National Institute Of Materials Physics - Romania

Thin films with nominal composition Fe(78)Si(9)B(13) generated by radio-frequency (rf) sputtering on Si substrates with (100) orientation were irradiated with an eyeliner laser with 130mJ/cm(2) fluence. Information on the induced structural changes were obtained by X-ray diffraction and scanning electron microscopy, while the magnetization curves were determined by the magneto-optical Kerr effect. The effects of the irradiation consist in the growing of crystallites,appearence of melted and ultra-fast frozen zones, surface oxidation and changes in the magnetic properties of the thin films.

Strong, artificial pinning centres are required in superconducting films of large thickness for power applications in high magnetic fields. One of the methods for the introduction of pinning centres in such films is substrate decoration, i.e., growing nanoscale islands of certain materials on the substrate prior to the deposition of the superconducting film. Two other methods are building up a layered distribution of a second phase and homogeneous incorporation of second phase inclusions from a compositional target. In this paper, we compare the effectiveness of these methods in terms of the type of the self-assembly of nanoparticles. The comparison is made over a large set of YBa2Cu3O7 films of thickness up to 6.6 mu m deposited with Au, Ag, Pd, LaNiO3, PrBa2Cu3O7, YBCO, BaZrO3 and Gd2Ba4CuWOy nanoparticles. It is found that substrate-decoration self-assembly is able to provide higher critical current in low magnetic field than the incorporation of homogeneous second phase in the sample microstructure. By specific modification of substrate decoration we achieved the self-field critical current per centimetre of width of 896 A/cm at 77.3 K and 1620 A/cm at 65 K in a film of thickness of 4.8 mu m.

The influence of different crystallization conditions on the microstructure and magnetic hardening of RE-Fe-B amorphous ribbons with different Fe concentrations and Pr and Nd as rare earth elements was analyzed. The microalloying effects of Zr and Ti substitution on the evolution of crystallization process and the magnetic hardening was also discussed. The final aim of this experimental study was to obtain performing exchange spring magnets with as much as lower content of the expensive RE material.

The relaxation of the irreversible magnetization in optimally doped YBCO films with natural and artificial pinning centres was measured in zero-field cooling conditions using SQUID magnetometry. The external magnetic field H was oriented along the c axis. An appropriate method for the determination of the characteristic vortex pinning energy from the normalized vortex-creep activation energy is discussed. This is based on the existence of a crossover elastic (collective) vortex creep at low temperatures T - plastic vortex creep at high T, caused by the T dependent macroscopic currents induced in the sample during magnetization measurements.

Optical and electrical behavior of Giant Magnetoresistivc (GMR)/Tunneling Magnetoresistive (TMR) multilayer thin films were controlled adjusting thermionic vacuum arc plasma parameters. Smooth, dense and adherent films were prepared using optimized plasma conditions. A new setup for simultaneous depositions of pure materials was made, having inside the vacuum chamber three simultaneous discharges. To obtain the discharge all of the tungsten filaments of the thermionic vacuum arc guns were heated by a 45-55 A current. The emitted electrons were focused on a tungsten coated carbon crucible anode, filled with Cu, and the others crucible anodes filed with MgO, Co and Ag, respectively. The anode applied voltages for Cu, MgO Co and Ag were up to 1500 V, 1700V 1800V and 820V respectively. An optimization of the geometrical parameters as the distance between the cathode and the anode, and of the external parameters as the discharge voltage and the heating current of tungsten current filament was performed. Structural and morphological properties of the prepared films were analyzed by scanning electron microscopy (SEM). The behavior of the obtained films was first analyzed in a Magneto-Optical Kerr Effect (MOKE) experiment. The electrical resistance behavior of the prepared films was determined in a magnetic field which values varied from 0.4 T to 0.4 T. The important changes in the values of the electrical resistance of the films were observed and correlated to the plasma conditions.

The two crystallographic modifications of AuSe, alpha-AuSe and beta-AuSe, were studied by Au-197 Mossbauer spectroscopy. Despite the different crystal structures, the Mossbauer spectra of the two phases are nearly the same, and reveal the presence of two gold sites with Mossbauer parameters typical for Au-I and Au-III. The Mossbauer-Lamb factor of the Au-I is less than half of that of the Au-III. Calculations using the WIEN2k program reproduce the experimental results quite well and also yield the signs of the electric field gradients.

The transport properties of the gamma-irradiated CrO2-polymer composites were investigated. The resistance R is strongly current I dependent, except at high temperature where it displays a semiconductor-like temperature dependence for all currents. At low currents, I <= 10 mu A, as the temperature decreases far below the ferromagnetic transition, the resistance decreases first, then reaches a minimum, and at last increases again toward a second peak. At even lower temperatures, the second peak is followed by a metallic-like temperature dependence of R that ends at a cusp point marking the metal-insulator transition. The increase of the current shifts the cusp toward lower temperatures and the first minima toward higher temperatures. The resistance increases with the increase in current for all currents in the range 0.2 <= I <= 10 mu A. For I >= 50 mu A, the resistance increases monotonically with the decrease in temperature in the whole temperature range but obeys different laws at low and high temperatures. An explanation attempt in terms of spin transport, disorder, and thermal effects is proposed. (C) 2009 Elsevier B.V. All rights reserved.

Fe@C) nanoparticles have been successfully synthesized using the laser pyrolysis method and variable nozzle geometries. At large nozzle diameters, XRD and SAED analysis clearly identified distinct alpha-Fe and Fe3C phases. TEM and HRTEM indicated that these Fe-based nanoparticles have an average grain size of 3.5-10.2 nm. Temperature dependent Mossbauer spectra further confirm their distinct nanophases. By using a multi-step reduction procedure, Fe@C powders can be disaggregated into stable, water soluble nanoparticles.

The difficulty to stabilize a definite crystalline structure or phase, due to a large gradient of defects along the particle diameter, is unfortunately specific to particles of nanometer size. Therefore, atypical spin configurations and magnetic anisotropies as well as enhanced magnetic relaxations via thermal excitations are expected in systems of fine nano-particles. The actual work reports on various possibilities for a comprehensive characterization of the magnetic configuration and magnetic relaxation mechanisms of nano-particles, by corroborating the powerful method of Mossbauer spectroscopy with complementary magnetic and structural techniques. The capabilities of temperature and field dependent Mossbauer spectroscopy to provide valuable information about spin blocking temperatures, effective anisotropy constants, amount of uncompensated spins inside of particle and magnetic phase composition are critically discussed for both powder like systems and nano-particles dispersed in different solvents.

Computer Material Science is a branch of material sciences that uses theoretical concepts and models from chemistry, physics, geology, mineralogy, as well as biology and includes them into software applications in order to calculate the structure and the properties of molecules, gases, liquids, soft and condensed matter. This paper presents the estimated performances of the HPC-ICF cluster of the Institute of Physical Chemistry "Ilie Murgulescu" of the Romanian Academy and the NCIT-UPB Cluster developed by the Computer Science Department of the University Politehnica of Bucharest, with the run in the parallel mode on these systems of the HPL - High Performance Linpack Benchmark as well as of the electronic structure codes GAMESS and ONETEP. The results of the study show that both clusters break the Teraflop barrier, thus making available a valuable tool for the Romanian scientific community in Computing Materials Science.