Spin and orbital magnetic moments of free nanoparticles

Direct probing of low-energy intra d-band transitions in gas-phase cobalt clusters

The interplay between constituent localized and itinerant electrons of metal clusters defines their physical and chemical properties. In turn, the electronic and geometrical structures are strongly entwined and exhibit strong size-dependent variations. Current understanding of low-energy excited states of metal clusters relies on stand-alone theoretical investigations and few comparisons with measured properties, since direct identification of low-lying states is lacking hitherto. Here, we report on the measurement of low-lying electronic transitions in cationic cobalt clusters using infrared photofragmentation spectroscopy. Broad and size-dependent absorption features were observed within 0.056 - 0.446 eV, well above the energies of the sharp absorption bands caused by cluster vibrations. Complementary time-dependent density functional theory calculations reproduce the main observed absorption features, providing direct evidence that they correspond to transitions between electronic states of mainly d-character, arising from the open d-shells of the Co atoms and the high spin multiplicity of the clusters.

Dynamics of the Oxygen Atom Transfer Reaction between Carbon Dioxide and the Tantalum Cation

The understanding of fundamental atomic-level processes often requires well-defined model systems. The oxygen atom transfer from CO₂ to a transition metal cation in the gas phase presents such a model system. We investigate the reaction of Ta⁺ + CO₂ for which the formation of TaO⁺ is highly efficient and attributed to multistate reactivity. Here, we study the atomistic dynamics of the oxygen atom transfer reaction by recording experimental energy and angle differential cross sections by crossed beam velocity map imaging supported by ab initio quantum chemical calculations. Product ion velocity distributions are dominated by signatures for indirect dynamics, despite the reaction being highly exothermic. Product kinetic energy distributions show little dependence on additional collision energy even with only four atoms involved, which hints at dynamical trapping behind a submerged barrier.

Probing the binding and activation of small molecules by gas-phase transition metal clusters via IR spectroscopy

Isolated transition metal clusters have been established as useful models for extended metal surfaces or deposited metal particles, to improve the understanding of their surface chemistry and of catalytic reactions. For this objective, an important milestone has been the development of experimental methods for the size-specific structural characterization of clusters and cluster complexes in the gas phase. This review focusses on the characterization of molecular ligands, their binding and activation by small transition metal clusters, using cluster-size specific infrared action spectroscopy. A comprehensive overview and a critical discussion of the experimental data available to date is provided, reaching from the initial results obtained using line-tuneable CO₂ lasers to present-day studies applying infrared free electron lasers as well as other intense and broadly tuneable IR laser sources.

The spin magnetic order of Con+ (n ≤ 5) clusters

The magnetism of transition metal clusters has been for decades a complicated puzzle, with experimental results disagreeing with calculations performed within the density functional theory formalism. In this work, we provide a key to this puzzle by investigating the lowest-energy spin states of cobalt cluster, Co_n⁺ (n ≤ 5), using CASSCF/NEVPT2 calculations with very large active spaces. The geometries as well as the spin configurations adopted by the clusters in their ground-state are known from experiments, making Co_n⁺ clusters an ideal model system for theoretical investigation. Here, using the experimentally known geometries determined by far-infrared spectroscopy as inputs, we calculated the lowest-energy spin configurations of the clusters, revealing that the CASSCF/NEVPT2 formalism correctly predicts the preferred electronic configuration of the clusters known experimentally. This is in contrast to the widely used density functional theory, with results that depend on the selected exchange-correlation functional. The reasons for the failure of density functional theory, in opposition to CASSCF/NEVPT2, are discussed, providing a solid framework for investigating other transition metal and transition metal oxide clusters.

