Angle resolved-Auger-photoelectron coincidence spectroscopy (AR-APECS) has been exploited to investigate the role that electron correlation plays in the exchange-coupling at the ferromagnetic/antiferromagnetic interface of a Fe/CoO bilayer growth on Ag(001). The effective correlation energy U$_{\textrm{eff}}$, usually employed to assess the energy distribution of core-valence-valence Auger spectra, has been experimentally determined for each possible combination of the orbital (e$_g$ or t$_{2g}$) and the spin (majority or minority) of the two valence electrons involved in the Auger decay. Coulomb and exchange interactions have been identified and compared with the result obtained on the Fe/Ag system. The presented analysis reveals in the Fe/CoO interface an enhancement of the Coulomb interaction for the e$_g$ orbital and of the exchange interaction for the t$_{2g}$ orbital with respect to the Fe/Ag case that can be associated to the stronger electron confinement and to the exchange coupling between the two layers, respectively.

A set of electron-correlation energies as large as 10 eV have been measured for a magnetic 2 ML Fe film deposited on Ag(001). By exploiting the spin selectivity in angle-resolved Auger-photoelectron coincidence spectroscopy and the Cini-Sawatzky theory, the core-valence-valence Auger spectrum of a spin-polarized system have been resolved: correlation energies have been determined for each individual combination of the two holes created in the four subbands involved in the decay: majority and minority spin, as well as e_{g} and t_{2g}. The energy difference between final states with parallel and antiparallel spin of the two emitted electrons is ascribed to the spin-flip energy for the final ion state, thus disentangling the contributions of Coulomb and exchange interactions.

Even though DMFT and DFT-LDA calculations allow to properly predict the itinerant-like electronic properties of transition metals and their oxides, as well as magnetic moments in ferromagnetic (FM) and antiferromagnetic (AFM) systems, the interplay between band structure, magnetism and many-body correlations is still intriguing and debated [1]. In particular, the relevance of the on-site Coulomb interaction in the d orbitals (Hubbard U) as compared to the nonlocal exchange interaction (J) is far from being clarified in depth even at the experimental level [2]. Furthermore, possible size effects are usually not accounted for by current theories, thus overlooking the role played by the electronelectron interaction in the early stages of growth or synthesis of magnetic thin films and nanostructures. Core-valence-valence (CVV) Auger decays can potentially provide information about an effective Ueff through the distribution of two-hole local density of states 2LDOS, so have the unique capability to probe intimately the electronelectron interaction in the valence band. Unfortunately, for most of the relevant systems, Auger spectra are featureless, thus leading to the astonishing conclusion that the behavior of some of the major magnetic materials, like Fe, is bandlike due to a Ueff small as compared to the band-width. All of this in sharp disagreement with the magnetic properties of the samples. In trying to resolve this puzzling contradiction, we resorted on Angle Resolved Auger-PhotoElectron Coincidence Spectroscopy (AR-APECS) which, by accessing to the spin coupling of the two valence hole final state, is capable to unravel the otherwise blurred multiplet structure in Auger spectra, and hence yields a direct measure of Coulomb and exchange interactions for each individual multiplet component [3]. We report on Auger spectra measured by AR-APECS with unprecedented accuracy on FM (Fe, Co and Ni), AFM (CoO and NiO) materials and FM/AFM (Fe/CoO) and FM/diamagnetic (Fe/Ag) interfaces. This novel investigation provides a completely new insight in the role of the electronic correlation in magnetic systems. The major results are summarized in the following:

Spin selectivity in angle-resolved Auger photoelectron coincidence spectroscopy (AR-APECS) is used to probe electronic structure in antiferromagnetic thin films. In particular, exploiting the AR-APECS capability to discriminate Auger electron emission events characterized by a different spin of the ion in its final state, a sharp multiplet structure in the Ni MVV Auger line shape of NiO/Ag(001) thin films is measured below the critical Neel temperature. The assignment of multiplet terms follows from a close comparison of the experimental AR-APECS line shapes with the predictions based on semiempirical calculations on a cluster model and an open-band extension of the Cini-Sawatzky approach. In analogy to CoO, also in NiO, above the Neel temperature a more featureless Auger spectrum appears and AR-APECS does not disentangle anymore high-spin and low-spin contributions to the total Auger intensity. Such a behavior, which seems to be a general result for metal oxide antiferromagnetic systems, is discussed.

