increased J(c) only in high fields. We have also investigated the frequency dependence of J(c) from AC susceptibility studies and found that the pinning potential is well described by a logarithmic dependence on current density. Pinning potentials in PrBCO/YBCO quasi-multilayers also proved to be higher than in the reference sample at high fields. From angle-dependent transport measurements we have found indications of strong pinning centres induced by the (PrBCO) nanodots, both isotropic and c-axis correlated. (C) 2009 Elsevier B.V. All rights reserved.

A THz Time Domain Spectroscopy experimental set-up was built for studying the biomolecules absorption spectra in the THz range and obtaining information on their collective motions. Details on the set-up calibration and performances are presented, together with the used noise and artefacts reduction data processing techniques. Preliminary results on the absorbance spectrum of the solvated Bovine Serum Albumin (BSA) for different water concentration are given.