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Imogolite is an inorganic nanotube, that forms naturally in weathered volcanic ashes, and it can be synthesized in nearly monodisperse diameters. However, long after its successful synthesis, the details of the way it is achieved are not fully understood. Here we elaborate on a model of its synthesis, which starts with a planar aluminosilicate sheet that is allowed to evolve freely, by means of classical molecular dynamics, until it achieves its minimum energy configuration. The minimal structures that the system thus adopts are tubular, scrolled, and more complex conformations, depending mainly on temperature as a driving force. Here we focus on the effect that the arrangement of the hydroxyl groups in the inner wall of the nanotube have on the minimal nanotubular configurations that we obtain are monodispersed in diameter, and quite similar to both from the those of weathered natural volcanic ashes, and to the ones that are synthesized in the laboratory. In this contribution we expand on the atomic mechanisms behind those behaviors.

The discovery and development of Carbon Nanotubes (CNTs) at the beginning of the 1990s has driven a major part of solid state research. The electronic properties of the CNTs have generated a large number of ideas, as building coaxial nanocables. In this work we propose a possible type of such nanocables, which is formed by three nanostructures: two conducting CNTs, where one of them is covered by an insulator (an inorganic oxide nanotube: the imogolite aluminosilicate). The theoretical calculations were carried out using the density functional tight-binding formalism, by means of the DFTB+ code. This formalism allows to calculate the band structure, which compares favorably with DFT calculations, but with a significantly lower computational cost. As a first step, we reproduce the calculations of already published results, where the formation of a nanocable composed by one CNT and the imogolite as an insulator. Afterwards, we simulate the band structure for the proposed structure to study the feasibility of the coaxial nanocable. Finally, using classical MD simulations, we study the possible mechanisms of formation of these nanocables.

The cubane molecule, and a few representative oligomers built with cubane units, are examined in order to investigate the feasibility and likelihood of creating covalently bonded arrays with cubane units as building blocks. In this way, oligomers with a variety of structures were obtained, including rods and surface-like networks. B3LYP/6-311G** calculations indicate that, in the bulk limit, the cubane structures do not accept electrons. The electron transfer within the cubane network may occurs only through doping or substitution with donor/acceptor groups. q 2006 Elsevier B.V. All rights reserved.

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.

The systematic replacement of hydrogen by fluorine atoms in disilane ( Si 2 H 6 ) is investigated using density functional theory (DFT). For every molecule in the Si 2 H 6−x F x family, with 0⩽x⩽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.

The incomplete ferromagnetic domain-wall model we proposed recently is solved analytically. We derive the dependence of the exchange bias field ( H EB ) on the different parameters that characterize the magnetic bilayer system. Excellent agreement with the numerical solutions is achieved. Moreover, the model yields a crossover from a t F −1 dependence of H EB for thin ferromagnetic films, to a t F −1.9 dependence for thick films, where t F is the ferromagnetic film thickness. Our results are in agreement with experiment.

The deformation process of Fe encapsulated in a carbon nanotube (CNT) is investigated by means of classical molecular dynamics. The [100], [110], and [111] Fe crystal orientations parallel to the CNT symmetry axis, as well as the temperature dependence, are studied. The system encompasses approximately 80 000 atoms. While crystal orientation and temperature determine the system's response, the results are almost independent of the strain rate that is applied. This behavior is only slightly modified by the Fe encapsulation in the CNT. The principal energy release mechanism is the generation of dislocations and twin boundaries, at low and intermediate temperatures (T ≤ 600 K). The dislocations and twin boundaries interact, but do not interlock. For large temperatures (T ∼ 1000 K), a different reaction to deformation sets in, and no elastic response of the Fe–CNT system is observed.

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 HE

Exchange bias (EB) is a shift of the hysteresis loop from its normal position, symmetric around H = 0, to HE ≠ 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) HE is of similar magnitude for compensated and uncompensated AF interface layers; iii) the sign of HE can assume both positive and negative values; and, iv) the magnetization |M(H − Hc)| ≠ |M(H + Hc)| , where Hc 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.

