Papers by Faustino Aguilera-Granja
Revista Mexicana De Fisica, 1998
The Journal of chemical physics, Jan 21, 2016
We present, based on state of the art density functional theoretic calculations, a new putative g... more We present, based on state of the art density functional theoretic calculations, a new putative ground state (GS) for the cluster (TiO2)10, which results more than 1 eV lower in energy than all those previously reported in the literature. The geometric and electronic properties of this new cluster are discussed in detail and in comparison with the rest. We analyze the implications of the new GS in the context of recent experiments of reactivity regarding oxygen exchange with gaseous CO2 in TiO2 nanostructures, and also in connection with a recent interpretation of photoelectron spectroscopic measurements of the band gap of gas phase TiO2 (-) clusters.
Lectures on Surface Science, 1987
Current Problems in Condensed Matter, 1998
We study the magnetic surface enhancement and the Curie temperature in ferromagnetic Ising thin f... more We study the magnetic surface enhancement and the Curie temperature in ferromagnetic Ising thin films as a function of the surface orientations and structure. We use a simplified version of the cluster variation method in the pair approximation that only considers as inequivalent those sites on the surface, while all the internal sites are considered as equivalent regardless of the position. We calculate the critical value of the surface interaction J SC , beyond which the surface dominates the magnetic properties of the system. Our results are in agreement with those of the reaction field approximation and the renormalization group theory for the case of simple cubic lattice thin films.
Physics Letters A, 1998
We present a simple model for the study of ferromagnetic transition metal clusters. Based on Frid... more We present a simple model for the study of ferromagnetic transition metal clusters. Based on Fridel's model and on the geometrical characteristics, we obtain analytical expressions for the cluster average magnetic moment (μN) as a function of the number of atoms in bcc structures. The oscillatory behavior of μN as a function of cluster size reported experimentally in Fe clusters is reproduced by considering spherical bcc clusters. We briefly comment on the influence of ellipsoidal shape clusters on the magnetic moment measurements.
Physical Chemistry Chemical Physics, 2021
Unexpected reentrance of magnetism in iron oxide nanoparticles at high oxidation ratios.
Resumen del trabajo presentado al IPC–PAMS Theory Workshop: "Towards reality in modelling of... more Resumen del trabajo presentado al IPC–PAMS Theory Workshop: "Towards reality in modelling of molecular electronics", celebrado en Donostia-San Sebastian (Espana) del 13 al 17 de junio de 2016.
Physical Chemistry Chemical Physics, 2019
Ni3Bz3 molecule shows a large magnetic anisotropy energy of 8 meV, with the easy axis perpendicul... more Ni3Bz3 molecule shows a large magnetic anisotropy energy of 8 meV, with the easy axis perpendicular to the plane of Ni metal atoms. Note that the corresponding bare Ni3 cluster has an in-plane easy axis.
Computational Materials Science, 2018
Using a basin-hopping global optimization method, an exhaustive global search of the potential en... more Using a basin-hopping global optimization method, an exhaustive global search of the potential energy surface (PES) of trimetallic Ag-Au-Pt clusters is performed at the empirical potential (Gupta) level. The clusters contain a total of N = 13, 19, 33, 38 atoms while the Ag, Au, Pt content range from 0% to 100%. A broad region of the PES of these systems is sampled, finding a menagerie of geometries, which are catalogued into structural families according to geometrical considerations. The structural families so defined contain a fixed number of Pt atoms while the loading of Ag and Au atoms varies, and indicate that selective Pt doping can stabilize Ag-Au subnanometer clusters. Structural stability trends are identified thus obtaining the optimal composition for each cluster size. Segregation is analyzed through a chemical order parameter, and found to exhibit a size-dependent behavior, with a transition from mixed to segregated structures clearly evidenced at 38-atom cluster size. Optimal composition structures of the Gupta global minima and a few isomers are subjected to DFT reoptimization, which is found to predict qualitatively similar atomic arrangements with a certain degree of deformation, especially for 38-atom clusters, and to introduce changes in the energy ordering among structures with respect to the Gupta potential predictions, although the stabilizing effect of Pt and a cross-over character of 38-atom size seem confirmed.
Nanotechnology, 2015
Downsizing to the nanoscale has opened up a spectrum of new magnetic phenomena yet to be discover... more Downsizing to the nanoscale has opened up a spectrum of new magnetic phenomena yet to be discovered. In this context, we investigate the magnetic properties of Ni clusters embedded in a metallic Ag matrix. Unlike in Ni free-standing clusters, where the magnetic moment increases towards the atomic value when decreasing the cluster size, we show, by tuning the Ni cluster size down to the sub-nanoscale, that there is a size limit below which the clusters become non-magnetic when embedded in Ag. To this end, we have fabricated by DC-sputtering a system composed of sub-nanometer sized and non interacting Ni clusters embedded into a Ag matrix. A thorough experimental characterization by means of structural techniques (x-ray diffraction, x-ray absorption spectroscopy) and DC-magnetization confirms that the cluster size is in the sub-nanometric range and shows that the magnetization of the system is dramatically reduced, reaching only 38% of the bulk value. The experimental system has been reproduced by density functional theory calculations on Ni m clusters (m = 1-6, 10 and 13) embedded in Ag. The combination of the experimental and theoretical analysis points out that there is a breakdown of magnetism occurring below a cluster size of six atoms. According to our results, the loss of magnetic moment is not due to Ag-Ni hybridization but to charge transfer between the Ni sp and d orbitals, and the reduced magnetization observed experimentally is explained on the basis of the presence of a narrow cluster size-distribution where magnetic and non-magnetic clusters coexist.
