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- Bhowmick, Somnath, et al.
(författare)
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X-ray absorption spectra : Graphene, h-BN, and their alloy
- 2013
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 87:15, s. 155108
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Tidskriftsartikel (refereegranskat)abstract
- Using first-principles density functional theory calculations, in conjunction with the Mahan-Nozieres-de Dominicis theory, we calculate the x-ray absorption spectra of the alloys of graphene and monolayer hexagonal boron nitride on a Ni (111) substrate. The chemical neighborhood of the constituent atoms (B, C, and N) inside the alloy differs from that of the parent phases. In a systematic way, we capture the change in the K-edge spectral shape, depending on the chemical neighborhood of B, C, and N. Our work also reiterates the importance of the dynamical core-hole screening for a proper description of the x-ray absorption process in sp(2)-bonded layered materials.
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| 5. |
- Lidbaum, H, et al.
(författare)
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EMCD in the TEM - Optimization of signal acquisition and data evaluation
- 2008
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Konferensbidrag (refereegranskat)abstract
- The method of electron magnetic circular dichroism (EMCD) was recently proposed anddemonstrated by Peter Schattschneider et al. [I]. The EMCD signal consists in differences of L3 andL2 edge intensities of ferromagnetic materials at specific diffraction vectors. EMCD is thereforeelement specific. Furthermore. it was shown recently that sum rules apply to the EMCD methodwhich is an essential progress to obtain quantitative magnetic information from this method [2-3].Though. the theoretical derivations of sum rule suppose that the obtained spin- to orbital magneticmoments do not depend on the choice of scattering vector. This is a simplification which must beverified both by simulations and experiments to make EMCD a quantitative method for the study ofmagnetic moments.In the experiment. the dichroic signal. i.e. the change ofLJ and L2 edges at different diffractionvectors is small. i.e. of the order of 5-15% of the total signal intensity. Therefore. the acquisitionconditions must be optimized to both approach a di ffraction geometry where sum rules can beapplied and to maximize the signal to noise ratio.In this work. we optimize both. the signal and the signal/noise ratio. The experiments were carriedout on a FEI Tecnai F30 equipped with a Gatan GIF2002 energy filter. Instead of recording k-vectordependent single spectra as in the original work [I], we acquire energy filtered diffraction patternsin the energy interval around the transition metal L-edge. This allows for extraction of the EMCDsignal at k-vectors where the signal contains quantitative magnetic information. All measurementsof2 dimensional k-space maps of the EMCD signal are compared with simulations of the EM CDsignal. We find very good agreement between theoretical predictions and experimental values forboth. the k-space evolution and quantitative agreement the EMCD signal. The data evaluationincludes a careful normalization procedure. a statistical optimization of the signal to noise ratio aswell as the consideration of the entire edge intensity. Finally. we demonstrate the quantitativeprecision of the proposed method at the example ofa slightly textured iron layer (figures I and 2).References[I] P Schattschneider et aJ.. Nature 44 I (2006) 486.[2] J. Rusz, O. Eriksson, P. Novak, P. M. Oppeneer. Phys. Rev. B 76 (2007) 060408.[3] L. Calmels et aJ.. Phys. Rev. B 76 (2007) 060409.
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| 6. |
- Lidbaum, Hans, et al.
(författare)
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Experimental conditions and data evaluation forquantitative EMCD measurements in the TEM
- 2008
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Ingår i: European Microscopy Conference.
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Konferensbidrag (refereegranskat)abstract
- The recently demonstrated technique electron energy-loss spectroscopy (EMCD) [I)opens new routes for characterization of magnetic materials using transmission electronmicroscopy. The technique enables quantitative measurements of orbital to spinmagnetic moments with element specificity, according to the recently derived sum rules[2). Electron energy-loss spectra is obtained at well defined scattering geometries, seefigure I.The principle of the technique having been demonstrated, further progress isrequired to obtain reliable quantitative information about the magnetic properties of thesample. By using energy filtered diffraction patterns, the distribution of the EM CDsignal in reciprocal space is obtained. The ava,lability of these data sets from a fullreciprocal plane allows for the optimisation of the data treatment. We study the theinfluence of experimental geometries on the EMCD signal and optimise data analysis ofthe probed reciprocal plane. This is essential to obtain correct and reliable magneticinformation. Especially normalization, signal to noise optimization and consideration ofthe entire edge intensities are important. The data cubes consisting of the reciprocalplane and energy-loss were acquired using a FEI Tecnai F30ST microscope equippedwith a Gatan GIF2002 spectrometer. In figure 2, two spectra that were extracted at theP+ and P- positions are shown. The experimental results are compared with calculationsof the EMCD signal for a thin Fe film, showing very good agreement.I. P. Schattschneider, S. Rubino, C. H~bert, J. Rusz, J. Kune~, P Novak, E. Carlino,M. Fabrizioli, G. Panaccione and G. Rossi, Nature 441 (2006), p. 486-488.2. J. Rusz, O. Eriksson, P. Novak and P.M. Oppeneer, Phys. Rev. B 76 (2007),060408(R).