Molecular dynamics simulations of a Xe monolayer sliding on Ag(001) and Ag(111) are carried out in order to ascertain the microscopic origin of friction. For several values of the electronic contribution to the friction of individual Xe atoms, the intra-overlayer phonon dissipation is calculated as a function of the corrugation amplitude of the substrate potential, which is a pertinent parameter to consider. Within the accuracy of the numerical results and the uncertainty with which the values of the relevant parameters are known at present, we conclude that electronic and phononic dissipation channels are of similar importance. While phonon friction gives rise to the rapid variation with coverage, the electronic friction provides a roughly coverage-independent contribution to the overall sliding friction., 11 pages, 8 figures

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 σ XeAl , for several sizes of the Xe island adsorbed on Al. This way a critical value of σ 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.

An analytic solution for the Heisenberg Hamiltonian with arbitrarily long-range, ferromagnetic and/or antiferromagnetic, interactions on a Bethe lattice is obtained in the semiclassical approximation (S\ensuremath{\rightarrow}\ensuremath{\infty}). The density of states of the excitations is evaluated in the one-spin-wave approximation. The stability of a general helical ground state configuration, against variations in the magnitude and sign of the long-range interactions, is investigated for several illustrative examples, generating a wealth of sharp and smooth parametric transitions.

The structural configurations and magnetic properties of zig-zag nanoribbons and nanowires of Fe 8− N Co N , for 0 ≤ N ≤ 8 , are calculated within the density functional theory. Both, for the zig-zag nanoribbons and the nanowires, there is a tendency towards forming Fe–Co bonds, while segregation of the Fe and Co is energetically unfavorable. For the nanowire structures a transition from bcc Fe to hcp Co spatial arrangements is observed when N is increased from 4 to 6, in spite of the small size of the systems under investigation. The energy minimization was performed taking into consideration the electronic and magnetic structures, since for each crystalline structure, chemical composition, and short range order, particular magnetic properties of these systems do correspond. The magnetocrystalline anisotropy energy is calculated, and it is found that the easy axis changes from a transverse direction in Fe-rich systems, to the axial direction as the Co concentration increases. It is also found that although there are important variations of the local magnetic moment of the components, and their particular location in the system, the average magnetic moment is an almost linear function of N .

The collision dynamics of one Ag or Cu atom impinging on a Au12 cluster is investigated by means of DFT molecular dynamics. Our results show that the experimentally confirmed 2D to 3D transition of Au12-Au13 is mostly preserved by the resulting planar Au12Ag and Au12Cu minimum energy clusters, which is quite remarkable in view of the excess energy, well larger than the 2D-3D potential barrier height. The process is accompanied by a large sd hybridization and charge transfer from Au to Ag or Cu. The dynamics of the collision process mainly yields fusion of projectile and target, however scattering and cluster fragmentation also occur for large energies and large impact parameters. While Ag projectiles favor fragmentation, Cu favors scattering due to its smaller mass. The projectile size does not play a major role in favoring the fragmentation or scattering channels. By comparing our collision results with those obtained by an unbiased minimum energy search of 4483 Au12Ag and 4483 Au12Cu configurations obtained phenomenologically, we find that there is an extra bonus: without increase of computer time collisions yield the planar lower energy structures that are not feasible to obtain using semi-classical potentials. In fact, we conclude that phenomenological potentials do not even provide adequate seeds for the search of global energy minima for planar structures. Since the fabrication of nanoclusters is mainly achieved by synthesis or laser ablation, the set of local minima configurations we provide here, and their distribution as a function of energy, are more relevant than the global minimum to analyze experimental results obtained at finite temperatures, and is consistent with the dynamical coexistence of 2D and 3D liquid Au clusters conformations obtained previously.

The dynamics of the collision process of a titanium dimer against a C20 nanocluster in the bowl configuration is simulated by means ab initio molecular dynamics, focusing our interest on the first steps to synthesize Ti2C20 clusters, characterizing the relevant structures generated during the collision process. The electronic localization function is also calculated. The early stages of the dynamics are critical to determine the outcome because of the large kinetic energy of the Ti dimer, which allows it to wander around the C20 cluster.

We present a comparison of the strength of the classical dipolar interaction, relative to quantum-mechanical coupling mechanisms like RKKY and complete confinement, between two ferromagnetic films separated by a paramagnetic spacer. The classical dipolar coupling, which vanishes if the two interfaces are perfectly continuous and flat, builds up strength as the interface roughness grows for several models of interface topography. These numerical estimates, carried out for a Co/Cu/Co trilayer show that, in the presence of substantial surface roughness, the dipole-dipole interaction strength is comparable, and at times even larger, than those obtained using other well established mechanisms. These results are also in qualitative agreement with experimental measurements in a variety of multilayer systems. Thus, for rough interfaces, the dipolar interaction cannot be ignored.

