Structural transition and magnetic properties of clusters (original) (raw)

Structural motifs, mixing, and segregation effects in 38-atom binary clusters

The Journal of Chemical Physics, 2008

± 38-atom binary clusters composed of elements from groups 10 and 11 of the periodic table mixing a 2 nd -row with a 3 rd -row transition metal (TM) (i.e., clusters composed of four binary pairs: Pd-Pt, Ag-Au, Pd-Au, Ag-Pt) are studied through a combined empirical-potential (EP) / density-functional theory (DFT) method. À`s ystem comparison'' approach is adopted in order to analyze a wide diversity of structural motifs. The energy competition between the different structural motifs, as well as surface segregation effects of the different metals involved, is studied at the DFT level. The results substantially confirm the EP predictions about the stability of crystalline fcc type structures at this size, but decahedral and mixed 5-fold-symmetric/closed-packed structures are also found in close competition with the fcc motifs.

Magnetic properties of small Pt-capped Fe, Co, and Ni clusters: A density functional theory study

Physical Review B, 2010

Theoretical studies on M 13 (M = Fe, Co, Ni) and M 13 Pt n (for n = 3, 4, 5, 20) clusters including the spin-orbit coupling are done using density functional theory. The magnetic anisotropy energy (MAE) along with the spin and orbital moments are calculated for M 13 icosahedral clusters. The angle-dependent energy differences are modelled using an extended classical Heisenberg model with local anisotropies. From our studies, the MAE for Jahn-Teller distorted Fe 13 , Mackay distorted Fe 13 and nearly undistorted Co 13 clusters are found to be 322, 60 and 5 µeV/atom, respectively, and are large relative to the corresponding bulk values, (which are 1.4 and 1.3 µeV/atom for bcc Fe and fcc Co, respectively.) However, for Ni 13 (which practically does not show relaxation tendencies), the calculated value of MAE is found to be 0.64 µeV/atom, which is approximately four times smaller compared to the bulk fcc Ni (2.7 µeV/atom). In addition, MAE of the capped cluster (Fe 13 Pt 4 ) is enhanced compared to the uncapped Jahn-Teller distorted Fe 13 cluster.

Theoretical investigation of the structures of unsupported 38-atom CuPt clusters

European Physical Journal B, 2018

A genetic algorithm has been used to perform a global sampling of the potential energy surface in the search for the lowest-energy structures of unsupported 38-atom Cu-Pt clusters. Structural details of bimetallic Cu-Pt nanoparticles are analyzed as a function of their chemical composition and the parameters of the Gupta potential, which is used to mimic the interatomic interactions. The symmetrical weighting of all parameters used in this work strongly influences the chemical ordering patterns and, consequently, cluster morphologies. The most stable structures are those corresponding to potentials weighted toward Pt characteristics, leading to Cu-Pt mixing for a weighting factor of 0.7. This reproduces DFT results for Cu-Pt clusters of this size. For several weighting factor values, the Cu30Pt8 cluster exhibits slightly higher relative stability. The copper-rich Cu32Pt6 cluster was reoptimized at the DFT level to validate the reliability of the empirical approach, which predicts a Pt@Cu core-shell segregated cluster. A general increase of interatomic distances is observed in the DFT calculations, which is greater in the Pt core. After cluster relaxation, structural changes are identified through the pair distribution function. For the majority of weighting factors and compositions, the truncated octahedron geometry is energetically preferred at the Gupta potential level of theory.

On the metallic behavior of Co clusters

Solid State Communications, 1999

The role of structure in the nonmetal-metal transition of Co clusters is investigated by performing calculations for different symmetries: hexahedral, octahedral and decahedral. This transition occurs when the density of states at the Fermi level exceeds 1/k B T and the discrete energy levels begin to form a quasi-continuous band. The electronic structure is calculated including spd orbitals and spillover effects in a Hubbard Hamiltonian solved within the unrestricted Hartree-Fock approximation. We find that in small clusters N Յ 40 the metallic behavior is strongly related to the geometrical structure of the cluster. We compare our results with those coming out of a simple Friedel's model. ᭧

