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ABSTRACT: The effect of magnetism on the ground state structures has been investigated by means of first-principles, density- functional calculations. In addition to the experimentally observed CsCl-type ground-state structure for FeCo stoichiometric alloys, our spin-polarized calculations predict several new ordered structures for iron-rich alloys. In particular, our results show that Fe3Co alloys are energetically stable for a number of ordered structures.
Journal of Magnetism and Magnetic Materials 01/2004; 272-276:780. · 1.78 Impact Factor
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ABSTRACT: The effect of hydrostatic and axial deformations on the magnetic and electronic properties of small Rh clusters was studied. The electronic properties were calculated using a self-consistent spd tight-binding Hamiltonian within the unrestricted Hartree–Fock approximation. For hydrostatic deformations, the magnetic moment of the clusters increases with bond-length in a steplike fashion. On the other hand, axial distortions affect the magnetism of the clusters in a more complex manner that cannot always be related in a simple way to geometrical parameters. An analysis of the paramagnetic density of states is performed in order to elucidate the physical origin of our results.
Journal of Alloys and Compounds 01/2004; 369:81. · 2.29 Impact Factor
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ABSTRACT: The magnetization reversal and magnetoresistance of two-dimensional arrays of aligned Fe-filled carbon nanotubes have been investigated. Our results show a linear temperature dependence of the coercivity above 10 K and a relative large hysteresis for the applied field perpendicular to the nanowires axes. A continuous decrease of the magnetoresistance for both field directions is observed. We attribute this behavior to the anisotropic nature of the system.
Journal of Magnetism and Magnetic Materials 01/2004; 272-276:e1255. · 1.78 Impact Factor
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ABSTRACT: We studied the magnetic and transport properties of the orthorhombic compounds R2Ni2Pb (R = Gd, Tb and Y) by means of magnetization and magnetoresistivity measurements. These compounds form a new family of naturally layered intermetallics with a magnetic structure that can be tailored by changing the rare-earth element. Our results show a subtle interdependence between the magnetoresistance and the metamagnetic behavior in these systems. We attribute such anomalous behavior to the layered structure of the compounds, and to the intricacies introduced by the complex magnetic structure of rare earths. First-principles density-functional calculations for the electronic structure of Y2Ni2Pb are presented to support this idea.
Journal of Alloys and Compounds 01/2004; 369:260. · 2.29 Impact Factor
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ABSTRACT: The magnetic behavior of small rare-earth clusters was investigated based on a quantum Heisenberg model. Clusters with a fixed number of atoms and different geometry are found to have drastically different magnetic behavior as temperature is increased. A quantum mechanical treatment of the Heisenberg model explains in a natural way the seemingly controversial experimental results in Gd clusters.
Physical Review B 10/2002; 66:144406. · 3.69 Impact Factor
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ABSTRACT: Phase equilibrium in confined Ising antiferromagnets was studied as a function of the coupling (v) and a magnetic field (h) at the surfaces, in the presence of an external field H. The ground state properties were calculated exactly for symmetric boundary conditions and nearest-neighbor interactions, and a full zero-temperature phase diagram in the plane v-h was obtained for films with symmetry-preserving surface orientations. The ground-state analysis was extended to the H-T plane using a cluster-variation free energy. The study of the finite-T properties (as a function of v and h) reveals the close interdependence between the surface and finite-size effects and, together with the ground-state phase diagram, provides an integral picture of the confinement in anisotropic antiferromagnets with surfaces that preserve the symmetry of the order parameter. Comment: 10 pages, 8 figures, Accepted in Phys. Rev. B
04/2000;
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ABSTRACT: Confinement effects on the phase transitions in antiferromagnets are studied as a function of the surface coupling v and the surface field h for bcc(110) films. Unusual topologies for the phase diagram are attained for particular combinations of v and h. It is shown that some of the characteristics of the finite-temperature behavior of the system are driven by its low-temperature properties and consequently can be explained in terms of a ground-state analysis. Cluster variation free energies are used for the investigation of the finite temperature behavior.
