National Institute Of Materials Physics - Romania

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903 articles found

671. New growth approach of high-quality oxide thin films for future device applications: independent control of supersaturation and migration

Authors: Endo, K; Badica, P; Sato, H; Akoh, H

Published: MAY 2006, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 19, S225, DOI: 10.1088/0953-2048/19/5/S12

In this paper we focus on supersaturation and migration of the atomic species involved in the growth of oxide films in order to search for new efficient methods for growth of flat, uniform and precipitate-free complex thin films with high quality. In this respect we have applied MOCVD to HTS thin films, 'interrupted growth' (consisting of on/off vapour deposition cycles, keeping the temperature and atmospheric conditions constant), similar to 'pulsed laser interval deposition' reported in the literature. We show that MOCVD can be easily and successfully applied to suppress island growth, contrary to the currently accepted idea that this approach is unique to PLD. This suggests that principles of supersaturation-migration-supersaturation cycles are universal for the films' growth. By this approach, high-quality Bi-2223 thin films with a uniform flat surface, having roughness less than half the c-axis unit cell, were grown by MOCVD. Using substrates with artificial steps of predefined width equal to 'twice the migration length' totally precipitate-free thin films were obtained: precipitates-segregates of impurity phases (Cu rich for HTS materials) are gathered at the step edges where the free energy is lowest. As-prepared films were used to fabricate intrinsic Bi-2223 and Bi-2212/Bi-2223 SIS-type Josephson junctions (IJJs).

672. Detailed study of the CePd2-xNixAl3 magnetic phase diagram around its critical concentration

Authors: Sereni, JG; Pedrazzini, P; Bauer, E; Galatanu, A; Aoki, Y; Sato, H

Published: APR 19 2006, JOURNAL OF PHYSICS-CONDENSED MATTER, 18, 3802, DOI: 10.1088/0953-8984/18/15/022

Thermal, magnetic and transport measurements on CePd2-xNixAl3 alloys within the 0 <= x <= 1 range are reported, including applied pressure (p) and magnetic field on some selected samples. The low temperature results indicate that long range antiferromagnetic order is robust up to x = 0.2, whereas between 0.25 and 0.5 magnetic fluctuations give rise to non-Fermiliquid (NFL) behaviour. In this critical region, the low temperature specific heat can be described as due to two components, the major showing a NFL C-p/T = gamma(0) - gamma(1) root T dependence, while the minor one includes a decreasing fraction of short range order degrees of freedom. The latter is only observed close to the critical point, x(cr) approximate to 0.35. Electrical resistivity (rho) studies performed under pressure for x = 0.5 allow us to investigate the evolution of the NFL state around and beyond x(cr), where the exponent of rho proportional to T-n increases from n = 1 (for p = 0) up to n = 2 (for p = 12 kbar). This exponent is also observed at normal pressure on the x = 1.0 sample. indicating the onset of the Fermi liquid behaviour. Doping and pressure effects are compared by fitting high temperature resistivity data employing a unique function which allows us to describe the evolution of the characteristic energy of this series along a large range of concentration and pressure.

673. FexOy-SiO2 nanocomposites studied by Mossbauer spectroscopy

Authors: Predoi, D; Kuncser, V; Zaharescu, M; Jitianu, A; Crisan, M; Keune, W; Sahoo, B; Filoti, G; Raileanu, M

Published: APR 2006, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 8, 522, DOI:

FexOy-SiO2 nanocomposites were obtained via the sol-gel method, by using three different precursors for the silica matrix. The morphology and the iron phase composition is strongly influenced by the precursors. The presence of magnetically ordered goethite and fayalite is evidenced by low temperature Mossbauer spectroscopy. Subsequent annealing treatments performed on the initial samples have induced the formation of hematite nanoparticles in the silica matrix, mainly on the account of the goethite decomposition. The total amount and the size distribution of the hematite nanoparticles can be controlled via the initial precursors and the subsequent annealing conditions. The simultaneous analysis of the iron phase composition and of the local magnetic interactions in the analysed samples is performed.