A new type of collective excitations, due to the topology of a complex random network that can be characterized by a fractal dimension DF, is investigated. We show analytically that these excitations generate phase transitions due to the non-periodic topology of the DF > 1 complex network. An Ising system, with long range interactions, is studied in detail to support the claim. The analytic treatment is possible because the evaluation of the partition function can be decomposed into closed factor loops, in spite of the architectural complexity. The removal of the infrared divergences leads to an unconventional phase transition, with spin correlations that are robust against thermal fluctuations.

We propose a probabilistic cellular automaton model with a rule that interchanges neighboring atoms to simulate the diffusion driven dynamics of binary alloys. In particular, we consider the ordering transition of two dimensional binary alloys on a square lattice by assuming antiferromagnetic couplings and measure the energy, specific heat, and correlation functions as functions of temperature. We perform damage spreading simulations and the phase space structure that emerges consists of a single paramagnetic thermodynamic state at high temperatures and a two-basin phase space at low temperatures. The results are compatible with an order-disorder phase transition at a temperature close to the Onsager exact value.

Collision processes between single gold and rhodium atoms and gold and rhodium nanoclusters, respectively, are used to study the interplay between the large elastic energies and the much smaller magnetic interactions. The latter, in spite of their small magnitude, often settle the configuration of crystals, clusters, and molecules. We implement ab initio molecular dynamics (DFT-MD) calculations to investigate the subtle balance between these two forces. The targets we consider are minimum energy 13 (12) atom gold (rhodium) clusters. Results for gold and rhodium are compared emphasizing the relevance of d-shell polarization. The use of these collision processes to generate putative nanocluster minimum energy configurations will be presented and illustrated with successful examples.

A two-dimensional molecular dynamics model is introduced in order to obtain a better understanding of interface amorphization. We start from a monodisperse system of particles arranged on a perfect two-dimensional triangular lattice, which is divided into two separate regions. In one of these regions the particle size is uniformly reduced, while in the other it is enlarged, preserving the total system 'volume'. Using a whole wealth of diagnostic tools, and visualizations of the particle distributions, a clear distinction between the bulk and the interface disordering transition emerges. Several interesting phenomena, such as the formation of defects, hysteresis of the transition, single-layer displacement and grain boundary development in the vicinity of the interface, show up in our simulations.

The zero-temperature minimal energy structure of small free-standing Pd clusters (14≤N≤21, where N is the number of atoms in the cluster), their characteristics and their magnetic configurations are investigated. Results obtained using five different phenomenological many-body potentials (implemented in combination with a genetic algorithm search) are refined by means of various density functional theory (DFT) techniques. The agreement and differences between the results obtained with our procedure, using these five potentials, are displayed in detail. While phenomenological potentials yield values that approach the minimal energies of larger clusters, as compared with DFT results, they fail to predict the right symmetry group for some of the clusters with N>14. We find that the minimal energy configurations are not necessarily associated with high symmetry of the atomic arrangement. Actually, several cases of previously overlooked low symmetry structures turn out to have lower energies than more symmetric ones.

Molecular-dynamics simulations are used to investigate lateral friction in contact- mode atomic force microscopy of tetracosane (n-C24H50) films. We find larger friction coefficients on the surface of monolayer and bilayer films in which the long axis of the molecules is parallel to the interface than on a surface of molecules with the long axis perpendicular to the surface, in agreement with experimental results. A major dissipation mechanism is the molecular flexibility as manifested in the torsional motion about the molecules' C-C bonds. The generation of gauche defects as a result of this motion does not appear to be in itself a major channel of energy dissipation. As previously reported in the literature, the layer density and thereby the strength of the attractive film-tip interaction is also an important factor in energy dissipation. Copyright c EPLA, 2011

The effects due to d-orbital polarization in collision processes between a single rhodium atom and a 12 atom rhodium cluster are investigated by means of ab initio molecular dynamics. For the initial configuration of the 12 atom rhodium targets we adopt two different low energy structures. The kinetic energy and impact parameter of the projectile are chosen in such a way that fusion, scattering and fragmentation of the cluster do occur. The collision is treated by means of density functional theory molecular dynamics (DFT-MD). Both spin unpolarized and polarized treatments are implemented in order to clearly distinguish the effects that are due to d-orbital polarization. We find a novel block dynamics, of parts of the cluster, which is due to the directional nature of d-bonds. In addition, the treatment of the collisions by means of high temperature DFT dynamics yields promising minimal energy configurations, which are target dependent but are difficult to obtain otherwise.

