Quadrupolar XMCD at the Fe K -edge in Fe phthalocyanine film on Au: Insight into the magnetic ground state

Under the terms of the Creative Commons Attribution license.-- et al.The observation of an anomalous quadrupolar signal in x-ray magnetic circular dichroism (XMCD) at the Fe K-edge of iron phthalocyanine (FePc) films is reported. All ground states previously suggested for FePc are incompatible with the experimental data. Based on ab initio molecular orbital multiplet calculations of the isolated FePc molecule, we propose a model for the magnetic ground state of the FePc film that explains the XMCD data and reproduces the observed values of the orbital moments in the perpendicular and planar directions.The financial support of the Spanish financial agency MINECO MAT2011-23791 and MAT2014-53921-R, Aragonese DGA-IMANA E34 (co-funded by Fondo Social Europeo), and European Union FEDER funds is acknowledged. The research at UCSD was supported by the Office of Basic Energy Science, US Department of Energy, BES-DMS, funded by the Department of Energy Office of Basic Energy Science, DMR, under Grant No. DE FG03 87ER-45332.Peer Reviewe

Molecular tilting and columnar stacking of Fe phthalocyanine thin films on Au(111)

Scanning tunneling microscopy and x-ray absorption spectroscopic results at the Fe K edge of Fe phthalocyanine (FePc) thin films grown on Au substrates, together with theoretical calculations, allow us to refine the structure of the film. In particular, we show that the columnar stacking of the FePc molecules is different from that found in bulk ¿ and ß phases. Moreover, the molecules do not lay parallel to the surface of the substrate. These structural findings are relevant to understand magnetism of FePc films.The financial support of the Spanish financial agency MINECO MAT2011-2379 and MAT2014-53921-R, Aragonese DGA-IMANA E34 (co-funded by European Social Fund), as well as European Union FEDER funds is acknowledged. The sample preparation and initial structural characterization were supported by the Office of Basic Energy Science, U.S. Department of Energy, BES-DMS funded by the Department of Energy’s Office of Basic Energy Science, DMR under Grant No. DE FG03 87ER-45332 and NSF DMR 0847552.Peer Reviewe

Projector augmented wave calculation of x-ray absorption spectra at the L2,3 edges

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).We develop a technique based on density functional theory and the projector augmented wave method in order to obtain the x-ray absorption cross section at a general edge, both in the electric dipole and quadrupole approximations. The method is a generalization of Taillefumier,. We apply the method to the calculation of the Cu L2,3 edges in fcc copper and cuprite (Cu 2O), and to the S L2,3 edges in molybdenite (2H-MoS 2). The role of core-hole effects, modeled in a supercell approach, as well as the decomposition of the spectrum into different angular momentum channels are studied in detail. In copper we find that the best agreement with experimental data is obtained when core-hole effects are neglected. On the contrary, core-hole effects need to be included both in Cu2O and 2H-MoS2. Finally we show that a non-negligible component of S L 2,3 edges in 2H-MoS2 involves transition to states with s character at all energy scales. The inclusion of this angular momentum channel is mandatory to correctly describe the angular dependence of the measured spectra. We believe that transitions to s character states are quantitatively significant at the L2,3 edges of third row elements from Al to Ar. © 2013 American Physical Society.One of the authors (O.B.) declares noncompeting financial support from the Agence nationale de la recherche (SWITCH project, of the ANR-BLANC-SIMI7-2010-0 program) project and equally from Ministerio de Economia y Competitividad (MINECO MAT2011/23791) and Aragonese IMANA projects, partially funded by the Fondo Europeo de Desarollo Regional program and the European Social Fund.Peer Reviewe

A robust expectation-maximization method for the interpretation of small-angle scattering data from dense nanoparticle samples

