Browsing by Author "Mejia-Lopez, J."

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    Effect of surface anisotropy on the magnetic properties of magnetite nanoparticles
    (2008) Mazo-Zuluaga, J.; Restrepo, J.; Mejia-Lopez, J.
    In this study, we analyze the effect of surface anisotropy on the magnetic properties of magnetite Fe3O4 nanoparticles on the basis of a core-shell model. Magnetization, magnetic susceptibility, and specific heat are computed over a wide range of temperatures. In our model, we stress on magnetite nanoparticles of 5 nm in diameter which consist of 6335 ions. Our theoretical framework is based on a three-dimensional classical Heisenberg Hamiltonian with the nearest magnetic neighbor interactions between iron ions involving tetrahedral (A) and octahedral (B) sites. Terms dealing with cubic magnetocrystalline anisotropy for core ions, a single-ion site surface anisotropy for those Fe ions belonging to the shell, and the interaction with a uniform external magnetic field are considered. To compute the equilibrium averages, a single-spin movement Monte Carlo-Metropolis dynamics was used. Results reveal the occurrence of low-temperature spin configurations different from those expected for a collinear single-domain ferrimagnetic state, depending on the magnitude and sign of the surface anisotropy constant. A transition to a spike state, with magnetization close to zero, is obtained beyond a certain critical positive surface anisotropy value. Such a transition is not observed for negative values. Moreover, a two-pole magnetic state is developed at sufficiently high negative values. Such differences are explained in terms of the interplay between the superexchange couplings and the easy directions imposed by the surface anisotropy vectors. Our results are summarized in a proposal of phase diagram for the different spin structures as a function of the surface-to-core anisotropy ratio. Lastly, hysteretic behavior is evaluated. Nanoparticles become magnetically harder as the surface anisotropy increases in magnitude, and the way in wich the coercive field changes with this quantity is explicitly shown. (C) 2008 American Institute of Physics.
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    Low-energy configurations of Pt6Cu6 clusters and their physical-chemical characterization: a high-accuracy DFT study
    (2022) Mejia-Lopez, J.; Velasquez, E. A.; Mazo-Zuluaga, J.
    Based on a combination of many-body potentials, an analysis of the inertia tensors and a Density Functional Theory framework, we use a method to harvest the lowest energy states of any set of cluster systems. Then, this methodology is applied to the Pt6Cu6 cluster case and the structural, chemical, electronic, anisotropy, magnetic and vibrational properties of the lowest energy isomers are studied. Unexpectedly, some tens of isomers with much lower energy than the precedent believed ground state [J. Chem. Phys., 131(4):044701] are found, which indicates the goodness of this methodology. Some of the isomers obtained present the point groups C-s, C-2v according to Schoenflies notation, while others do not exhibit specific symmetry operations. The global chemical descriptors as the ionization potential, the electron affinity and the chemical hardness have oscillating behaviors with overall decreasing trends as the energy of the isomer grows up, indicating a higher rate of deactivation by sintering processes and a higher strength of the adsorption of small molecules on these systems. We present interesting results of the electronic, magnetic, anisotropy, vibrational and thermal properties of these clusters and discuss them; what can be useful information for future experiments and technical applications in varied fields as catalysis, spintronics, molecular magnetism or magnetic storage information.
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    Structural stability, shape memory and mechanical properties of Fe/Ni core/shell nanorods
    (2021) Mejia-Burgos, D.; Berrios, S. A.; Mazo-Zuluaga, J.; Mejia-Lopez, J.
    During recent years, production and characterization of core-shell nanostructures have been in the center of attention due to their unique functional properties, which are useful for potential uses in technological devices. However, several issues regarding their basic physics remain unexplored. In this work, we report on an extensive molecular dynamics study of the thermomechanical properties of cylindrical Fe, Ni and Fe/Ni core/shell nanowires under uniaxial tensile strain. The mechanical properties are analyzed and the de -formation mechanisms, as well as the size and temperature effects, are studied and discussed. Results indicate that the nanowires are elastically softer than the bulk iron and a weakening effect is observed as increasing the diameter of the samples. The Fe/Ni core/shell systems exhibit shape memory effect when they are grown along the crystal directions considered here, what makes these systems potentially inter-esting for technical applications.
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    Surface states of FeF2 (110) and its uncompensated magnetization
    (2015) Munoz, F.; Romero, A. H.; Mejia-Lopez, J.; Roshchin, Igor V.; Gonzalez, R. I.; Kiwi, M.
    The (110) surface of iron fluoride (FeF2) is especially relevant to the understanding of the exchange bias phenomenon, which has important applications in the sensor industry, and has been extensively explored, both theoretically and experimentally. Here we investigate this FeF2 surface by means of oh mine techniques. We compute the (110) surface reconstruction, energetics, magnetic moments, band structure, charge density and electron localization function, for the two possible terminations (Fe and F). The surface reconstruction modifies the atomic and electronic structure of the free surface, yielding magnetism of a magnitude of 0.1 mu(B) per surface unit cell. Moreover, the charge density also changes, which alters the bonding in the vicinity of the surface. All these changes are expected to be relevant for exchange bias, that is once a ferromagnetic layer is deposited on the FeF2 surface. (C) 2015 Elsevier B.V. All rights reserved.
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    Tailoring Electronic Properties on Bi2O2Se under Surface Modification and Magnetic Doping
    (2024) Arias-Camacho, I. M.; Leon, A. M.; Mejia-Lopez, J.
    The search for a two-dimensional material that simultaneously fulfills some properties for its use in spintronics and optoelectronics, i.e., a suitable bandgap with high in-plane carrier mobility and good environmental stability, is the focus of intense current research. If magnetism is also present, its range of utility is considerably expanded. One of the promising materials fulfilling these features is Bi2O2Se, a non-van der Waals system whose monolayer has been recently obtained. This study addresses, within a theoretical framework, the structure and electronic properties of different monolayers that could be obtained experimentally. It is observed that these monolayers are very sensitive to the introduction of "extra" electrons, changing their electronic character from semiconductor to conductor. Furthermore, we investigate how the properties of each studied monolayer change when the system is doped with magnetic atoms. The result is that doping introduces bands of low dispersion caused by the d orbitals of the impurities that can hybridize with the oxygen and bismuth atoms in the monolayer. This strongly modifies the electronic properties of the material, producing changes in the valence of certain Bi atoms, which can induce symmetry breaking in the perpendicular plane. Such phenomena lead to metallic or semiconducting characteristics, depending on the metal doping.