The deposition of iron clusters on an iron substrate is investigated by means of classical molecular dynamics. The substrate dynamics is incorporated and shown to be a relevant ingredient, which leads to substantial changes of the transition temperatures, and of the thermodynamics of the cluster substrate interacting system. Fe 55 clusters on Fe (110) and (111) reconstructed surface orientations were studied, showing a stronger effect of the substrate dynamics for the (111) orientation. In fact, the melting temperature for a free Fe cluster, obtained by molecular dynamics simulations, is of the order of 1000 K, significantly lower than the 1812 K of bulk iron. When supported on a rigid Fe substrate it corresponds to ≈ 1400 K on both the (110) and (111) orientations. However, when the substrate dynamics is incorporated, the melting temperature changes markedly to ≈ 1200 K on the (110), and to ≈ 900 K on the (111), Fe substrate orientation.

We present a gauge-invariant reformulation of the Hubbard model, which assumes a set of arbitrarily chosen local spin quantization axes. This way, the macroscopic spin angular degrees of freedom are described by gauge fields. An effective interaction between fermions, with a long-range confining potential, is thus obtained. We compute the ground state energy of the gauged model in the one-dimensional version, for a classical configuration of the gauge sector. It is shown that gauge fields contribute to stabilize the antiferromagnetic configuration. A brief discussion on possibilities unveiled by this formulation is also given.

The magnetic ordering due to interactions between two ferromagnetic slabs, each of them a few monolayers thick, and separated by a nonmagnetic interlayer, is investigated. An RKKY like interaction is assumed to couple the ferromagnetic planes. The dependence of the ferro- or antiferromagnetic interaction strength on the spacer thickness, its nature and the roughness of the interface, is discussed, taking into consideration the discrete nature of the spin system. For values of the electron wavelength significantly smaller than the lattice parameter our model yields interesting interference effects, which do not appear in a pseudo one-dimensional treatment.

The determination of the spatial distributions that atoms adopt to form condensed matter is a problem of crucial importance, since most physical properties depend on the atomic arrangement. This is especially relevant for clusters, where periodicity is nonexistent. Several optimization procedures have been implemented to tackle this problem, with ever increasing success. Here we put forward a search scheme which preserves as large a diversity as allowed by the use of phenomenological potentials, generating in an unbiased fashion a bank of configurations to be explored; a procedure we denominate diversity driven unbiased search (DDUS). It consists in the generation, using phenomenological potentials, of a data bank of putative minima rather than a single, or just a few, configurations which are based on the conformational space annealing method (CSA). All of the configurations in the bank are thereafter refined by means of DFT computations. Certainly, in spite of our efforts to generate a bank as diverse as possible, not all relevant structures might be included in it, since quantum effects are ignored. The procedure is applied to several examples of rhodium, palladium, silver, platinum and gold clusters, between 5 and 23 atoms in size. The main conclusion we reach is that unbiased search, among a significant number of candidates, quite often leads to rather unexpectedly low symmetry configurations, which turn out to be the lowest energy ones within our scheme. (Some figures in this article are in colour only in the electronic version)

An Anderson-like Hamiltonian, which describes a cluster of a rare earth (Sm or Y) cation surrounded by six S anions, is used to model the electron-lattice interaction in mixed-valence systems. Coupling between the electronic and phononic variables is introduced, and two different phonon modes are considered: a breathing and an asymmetric one. The first, related to the ionic radius, is treated exactly. The asymmetric mode, which determines the sd-f hybridization, is dealt with in the Born-Oppenheimer approximation. Experimental results are adequately accounted for by this simple model.

