75 research outputs found

    Mapping the magnetic exchange interactions from first principles: Anisotropy anomaly and application to Fe, Ni, and Co

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    Mapping the magnetic exchange interactions from model Hamiltonian to density functional theory is a crucial step in multi-scale modeling calculations. Considering the usual magnetic force theorem but with arbitrary rotational angles of the spin moments, a spurious anisotropy in the standard mapping procedure is shown to occur provided by bilinear-like contributions of high order spin interactions. The evaluation of this anisotropy gives a hint on the strength of non-bilinear terms characterizing the system under investigation.Comment: 11 pages, 1 figur

    The combined effect of temperature and disorder on interlayer exchange coupling in magnetic multilayers

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    We study the combined effect of temperature and disorder in the spacer on the interlayer exchange coupling. The temperature dependence is treated on ab initio level. We employ the spin-polarized surface Green function technique within the tight-binding linear muffin-tin orbital method and the Lloyd formulation of the IEC. The integrals involving the Fermi-Dirac distribution are calculated using an efficient method based on representation of integrands by a sum of complex exponentials. Application is made to Co/Cu_{100-x}M_x/Co(001) trilayers (M=Zn, Au, and Ni) with varying thicknesses of the spacer.Comment: 5 pages, LaTeX, 1 figure. Submitted to Phil. Mag.

    Ballistic Spin Injection and Detection in Fe/Semiconductor/Fe Junctions

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    We present {\it ab initio} calculations of the spin-dependent electronic transport in Fe/GaAs/Fe and Fe/ZnSe/Fe (001) junctions simulating the situation of a spin-injection experiment. We follow a ballistic Landauer-B\"uttiker approach for the calculation of the spin-dependent dc conductance in the linear-responce regime, in the limit of zero temperature. We show that the bulk band structure of the leads and of the semiconductor, and even more the electronic structure of a clean and abrupt interface, are responsible for a current polarisation and a magnetoresistance ratio of almost the ideal 100%, if the transport is ballistic. In particular we study the significance of the transmission resonances caused by the presence of two interfaces.Comment: 13 pages, 9 figure

    Surface state scattering by adatoms on noble metals

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    When surface state electrons scatter at perturbations, such as magnetic or nonmagnetic adatoms or clusters on surfaces, an electronic resonance, localized at the adatom site, can develop below the bottom of the surface state band for both spin channels. In the case of adatoms, these states have been found very recently in scanning tunneling spectroscopy experiments\cite{limot,olsson} for the Cu(111) and Ag(111) surfaces. Motivated by these experiments, we carried out a systematic theoretical investigation of the electronic structure of these surface states in the presence of magnetic and non-magnetic atoms on Cu(111). We found that Ca and all 3dd adatoms lead to a split-off state at the bottom of the surface band which is, however, not seen for the spsp elements Ga and Ge. The situation is completely reversed if the impurities are embedded in the surface: Ga and Ge are able to produce a split-off state whereas the 3dd impurities do not. The resonance arises from the s-state of the impurities and is explained in terms of strength and interaction nature (attraction or repulsion) of the perturbing potential.Comment: 6 pages, 5 figure

    RKKY-like contributions to the magnetic anisotropy energy: 3d adatoms on Pt(111) surface

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    The magnetic anisotropy energy defines the energy barrier that stabilizes a magnetic moment. Utilizing density functional theory based simulations and analytical formulations, we establish that this barrier is strongly modified by long-range contributions very similar to Frieden oscillations and Rudermann-Kittel-Kasuya-Yosida interactions. Thus, oscillations are expected and observed, with different decaying factors and highly anisotropic in realistic materials, which can switch non-trivially the sign of the magnetic anisotropy energy. This behavior is general and for illustration we address transition metals adatoms, Cr, Mn, Fe and Co deposited on Pt(111) surface. We explain in particular the mechanisms leading to the strong site-dependence of the magnetic anisotropy energy observed for Fe adatoms on Pt(111) surface as revealed previously via first-principles based simulations and inelastic scanning tunneling spectroscopy (A. A. Khajetoorians et al. Phys. Rev. Lett. 111, 157204 (2013)). The same mechanisms are probably active for the site-dependence of the magnetic anisotropy energy obtained for Fe adatoms on Pd or Rh(111) surfaces and for Co adatoms on Rh(111) surface (P. Blonski et al. Phys. Rev. B 81, 104426 (2010)).Comment: published manuscript with additional figures and comment

