Browsing by Author "Kiwi, M"

Now showing 1 - 8 of 8
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    Calculation of the interface exchange coupling constants between Fe and FeF2-like fluorides
    (2001) Weissmann, M; Llois, AM; Kiwi, M
    The interface exchange coupling between ferromagnetic (F) and antiferromagnetic (AF) materials is interesting in itself and has also attracted recent attention in relation to the exchange bias phenomenon. A major difficulty in developing a reliable exchange bias theory lies in the fact that both the F and AF interface characteristics (geometry and physical parameters) are hard to determine experimentally and complicated to estimate theoretically. We adopt in this paper two alternative interface conrigurations to obtain upper and lower bounds for the computed values of the exchange coupling across the interface between metallic Fe and insulating FeF2, derived on the basis of ab initio calculations implemented for a periodic supercell. Electronic structures and total energies were computed within density functional theory using the generalized gradient approximation for the exchange correlation potential. We expect the results obtained to be useful in model simulations with larger unit cells and non-collinear spins. (C) 2001 Published by Elsevier Science B.V.
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    Density functional theory study of the Si2H6-xFx series of molecules
    (2003) Valencia, F; Romero, AH; Kiwi, M; Ramírez, R; Toro-Labbé, A
    The systematic replacement of hydrogen by fluorine atoms in disilane (Si2H6) is investigated using density functional theory (DFT). For every molecule in the Si2H6-xFx family, with 0 less than or equal to x less than or equal to 6; we find the energetically most favorable configurations. Properties such as chemical potential, chemical hardness, polarizability, electrophilicity and the infrared vibration modes and frequencies are evaluated and analyzed in terms of the fluorine substitution. The molecular. response to twisting the silyl groups at each end of the Si-Si axis, relative to each! other; is also investigated. A consistent picture emerges which is in good agreement with the available experimental results:. (C) 2003 Elsevier Science B.V. All rights reserved.
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    Exchange bias model for Fe/FeF2
    (1999) Kiwi, M; Mejía-López, J; Portugal, RD; Ramírez, R
    Exchange bias (EB) is a shift of the hysteresis loop from its normal position, symmetric around H = 0, to HE not equal 0. It occurs when thin ferromagnetic (F) films are deposited on a variety of antiferromagnetic (AF) materials. EB is also associated with several additional remarkable features: i) the bulk magnetizations of the F is orthogonal to the AF easy axis; ii) H-E is of similar magnitude for compensated and uncompensated AF interface layers; iii) the sign of H-E can assume both positive and negative values; and, iv) the magnetization \M(H << -H-c)\ not equal \M(H >> +H-c)\ where H-c is the coercive field. Here we propose a model that describes the EB phenomenon for a compensated interface. Based on the experimental evidence, and extensive computer simulations, we suggest that close to the Neel temperature a canted spin configuration in the AF interface freezes into a metastable state. As a consequence, the EB energy is reversibly stored in a spring-like magnet, or incomplete domain wall (IDW), in the F slab. The results we extract from our model, both analytically and through simulations, are qualitatively and quantitatively compatible with the available experimental information.
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    Internal rotation of disilane and related molecules
    (2003) Valencia, F; Romero, AH; Kiwi, M; Ramírez, R; Toro-Labbé, A
    DFT calculations performed on Si2H6, Si2F6, Si2Cl6 and Si2Br6 are reported. The evolution of the energy, the chemical potential and the molecular hardness, as a function of torsion angle, is studied. Results at the DFT-B3LYP/6-311++G** level show that the molecules always favor the stable staggered conformations, with low but significant energy barriers that hinder internal rotation. Internal rotation is always accompanied by weakening and lengthening of the central Si-Si bond. In most cases this lengthening seems to be due to an interplay of electrostatic and hyperconjugative interactions. The chemical potential and hardness Of Si2H6 remains quite constant as the sylil groups rotate around the Si-Si axis, whereas the other systems exhibit different degrees of rearrangement of the electronic density as a function of the torsion angle. A qualitative analysis of the frontier orbitals shows that the effect of torsional motion on electrophilic attack is negligible, whereas this internal rotation may generate different specific mechanisms for nucleophilic attack. (C) 2003 Elsevier Science B.V. All rights reserved.
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    Molecular dynamics study of physisorbed xenon on Al(1 1 0)
    (1997) Goncalves, S; Ramirez, R; Kiwi, M
    The adsorption of Xe on the (1 1 0) face of an Al substrate is investigated by means of molecular dynamics. A thermodynamic transformation at constant temperature is studied as a function of the Lennard-Jones potential range parameter sigma(Xe-Al), for several sizes of the Xe island adsorbed on Al. This way a critical value of sigma(Xe-Al), for which the island undergoes a commensurate-incommensurate transition, is identified. The implications of our results are discussed in the context of experimental interpretation and compared with published Monte Carlo work on this system. (C) 1997 Published by Elsevier Science Ltd.
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    Positive exchange bias model
    (2000) Kiwi, M; Mejía-López, J; Portugal, RD; Ramírez, R
    Positive exchange bias (PEB) is a remarkable phenomenon, which was recently observed experimentally. Normal (negative) exchange bias (NEB) was discovered more than 40 years ago. Its signature is the shift of the hysteresis loop along the applied field axis by H-E < 0, in systems where a ferromagnet (FM) is in close contact with an antiferromagnet (AFM). This occurs after the system is cooled below the Neel temperature in an external field H-cf of a few kOe. As H-cf is substantially increased H-E adopts positive values. Here we explain this rather unexpected behavior on the basis of an incomplete domain wall model that develops in the FM, for Fe/FeF2 and Fe/MnF2 systems. A consistent and unified picture of both NEB and FEB, and satisfactory quantitative agreement with experimental results are obtained on the basis of our theory. (C) 2000 Published by Elsevier Science Ltd.
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    Small Pd clusters
    (2005) Rogan, J; García, G; Valdivia, JA; Orellana, W; Romero, AH; Ramírez, R; Kiwi, M
    The lowest-energy structures of small Pd clusters (2 <= N <= 13) are computed by means of available phenomenological many-body potentials and by ab initio methods. The lowest-energy configuration is found by means of a genetic algorithm search. Satisfactory agreement between the results of the several methods implemented is achieved. Of special interest is the fact that all phenomenological potentials yield the same symmetry group for the lowest-energy cluster geometries, which moreover are identical with ab initio results. This constitutes an indication that the most common many-body empirical potentials can be trusted to yield reliable results.
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    Structural similarities between Ti metal and titanium oxides
    (2004) Vegas, A; Mejía-López, J; Romero, AH; Kiwi, M; Santamaría-Pérez, D; Baonza, VG
    The stabilities of the body-centered-tetragonal and distorted-diamond phases of titanium are investigated by first-principles methods. Our results, together with previous experimental and theoretical work, confirm two interesting observations. First, that the metal arrays in oxides correspond to stable or metastable phases of the parent metal; and second, that oxygen provides the pressure medium that stabilizes these phases. In addition, we have confirmed that the bulk modulus of oxygen matrices follows a nearly universal behavior with pressure, and that pressure-induced phase transitions tend to occur when the compressibility of the oxygen matrix reaches the compressibility of the high-pressure phase of the oxide. (C) 2004 Elsevier SAS. All rights reserved.