Model predictions and experimental characterization of Co-Pt alloy clusters

The European Physical Journal D, 2007

Model and real cobalt-platinum alloy clusters are compared in terms of structure, composition and segregation. Canonical and semi grand canonical Metropolis Monte Carlo simulations are performed to model these clusters, using embedded atom (EAM) and modified embedded atom (MEAM) potentials. All of them correctly predict the bulk L12 Co3Pt and CoPt3 structures as well as the L10 CoPt phase. However, the lattice parameters, phase stability and the L10-fcc order-disorder transition temperature are at variance. Segregation predictions with EAM and MEAM potentials are contradictory. Experimentally, mixed clusters with various compositions were deposited by Low Energy Cluster Beam on amorphous carbon at room temperature. Their size distribution, crystalline structure and composition were examined by Transmission Electron Microscopy (TEM). Clusters with the same size distributions were modelled. Both experiment and modelling show their crystallographic parameters to continuously correspond to the fcc CoPt chemically disordered phase. Diffraction measurements indicate surface segregation of the specie in excess, in agreement with EAM predictions for the Co-rich phase. The consequences on magnetic properties are discussed.

Electronic and geometric properties of M@Pt12 bimetallic clusters (M = Li,

2024

Using density functional theory, this study investigates the changes that occur to the electronic and geometric properties of alkali metals doped with Pt12 clusters. The research shows that doping causes enormous changes in the geometric structure of the clusters, which makes them stable. This is particularly noticeable for K@Pt12 clusters. This research investigates frontier molecular orbitals and finds that Li@Pt12 and Na@Pt12 clusters have small Eg of 0.13 eV and 0.19 eV, respectively, which could make them better for electronic and photovoltaic uses. Analysis of spin charge density indicates that in Li@Pt12 and Na@Pt12 clusters, the Pt12 cage contributes the most spin to the electronic structures, with little to no spin contribution from Li and Na atoms, while in K@Pt12, both the K+ ion and the Pt12 cage build up the electronic structure. Charge density analysis shows alkali metals transfer electrons to the Pt12 cage, making ionic bonds in Li@Pt12 and metallic bonds in Na@Pt12, and K@Pt12 clusters, respectively. However, density of states (DOS) shows how new electronic states are created upon doping, altering the cluster’s electronic properties. Furthermore, the calculation of global reactivity descriptors indicates that doping alters the chemical reactivity and kinetic stability. Clusters of K@Pt12 show enhanced hardness and stability. The results of this research provide novel insight into Pt12 clusters doped with Li, Na, and K. Furthermore, it expands the abilities of computational chemists to design materials that propel technological advancement.

The Behavior of Magnetic Properties in the Clusters of 4d Transition Metals

The current focus of material science researchers is on the magnetic behavior of transition metal clusters due to its great hope for future technological applications. It is common knowledge that the 4d transition elements are not magnetic at their bulk size. However, studies indicate that their magnetic properties are strongly dependent on their cluster sizes. This study attempts to identify magnetic properties of 4d transition metal clusters. Using a tight-binding Friedel model for the density of d-electron states, we investigated the critical size for the magnetic-nonmagnetic transition of 4d transition-metal clusters. Approaching to the critical point, the density of states of the cluster near the Fermi level is higher than 1/J and the discrete energy levels form a quasi-continuous band. Where J is correlation integral. In order to determine the critical size, we considered a square shape band and fcc, bcc, icosahedral and cuboctahedral close-packed structures of the clusters. We also investigated this size dependent magnetic behavior using Heisenberg model. Taking some quantum mechanical approximations in to consideration, we determined magnetic behavior of the clusters. For practicality, we considered three clusters of transition metals (Ru, Rh and Pd) and the obtained results are in line with the results of previous studies.