New Trends in Magnetism, Magnetic Materials, and Their Applications, 1994
The segregation effects on the magnetic properties of a system consisting of a magnetic slab on t... more The segregation effects on the magnetic properties of a system consisting of a magnetic slab on top of other that is paramagnetic are calculated. The Hamiltonian includes magnetic and chemical interactions and it is solved in the Bethe approximation. We discuss only the case in which the two components phase-separate in the bulk (i.e., CuNi, CuCo, etc.). One observes that the magnetic properties are strongly modified as compared to the case in which interdiffusion is not allowed. We present results for the CuNi system.
New Trends in Magnetism, Magnetic Materials, and Their Applications, 1994
The magnetic properties of the ternary Heusler alloys are modeled by a Heisenberg Hamiltonian tha... more The magnetic properties of the ternary Heusler alloys are modeled by a Heisenberg Hamiltonian that includes first, second and third-nearest-neighbor interactions. The magnetic ground state structures at zero magnetic field and their range of stability with regard to the interaction parameters are obtained using the method of linear inequalities. The Curie and Neel temperatures are calculated within the mean field theory. The main characteristics of the evolution from the antiferromagnetic phase AF2, to the antiferromagnetic phase AF3, to the ferromagnetic state observed in Pd2MnSb x In1−x as a function of concentration, are obtained.
Physical Review B, 2003
The values for the calculated in the rows labeled ''present work'' of Table I, p. 224410-6, are i... more The values for the calculated in the rows labeled ''present work'' of Table I, p. 224410-6, are incorrect. The correct values must be read from the second column of Table II, p. 224410-7.
The Journal of Physical Chemistry A, 2011
Solid State Communications, 1998
The thermodynamics of ordering binary alloys with film geometry is studied for the case of symmet... more 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 on film thickness for the case of vanishing surface fields (ordinary transitions). We focus on the interplay between ordering and segreg,ation and its effects on the phase equilibria of b.c.c.(l 1 0) films as well as on the low-temperature behavior of the critical curve. At T = 0 the critical curve of a b.c.c. ordering binaryalloy film ends at a finite concentration. It is shown that it is useful to interpret the zero temperature transitions as percolation-like transitions. Calculations by the cluster v.ariation method in the pair and tetrahedron approximations are presented.
Solid State Communications, 1997
We have studied the nonmetal-metal transition in Ni clusters. A self-consistent tight-binding met... more We have studied the nonmetal-metal transition in Ni clusters. A self-consistent tight-binding method for the s, p and d valence electrons has been used to calculate the density of states and the prescription employed to identify the nonmetal-metal transition was Kubo's criterion that the density of states at the Fermi level should exceed 1kBT. The calculated critical cluster size decreases
Nanostructured Materials, 1993
... Manuel Sandoval Vallarta" Universidad Aut6noma de San Luis Potosi 78000 San Luis... more ... Manuel Sandoval Vallarta" Universidad Aut6noma de San Luis Potosi 78000 San Luis Potosi, SLP, Mexico J. Ortiz-Saavedra Escuela de Fisica Universidad Auton6ma de Zacatecas Zacatecas,Zac., Mexico Abstract ... For the parametrized Slater-Koster hopping integrals ti7lJ. ...
Nanostructured Materials, 1997
The critical size for the magnetic non-magnetic transition in non-magnetic 3d and 4d metal cluste... more The critical size for the magnetic non-magnetic transition in non-magnetic 3d and 4d metal clusters is studied by means of a tight-binding Friedel model and the Stoner criterion. The density of states is modeled by geometric shapes that include a dependence on the number of electrons. The second-moment approximation is used to introduce a cluster size-dependent band-width. We apply this model to estimate the critical sizes for the magnetic transition.
Physica B: Condensed Matter, 2004
The magnetic properties of free-standing Co n Rh m clusters (N ¼ n þ m % 110 and n % m) of three ... more The magnetic properties of free-standing Co n Rh m clusters (N ¼ n þ m % 110 and n % m) of three different symmetries: cubo-octahedral, icosahedral and HCP, were investigated in the segregated case. The initial geometrical structures constructed at Rh bulk distances were relaxed with a many-body Gupta potential to obtain the cluster geometries and energies. We find that the lowest energy is associated with the HCP structure. The relaxed interatomic distance for all the structures is slightly lower than the Rh bulk distance. The spin-polarized electronic structure and related magnetic properties of these optimized geometries were calculated by solving self-consistently a spd tightbinding Hamiltonian. The magnetic moment of the Rh atoms shows strong dependence on the position and environment, whereas the Co atoms show a smoother dependence. Results are compared with the experimental data and with other theoretical calculations available in the literature.
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Papers by Faustino Aguilera-Granja