S (in order of presentation) W01 Inorganic molecules for molecular nanoscience Colette Boskovic School of Chemistry, University of Melbourne, Australia Ongoing work in our group is directed towards the development of two very different classes of functional metal-based molecules for incorporation into new molecular nanomaterials. Stimulated intramolecular electron transfer in metal complexes with redox-active metals and ligands can provide a mechanism for switching between distinct valence tautomeric forms. We have recently synthesised a family of dinuclear Co complexes bridged by redox-active bis(dioxolene) ligands that show promise as molecular systems that can be interconverted between three different states, potentially paving the way to logic gates more complex than simple "on/off" switches on the molecular scale [1]. Single-molecule magnet (SMM) and luminescence properties of lanthanoid complexes with organic ligands are well-established. Polyoxometalates (POMs) approximate fragments of metal oxide and represent an important alternative as inorganic ligands for lanthanoid complexes with novel properties. We have developed new families of POMsupported lanthanoid complexes that are luminescent, photochromic or function as SMMs, which is promising for the development of bifunctional SMMs [2]. [1] K.G. Alley, G. Poneti, J. B. Aitken, R.K. Hocking, B. Moubaraki, K.S. Murray, B.F. Abrahams, H.H. Harris, L. Sorace, C. Boskovic, Inorg. Chem. 51 (2012) 3944. [2] (a) C. Ritchie, M. Speldrich, R.W. Gable, L. Sorace, P. Kögerler, C. Boskovic, Inorg. Chem. 50 (2011) 7004 (b) C. Ritchie, E.G. Moore, M. Speldrich, P. Kögerler, C. Boskovic, Angew. Chemie Int. Ed. 49 (2010) 7702. W02 Multifunctional nanomagnets: from hard to soft materials Marco Affronte University of Modena and Reggio Emilia and CNRNANO (Istituto Nanoscienze Consiglio Nazionale delle Ricerche) Institute, Italy Examples of nano-fabrication of magnetic devices and sensors obtained by using Focussed Ion Beam and Electron beam lithography will be presented as well as some experiments performed in our low temperature laboratory. Functionalities of molecular nanoMagnets including magnetothermal properties and topical issues on the quantum magnetism (continues on next page)

Spin selectivity in angle-resolved Auger photoelectron coincidence spectroscopy (AR-APECS) is used to probe electron correlation in ferromagnetic thin films. In particular, exploiting the AR-APECS capability to discriminate Auger electron emission events characterized by valence hole pairs created either in the high or in the low total spin state, a strong correlation effect in the Fe M(2,3)VV Auger line shape (measured in coincidence with the Fe 3p photoelectrons) of Fe/Cu(001) thin films is detected and ascribed to interactions within the majority spin subband. Such an assignment follows from a close comparison of the experimental AR-APECS line shapes with the predictions of a model based on spin polarized density functional theory and the Cini-Sawatzky approach.

The absence of sharp structures in the Auger line shapes of partially filled bands has severely limited the use of electron spectroscopy in magnetic crystals and other correlated materials. By a novel interplay of experimental and theoretical techniques we achieve a combined understanding of the photoelectron, Auger, and Auger-photoelectron coincidence spectra (APECS) of the antiferromagnetic CoO. A recently discovered dichroic effect in angle resolved (DEAR) APECS reveals a complex pattern in the Auger line shape, which is here explained in detail, labeling the final states by their total spin. Since the dichroic effect exists in the antiferromagnetic state but vanishes at the Néel temperature, the DEAR-APECS technique detects the phase transition from its local effects, thus providing a unique tool to observe and understand magnetic correlations where the usual methods are not applicable.

Auger-photoelectron coincidence spectroscopy is used for investigating the electronic properties of a CoO thin film above and below the magnetic transition temperature (T-N). By using the dichroic effect in angle-resolved measurements, we identify and assign well-defined high-spin and low-spin structures in spite of the otherwise featureless Auger singles spectra, typically found for open-band systems. The disappearance of the dichroism for temperatures just above T-N indicates a collapse of the surface short-range magnetic order, presumably due to a strongly reduced exchange field in the surface compared to that in the bulk. Copyright (C) EPLA, 2011

The electronic properties of Cu-phthalocyanine (CuPc) molecules flat lying along the channels of the Au(110) reconstructed surface have been investigated by means of ultraviolet and x-ray photoelectron spectroscopy. The ordered chains give rise to a highly ordered single-layer structure with a (5x3) symmetry. Although from the core-level analysis not any significant charge transfer between the molecules and the underlying Au surface is observed, the valence band photoemission data bring to light CuPc-induced features localized at the interface. In particular, energy versus momentum dispersion of an interface state reveals a bandwidth of about 90 meV along the enlarged Au channels, where the CuPc chains are formed, with a defined fivefold symmetry well fitting the CuPc intermolecular distance.

