K. Wildberger

Forschungszentrum Jülich, Jülich, North Rhine-Westphalia, Germany

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Publications (31)89.17 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We present ab initio calculations for the electronic structure of 3d, 4d, and 5d transition metal impurities at the (001) surface of Cu and Ag. Our focus is on the surface enhancement of the local impurity moments. The calculations are performed within local density functional theory and use a KKR Green's function method for impurities at surfaces. For 3d impurities we find a sizeable enhancement of the local moments, being most important for V and Cr. Extremely large effects are found for 4d and 5d impurities, which in general are nonmagnetic in the bulk. On the Ag(001) surface we find that Zr, Nb, Mo, Tc, Ru, Ta, W, Re, and Os are magnetic. Some of adatoms (Nb, Mo, Tc, W, Re) have "giant" magnetic moments between 3 and 4 μB.
    Surface Review and Letters 01/2012; 01(04). · 0.28 Impact Factor
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    ABSTRACT: We present a detailed ab-initio study of direct exchange and interaction processes of 3d atoms on the Fe(001) surface. The calculations are based on local density functional theory and apply a KKR-Green's function method for impurities on surfaces. For practically all 3d transition metal impurities on Fe(001) we find a strong tendency for a direct exchange mechanism into the first surface layer. The early 3d impurities V, Cr and Mn strongly repel each other on neighbouring positions within the first layer, while Ni and Cu atoms show a moderate repulsion. The ab-initio results are in good agreement with STM studies for Cr/Fe(001) by the NIST group and present valuable predictions for all 3d/Fe(001) systems.
    Journal of Magnetism and Magnetic Materials 01/1999; 198:548-550. · 2.00 Impact Factor
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    ABSTRACT: We study the reflection and transmission of a spacer Bloch-wave at a magnetic layer of finite thickness. The layer has a composite structure obtained by attaching an adlayer to the magnetic layer. The emphasis of this paper will lay on the change of the reflectivity induced by the adlayer. In particular, we calculate the reflection coefficients for (001) layers of Fe, Co and Ni in Cu as well as such layers with one or more admonolayers of 3d elements. The calculations are performed by the KKR-Green's function method for layered systems. We discuss in detail the relation between the q‖-dependent density of states of the composite magnetic layer and the reflectivity. We show that the reflectivity of a magnetic layer in Cu can change considerably due to the introduction of adlayers.
    Journal of Magnetism and Magnetic Materials 01/1999; 198:570-572. · 2.00 Impact Factor
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    ABSTRACT: Ab initio calculations are presented for magnetic properties of small islands of 3d, 4d, and 5d adatoms on the Ag(001) surface, intermixed with Ag substrate atoms. We show that the intermixing of small 4d and 5d clusters can lead to an unexpected enhancement of the local moments and is very different from the intermixing behavior found in the monolayer regime.
    Physical review. B, Condensed matter 01/1999; 59(3):1681-1684. · 3.77 Impact Factor
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    ABSTRACT: Magnetic properties of free and supported Vanadium clusters of up to four atoms have been calculated self-consistently using the density functional theory. For the free clusters we have used the self-consistent field - linear combination of atomic orbitals - molecular orbital theory with a Gaussian basis for the atom. The geometries, together with the preferred spin multiplicities, were optimized by using the method of steepest descent. For supported clusters on Cu(001) and Ag(001), we have used the self-consistent Korringa - Kohn - Rostoker (KKR)-Green's function approach. Both free and supported clusters are found to be magnetic, although the magnetic moments depend strongly on the cluster size. While free clusters have ferromagnetic ground states, the supported V clusters in general prefer antiferromagnetic configurations. The role of inter-atomic distances, coordination, and surface morphology on the magnetic properties of V clusters are discussed.
    Journal of Physics Condensed Matter 12/1998; 9(48):10739. · 2.22 Impact Factor
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    ABSTRACT: The reflection at the interface of a nonmagnetic and a magnetic layer represents the key quantity for the understanding of interlayer exchange coupling and quantum-well states (QWS). We give a general analysis of the scattering in one dimension in terms of scattering phase shifts and present analytical formulas for the reflection coefficients and the integrated density of states. We discuss the connection between reflectivity and QWS and calculate from first principles the reflection coefficient of a Cu Bloch electron at the (001) interface of fcc Fe, Co, and Ni layers of various, finite thicknesses. We discuss the effect of d resonances in the density of states of the magnetic layer on the reflectivity and the transition from finite to infinite layer thicknesses. We further demonstrate the close relation between the complex reflection coefficients and the width and position of QWS.