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| 7. |
- Lidbaum, Hans, et al.
(författare)
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Quantitative magnetic information from reciprocal space maps in transmission electron microscopy
- 2009
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Ingår i: Physical Review Letters. - 0031-9007. ; 102:3, s. 037201
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Tidskriftsartikel (refereegranskat)abstract
- One of the most challenging issues in the characterization of magnetic materials is to obtain a quantitative analysis on the nanometer scale. Here we describe how electron magnetic circular dichroism (EMCD) measurements using the transmission electron microscope can be used for that purpose, utilizing reciprocal space maps. Applying the EMCD sum rules, an orbital to spin moment ratio of mL/mS=0.08±0.01 is obtained for Fe, which is consistent with the commonly accepted value. Hence, we establish EMCD as a quantitative element-specific technique for magnetic studies, using a widely available instrument with superior spatial resolution.
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| 8. |
- Lidbaum, Hans, et al.
(författare)
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Reciprocal and real space maps for EMCD experiments
- 2010
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Ingår i: Ultramicroscopy. - 0304-3991. ; 110:11, s. 1380-1389
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Tidskriftsartikel (refereegranskat)abstract
- Electron magnetic chiral dichroism (EMCD) is an emerging tool for quantitative measurements of magnetic properties using the transmission electron microscope (TEM), with the possibility of nanometer resolution. The geometrical conditions, data treatment and electron gun settings are found to influence the EMCD signal. In this article, particular care is taken to obtain a reliable quantitative measurement of the ratio of orbital to spin magnetic moment using energy filtered diffraction patterns. For this purpose, we describe a method for data treatment, normalization and selection of mirror axis. The experimental results are supported by theoretical simulations based on dynamical diffraction and density functional theory. Special settings of the electron gun, so called telefocus mode, enable a higher intensity of the electron beam, as well as a reduction of the influence from artifacts on the signal. Using these settings, we demonstrate the principle of acquiring real space maps of the EMCD signal. This enables advanced characterization of magnetic materials with superior spatial resolution.
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| 9. |
- Oppeneer, Peter, et al.
(författare)
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Fermiology of PuCoGa5 and of related Pu-115 compounds
- 2007
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Ingår i: Journal of Alloys and Compounds. - 0925-8388. ; 444, s. 109-113
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Tidskriftsartikel (refereegranskat)abstract
- We report computational investigations of the electronic structures of the superconducting Pu-compounds PuCoGa5 as well as of the non-superconducting compounds PuFeGa5 and PuNiGa5. To capture the localization behavior of the Put 5f electrons, we apply two computational approaches, which are both rooted in the density-functional theory: the local spin-density approximation (LSDA) and the around mean field (AMF) LSDA + U approach. The latter is applicable to moderately localized 5f electrons while the former is applicable to delocalized 5f electrons. Our LSDA calculations show that the Fermi surfaces of the three Pu-115 compounds are sensitive to the amount of band filling, i.e., the number of electrons of the 3d element. Precisely at the electron filling corresponding to PuCoGa5 the Fermi surface has a particularly two-dimensional shape. AMF-LSDA+U calculations (with a Coulomb U of about 3 eV and exchange J of 0.6 eV) lead to a non-magnetic ground state for PuCoGa5, in which the 5f states are shifted to a higher binding energy, in better agreement with photoemission data. The Fermi surface of PuCoGa5 computed with the AMF-LSDA+U approach is nonetheless rather two-dimensional and similar to the LSDA Fermi surface. The AMFLSDA+U approach with a Coulomb U of approximate to 3 eV would thus predict an electronic structure for PuCoGa5 in accord with several experimental data.
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