National Institute Of Materials Physics - Romania

Publications

5975 articles found

4891. Collapse of the magnetic hyperfine structure of barium hexaferrite by mechanochemical activation

Authors: Sorescu, M; Grabias, A; Diamandescu, L; Tarabasanu, D

Published: MAR 2000, JOURNAL OF MATERIALS SYNTHESIS AND PROCESSING, 8, 72, DOI: 10.1023/A:1026565802246

The collapse of the magnetic hyperfine structure of barium hexaferrite as an effect of mechanochemical activation was investigated. The various inequivalent sites and phases present in the milled material were studied by transmission Mossbauer spectroscopy. Barium hexaferrite nanoparticles were obtained after only 2 h of milling time and were found to coexist with iron oxide particles for longer times of exposure to mechanochemical activation. The milled samples were then subjected to thermal annealing, in order to test the reversibility of the collapse of the hyperfine structure. The thermal annealing performed (700 degreesC, 1 h) was partly successful in restoring the magnetic hyperfine structure of milled hexaferrite. Complementary information was obtained using transmission electron microscopy (TEM) and electron diffraction (ED).

4892. Influence of the Jahn-Teller effect on the linewidth of the Mossbauer spectra of natural spinels

Authors: Kuncser, V; Russo, U; Graziani, R; Della Giusta, A; Filoti, G

Published: FEB 21 2000, JOURNAL OF PHYSICS-CONDENSED MATTER, 12, 1460, DOI: 10.1088/0953-8984/12/7/326

The temperature dependence of the Mossbauer doublet linewidth of tetrahedrally coordinated Fe2+ ions, distorted by the Jahn-Teller effect, was analysed by means of a new mathematical approach. The idea was that the linewidth depends on a distribution of quadrupole splittings that are in turn dependent on temperature. The variation of the quadrupole splitting was supposed to be connected with the temperature dependent population of the ground state E-g orbitals, split by the local Jahn-Teller effect. The model was applied to three sets of experimental data. The barrier splitting Delta between the two E-g electronic states as well as their widths, delta Delta, were obtained.

4893. Magnetization in short-period mesoscopic electron systems

Authors: Gudmundsson, V; Erlingsson, SI; Manolescu, A

Published: FEB 15 2000, PHYSICAL REVIEW B, 61, 4843, DOI: 10.1103/PhysRevB.61.4835

We calculate the magnetization of the two-dimensional electron gas in a short-period lateral superlattice, with the Coulomb interaction included in Hartree and Hatree-Fock approximations. We compare the results for a finite, mesoscopic system modulated by a periodic potential, with the results for the infinite periodic system. In addition to the expected strong exchange effects, the size of the system, the type and the strength of the lateral modulation leave their fingerprints on the magnetization.

4894. Icosahedral Al-Mn phases grown in diffusion-limited conditions

Authors: Popescu, R; Barna, PB; Constantin, M; Devenyi, A; Manaila, R

Published: FEB 1 2000, THIN SOLID FILMS, 360, 51, DOI: 10.1016/S0040-6090(99)01085-8

Sequential deposition of Mn on a polycrystalline Al layer at 523-573 K was found to result in an anomalous icosahedral (i) Al-Mn phase, characterized by alterations of intensity- and linewidth ratios. The anomalies suggest a non-equilibrium atom distribution, related to the limited diffusion of Al through the already-formed layer of i-AlMn. The activation energy of Al diffusion was inferred from X-ray diffraction (XRD) data taken for different deposition temperatures. (C) 2000 Elsevier Science S.A. All rights reserved.