674. Mossbauer spectroscopical investigation of the exchange biased Fe/MnF2 interface

Authors: Sahoo, B; Macedo, WAA; Keune, W; Kuncser, V; Eisenmenger, J; Nogues, J; Schuller, IK; Felner, I; Liu, K; Rohlsberger, R

Published: APR 2006, HYPERFINE INTERACTIONS, 169, 1377, DOI: 10.1007/s10751-006-9453-8

Two different Fe/MnF2 samples have been prepared by e-beam evaporation on MgO(001) substrates. The Fe layer in the samples includes a 10 angstrom thick Fe-57 probe layer either at the Fe/MnF2 interface (interface sample) or 35 angstrom away from the interface (center sample). The samples are characterized by X-ray diffraction, conversion electron Mossbauer spectroscopy (CEMS) and SQUID magnetometry. Fe-57 CEMS has been employed to study the depth dependent hyperfine interactions in Fe/MnF2 as a function of temperature between 18 K to 300 K. The hyperfine field B-hf has been obtained for the interfacial and off-interfacial Fe-57 layers. At the interface, besides B-hf of bcc-Fe, the presence of a component with a distribution P(B-hf ) is observed. The latter is assigned to interfacial Fe-57 atoms, indicating some (similar to 15%, equivalent to similar to 1 Fe atomic layer) intermixing at the Fe/MnF2 interface and a decrease of the average < B-hf > by 21%. The influence of the interface disappears as the Fe-57 probe layer is placed away from the interface. The temperature dependence of the average < B-hf > of the interface has been measured. The Fe spins, at remanence, are found to lie in the film plane.

675. The 'in-plane' angular spin distribution in layered systems as obtained by Fe-57 Mossbauer spectroscopy (vol 196, pg 135, 2002)

Authors: Kuncser, V; Keune, W; Vopsaroiu, M; Bissell, PR

Published: APR 2006, NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS, 245, 542, DOI: 10.1016/j.nimb.2006.01.022

676. Spin configurations in exchange coupled magnetic phases studied by Mossbauer spectroscopy

Authors: Kuncser, V; Keune, W; Stromberg, F; Schinteie, G; Filoti, G

Published: APR 2006, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 8, 517, DOI:

The ability of the Fe-57 Mossbauer technique to reveal the spin structures in exchange coupled magnetic phases containing iron is emphasized. Experimental results obtained on layered exchange-bias systems are presented. The influence of the ferromagnetic top layer on the out-of-plane spin component of the pinning antiferromagnetic-like layer, in both Fe/FeSn2 and Fe/Fe-Gd-B exchange bias systems, is discussed.

677. The magnetic effects in Sr2FeMoO6 perovskite type compound obtained by soft chemistry

Authors: Aldica, G; Plapcianu, C; Badica, P; Valsangiacom, C; Popescu, B

Published: APR 2006, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 8, 463, DOI:

Recently, a double perovskite oxide, Sr2FeMoO6, has been reported to show a very sharp magnetoresistance (MR) response at relatively small-applied fields and at high temperatures. The half-metallic state of this compound ensures a theoretical magnetic moment of 4 mu(B) per formula unit. Ordering control is expected to be influenced by the synthesis route. In this regard, in this article, perovskite samples were obtained from the liquid phase, using oxalic acid as complexing agent. Application of a field above 0.08 T increases the zero-field-cooled (ZFC) magnetization. Below this field, for the ZFC-curves measured from 2 K up to 400 K, magnetization increases, reaching a peak at T-cups(ZFC), before decreasing cups gradually to a paramagnetic state. Saturation moment is relatively low (2.1-2.2 mu(B)), probably due to anti-site defects, as suggested in the literature.

678. Mossbauer study of Mg-Ni(Fe) alloys processed as materials for solid state hydrogen storage

Authors: Palade, P; Principi, G; Sartori, S; Maddalena, A; Lo Russo, S; Schinteie, G; Kuncser, V; Filoti, G

Published: FEB 2006, HYPERFINE INTERACTIONS, 168, 1035, DOI: 10.1007/s10751-006-9389-z