Kinetics of stepwise nitrogen adsorption by size-selected iron cluster cations: Evidence for size-dependent nitrogen phobia

We present a study of stepwise cryogenic N₂ adsorption on size-selected Fe_n ⁺ (n = 8-20) clusters within a hexapole collision cell held at T = 21-28 K. The stoichiometries of the observed adsorption limits and the kinetic fits of stepwise N₂ uptake reveal cluster size-dependent variations that characterize four structural regions. Exploratory density functional theory studies support tentative structural assignment in terms of icosahedral, hexagonal antiprismatic, and closely packed structural motifs. There are three particularly noteworthy cases, Fe₁₃ ⁺ with a peculiar metastable adsorption limit, Fe₁₇ ⁺ with unprecedented nitrogen phobia (inefficient N₂ adsorption), and Fe₁₈ ⁺ with an isomeric mixture that undergoes relaxation upon considerable N₂ uptake.

Mn12 -Acetate Complexes Studied as Single Molecules

The phenomenon of single molecule magnet (SMM) behavior of mixed valent Mn₁₂ coordination clusters of general formula [Mn^III₈Mn^IV₄O₁₂(RCOO)₁₆(H₂O)₄] had been exemplified by bulk samples of the archetypal [Mn^III₈Mn^IV₄O₁₂(CH₃COO)₁₆(H₂O)₄] (4) molecule, and the molecular origin of the observed magnetic behavior has found support from extensive studies on the Mn₁₂ system within crystalline material or on molecules attached to a variety of surfaces. Here we report the magnetic signature of the isolated cationic species [Mn₁₂O₁₂(CH₃COO)₁₅(CH₃CN)]⁺(1) by gas phase X‐ray Magnetic Circular Dichroism (XMCD) spectroscopy, and we find it closely resembling that of the corresponding bulk samples. Furthermore, we report broken symmetry DFT calculations of spin densities and single ion tensors of the isolated, optimized complexes [Mn₁₂O₁₂(CH₃COO)₁₅(CH₃CN)]⁺(1), [Mn₁₂O₁₂(CH₃COO)₁₆] (2), [Mn₁₂O₁₂(CH₃COO)₁₆(H₂O)₄] (3), and the complex in bulk geometry [Mn^III₈Mn^IV₄O₁₂(CH₃COO)₁₆(H₂O)₄] (5). The found magnetic fingerprints – experiment and theory alike – are of a remarkable robustness: The Mn^IV₄ core bears almost no magnetic anisotropy while the surrounding Mn^III₈ ring is highly anisotropic. These signatures are truly intrinsic properties of the Mn₁₂ core scaffold within all of these complexes and largely void of the environment. This likely holds irrespective of bulk packing effects.

Advancing Inorganic Coordination Chemistry by Spectroscopy of Isolated Molecules: Methods and Applications

A unique feature of the work carried out in the Collaborative Research Center 3MET continues to be its emphasis on innovative, advanced experimental methods which hyphenate mass-selection with further analytical tools such as laser spectroscopy for the study of isolated molecular ions. This allows to probe the intrinsic properties of the species of interest free of perturbing solvent or matrix effects. This review explains these methods and uses examples from past and ongoing 3MET studies of specific classes of multicenter metal complexes to illustrate how coordination chemistry can be advanced by applying them. As a corollary, we will show how the challenges involved in providing well-defined, for example monoisomeric, samples of the molecular ions have helped to further improve the methods themselves thus also making them applicable to many other areas of chemistry.

Identification of Active Sites and Structural Characterization of Reactive Ionic Intermediates by Cryogenic Ion Trap Vibrational Spectroscopy

Cryogenic ion trap vibrational spectroscopy paired with quantum chemistry currently represents the most generally applicable approach for the structural investigation of gaseous cluster ions that are not amenable to direct absorption spectroscopy. Here, we give an overview of the most popular variants of infrared action spectroscopy and describe the advantages of using cryogenic ion traps in combination with messenger tagging and vibrational predissociation spectroscopy. We then highlight a few recent studies that apply this technique to identify highly reactive ionic intermediates and to characterize their reactive sites. We conclude by commenting on future challenges and potential developments in the field.

Reply to the 'Comment on "Magnetostructural correlations in isolated trinuclear iron(iii) oxo acetate complexes"' by M. Antkowiak, G. Kamieniarz and W. Florek, Phys. Chem. Chem. Phys., 2018, 20, DOI: 10.1039/C8CP04691C

This reply acknowledges comments by Antkowiak et al. and it announces the outcome of new experiments which are in support of the initial findings by Lang et al.

