Joseph Morillo

Calculation of magnetic and structural properties of small Co-Rh clusters

Structural and magnetic properties of CoM RhN clusters with M þ N 6 4atoms are determined by performing firstprinciples calculations in the framework of the density functional theory in a generalized gradient-corrected approximation and within the projector-augmented wave method. The role of magnetism on the most stable structures and on the energy differences among the low-lying isomers is quantified by comparing magnetic and non-magnetic solutions of the Kohn–Sham equations. As usual in small clusters, CoM RhN clusters show contracted interatomic distances with respect to the bulk. They exhibit ferromagnetic-like order with environment-dependent local magnetic moments lðiÞ. The average magnetic moments per atom � lðM; NÞ and lðiÞ are generally more than a factor 2 larger than in macroscopic alloys of similar concentration. For a given cluster size M þ N, � l increases with increasing number of Co atoms M. The lðiÞ at Rh atoms are remarkably enhanced by the presence of Co nearest neighbors, while the lðiÞ at Co atoms are not much affected by the presence of Rh atoms. The correlation between structure, chemical order, and environment-dependent magnetic properties is discussed. 2003 Elsevier Science B.V. All rights reserved.

First-principles study of binary transition-metal clusters and alloys

Small CoMRhN clusters have been studied by performing first-principles calculations in the framework of the density functional theory. In order to understand the interplay between their structural order, chemical order and magnetic properties, the corresponding bulk alloys in various hypothetically ordered phases were also calculated. In the bulk alloys and in most of the clusters there is an induced magnetic moment on the Rh atoms with increasing Co concentration which results in an increase in the average magnetic moment per atom. This effect, when present, may be twice as large in the clusters than in the corresponding macroscopic alloys. For low-symmetry clusters structures, the substitution of Rh by Co atoms results in a remarkable enhancement of the local magnetic moment at the Rh neighbours even at low Co concentration. On the other hand, the Co doping effect can be masked in cluster geometries of higher symmetry in which pure Rh clusters already show a high magnetic moment. In addition it is shown that local coordination and magnetic energy are the leading parameters for the determination of the stability and magnetic enhancement, whereas chemical order seems less important.

Adsorption and diffusion of Mg, O, and O2 on the MgO(001) flat surface.

A thorough investigation of the adsorption and diffusion of Mg, O, and O(2) on MgO(001) terraces is performed by first-principles calculations. The single Mg adatom weakly binds to surface oxygens, diffuses, and evaporates easily at room temperatures. Atomic O strongly binds to surface oxygens, forming peroxide groups. The diffusion of the O adatom is strongly influenced by the spin polarization, since energy barriers are significantly different for the singlet and triplet states. The crossing of the two Born-Oppenheimer surfaces corresponding to the distinct spin states is also analyzed. Although the O(2) molecule does not stick to the perfect surface, it chemisorbs on surface nonstoichiometric point defects such as O vacancies or Mg adatoms, forming in the latter case new chemical species on the surface. We show that the oxidation rate limiting factor in an O(2) atmosphere is the concentration of point defects (O vacancies and Mg adatoms) in the growing surface. The simulated O core-level shifts for the various adsorption configurations enable a meaningful comparison with the measured values, suggesting the presence of peroxide ions on growing surfaces. Finally, the computed energy barriers are used to estimate the Mg and O surface lifetimes and diffusion lengths, and some implications for the homoepitaxial growth of MgO are discussed.

Density functional theory investigations of titanium γ-surfaces and stacking faults

Properties of hcp-Ti such as elastic constants, stacking faults and gamma-surfaces are computed using Density Functional Theory (DFT) and two central force Embedded Atom interaction Models (EAM). The results are compared to previously published calculations and to predicting models. Their implications on the plastic properties of hcp-Ti are discussed.

Core structure of screw dislocations in hcp Ti: an ab initio DFT study

Abstract The core structure of screw dislocations in -Ti was studied in the cluster approach with ab initio DFT-GGA, and in both the cluster and quadrupole approaches with a recently highly optimized EAM central force potential. With the EAM potential we have shown that finite-size effects, in the cluster approach, are negligible down to the size studied in the ab initio DFT calculations that have shown unambiguously a preferential prismatic core spreading for the dislocation. Our results are in agreement with previously published approximated calculations using empirical or semi-empirical interaction models: only approximated interaction models, taking explicitly into account the covalent directional bonding of the d electrons, can properly account for the preferential prismatic core spreading against the basal one; and empirical interaction models without angular force components are inadequate. Interestingly, at first sight, the relaxed core structures (basal or prismatic) obtained with empirical or semi-empirical interaction models are almost identical to the ones obtained with the ab initio DFT calculations.