The electron-phonon interaction in mixed-valence systems is modeled on the basis of an Anderson-like Hamiltonian that describes a cluster of one metallic rare-earth cation surrounded by six anions. Coupling between the electronic and phononic variables is introduced, keeping two different phonon modes: a breathing and an asymmetric mode. The first, related to the ionic radius, is treated exactly. The asymmetric mode, which determines the sd-f hybridization, is dealt with in the Born-Oppenheimer approximation. A variety of experimental results are adequately accounted for by this simple model, like the anomalous thermal expansion, the Debye-Waller factor, the phonon softening and broadening, and the charge-distance correlation.

The molecular structure of [Cu2(dpm)2L(OH)][CIO4]3·2H2O [dpm = di(2-pyridyl)methane; L = 1,1,2,2-tetrakis(2-pyridyl)ethylene] has been determined by single-crystal X-ray diffraction: triclinic, space group P, a= 11.685(3), b= 14.569(2), c= 15.509(4)A, α= 87.97(2), β= 73.34(2), γ= 74.15(1)° and Z= 2. The copper co-ordination geometry can be described as a square-based pyramid with the two copper(II) centres bridged by two groups, N–C–CC–C–N from L and the hydroxide ion. The bridging angle Cu–O–Cu is 137.9(4)° and the copper centres are separated by 3.663(3)A. Magnetic susceptibility data collected as a function of temperature (5–400 K) are indicative of a relatively strong antiferromagnetic interaction (2J≈–360 cm–1).

The origin of the exchange bias phenomenon is investigated on the basis of a quantum mechanical model. In particular, the mechanisms that determine the magnetic structure in the vicinity of an antiferromagnetic–ferromagnetic interface are reexamined. This way we establish how the breaking of translational invariance modifies quantum spin fluctuations. It is found that non-uniform fluctuations induce uncompensated spins in the antiferromagnet, which in turn give rise to a dipole field that couples to the magnetization of the ferromagnet. This coupling yields an exchange bias field that is of the order of magnitude of the one observed experimentally. A net surface magnetization should also be experimentally observable in a clean antiferromagnetic surface.

An alternative strategy to find the minimal energy structure of nanoclusters is presented and implemented. We use it to determine the structure of metallic clusters. It consists in an unbiased search, with a global minimum algorithm: conformational space annealing. First, we find the minima of a many-body phenomenological potential to create a data bank of putative minima. This procedure assures us the generation of a set of cluster configurations of large diversity. Next, the clusters in this data bank are relaxed by ab initio techniques to obtain their energies and geometrical structures. The scheme is successfully applied to magic number 13 atom clusters of rhodium, palladium, and silver. We obtained minimal energy cluster structures not previously reported, which are different from the phenomenological minima. Moreover, they are not always highly symmetric, thus casting some doubt on the customary biased search scheme, which consists in relaxing with density functional theory global minima chosen among high symmetry structures obtained by means of phenomenological potentials.

The pioneer work of Krim and Widom [Phys. Rev. B 38, 12184 (1988)] unveiled the origin of the viscous nature of friction at the atomic scale. This generated extensive experimental and theoretical activity. However, fundamental questions remain open like the relation between sliding friction and the topology of the substrate, as well as the dependence on the temperature of the contact surface. Here we present results, obtained using molecular dynamics, for the phononic friction coefficient $({\ensuremath{\eta}}_{\mathit{ph}})$ for a one-dimensional model of an adsorbate-substrate interface. Different commensuration relations between adsorbate and substrate are investigated as well as the temperature dependence of ${\ensuremath{\eta}}_{\mathit{ph}}$. In all the cases we studied ${\ensuremath{\eta}}_{\mathit{ph}}$ depends quadratically on the substrate corrugation amplitude, but is a nontrivial function of the commensuration ratio between substrate and adsorbate. The most striking result is a deep and wide region of small values of ${\ensuremath{\eta}}_{\mathit{ph}}$ for substrate-adsorbate commensuration ratios between $\ensuremath{\approx}0.65$ and 0.9. Our results shed some light on contradictory results for the relative size of phononic and electronic friction found in the literature.