Structural determination of neutral Co n clusters (n = 4-10,13) through IR-UV two-color vibrational spectroscopy and DFT calculations

We recorded IR spectra for neutral cobalt clusters via two-color IR-UV ionization, using the Free Electron Laser for intracavity experiments (FELICE). Well-resolved IR spectra are presented for [Formula: see text] (n  =  4-10, 13) and analyzed with the help of Density Functional Theory calculations using two different correlation exchange functionals: the revisited Tao-Perdew-Staroverov-Scuseria (revTPSS) and the frequently used Perdew-Burke-Ernzerhof (PBE) approaches. Although we have not performed an extensive structure search, we tentatively assign the spectra for all cluster sizes except for n  =  7, and n  =  10. We find that neither of the two functionals chosen clearly outperforms the other in predicting IR spectra, and that relatively low scaling factors of 0.82 (PBE) and 0.8 (revTPSS) are required. In contrast to the magnetic moments, the calculated electric dipole moments fluctuate strongly as a function of cluster size and could therefore be used as an indirect probe to the cluster structure.

Structures and magnetic properties of small [Formula: see text] and Co n-1Cr+ (n = 3-5) clusters

Small cobalt clusters [Formula: see text] and their single chromium atom doped counterparts Co _n-1Cr⁺ (n  =  3-5) were studied mass spectrometrically by measuring the infrared multiple photon dissociation (IRMPD) spectra of the corresponding argon tagged complexes. The geometric and electronic structures of the [Formula: see text] and Co _n-1Cr⁺ (n  =  3-5) clusters as well as their Ar complexes were optimized by density functional theory (DFT) calculations. The obtained lowest energy structures were confirmed by comparing the IRMPD spectra of [Formula: see text] and [Formula: see text] (n  =  3-5, m  =  3 and 4) with the corresponding calculated IR spectra. The calculations reveal that the doped Co _n-1Cr⁺ clusters retain the geometric structures of the most stable [Formula: see text] clusters. However, the coupling of the local magnetic moments within the clusters is altered in a size-dependent way: the Cr atom is ferromagnetically coupled in Co₂Cr⁺ and Co₃Cr⁺, while it is antiferromagnetically coupled in Co₄Cr⁺.

Large orbital magnetic moments of small, free cobalt cluster ions Co[Formula: see text] with n [Formula: see text]

The size dependent electronic structure and separate spin and orbital magnetic moments of free Co[Formula: see text] ([Formula: see text]) cluster ions have been investigated by x-ray absorption and x-ray magnetic circular dichroism spectroscopy in a cryogenic ion trap. A very large orbital magnetic moment of [Formula: see text] per atom was determined for Co[Formula: see text], which is one order of magnitude larger than in the bulk metal. Large orbital magnetic moments per atom of  ≈1 [Formula: see text] were also found for Co[Formula: see text], Co[Formula: see text], and Co[Formula: see text]. The orbital contribution to the total magnetic moment shows a non-monotonic cluster size dependence: The orbital contribution increases from a local minimum at n  =  2 to a local maximum at n  =  5 and then decreases with increasing cluster size. The 3d spin magnetic moment per atom is nearly constant and is solely defined by the number of 3d holes which shows that the 3d majority spin states are fully occupied, that is, 3d hole spin polarization is 100%.

Long-Term Monitoring of the Internal Energy Distribution of Isolated Cluster Systems

A method is presented to monitor the internal energy distribution of cluster anions via delayed electron detachment by pulsed photoexcitation and demonstrated on Co_{4}^{-} in an electrostatic ion beam trap. In a cryogenic operation, we calibrate the detachment delay to internal energy. By laser frequency scans, at room temperature, we reconstruct the time-dependent internal energy distribution of the clusters. The mean energies of ensembles from a cold and a hot ion source both approach thermal equilibrium. Our data yield a radiative emission law and the absorptivity of the cluster for thermal radiation.