Benchmarking Density Functional Based Tight-Binding for Silver and Gold Materials: From Small Clusters to Bulk

We benchmark existing and improved self-consistent-charge density functional based tight-binding (SCC-DFTB) parameters for silver and gold clusters as well as for bulk materials. In the former case, our benchmarks focus on both the structural and energetic properties of small-size AgN and AuN clusters (N from 2 to 13), medium-size clusters with N = 20 and 55, and finally larger nanoparticles with N = 147, 309, and 561. For bulk materials, structural, energetics and elastic properties are discussed. We show that SCC-DFTB is quite satisfactory in reproducing essential differences between silver and gold aggregates, in particular their 2D-3D structural transitions, and their dependency upon cluster charge. SCC-DFTB is also in agreement with DFT and experiments in the medium-size regime regarding the energetic ordering of the different low-energy isomers and allows for an overall satisfactory treatment of bulk properties. A consistent convergence between the cohesive energies of the largest investigated nanoparticles and the bulks is obtained. On the basis of our results for nanoparticles of increasing size, a two-parameter analytical extrapolation of the cohesive energy is proposed. This formula takes into account the reduction of the cohesive energy for undercoordinated surface sites and converges properly to the bulk cohesive energy. Values for the surface sites cohesive energies are also proposed.

Screw dislocation in hcp Ti:DFT dislocation excess energies and metastable core structures

An extensive DFT search of (meta)stable structures of the a � 11 ¯ 20 � screw dislocation in hcp-Ti is presented. It reveals that the stable core structures are never basal but always prismatic. This prismatic core dissociates into two partial dislocations in the same or neighboring prismatic planes depending on the initial position of the dislocation line, leading to either a symmetric or an asymmetric core. An alternative way of defining the core region from an electronic structure point of view is also proposed. It evidences clearly the symmetric or asymmetric character of the cores. We then introduce an ansatz for a straightforward and fast calculation of the excess energy, per unit length of dislocation, of a screw dislocation applicable to DFT calculations, in the cluster approach. The method is first validated on calculations of a a � 11 ¯ 20 � screw dislocation in hcp-Ti, performed with an EAM potential from which exact excess energies can be extracted. Then, it is shown that it does work in a DFT calculation, through its application to the same screw dislocation in hcp-Ti with an accuracy of 8.4 meV/A (1.8% of the excess energy for a cluster of 126 atoms per plane normal to the dislocation line). The comparison of the excess energies of the symmetric and assymmetric cores, calculated with the proposed ansatz, reveals that their energy difference is within the uncertainty

Competing mechanisms in the atomic diffusion of a MgO admolecule on the MgO(001) surface

The diffusion mechanism of a MgO admolecule on a flat MgO(001) surface has been investigated by equilibrium molecular dynamics simulation. Care has been taken in the choice of the phenomenological interionic potential used. Four distinct mechanisms have been found and the corresponding dynamical barriers determined at high temperature. Some static barriers have also been computed for comparison and all intermediate configurations have been obtained with the same phenomenological potential and also by the DFT-GGA approach. The hopping mechanisms involving the Mg adatom, although dominant, must be combined with the infrequent mechanisms involving displacements of O adatoms in order to provide the mass transport on the surface, which is crucial for crystal growth both in the nucleation and step-flow regimes.

Towards atomic-scale design: A theoretical investigation of magnetic nanoparticles and ultrathin films

This paper reports recent theoretical work on nanostructured materials including magnetic alloyed CoRh nanoparticles that are good candidates to combine both a large magnetic moment and a high magnetic anisotropy, and nanoscale Mn films adsorbed on W substrates as an example of artificial magnetic material. It illustrates how modern atomistic modeling and simulation can fruitfully supplement the experiments performed in the laboratory by helping to resolve and understand the experimental information, by predicting new phenomena and providing useful hints to guide the development of innovative materials with original, specifically tailored properties.

Atomic Organization In Magnetic Bimetallic Nanoparticles: An Experimental And Theoretical Approach

Ultrafine bimetallic CoRh nanoparticles synthesized by a soft chemical route with compositions ranging from pure cobalt to pure rhodium are investigated using high-resolution and energy filtering transmission electron microscopy techniques as well as wide angle x-ray scattering. In parallel, they are simulated with the use of an n-body semi-empirical interaction model: quenched molecular dynamics and Monte-Carlo Metropolis simulated annealing are performed on these nanoparticles in order to find their most stable isomers as a function of composition and size. A progressive evolution from an original polytetrahedral structure to the face-centered cubic structure with increasing Rh content is observed in these particles. Strong tendency to Co surface segregation is both experimentally evidenced and confirmed by the simulations.