International audience; The structural arrangements and magnetic properties of iron encapsulated in single wall carbon nanotubes (SWCNT) are investigated. Fe nanowires are of interest because of their potential use in spintronics. They have also been fabricated inside carbon nanotubes, where occasionally exchange bias has also beendetected. An additional motivation to study these systems is to contribute to the understanding of how the iron-carbon interactiondetermines the magnetic ordering. Here we investigate, using ab-initio methods,the geometry and magnetic structure of freestanding and encapsulated Fe nanowires. In addition, we also investigate the energetics and magnetic properties of the nanowire-nanotube system when defects are present in the single wallcarbon nanotube. When the ratio of the nanowire to nanotube diameter is small the system is stable and the spin polarization at the Fermienergy is large, thus making the system potentially interesting for spintronics. When this ratio is close to one the system is lessstable and a tendency towards antiferromagnetic ordering is occasionally observed.

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.

An interesting carbon structure is cubane $({\mathrm{C}}_{8}{\mathrm{H}}_{8})$. This molecule is a cube formed by carbons and $s{p}^{3}$ bonds saturated with hydrogens. The fact that the bonding angle is 90\ifmmode^\circ\else\textdegree\fi{}, rather than the ideal 109.5\ifmmode^\circ\else\textdegree\fi{} value found in diamond, gives to this structure rather unusual properties. Here we report an electronic and structural characterization of polycubane chains linked with ${\mathrm{C}}_{2}$, ${\mathrm{N}}_{2}$, NO, and NS, and show that the electronic properties can be modified to obtain oligomeric chains with insulating, semiconducting and metallic behavior.

The magnetic arrangement in the vicinity of the interface between a ferromagnet and an antiferromagnet is investigated, in particular its dependence on the exchange couplings and the temperature. Applying a Heisenberg model, both sc(001) and fcc(001) lattices are considered and solved by a mean field approximation. Depending on the parameter values a variety of different magnetic configurations emerge. Usually the subsystem with the larger ordering temperature induces a magnetic order into the other one (magnetic proximity effect). With increasing temperature a reorientation of the magnetic sublattices is obtained. For coupled sc(001) systems both FM and AFM films are disturbed from their collinear magnetic order, hence exhibit a similar behavior. This symmetry is absent for fcc(001) films which, under certain circumstances, may exhibit two different critical temperatures. Analytical results are derived for simple bilayer systems., accepted for publication in Eur. Phys. J. B

Ground-state fluctuations reduce the zero-temperature magnetic moments of the spins in a quantum antiferromagnet. In the neighborhood of surfaces, interfaces, and other defects which break translational symmetry, these fluctuations are not uniform. Because of this the magnetic moments of up and down spins do not exactly compensate each other--as they do in a bulk antiferromagnet. At a surface or interface this leads to a small magnetic dipole density. The corresponding dipole field can account for the magnitude of observed exchange fields. At finite temeperatures localized surface (interface) excitations are populated and change the dipole density. This gives rise to the temperature-dependence of the exchange field. We expect the fluctuation-induced surface dipole density to play a role in the magnetic properties of antiferromagnetic nano-particles, as well.

The spatial arrangements and physical properties of one- and two-dimensional structures, based on the amazing cubane (C(8)H(8)) molecule, are investigated in detail. In particular, we compute the electronic structure, both by first principle calculations and by semiempirical methods. The elastic and vibrational properties are evaluated as well. All these results are compared with those of the single cubane molecule, in order to elucidate the influence of dimensionality.

Several properties of hybrid systems made of iron nanowires coated with carbon are computed from first principles. In particular, we focus on how the presence of carbon determines the magnetic ordering. A quasione-dimensional fcc (or hcp) Fe structure favors ferromagnetic ordering, but when encapsulated into a C tube, antiferromagnetic ordering can become favorable. The spin polarization at the Fermi level is large for the bare nanowires, but it decreases due to hybridization with the carbon coating. Implications of these results for the fabrication of nanodevices, as well as for the appearance of exchange bias, are discussed.

The magnetic proximity effect (MPE) has attracted the attention of theorists and experimentalists for at least three decades. Lately, the relevance of the effect for the development of nanodevices has revived interest on the subject. Here we review how the field has evolved, centering our attention on metal-metal and metal-insulator systems. We describe, and critically compare, the different theoretical approaches that have been put forward, as well as their limitations. An evaluation of the relationship between existing theories and available experimental results is also attempted.