First-principle study of structural, electronic and magnetic properties of (FeC)n (n = 1-8) and (FeC)8TM (TM = V, Cr, Mn and Co) clusters

The structural, electronic and magnetic properties of the (FeC)_n (n = 1–8) clusters are studied using the unbiased CALYPSO structure search method and density functional theory. A combination of the PBE functional and 6–311 + G* basis set is used for determining global minima on potential energy surfaces of (FeC)_n clusters. Relatively stabilities are analyzed via computing their binding energies, second order difference and HOMO-LUMO gaps. In addition, the origin of magnetic properties, spin density and density of states are discussed in detail, respectively. At last, based on the same computational method, the structures, magnetic properties and density of states are systemically investigated for the 3d (V, Cr, Mn and Co) atom doped (FeC)₈ cluster.

Enhanced Magnetization of Cobalt Defect Clusters Embedded in TiO2-δ Films

High magnetizations are desirable for spintronic devices that operate by manipulating electronic states using built-in magnetic fields. However, the magnetic moment in promising dilute magnetic oxide nanocomposites is very low, typically corresponding to only fractions of a Bohr magneton for each dopant atom. In this study, we report a large magnetization formed by ion implantation of Co into amorphous TiO_2-δ films, producing an inhomogeneous magnetic moment, with certain regions producing over 2.5 μ_B per Co, depending on the local dopant concentration. Polarized neutron reflectometry was used to depth-profile the magnetization in the Co:TiO_2-δ nanocomposites, thus confirming the pivotal role of the cobalt dopant profile inside the titania layer. X-ray photoemission spectra demonstrate the dominant electronic state of the implanted species is Co⁰, with a minor fraction of Co²⁺. The detected magnetizations have seldom been reported before and lie near the upper limit set by Hund's rules for Co⁰, which is unusual because the transition metal's magnetic moment is usually reduced in a symmetric 3D crystal-field environment. Low-energy positron annihilation lifetime spectroscopy indicates that defect structures within the titania layer are strongly modified by the implanted Co. We propose that a clustering motif is promoted by the affinity of the positively charged implanted species to occupy microvoids native to the amorphous host. This provides a seed for subsequent doping and nucleation of nanoclusters within an unusual local environment.

Puzzle of magnetic moments of Ni clusters revisited using quantum Monte Carlo method

The puzzle of the magnetic moments of small nickel clusters arises from the discrepancy between values predicted using density functional theory (DFT) and experimental measurements. Traditional DFT approaches underestimate the magnetic moments of nickel clusters. Two fundamental problems are associated with this puzzle, namely, calculating the exchange-correlation interaction accurately and determining the global minimum structures of the clusters. Theoretically, the two problems can be solved using quantum Monte Carlo (QMC) calculations and the ab initio random structure searching (AIRSS) method correspondingly. Therefore, we combined the fixed-moment AIRSS and QMC methods to investigate the magnetic properties of Ni_n (n = 5-9) clusters. The spin moments of the diffusion Monte Carlo (DMC) ground states are higher than those of the Perdew-Burke-Ernzerhof ground states and, in the case of Ni_8-9, two new ground-state structures have been discovered using the DMC calculations. The predicted results are closer to the experimental findings, unlike the results predicted in previous standard DFT studies.

Size-Dependent Ligand Quenching of Ferromagnetism in Co3(benzene)n+ Clusters Studied with X-ray Magnetic Circular Dichroism Spectroscopy

Cobalt-benzene cluster ions of the form Co₃(bz)_n⁺ (n = 0-3) were produced in the gas phase, mass-selected, and cooled in a cryogenic ion trap held at 3-4 K. To explore ligand effects on cluster magnetic moments, these species were investigated with X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) spectroscopy. XMCD spectra yield both the spin and orbital angular momenta of these clusters. Co₃⁺ has a spin magnetic moment of μ_S = 6 μ_B and an orbital magnetic moment of μ_L = 3 μ_B. Co₃(bz)⁺ and Co₃(bz)₂⁺ complexes were found to have spin and orbital magnetic moments identical to the values for ligand-free Co₃⁺. However, coordination of the third benzene to form Co₃(bz)₃⁺ completely quenches the high spin state of the system. Density functional theory calculations elucidate the spin states of the Co₃(bz)_n⁺ species as a function of the number of attached benzene ligands, explaining the transition from septet to singlet for n = 0 → 3.