03/2000;
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ABSTRACT: A complete compositional and thermal characterization of celotex was carried out using energy dispersive spectroscopy, inductively coupled plasma-mass spectroscopic analysis, ion chromatography, and thermogravimetric analysis. Both fluoride and sulfate anions in addition to chloride as well as several metal ions are found to be present. The chloride present in celotex is lost at moderate temperatures 200<T<400C, which may have important consequences on the formation of corrosion precursors. Equilibrium thermodynamic calculations on the stability of beryllium oxide in chlorinated environments were performed for a model system composed by beryllium, carbon, chlorine, hydrogen and oxygen. The calculations were aimed at describing the equilibrium, long-term behavior of a beryllium surface and its interaction with chlorine in a closed environment. We found that (i) carbon is essential for the formation of BeCl 2 , and (ii) the chlorine-to-hydrogen ratio in chlorinated solvents is an importa...
10/1999;
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ABSTRACT: The importance of safe and long-term storage of nuclear weapons components has drawn attention towards the characterization and understanding of the corrosion precursors formed on beryllium. A complete compositional and thermal characterization of celotex was carried out using energy dispersive spectroscopy, inductively coupled plasma-mass spectroscopic analysis, ion chromatography, and thermogravimetric analysis. Both fluoride and sulfate anions in addition to chloride as well as several metal ions are found to be present. The chloride present in celotex is lost at moderate temperatures 200 < T < 400 C, which may have important consequences on the formation of corrosion precursors. Equilibrium thermodynamic calculations on the stability of beryllium oxide in chlorinated environments were performed for a model system composed by beryllium, carbon, chlorine, hydrogen and oxygen. The calculations were aimed at describing the equilibrium, long-term behavior of a beryllium surface and its interaction with chlorine in a closed environment. The authors found that (i) carbon is essential for the formation of BeCl{sub 2}, and (ii) the chlorine-to-hydrogen ratio in chlorinated solvents is an important quantity since for solvents with chlorine-to-hydrogen ratio higher than 1, BeCl{sub 2} is not formed independently of the amount of solvent.
03/1999;
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ABSTRACT: The phase transitions in Ising thin films with antiferromagnetic interactions are studied as a function of the surface field h for symmetry-preserving surface orientations. It is found that for very large h an N-layer system undergoes a separation between its surfaces and the (N-2) inner layers. The system comprised of the surfaces and the inner layers displays a rather unusual phase diagram with a disordered phase lying between the ordered phases down to zero temperature. For large but moderate h, both ordered regions merge at high temperatures, isolating a low-temperature disordered region. Cluster variation calculations are used to compute the finite-temperature behavior.
Physical Review Letters 08/1998; 81:1146. · 7.37 Impact Factor
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ABSTRACT: The thermodynamics of ordering binary alloys with film geometry is studied
for the case of symmetry-preserving surfaces. In particular, we analyze the
dependence of the phase diagrams with film thickness for the case of
vanishing surface fields (ordinary transitions). We focus on the interplay
between ordering and segregation and its effects on the phase equilibria of
bcc(110) films as well as on the low-temperature behavior of the critical
curve. At T=0 the critical curve of a bcc ordering binary-alloy film ends
at a finite concentration. It is shown that it is useful to interpret the
zero temperature transitions as percolationlike transitions. Calculations
in the pair and tetrahedron approximations of the cluster variation method
are presented.
Solid State Communications 01/1998; 107(6):285. · 1.65 Impact Factor
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ABSTRACT: The thermodynamics of ordering binary alloy thin films with non-neutral or "polarizing" boundary conditions is studied within the frame of an Ising-like Hamiltonian with nearest-neighbor interactions. The finite-temperature behavior is obtained by means of the pair approximation of the cluster variation method which is the lowest approximation that takes into account short-range correlations. The analysis focuses on the dependence upon film thickness of the phase diagrams and on the nature of the surface field which acts close to the surface and leads to segregation of one of the components. The finite-size and surface effects along with the discreteness of the system entail novel features on the phase diagrams that are not present in continuous theories and/or in the usual mean-field theory (Bragg-Williams) that neglects short-range order correlations. Temperature-chemical potential and temperature-concentration phase diagrams are presented as well as concentration and long-range order profiles
Computational Materials Science 01/1997; 8:79. · 1.57 Impact Factor
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ABSTRACT: The Curie temperature of Ising thin films with various crystalline structures and surface orientations is calculated within the Bragg-Williams and the Bethe approximations. Tc is calculated as a function of the number of layers N and the scaling relation Delta tau = 1 -Tc(N)/Tc(infinity)= A N^-lambda is obtained. It is analyzed in detail how the coefficient A and the exponent lambda depend on the approximation used, on the crystalline structure, on the ratio between surface and bulk coordination numbers, and how they tend to the asymptotic values.
Solid State Communications 01/1994; 91(6):435. · 1.65 Impact Factor
