National Institute Of Materials Physics - Romania

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892 articles found

531. OPTIMIZATION OF THERMIONIC VACUUM ARC PLASMA USED FOR MULTILAYER GMR/TMR FILMS PREPARATION

Authors: Jepu, I; Porosnicu, C; Mustata, I; Lungu, CP; Kuncser, V; Miculescu, F

Published: 2010, ROMANIAN REPORTS IN PHYSICS, 62, 779, DOI:

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.

532. Au-197 Mossbauer study of gold selenide, AuSe

Authors: Wagner, FE; Palade, P; Friedl, J; Filoti, G; Wang, N

Published: 2010, INTERNATIONAL CONFERENCE ON THE APPLICATIONS OF THE MOSSBAUER EFFECT (ICAME 2009), 217, DOI: 10.1088/1742-6596/217/1/012039

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.

533. Magnetism and transport properties of gamma-irradiated polymer-CrO2 composites

Authors: Sandu, V; Popa, S; Ivan, I; Sandu, E; Hurduc, N; Nor, I

Published: MAY-JUN 2010, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 322, 1408, DOI: 10.1016/j.jmmm.2009.09.045

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.

534. Development Of Magnetic Fe@C Nanocomposites Obtained Via The Laser Pyrolysis: Structural And Disaggregation Properties

Authors: Morjan, I; Alexandrescu, R; Dumitrache, F; Fleaca, C; Birjega, R; Soare, I; Luculescu, CR; Prodan, V; Kuncser, V; Filoti, G; Xu, H; Wang, D

Published: 2010, BONSAI PROJECT SYMPOSIUM: BREAKTHROUGHS IN NANOPARTICLES FOR BIO-IMAGING, 1275, +, DOI: 10.1063/1.3505074

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.

535. Magnetic Configuration and Relaxation in Iron Based Nano-Particles: A Mossbauer Approach

Authors: Kuncser, V; Schinteie, G; Alexandrescu, R; Morjan, I; Vekas, L; Filoti, G

Published: 2010, TRENDS IN NANOPHYSICS: THEORY, EXPERIMENT AND TECHNOLOGY, 314, DOI: 10.1007/978-3-642-12070-1_13

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.

536. HIGH PERFORMANCE COMPUTING INFRASTRUCTURE FOR MATERIALS SCIENCE

Authors: Chihaia, V; Slusanschi, E; Herisanu, A; Palade, P; Tapus, N

Published: NOV-DEC 2010, REVUE ROUMAINE DE CHIMIE, 55, +, DOI:

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.

537. Q-switched Nd lasers pumped directly into the F-4(3/2) emitting level

Authors: Pavel, N; Dascalu, T; Salamu, G; Sandu, O; Leca, A; Lupei, V

Published: DEC 15 2009, OPTICS COMMUNICATIONS, 282, 4754, DOI: 10.1016/j.optcom.2009.08.067

The influence of the direct pumping into the F-4(3/2) emitting level on the output characteristics of continuous-wave (CW) pumped, passively or actively (acoustooptic, AO) Q-switched Nd lasers is discussed. In case of passive Q-switching by Cr4+: YAG saturable absorber (SA) crystal, the change of pumping wavelength from 0.81 mu m into the highly-absorbing F-4(5/2) level to 0.88 mu m into the F-4(3/2) level of Nd does not modify the energy of the Q-switch pulse, but increases the pulse repetition rate and the laser average power for the same absorbed pump power. This is demonstrated with 0.81 and 0.88 gm CW laser diode-pumped Nd:YAG and Nd-vanadate lasers with average output power in the watt-level range at 1.06 mu m. The effect is explained by the control of passive Q-switching by the intracavity photon flux that is influenced by the pump wavelength and by the initial transmission of the SA crystal. On the other hand, it is discussed and experimentally proved that due to the possibility to control externally the frequency of switching, in case of the AO Q-switched Nd laser the change of the pump wavelength from 0.81 to 0.88 pm increases the pulse energy for a fixed frequency, leading to a corresponding increase of the average laser power. (C) 2009 Elsevier B.V. All rights reserved.

538. YBa2Cu3O7-x Thin Films by Citrate-Based Non-fluorine Precursor

Authors: Cui, W; Mikheenko, P; Yu, LM; Button, TW; Abell, JS; Crisan, A

Published: NOV 2009, JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM, 22, 815, DOI: 10.1007/s10948-009-0504-7

A new fluorine-free chemical solution deposition route has been developed for YBa2Cu3O7-x (YBCO) thin films. The precursor was formed by dissolving metal nitrates and citric acid in ethylene glycol. Polyethyleneimine was used to modify the solution. The preparation of YBCO thin films consisted of repeated dip coating, drying, heat-treating in air and annealing in flowing argon with a small addition of oxygen and with final treatment in pure oxygen. The films were characterized by XRD and SEM and show dominating and well-textured YBCO phase. The AC susceptibility and magnetization measurements reveal a critical temperature about 90 K and a high critical current density, comparable with that of pulse laser deposited films. The simplicity and high performance of the method may be attractive for large-scale superconducting applications.

539. Improvement of Pinning Force and Critical Current Density in Thick YBa (2) Cu (3) O (7-delta) Films Grown on SrTiO (3) Substrates Decorated with LaNiO (3) Nanodots

Authors: Crisan, A; Sarkar, A; Mikheenko, P; Dang, VS; Kechik, MMA; Abell, JS

Published: OCT 2009, JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM, 22, 636, DOI: 10.1007/s10948-009-0452-2

We have investigated by DC magnetization measurements and frequency-dependent AC susceptibility the critical current density (J (c)), pinning force (F (p)) and pinning potential in thick (1.3-1.6 mu m) YBa2Cu3O7-delta (YBCO) films grown by Pulsed Laser Deposition on SrTiO3 substrates decorated with LaNiO3 nanodots deposited by a few (5-15) laser pulses, in comparison with those of a 1 mu m thick YBCO reference sample. Experiments show that the highest improvement of superconducting properties was achieved for films grown on substrates decorated with 10 laser pulses on the LaNiO3 target, which have, at 77.3 K, a J (c) of 40-125% higher than in pure YBCO in fields between 1 and 2 T, and F (p) increased by 40%. These results could be important for further improvement of current-carrying capability of coated conductors for in-field power applications.

540. c-Axis correlated extended defects and critical current in YBa2Cu3Ox films grown on Au and Ag-nano dot decorated substrates

Authors: Mikheenko, P; Sarkar, A; Dang, VS; Tanner, JL; Abell, JS; Crisan, A

Published: JUL 15 2009, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 469, 804, DOI: 10.1016/j.physc.2009.05.001

We report measurements of critical current in YBa2Cu3Ox films deposited on SrTiO3 substrates decorated with silver and gold nanodots. An increase in critical current in these films, in comparison with the films deposited on non-decorated substrates, has been achieved. We argue that this increase comes from the c-axis correlated extended defects formed in the films and originated from the nanodots. Additionally to creating extended defects, the nanodots pin them and prevent their exit from the sample during the film growth, thus keeping a high density of defects and providing a lower rate of decrease of the critical current with the thickness of the films. The best pinning is achieved in the samples with silver nanodots by optimising their deposition temperature. The nanodots grown at a temperature of a few hundred degrees C have a small diameter of a few nanometres and a high surface density of 10(11)-10(12) particles/cm(2). We give evidence of c-axis correlated extended defects in YBa2Cu3Ox films by planar and cross-sectional atomic force microscopy, transmission electron microscopy and angle-dependent transport measurements of critical current. (C) 2009 Elsevier B.V. All rights reserved.