Multiple-decker and ring sandwich formation of manganese-benzene organometallic cluster anions: MnnBzn- (n = 1-5 and 18)

Organometallic multiple-decker sandwich clusters are topics of great interest due to their unique electronic and magnetic properties originating from anisotropic structures. We report a joint anion photoelectron spectroscopic and computational study on a new family of manganese (Mn)-benzene (Bz) anionic clusters Mn_nBz_n^-. In stark contrast to the most widely studied vanadium-Bz sandwich clusters, it is found that Mn_nBz_n^- (n = 1-5) clusters exhibit unprecedented multiple-decker structures with a tilted Mn-Bz stacking and a monotonically increasing behavior of their high spin multiplicities. Furthermore, a couple of closed ring forms of Mn₁₈Bz₁₈^- and its neutral state are computationally anticipated as an intriguing "cluster of Mn₁Bz₁ clusters" in which the neutral Mn₁₈Bz₁₈ has extremely high C_18h symmetry with an uncommon spin state of 2S + 1 = 55. The extensively delocalized electron environment of Mn₁₈Bz₁₈ allows the simple Hückel model to reveal the strong intra-atomic exchange interactions within the Mn 3d electrons.

Large Orbital Magnetic Moment Measured in the [TpFe(III)(CN)3](-) Precursor of Photomagnetic Molecular Prussian Blue Analogues

Photomagnetism in three-dimensional Co/Fe Prussian blue analogues is a complex phenomenon, whose detailed mechanism is not yet fully understood. Recently, researchers have been able to prepare molecular fragments of these networks using a building block synthetic approach from mononuclear precursors. The main objective in this strategy is to isolate the smallest units that show an intramolecular electron transfer to have a better understanding of the electronic processes. A prior requirement to the development of this kind of system is to understand to what extent electronic and magnetic properties are inherited from the corresponding precursors. In this work, we investigate the electronic and magnetic properties of the FeTp precursor (N(C4H9)4)[TpFe(III)(CN)3], (Tp being tris-pyrazolyl borate) of a recently reported binuclear cyanido-bridged Fe/Co complex. X-ray absorption spectroscopy and X-ray magnetic circular dichroism measurements at the Fe L2,3 edges (2p → 3d) supported by ligand field multiplet calculations have allowed to determine the spin and orbit magnetic moments. Inaccuracy of the spin sum rule in the case of low-spin Fe(III) ion was demonstrated. An exceptionally large value of the orbital magnetic moment is found (0.9 μB at T = 2 K and B = 6.5 T) that is likely to play an important role in the magnetic and photomagnetic properties of molecular Fe/Co Prussian blue analogues.

The spin and orbital contributions to the total magnetic moments of free Fe, Co, and Ni clusters

We present size dependent spin and orbital magnetic moments of cobalt (Con (+), 8 ≤ n ≤ 22), iron (Fen (+), 7 ≤ n ≤ 17), and nickel cluster (Nin (+), 7 ≤ n ≤ 17) cations as obtained by X-ray magnetic circular dichroism (XMCD) spectroscopy of isolated clusters in the gas phase. The spin and orbital magnetic moments range between the corresponding atomic and bulk values in all three cases. We compare our findings to previous XMCD data, Stern-Gerlach data, and computational results. We discuss the application of scaling laws to the size dependent evolution of the spin and orbital magnetic moments per atom in the clusters. We find a spin scaling law "per cluster diameter," ∼n(-1/3), that interpolates between known atomic and bulk values. In remarkable contrast, the orbital moments do likewise only if the atomic asymptote is exempt. A concept of "primary" and "secondary" (induced) orbital moments is invoked for interpretation.