International audienceThe Local Monodisperse Approximation (LMA) is a two-parameter model commonly employed for the retrieval of size distributions from the small angle scattering (SAS) patterns obtained on dense nanoparticle samples (e.g. dry powders and concentrated solutions). This work features a novel implementation of the LMA model resolution for the inverse scattering problem. Our method is based on the Expectation Maximiza-tion iterative algorithm and is free from any fine tuning of model parameters. The application of our method on SAS data acquired in laboratory conditions on dense nanoparticle samples is shown to provide good results

Resonant X-ray Diffraction: basic theoretical

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Large orbital magnetic moment in Pt13 clusters

We present an extensive study of Pt13 clusters embedded in a Na-Y zeolite, by comparing calculations for isolated clusters to experimental data. We perform structural refinements for various geometries involving the isolated clusters and calculate the corresponding x-ray absorption and magnetic circular dichroism spectra, from the joint perspective of pseudopotential plane wave calculations and real space multiple scattering theory. Taking into account the spin-orbit coupling significantly improves the previous scalar relativistic predictions of magnetic properties. The ensemble of embedded Pt13 is found to be dominated by a non-magnetic cuboctahedral geometry. One of the implications is that the ground state of Pt13 clusters in the zeolite environment is different from that of isolated particles. We investigate several isomers that yield a magnetic signature. Furthermore, their abundance was estimated by direct comparison with experiment. We found that one third of the magnetic moment of Pt13 comes from the orbital contribution, in agreement with the experimental value. We therefore provide theoretical proof of the extraordinary orbital magnetization in Pt13 clusters. © 2014 IOP Publishing Ltd.We acknowledge financial support from Ministerio de Economia y Competitividad (MINECO MAT2011/23791) and Aragonese IMANA projects, partially funded by the Fondo Europeo de Desarollo Regional program and the European Social Fund.Peer Reviewe

Structural and magnetic properties of granular Co-Pt multilayers with perpendicular magnetic anisotropy

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).-- et al.We present a study of granular Co-Pt multilayers by means of high-resolution transmission electron microscopy (HRTEM), extended x-ray absorption fine structure (EXAFS), SQUID-based magnetic measurements, anomalous Hall effect (AHE), and x-ray magnetic circular dichroism (XMCD). We describe these granular films as composed of particles with a pure cobalt core surrounded by an alloyed Co-Pt interface, embedded in a Pt matrix. The alloy between the Co and Pt in these granular films, prepared by room temperature sputter deposition, results from interdiffusion of the atoms. The presence of this alloy gives rise to a high perpendicular magnetic anisotropy (PMA) in the granular films, as consequence of the anisotropy of the orbital moment in the Co atoms in the alloy, and comparable to that of highly-ordered CoPt L10 alloy films. Their magnetic properties are those of ferromagnetically coupled particles, whose coupling is strongly temperature dependent: at low temperatures, the granular sample is ferromagnetic with a high coercive field; at intermediate temperatures the granular film behaves as an amorphous asperomagnet, with a coupling between the grains mediated by the polarized Pt, and at high temperatures, the sample has a superparamagnetic behavior. The coupling/decoupling between the grains in our Co-Pt granular films can be tailored by variation of the amount of Pt in the samples.The financial support of the Spanish Ministerio de Economía y Competitividad (MINECO)MAT2011-23791 and Aragonese DGA-IMANA E34 (cofunded by Fondo Social Europeo) projects is acknowledged. Authors also acknowledge the Servicio General de Apoyo a la Investigacón-SAI, Universidad de Zaragoza. A. I. F. acknowledges a Spanish Consejo Superior de Investigaciones Científicas (CSIC) JAE2008-Predoc grant.Peer Reviewe

Quadrupolar XMCD at the Fe K

The observation of an anomalous quadrupolar signal in x-ray magnetic circular dichroism (XMCD) at the Fe K-edge of iron phthalocyanine (FePc) films is reported. All ground states previously suggested for FePc are incompatible with the experimental data. Based on ab initio molecular orbital multiplet calculations of the isolated FePc molecule, we propose a model for the magnetic ground state of the FePc film that explains the XMCD data and reproduces the observed values of the orbital moments in the perpendicular and planar directions