Structure and Magnetism of Neutral and Anionic Palladium Clusters

Physical Review Letters, 2001

The properties of neutral and anionic PdN clusters were investigated with spin-density-functional calculations. The ground state structures are three-dimensional for N >3 and they are magnetic with a spin-triplet for 2≤N ≤7 and a spin nonet for N =13 neutral clusters. Structural-and spin-isomers were determined and an anomalous increase of the magnetic moment with temperature is predicted for a Pd7 ensemble. Vertical electron detachment and ionization energies were calculated and the former agree well with measured values for Pd − N .

Magnetic and structural properties of isolated and assembled clusters

Surface science …, 2005

Within the last years, a fundamental understanding of nanoscaled materials has become a tremendous challenge for any technical applications. For magnetic nanoparticles, the research is stimulated by the effort to overcome the superparamagnetic limit in magnetic storage devices. The physical properties of small particles and clusters in the gas phase, which are considered as possible building blocks for magnetic storage devices, are usually size-dependent and clearly differ from both the atom and bulk material. For any technical applications, however, the clusters must be deposited on surfaces or embedded in matrices. The contact to the environment again changes their properties significantly. Here, we will mainly focus on the fundamental electronic and magnetic properties of metal clusters deposited on surfaces and in matrices. This, of course, requires a well-defined control on the production of nanoparticles including knowledge about their structural behaviour on surfaces that is directly related to their magnetic properties. We describe two different approaches to produce magnetic nanoparticles: (i) cluster aggregation on reconstructed single crystal surfaces and (ii) deposition of mass-filtered clusters from the gas phase onto surfaces and into matrices. The process of cluster deposition offers the possibility of creating new materials in non-equilibrium conditions with tailored properties. A theoretical description of the evolution of cluster magnetism is given with respect to contributions from magnetic anisotropy effects and the local atomic environment. Especially small clusters show size-dependent magnetic orbital and spin moments that can experimentally be accessed by the element-specific technique of X-ray magnetic circular dichroism (XMCD). The magnetic properties of self-organized iron and cobalt clusters on Au(1?1?1) surfaces are discussed with respect to growth conditions. For deposited Fe clusters on surfaces increased orbital moments have been found even for large particles. Additionally, the influence of capping layers and deposition into matrices is discussed. While investigations on relative simple structures in pure 3d metal particles yield insight into the basic mechanisms of magnetism, alloy nanoparticles seem to be more promising in terms of technical application since they offer the possibility to adjust the magnetic properties by varying the stoichiometry. Alloys consisting of 3d metals (e.g. FexCo1-x alloys) have usually very high magnetic moments and are soft-magnetic. Binary clusters consisting of a 3d metal (e.g. Co) in combination with a heavy element (Sm, Ag or Pt) are candidates for materials with high magnetic anisotropies and increased blocking temperatures. The magnetic properties are directly related to their structural order. Here, we show first results for such alloy nanoparticles.

Magnetic anisotropies of late transition metal atomic clusters

Physical review letters, 2007

We analyze the impact of the magnetic anisotropy on the geometric structure and magnetic ordering of small atomic clusters of palladium, iridium, platinum and gold, using Density Functional Theory. Our results highlight the absolute need to include self-consistently the spin orbit interaction in any simulation of the magnetic properties of small atomic clusters, and a complete lack of universality in the magnetic anisotropy of small-sized atomic clusters. PACS numbers: 36.40.Cg, 71,70.Ej, 75.30.Gw Nanostructures of all kinds display a wealth of fascinating geometric, mechanical, electronic, magnetic or optical properties. The exploding field of Nanoscience pretends to understand, handle and tailor these properties for human benefit. Atomic clusters and chains, molecular magnets, and a number of organic molecules like for instance metallocenes indeed show novel magnetic behaviors, that include the enhancement of magnetic moments due to the reduced coordination and symmetry of the geometry[1], and a rich variety of new non-collinear magnetic structures that are absent in bulk materials . Among all these devices, metallic atomic clusters (MACs)[3] stand out since, on the one hand they represent the natural bridge between atomic and materials physics and, in the other, they can be grown, deposited on surfaces or embedded in diverse matrices, and characterized with relatively well-established techniques. Further, the magnetic properties of MACs show promise for a wide spectrum of applications, ranging from medicine to spintronics.