Orbit and spin resolved magnetic properties of size selected [ConRh]⁺ and [ConAu]⁺ nanoalloy clusters

Bi-metallic nanoalloys of mixed 3d-4d or 3d-5d elements are promising candidates for technological applications. The large magnetic moment of the 3d materials in combination with a high spin-orbit coupling of the 4d or 5d materials give rise to a material with a large magnetic moment and a strong magnetic anisotropy, making them ideally suitable in for example magnetic storage devices. Especially for clusters, which already have a higher magnetic moment compared to the bulk, these alloys can profit from the cooperative role of alloying and size reduction in order to obtain magnetically stable materials with a large magnetic moment. Here, the influence of doping of small cobalt clusters on the spin and orbital magnetic moment has been studied for the cations [Co(8-14)Au](+) and [Co(10-14)Rh](+). Compared to the undoped pure cobalt [Co(N)](+) clusters we find a significant increase in the spin moment for specific Co(N-1)Au(+) clusters and a very strong increase in the orbital moment for some Co(N-1)Rh(+) clusters, with more than doubling for Co12Rh(+). This result shows that substitutional doping of a 3d metal with even just one atom of a 4d or 5d metal can lead to dramatic changes in both spin and orbital moment, opening up the route to novel applications.

Nickel clusters embedded in carbon nanotubes as high performance magnets

Ensembles of fcc nickel nanowires have been synthesized with defined mean sizes in the interior of single-wall carbon nanotubes. The method allows the intrinsic nature of single-domain magnets to emerge with large coercivity as their size becomes as small as the exchange length of nickel. By means of X-ray magnetic circular dichroism we probe electronic interactions at nickel-carbon interfaces where nickel exhibit no hysteresis and size-dependent spin magnetic moment. A manifestation of the interacting two subsystems on a bulk scale is traced in the nanotube’s magnetoresistance as explained within the framework of weak localization.

Size effects in the magnetic anisotropy of embedded cobalt nanoparticles: from shape to surface

Strong size-dependent variations of the magnetic anisotropy of embedded cobalt clusters are evidenced quantitatively by combining magnetic experiments and advanced data treatment. The obtained values are discussed in the frame of two theoretical models that demonstrate the decisive role of the shape in larger nanoparticles and the predominant role of the surface anisotropy in clusters below 3 nm diameter.

Infrared spectroscopy of N2 adsorption on size selected cobalt cluster cations in isolation

We report IR active N2 stretching frequencies in isolated and size selected cobalt cluster nitrogen adsorbate complexes, [Con(N2)1](+) as recorded by virtue of InfraRed Photon Dissociation (IRPD) spectroscopy. The observed frequencies of the [Con(N2)1](+) complexes (n = 8-17) are significantly redshifted (2180 to 2290 cm(-1)) with respect to the IR inactive vibrations of free N2 (2359 cm(-1)). These bands are assigned to a μ1 head-on type of coordination of the N2 to the cobalt cluster surface, revealing remarkable cluster size dependent features to interpret.

Magnetic moments of chromium-doped gold clusters: the Anderson impurity model in finite systems

The magnetic moment of a single impurity atom in a finite free electron gas is studied in a combined x-ray magnetic circular dichroism spectroscopy, charge transfer multiplet calculation, and density functional theory study of size-selected free chromium-doped gold clusters. The observed size dependence of the local magnetic moment can be understood as a transition from a local moment to a mixed valence regime. This shows that the Anderson impurity model essentially describes finite systems even though the discrete density of states introduces a significant deviation from a bulk metal, and the free electron gas is only formed by less than 10 electrons. Electronic shell closure in the gold host minimizes the interaction of localized impurity states with the confined free electron gas and preserves the magnetic moment of 5 μ_{B} fully in CrAu_{2}^{+} and almost fully in CrAu_{6}^{+}. Even for open-shell species, large local moments are observed that scale with the energy gap of the gold cluster. This indicates that an energy gap in the free electron gas stabilizes the local magnetic moment of the impurity atom.