4895. The Pb(2+)(Li) and Pb(2+)(Na) centres in KCl crystals

Authors: Tsuboi, T; Polosan, S; Topa, V

Published: FEB 2000, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 217, 980, DOI: 10.1002/(SICI)1521-3951(200002)217:2<975::AID-PSSB975>3.0.CO;2-U

Emission, absorption and magnetic circular dichroism spectra have been studied on Li(+)- and Pb(2+)-codoped KCl crystals. Two emission bands (one is the A(T)-like emission and the other the A(X)-like emission) have been observed at 350 and 453 nm at room temperature, respectively These emissions have been observed by excitation in the absorption bands with peaks at 270 and 281 nm. These emission bands are attributed to the Pb(2+)(Li) centre, where a Pb(2+) ion has a Li(+) ion at one of the six nearest neighbouring cation sites. Measurements have been also made on Na(+)- and Pb(2+)-codopcd KCl crystals. Unlike the Pb(2+)(Li) centre, only the A(T)-like emission band with a peak at 347 nm due to the Pb(2+)(Na) centre has been observed. It is suggested that such a difference is due to a difference in the position of the impurity alkali ion substituted for the K(+) ion.

4896. Optical absorption of Ag- centres in KCl: line shape calculation

Authors: Zota, S; Enculescu, M

Published: FEB 2000, PHYSICA B, 275, 343, DOI: 10.1016/S0921-4526(99)00762-0

The line shapes of the absorption spectra of KCl: Ag- are calculated at several temperatures in the range 80-300 K and are compared with the experimental results. The complete 12 x 12 Hamiltonian matrix of the system, which includes the linear electron-lattice interaction, was diagonalized directly and the integration has been performed by the Gaussian quadrature method. The results explain most of the characteristic features experimentally observed. (C) 2000 Elsevier Science B.V. All rights reserved.

4897. Diffraction-line shift caused by residual stress in polycrystal for all Laue groups in classical approximations

Authors: Popa, NC

Published: FEB 2000, JOURNAL OF APPLIED CRYSTALLOGRAPHY, 33, 107, DOI: 10.1107/S0021889899013357

Analytical formulae for all Laue groups are derived, giving the dependence of the residual elastic strain measured by X-ray and neutron diffraction on the direction in the sample and on the Miller indices. These formulae are valid for isotropic polycrystals in the limits of the Reuss and Voigt approximations and are appropriate for Rietveld refinement.

4898. Metal clusters in metal/C-60 thin film nanosystems

Authors: Popescu, R; Macovei, D; Devenyi, A; Manaila, R; Barna, PB; Kovacs, A; Labar, JL

Published: FEB 2000, EUROPEAN PHYSICAL JOURNAL B, 13, 743, DOI: 10.1007/s100510050093

Structure of metal clusters and of the C-60 matrix in Au/C-60 and Cu/C-60 nanosystems was investigated by X-ray diffraction. Results support a charge-transfer-type interaction at the metal-C-60 interface, which affects the size distribution of metal clusters: favouring interstitial location of metal ions in the fullerite lattice.

4899. Finite-size effects in the magnetization of periodic mesoscopic systems

Authors: Erlingsson, SI; Manolescu, A; Gudmundsson, V

Published: FEB 2000, PHYSICA E, 6, 766, DOI: 10.1016/S1386-9477(99)00198-8

We calculate the orbital magnetization of a confined 2DEG as a function of the number of electrons in the system. Size effects are investigated by systematically increasing the area of the confining region. The results for the finite system are compared to an infinite one, where the magnetization is calculated in the thermodynamic limit. In all calculations the electron-electron interaction is included in the Hartree approximation. (C) 2000 Elsevier Science B.V. All rights reserved.

4900. Optical and structural differences between RF and DC AlxNy magnetron sputtered films

Authors: Dumitru, V; Morosanu, C; Sandu, V; Stoica, A

Published: JAN 24 2000, THIN SOLID FILMS, 359, 20, DOI: 10.1016/S0040-6090(99)00726-9

AlN films have been deposited by RF (1.78 MHz) and DC reactive magnetron sputtering (argon and nitrogen) on glass substrates at low temperatures (< 150 degrees C) and with 16, 21 and 30% nitrogen ratios. We have found optically and structurally distinguishable characteristics for films (both opaque and transparent), obtained using these two experimental methods. We suggest that these differences are due to plasma effects in the substrate during growth, resulting from the different ionic bombardment rates of the two sputtering modes. (C) 2000 Elsevier Science S.A. All rights reserved.