The capability of the recently observed dichroic effect in angle-resolved Auger-photoelectron coincidence spectroscopy (DEAR-APECS) to disentangle individual multiplet terms has been exploited to study the lineshape of the M3M45M45 Auger spectrum measured in coincidence with the 3p(3/2) photoelectrons from the Cu(111) surface. The relevant multiplet structure of the two hole final state is determined with an unprecedented sensitivity, including a reliable experimental estimation of the energy of the D-1 multiplet term. Spectroscopic data for the 3p photoemission feature are also given and energy conservation applied to the photoelectron-Auger-electron pair has been successfully used in order to quantitatively explain energy shifts in coincidence spectra. Multiple-scattering calculations prove that the DEAR-APECS effect is not destroyed by diffraction effects and a simple model which combines atomic angular distributions and electron-diffraction modulations is provided in order to obtain a detailed understanding of the multiplet energy and intensity distributions in Auger spectra.

The structure of ultrathin Cu-phthalocyanine (Cu-Pc) films on the (1 x 2)-Au(I 10) surface has been studied. The overlayer deposition has been monitored in real time by helium atom scattering (HAS) and low energy electron diffraction (LEED). Throughout the monolayer regime the Cu-Pc molecules are systematically observed to line-up edge-to-edge along the [110] direction of the Au substrate, yielding a commensurate 5-fold periodicity (14.4 angstrom). Cu-Pc chains deconstruct the 2-fold Au missing row order in the early stage of deposition. A set of higher order periodicities (5-, 7-, and 3-fold) are progressively observed along [001] with increasing CuPc deposition, the 3-fold phase appearing at the monolayer saturation coverage. The corresponding molecular orientation has been studied by variable polarization absorption spectroscopy (XAS), whereas the Au substrate structure has been determined by out-of-plane surface X-ray diffraction. The (5 x 5) phase is found to be rather corrugated, and it exhibits a high degree of long-range order yielding the most prominent diffraction pattern. In the (5 x 5) phase, the Cu-Pc chains are found to lift the underneath missing row reconstruction, being separated by residual Au rows. Similarly, in the more compressed 3-fold monolayer phase, the Cu-Pc molecules were formerly found to lie within a shallow (1 x 3) An reconstruction [Cossaro, A.; et al. J. Phys. Chem. B 2004, 108, 14671]. From comparison of the different deposition stages, as measured in real time by HAS, we can draw a comprehensive picture of the system evolution. In fact, the observed periodicities at different coverage are always formed by an array of Cu-Pc chains in shallow troughs that are equally spaced by a number of uncovered Au rows, as dictated by the Cu-Pc coverage. The growth of Cu-Pc arrays in the submonolayer range is thus driven by an interchain repulsion mechanism.

Electronic and structural properties of the CuPc/Al(100) organic−inorganic interface were investigated by means of a multitechnique experimental approach based on synchrotron radiation. The chemical selectivity of X-ray photoelectron spectroscopy (XPS) was used to investigate the electronic structure of copper-phthalocyanine (CuPc) as a function of the molecular thickness ranging from the submonolayer to 40 A. Photoemission from core levels shows a dramatic alteration of the electronic structure of molecules localized at the interface. At the lowest CuPc coverages, the complete reduction of the oxidation state of copper was observed, while C 1s and N 1s shake-up satellites were no longer visible. Both findings are explained with a sizable charge transfer from the substrate to the molecule involving the b1g (Cu 3d-derived) and the LUMO hybridized with the substrate conduction band. The linear polarization of the synchrotron light was employed in X-ray absorption near-edge spectroscopy (XANES) to determine ...