    Quantum well states and amplified spin-dependent Friedel oscillations in thin films

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    Electrons mediate many of the interactions between atoms in a solid. Their propagation in a material determines its thermal, electrical, optical, magnetic and transport properties. Therefore, the constant energy contours characterizing the electrons, in particular the Fermi surface, have a prime impact on the behavior of materials. If anisotropic, the contours induce strong directional dependence at the nanoscale in the Friedel oscillations surrounding impurities. Here we report on giant anisotropic charge density oscillations focused along specific directions with strong spin-filtering after scattering at an oxygen impurity embedded in the surface of a ferromagnetic thin film of Fe grown on W(001). Utilizing density functional theory, we demonstrate that by changing the thickness of the Fe films, we control quantum well states confined to two dimensions that manifest as multiple flat energy contours, impinging and tuning the strength of the induced charge oscillations which allow to detect the oxygen impurity at large distances (\approx 50nm).Comment: This paper has an explanatory supplemen

    Räumliche Organisationsstrukturen und Standortanforderungen im deutschen Online-Lebensmitteleinzelhandel: Beispiele aus ergänzendem, reinen und kombinierten Onlinehandel

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    Nicht erst seit der Covid-19-Pandemie nimmt der Online-Lebensmitteleinzelhandel in Deutschland zu und bringt neue, teilweise hybride, Betriebsformen und Vertriebsmodelle hervor. Hiermit gehen bisher kaum untersuchte räumliche Veränderungen der einzelnen Wertschöpfungsschritte einher, beispielsweise in den Bereichen vorgelagerte Logistik, Filialstruktur und Warenübergabe. Anhand von drei ausgewählten Fallbeispielen (Picnic, Wochenmarkt24 und Rewe) wurden neuere Betriebsformen und deren räumliche Logistik- und Vertriebsstrukturen identifiziert und unterschiedliche Standortfaktoren aufgeführt. Diese beinhalten neben den typischen Faktoren der Standortwahl für Distributionslager (Nähe zu Kunden, Arbeitskräften und Lieferanten) auch spezifische betriebsformen- und vertriebsmodellabhängige Faktoren, wie eine stärkere Verkürzung der "letzten Meile", eine Mindest- oder Maximalverdichtung von Haushalten im Einzugsgebiet oder die Nähe zu einer (landwirtschaftlichen) Erzeugerstruktur.Even before the current covid-19-pandemic, online grocery retailing in Germany is achieving a high level of momentum, giving rise to new and increasingly hybrid forms of operation and distribution models in the retail sector. These new operational forms of complementary, pure and combined e-commerce are accompanied by spatial changes of the individual value-added steps, e.g. in the areas of logistics, store structure and transfer of goods, which have hardly been investigated so far. Using three selected cases studies (Picnic, Wochenmarkt24 and Rewe) newer types of spatial logistics and distribution structures were identified and different location factors of these were listed. Thus, the paper illustrates that, in addition to the typical factors of location selection for distribution warehouses (proximity to customers, employees. and suppliers) in food retailing, other aspects, such as a shortening of the last mile, a minimum or maximum density of households in the surrounding area or the proximity to an (agricultural) producer structure, can also be of importance for the new forms of operation

    Theory of real space imaging of Fermi surfaces

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    A scanning tunneling microscope can be used to visualize in real space Fermi surfaces with buried impurities far below substrates acting as local probes. A theory describing this feature is developed based on the stationary phase approximation. It is demonstrated how a Fermi surface of a material acts as a mirror focusing electrons that scatter at hidden impurities.Comment: 10 pages, 4 figure
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