R. C. Longo

Structural and magnetic properties of X12Y (X, Y=Fe, Co, Ni, Ru, Rh, Pd, and Pt) nanoalloys

We perform extensive ab initio density-functional calculations to investigate the structures and magnetic moments of the binary clusters X12Y (X, Y=Fe, Co, Ni, Ru, Rh, Pd, and Pt). Although all the binary clusters Fe12Y, Co12Y, Ru12Y, and Rh12Y, plus Ni12Y (Y=Rh, Pd, and Pt) and Pt12Y (Y=Ru, Rh, and Pd), retain, with more or less distortions, the structures of the corresponding pure X13 clusters, the remaining binary clusters (i.e., a significant number of 12 of all the 42 cases) adopt geometries different from those of the corresponding pure clusters. Independent of the peculiarities of each family of binary clusters, the binding energies of all the binary clusters X12Ru are bigger than those of the pure X13 clusters, while the binding energies of all the binary clusters X12Pd are smaller. The clusters investigated exhibit a variety of magnetic behaviors. In the case of Ni12Rh, we predict a remarkable magnetic cooperative phenomenon that can be attributed to electronic effects associated to the chemical ...

Magnetism of substitutional Fe impurities in graphene nanoribbons.

Using the generalized gradient approximation to exchange and correlation, we perform density functional calculations on an Fe atom at a single vacancy of graphene nanoribbons. Our results show that, after relaxation, the Fe atom is magnetic, in contrast to the behavior recently found for Fe at a single vacancy of the graphene sheet.

A density-functional study of the possibility of noncollinear magnetism in small Mn clusters using SIESTA and the generalized gradient approximation to exchange and correlation.

We investigated the possibility of noncollinear magnetism in small Mn(n) clusters (n=2-6) using the density-functional method SIESTA with the generalized gradient approximation (GGA) to exchange and correlation. The lowest-energy states identified were collinear, with the atomic spin magnetic moments pointing in the same direction, for Mn(2) and Mn(3), and noncollinear for Mn(4), Mn(5) and, most decidedly, Mn(6). These SIESTA/GGA results, which are compared with those of an earlier SIESTA study that used the local spin density approximation, are qualitatively in keeping with the result obtained by VASP/GGA calculations.

Noncollinear magnetic order in the six-atom Mn cluster

We report ab initio calculations of the structures, binding energies, and magnetic moments of the lowest-energy isomers of the cluster Mn6 that were performed using SIESTA, a density-functional method that employs linear combinations of pseudoatomic orbitals as basis sets, nonlocal norm-conserving pseudopotentials, and the local spin-density approximation for exchange and correlation. Our results predict that ground-state Mn6 has a noncollinear magnetic configuration.

Structure and melting of small Ni clusters on Ni surfaces

Abstract Using the Voter and Chen (VC) version of the embedded atom model (EAM), we performed computer simulations to obtain the structures and binding energies of small Ni clusters on Ni(001), Ni(110) and Ni(111) surfaces. The predicted Ni cluster structures on Ni(001) and Ni(111) generally agree with the results obtained by Liu and Adams using the Foiles, Baskes and Daw (FBD) version of the EAM (the only exception is one structure formed on the Ni(001) surface), but the corresponding binding energies differ significantly. The temperature dependence of the behaviour of the seven-atom Ni cluster on the Ni(111) surface shows that the predicted cluster melting temperature also depends significantly on which version of the EAM is used. Hence EAM predictions of the properties of supported metal clusters depend crucially on the parameterization of the model, i.e. on the kind of data used in optimizing the embedding function and pair interaction. Although ab initio results for Ni clusters on Ni surfaces are not available for comparison, it seems plausible that the EAM description of supported transition metal clusters, like the EAM description of free transition metal clusters, may in general be more accurate if the VC version of the model is used rather than the FBD version since the former uses diatomic data as well as bulk properties in optimizing the EAM functions, and its parameterization should therefore be more appropriate for the cluster level.

Prediction of phonon thermal transport in thin GaAs, InAs and InP nanowires by molecular dynamics simulations: influence of the interatomic potential

A number of different potentials are currently being used in molecular dynamics simulations of semiconductor nanostructures. Confusion can arise if an inappropriate potential is used. To illustrate this point, we performed direct molecular dynamics simulations to predict the room temperature lattice thermal conductivity λ of thin GaAs, InAs and InP nanowires. In each case, simulations performed using the classical Harrison potential afforded values of λ about an order of magnitude smaller than those obtained using more elaborate potentials (an Abell-Tersoff, as parameterized by Hammerschmidt et al for GaAs and InAs, and a potential of Vashishta type for InP). These results will be a warning to those wishing to use computer simulations to orient the development of quasi-one-dimensional systems as heat sinks or thermoelectric devices.

Engineering the magnetic structure of Fe clusters by Mn alloying

We propose to tailor the magnetic structure of atomic clusters by suitable doping, which produces the nanometric equivalent to alloying. As a proof of principle, we perform a theoretical analysis of Fe(6-x)Mn(x) clusters (x = 0-5), which shows a modulation of the magnetic moment of the clusters as a function of Mn doping and, more importantly, a collinear to noncollinear transition at x = 4.

A density-functional study of the structures, binding energies and total spins of Ni-Fe clusters using nonlocal norm-conserving pseudopotentials and the generalized gradient approximation.

We report ab initio calculations of the structures, binding energies, and total spins of the clusters Ni(13), Ni(19), Ni(23), Ni(26), Ni(12)Fe, Ni(11)Fe(2), Ni(18)Fe, Ni(17)Fe(2), Ni(22)Fe, Ni(20)Fe(3), and Ni(25)Fe using a density-functional method that employs linear combination of atomic orbitals as basis sets, nonlocal norm-conserving pseudopotentials, and the generalized gradient approximation for exchange and correlation. Our results show that the Fe-doped Ni clusters, which have icosahedral or polyicosahedral ground-state structures similar to those of the corresponding pure Ni clusters, are most stable with the Fe atoms occupying internal positions, as has also been inferred from experimental results on the adsorption of molecular nitrogen on the cluster surfaces. We also rule out the possibility that the experimentally observed difference between the (nonpolyicosahedral) configurations of N(2)-saturated Ni(26) and N(2)-saturated Ni(25)Fe be due to the influence of the Fe atom on the energy of the underlying metal cluster.

Magnetic Cooperative Effects in Small Ni–Ru Clusters

We report ab initio calculations of the structures, magnetic moments, and electronic properties of Ni(7)-xRu(x) clusters (x = 0-7) using a density-functional method that employs linear combinations of pseudoatomic orbitals as basis sets, nonlocal norm-conserving pseudopotentials, and the generalized gradient approximation to exchange and correlation. The pure clusters Ni(7) and Ru(7) were predicted to have octahedral and cubic structures, respectively, and the binary clusters were found to be either decahedral (Ni(6)Ru, Ni(5)Ru(2), and Ni(4)Ru(3)) or cubic (Ni(3)Ru(4), Ni(2)Ru(5), and NiRu(6)). For Ni(5)Ru(2) and Ni(4)Ru(3) we found a magnetic cooperative phenomenon, which is due to both geometrical effects and electronic contributions through Ni-Ru hybridization.

A density-functional study of the vertical ionization potentials of the cluster Mn13

Using the density-functional method SIESTA, nonlocal norm-conserving pseudopotentials and the generalized gradient approximation to exchange and correlation, we show that the two ionization energies of the photoionization spectrum of Mn(13) can be attributed to the presence of the ground state and an excited spin isomer of this species.