DFT calculations performed on Si_2H_6, Si_2F_6, Si_2Cl_6, and Si_2Br_6 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. The chemical potential and hardness of Si_2H_6 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., Comment: LATeX file, 7 figures, uses elsart.cls, natbib, graphicx

A critical comparison is made between discrete and continuum treatments put forward to determine the magnetic ordering of exchange coupled superlattices. In particular, our interest is focused on the spatial patterns adopted by the coupling between ferromagnetic layers across the nonmagnetic spacers. We find that for values of the spacer electron Fermi wavevector kF≳π/a, where a is the lattice parameter, the continuum model breaks down. This gives rise to interesting interference effects, which emerge in the discrete three–dimensional treatment, but which are missed in a continuum pseudo one–dimensional approximation. The experimental evidence is discussed, and an analytic derivation of the critical kF value is also given.

Collision processes between two gold clusters are investigated using classical molecular dynamics in combination with embedded atom (EA) potentials, after checking the reliability of EA results by contrasting them with first principles calculations. The Au projectiles considered are both single atoms (N=1) and clusters of N=2, 12, 13 and 14 atoms. The targets contain N= 12, 13 and 14 gold atoms. The initial projectile energy E is in the range 0 < E < 1.5 eV/atom. The results of the collision processes are described and analyzed in detail., LATeX file, 8 figures, uses svjour.cls

We investigate the feasibility of using nuclear magnetic resonance (NMR) chemical shift calculations as a tool to provide structural information for C 20 fullerene type molecules. NMR chemical shifts are extremely sensitive to the local chemical environment of an atom, reflecting unambiguously its bond lengths and angles as well as its hybridization. Thus, they can distinguish between the different isomers that are candidates for the ground state of this molecule. We calculate the NMR shifts for several C 20 isomers and show that NMR constitutes a potential tool to discriminate and identify experimentally a particular C 20 molecular conformation, and also the level of theory which best describes the experimental structure.

We calculate from first principles the electronic ground and excited state, geometrical and vibrational properties of the staggered and eclipsed disilane (Si 2 H 6 ) conformations. We find that, due to the rotation of a silyl group around the Si-Si axis, significant changes of the physical properties are induced. Therefore, depending on the conformation, the reactivity of disilane may be strongly altered.

The inclusion of a Dzyaloshinskii-Moriya short range antisymmetric interaction induces spontaneous magnetization, at nonzero temperatures, in one and two dimensions. It is shown that quantum fluctuations are reduced by the Dzyaloshinskii-Moriya interaction, but short range correlations are increased, thereby allowing to obtain long range magnetic order in these low dimensional systems.

Two-dimensional magnetic garnets exhibit complex and fascinating magnetic domain structures, like stripes, labyrinths, cells and mixed states of stripes and cells. These patterns do change in a reversible way when the intensity of an externally applied magnetic field is varied. The main objective of this contribution is to present the results of a model that yields a rich pattern structure that closely resembles what is observed experimentally. Our model is a generalized two-dimensional Ising-like spin-one Hamiltonian with long-range interactions, which also incorporates anisotropy and Zeeman terms. The model is studied numerically, by means of Monte Carlo simulations. Changing the model parameters stripes, labyrinth and/or cellular domain structures are generated. For a variety of cases we display the patterns, determine the average size of the domains, the ordering transition temperature, specific heat, magnetic susceptibility and hysteresis cycle. Finally, we examine the reversibility of the pattern evolution under variations of the applied magnetic field. The results we obtain are in good qualitative agreement with experiment., 8 pages, 12 figures, submitted to Phys. Rev. B

Magnetism of artificial structures, multilayer arrays and granular systems has lately attracted wide attention. On the contrary, the concept of percolation, first introduced in 1957, has scarcely been related to the former, in spite of the fact that it has been applied to many problems in completely different fields. In physics, percolation has been useful to study both macroscopic and microscopic phenomena, such as the electrical conduction of a mixture and the localization and hopping of electrons, respectively. However, recent experimental studies of magnetic granular solids have clearly established a connection between percolation and magnetism. In fact, magnetic anomalies were observed in a percolating system where the coercivity Hc of Ni films exhibits a non-monotonic behavior, related to topological morphology changes, which coincides with the onset of percolation. In this contribution we propose a theoretical model that describes the behavior of the coercivity Hc, of granular Ni films, as a percolation phenomenon. We show that the non-monotonic dependence of Hc, as a function of the occupation probability p, is due to the interplay of percolation and magnetism. Moreover, we also explain the dependence of Hc as a function of external magnetic field orientation.