Mg-Ni-Fe magnesium-rich intermetallic compounds were prepared following two distinct routes. A Mg88Ni11Fe1 sample (A) was prepared by melt spinning Mg-Ni-Fe pellets and then by high-energy ball milling for 6 h the obtained ribbons. A (MgH2)(88)Ni11Fe1 sample (B) was obtained by high-energy ball milling for 20 h a mixture of Ni, Fe and MgH2 powders in the due proportions. A SPEX8000 shaker mill with a 10:1 ball to powder ratio was used for milling in argon atmosphere. The samples were submitted to repeated hydrogen absorption/desorption cycles in a Sievert type gas-solid reaction controller at temperatures in the range 520 divided by 590 K and a maximum pressure of 2.5 MPa during absorption. The samples were analysed before and after the hydrogen absorption/desorption cycles by X-ray diffraction and Mossbauer spectroscopy. The results concerning the hydrogen storage properties of the studied compounds are discussed in connection with the micro-structural characteristics found by means of the used analytical techniques. The improved kinetics of hydrogen desorption for sample A, in comparison to sample B, has been ascribed to the different behaviour of iron atoms in the two cases, as proved by Mossbauer spectroscopy. In fact, iron results homogeneously distributed in sample A, partly at the Mg2Ni grain boundaries, with catalytic effect on the gas-solid reaction; in sample B, instead, iron is dispersed inside the hydride powder as metallic iron or superparamagnetic iron.

679. Growth of superconducting MgB2 films by pulsed-laser deposition using a Nd-YAG laser

Authors: Badica, P; Togano, K; Awaji, S; Watanabe, K

Published: FEB 2006, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 19, 246, DOI: 10.1088/0953-2048/19/2/016

Thin films of MgB2 on r-cut Al2O3 substrates have been grown by pulsed-laser deposition (PLD) using a Nd-YAG laser (fourth harmonic-266 nm) instead of the popular KrF excimer laser. The growth window to obtain superconducting films is laser energy 350-450 mJ and vacuum pressure with Ar-buffer gas of 1-8/10 Pa (initial background vacuum 0.5-1 x 10(-3) Pa). Films were deposited at room temperature and post-annealed in situ and ex situ at temperatures of 500-780 degrees C and up to 1 h. Films are randomly oriented with maximum critical temperature (offset of resistive transition) of 27 K. SEM/TEM/EDS investigations show that they are mainly composed of small sphere-like particles (<= 20 nm), and contain oxygen and some carbon, uniformly distributed in the flat matrix, but the amount of Mg and/or oxygen is higher in the aggregates-droplets (100-1000 nm) observed on the Surface of the film's matrix. Some aspects of the processing control and dependences on film characteristics are discussed. The technique is promising for future development of coated conductors.

680. Experimental evidence for a dimensional crossover of the vortex ensemble in BSCCO (Bi1.8Pb0.4Sr1.8Ba0.2Ca2Cu3Ox) by multi-harmonic ac susceptibility measurements

Authors: Mihalache, V; Popa, S; Di Gioacchino, D; Tripodi, P; Vinko, JD

Published: JAN 1 2006, PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS, 433, 233, DOI: 10.1016/j.physc.2005.11.005

We have studied a portion of H-T phase diagram of the anisotropic Bi2223 bulk system using multi-harmonic ac susceptibility measurements (chi(n)). Based on the behavior of the third harmonic modulus,vertical bar chi(3)vertical bar, the irreversibility line (IL) was obtained from the onset of vertical bar chi(3)vertical bar, and an anomalous peak effect (PE) was observed. It has been shown that the anomalous peak is due to the crossover between a three-dimensional (3D) and a quasi-two dimensional (quasi-2D) peculiarity of vortex dynamics, the crossover magnetic field being H-CR approximate to 0.1T. The obtained portion of B-irr(T) is well described by the melting line of the flux lattice. The 3D flux fluctuation part (low field and high temperature) is described by the nearly parabolic temperature dependence B=B-o(Tc/T - 1)" with n = 1.58 (sample 1) and n = 1.48 (sample 2). The other region (high field and low temperature) is well described by the temperature dependence of the quasi-2D flux fluctuations, T-m(H) = T-m(2D)[1 + b/(In B/B-cr )(1/v)] corresponding to the weak interaction between pancake vortices from adjacent planes. The 2D limit temperature was determined as T-m(2D) = 36.7K (sample 1) and T-m(2D) = 27.3K (sample 2). (c) 2005 Elsevier B.V. All rights reserved.