The evolution of the electronic properties of a thin film of copper phthalocyanine deposited on Al(100) and progressively intercalated with rubidium atoms was followed by photoemission and X-ray absorption spectroscopies. Electron donation from the Rb atoms to the C32H16N8Cu molecules results in the lifting of the degeneracy in the e(g) ligand-derived molecular orbital and the lowering of the molecular symmetry. For Rb similar to 2C32H16N8Cu, spectral evidence indicates that both donated electrons reside in the first split-off e(g)-derived level, thus creating an electronic inequivalence between the C atoms in the benzene rings. For higher Rb concentrations, a reduction of the Cu oxidation state is observed, together with a new Cu-derived state in valence-band photoemission spectra, testifying to the filling of the b(1g) orbital. Thus, even though b(1g) is the lowest unoccupied orbital of the neutral molecule, in the film, the Cu-derived b(1g)-derived states are occupied only after a partial filling of the e.-derived band has taken place. Despite the fact that the eg-derived spectral weight becomes larger as the rubidium content in the RbxC32H16N8Cu compound increases, no spectral density was observed at the Fermi level, showing that the film remains insulating for all of the investigated stoichiometries.

The M3M4,5M4,5 Auger transition from a Cu(111) surface is studied using Angular Resolved Auger-PhotoElectron Coincidence Spectroscopy (AR-APECS). In the experiment two different geometrical configurations of the electron analyzers allow us to sample different emission angles of the ejected electrons leading to different weights of the singlet and triplet contributions in the studied transition. The experimental spectra are modeled within a two-step approach using the Cini theory for the closed band case so as to properly consider the spin-orbit interaction and the hole-hole correlation energy. Ingredients for the theory, like density of states, are obtained fully ab-initio in the framework of density functional theory by performing all-electron calculations. The obtained results confirm the recently discovered selectivity of AR-APECS in the final spin-state.

We have developed a generalization of the multiple-scattering formalism to deal with Auger-photoelectron coincidence spectroscopy (APECS) in the solid state. We have merged the exact atomic treatment of the angular correlations between the two electrons and the single-particle approach, on which the multiple-scattering description of condensed matter relies. This allows the recovering, even in extended systems, of the entangled form of the electron-pair wave function characterizing the coincidence angular diffraction pattern. In the atomic limit our formalism correctly reproduces the cross section, as calculated within the statistical-tensors approach, usually employed in atomic physics. We have then performed numerical calculations for the Ge(100) L(3)M(45)M(45) APECS and compared the results with previous experiments. We found that, in the given geometry, the diffraction patterns in coincidence with different directions of the photoelectron keep little memory of the atomic anisotropy. We speculate on the conditions to be fulfilled in order to enhance the atomic-orbital sensitivity in APECS through solid-state diffraction effects.

We present and discuss X-ray absorption spectroscopy and resonant photoemission measurements on Fe nanostructures self-assembled on MgO(001). For Fe coverages below 1 ML equivalent we measured an increase of the Fe L-23 branching ratio and changes in the splitting, widths and relative intensities of the different final states in the L3M23M23 resonant Auger peak. Scanning tunnelling microscopy indicates that the average lateral dimensions of the self-aggregated structures decrease with decreasing Fe amount, from 12 nm at 15 ML nominal Fe amount to 5 nm at 2 ML Fe. This observation allows to interpret the observed changes in the 3d band electronic properties in terms of the evolution of the Fe local atomic coordination from a bulk-like situation to a configuration where low dimensionality effects are significant. (C) 2007 Elsevier B.V. All rights reserved.

The recently discovered capabilities of Auger-photoelectron coincidence spectroscopy (APECS), i.e., the emission depth selectivity and the final spin state selectivity, are reviewed and discussed as a tool for the study of complex systems like magnetic thin films and multilayers. After a brief overview of these recent findings and a description of a clear experimental evidence for a dichroic effect in angle resolved (DEAR) APECS, results of coincidence Auger line shape applied to Co film on Cu(001), are presented. The geometrical configurations used, achieved by means of the high degree of freedom available in the experimental apparatus, are discussed in detail, as well as all the possible combinations that can provide a spin final state selectivity are presented.Due to the very high statistics achieved in the measurement, some finer details can be resolved at energies corresponding to transitions involving electrons from the top and the bottom of the valence band, when comparing singlet versus triplet contributions. As a first attempt and in the simplest approach, the observed dichroism can be explained by appropriate convolutions of the majority and minority densities of states, possibly corrected by a Cini-Sawatzky model for taking into account spin dependent correlation effects. (c) 2006 Elsevier B.V. All rights reserved.