Emergence of antiferromagnetic ordering in Tbn (n = 2-33) clusters

Using the density functional theory (DFT) under both frameworks of the local density approximation (LDA) and the generalized gradient approximation (GGA), we show that the anomalous magnetic moment of Tbn (n = 2-20, 22, 33) clusters found in recent experiment [J. Appl. Phys. 2010, 107, 09B526] is due to the antiferromagnetic or ferromagnetic couplings between Tb atoms in clusters. The local spin magnetic moment of Tb atoms is in the range 5.1-5.7 μ(B), which is faintly influenced by the cluster sizes, geometrical structures, and spin arrangements. Emphasis is made on the Tb13 cluster to eliminate the large magnetic discrepancy between the experiment and calculation. Geometrical evolution shows that the square-antiprism motif with one centered atom is competitive with the icosahedral motif for small Tb clusters n = 9-12, whereas the close packed icosahedral growth is clearly favored for large clusters n = 13-20, 22, 33. The clusters containing 4, 7, 10, 13, and 19 atoms are found to be more stable than their neighboring sizes, in agreement with the early mass spectral measurements. The electronic properties including the HOMO-LUMO energy gaps, ionization potentials, electron affinities, and electric dipole moments are calculated, and more importantly, the variational trends of the magnetic moment and electric dipole moment qualitatively fit well with the experimental observations.

X-ray induced fragmentation of size-selected salt cluster-ions stored in an ion trap

A method combining X-ray synchrotron radiation and mass spectrometry is presented. Ammonium bisulphate cluster ions in an ion trap exhibit well-defined core-level absorption edges in the fragment-ion abundance spectra.

Antiferromagnetic-like coupling in the cationic iron cluster of thirteen atoms

We explore, within the density functional theory in the generalized gradient approximation to exchange and correlation, the map of spin isomers of the cationic Fe13(+) cluster in connection with recent X-ray magnetic circular dichroism spectroscopy experiments [M. Niemeyer et al., Phys. Rev. Lett. 2012, 108, 057201] which showed an anomalous low magnetic moment per number of 3d holes in this cluster. We systematically explore the low-lying magnetic excitations and correlate them with structural rearrangements and stability indicators. We obtain the observed low magnetic moment per 3d hole as the ground state of Fe13(+) and we demonstrate that, as supposed by the experimentalists, the cluster undergoes a magnetic transition from a ferromagnetic-like configuration to an antiferromagnetic-like one upon ionization. We unravel this unexpected magnetic behavior showing that it is concomitant with a Th-deformation of the icosahedral structure together with the electronic filling of this particular iron cluster. The spin-orbit interaction preserves this magnetic configuration which is essentially due to the spin. Our computed magnetic anisotropy energy supports the experimental interpretation of the cluster as fluxional due to the very weak coupling of the magnetic moment to an easy axis.

Size-dependent Auger spectra and two-hole Coulomb interaction of small supported Cu-clusters

Auger (L3M4,5M4,5) and X-ray photoionization spectra (2p, 3d) of mass-selected CuN-clusters supported by a thin natural silica layer are presented in the size range N = 8-55 atoms per cluster. The Auger spectra of all clusters are shifted to a lower kinetic energy with respect to the spectrum of the bulk. Furthermore the Auger energy decreases systematically with decreasing cluster size. The binding energies of the 2p and 3d valence states are higher than the corresponding bulk values. Using the energy of the Auger main line, the corresponding core hole peak and the centroid of the self-convoluted 3d valence band the on-site Coulomb interaction energy Udd of the two-hole final state as a function of cluster size has been determined.

Magnetic properties of small cobalt-copper clusters

Accurate first-principle calculations on bimetallic cobalt-copper clusters of up to six atoms (Pérez et al 2012 J. Nanopart. Res. 14 933) revealed a close similarity of the ground-state magnetic properties to the ultimate jellium model, provided that a 2D to 3D geometric transition was invoked. We discuss this relationship in terms of partial occupancies of the valence electrons in both cases, with the jellium results described by nonperturbative spherical wavefunctions. Based upon this, we propose a scheme to predict magnetic properties of cobalt-copper clusters of up to twenty atoms using arguments of dimensionality and charge localization, and confirm some of these results with other independent density-functional calculations and experimental available data. The comparison with experiments is carried out for neutral and singly ionized cobalt clusters. Furthermore, a many-body tight-binding pseudopotential is used with Monte Carlo techniques to verify the stability of these new first-principle solutions.