    Physical Review B 11/1998; 58(20). · 3.66 Impact Factor
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    ABSTRACT: Motivated by recent STM results for Cr on Fe(001) we present a detailed ab-initio study of the alloying process in the dilute limit for 3d atoms on the Fe(001) surface. The calculations are based on local density functional theory and apply a KKR--Green's function method for impurities on surfaces. For practically all 3d transition metal impurities on Fe(001) we find a strong tendency for a direct exchange mechanism into the first surface layer. The early 3d impurities V, Cr and Mn strongly repel each other on neighboring positions within the first layer, while Ni and Cu atoms show a moderate repulsion. The ab--initio results are in good agreement with STM studies for Cr/Fe(001) by the NIST group and present valuable predictions for all 3d/Fe(001) systems. Over the last years experimental methods like ion field and scanning tunneling microscopy became able to discern individual atoms on surfaces and to observe to a certain extent diffusion and formation processes on the surface of mate...
    Physical Review Letters 09/1998; · 7.73 Impact Factor
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    ABSTRACT: We apply the local approximation of the density-functional theory and Korringa-Kohn-Rostoker Green’s-function method to investigate magnetic properties of FeX (X=3d) clusters on Ag(001). Mixed dimers, impurities in small Fe clusters, plain islands, and cluster sandwiches are considered. In many cases we find both ferromagnetic and antiferromagnetic solutions with large magnetic moments. In particular, we discuss FeCr and FeMn supported clusters. Total-energy calculations show that the energy difference between magnetic configurations in these systems is small; therefore, magnetic fluctuations are possible in FeCr and FeMn nanostructures.
    Physical Review B 06/1998; 57(24). · 3.66 Impact Factor
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    ABSTRACT: Following a recent paper by Grnbeck and Rosen [1], we have re-examined the equilibrium geometries of free V3 and V4 clusters using the generalized gradient approximation within the density functional theory. In agreement with their results, we find that the V3 trimer has an isosceles triangular structure (two bonds of 2.26 and a short bond of 1.71 ) that lies 0.16 eV per atom lower than the equilateral triangle. Similarly, the V4 tetramer in the shape of a parallelogram (bond lengths 1.80 and 2.33 ) lies 0.04 eV per atom lower than the tetrahedral structure. The spin multiplicities of the trimers and tetramers in their ground state structures are 2 and 3, respectively. We would like to thank Professor A Rosen for bringing this to our attention. References [1] Grnbeck H and Rosen A 1997 J. Chem. Phys.107 10620
    Journal of Physics Condensed Matter 01/1998; 10(42):9591-9591. · 2.22 Impact Factor
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    ABSTRACT: We present ab initio calculations on the magnetic properties, in particular the surface enhancement, of single 4d transition-metal impurities on the (001) surfaces of the ferromagnets Fe and Ni. The calculations are based on local-density-functional theory and apply a KKR-Green’s function method for defects on surfaces. We calculate the local moments of impurities in the adatom and in-surface positions and compare with the results obtained for bulk impurities. Contrary to the large local moments found for 4d impurities on the surfaces of the noble metals, our calculations predict only a moderate magnetic enhancement at the surfaces of the ferromagnets.
    Physical Review B 12/1997; 57(1). · 3.66 Impact Factor
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    ABSTRACT: Relative stabilities of Ag clusters supported on a Ag(001) substrate have been studied using both the self-consistent Korringa-Kohn-Rostoker-Green's function technique as well as molecular dynamics. Total-energy calculations reveal that unlike in the gas-phase clusters, the relative stability of the supported clusters are governed by the underlying geometry of the substrate leading to completely different magic numbers in two-dimensional systems.
    Physical review. B, Condensed matter 08/1997; 56:6952-6957. · 3.77 Impact Factor
  • K. Wildberger, R. Zeller, P. H. Dederichs
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    ABSTRACT: A Green's-function method for layered systems based on screened Korringa-Kohn-Rostoker structure constants is presented. The screened structure constants are calculated in real space and the interlayer structure constants are obtained by a two-dimensional Fourier transform. As reference potentials we use repulsive muffin-tin and atomic-sphere potentials of various height. Performing test calculations for Cu(001) slabs, we demonstrate that for the angular momentum decomposed local charges a typical accuracy of 10-3 electrons can be obtained, so that for practical applications the screening approximation can be used without loss of numerical accuracy. This precision can only be obtained if the screened structure constants include coupling to next nearest neighbors and if the reference potentials are sufficiently repulsive. It is demonstrated in slab calculations that the numerical effort scales linearly with the number of layers, allowing accurate, self-consistent calculations for very large systems.
    Physical Review B 04/1997; 55(15). · 3.66 Impact Factor
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    ABSTRACT: Self-consistent first-principles calculations based on the molecular-orbital theory and the Korringa-Kohn-Rostoker Green’s-function method have been used to demonstrate that the magnetism of Rh clusters can be altered by either modifying their structure or depositing them on a suitable substrate. This ability to alter the magnetic properties of clusters can also have significant effect on their chemical reactivity, thus linking the field of magnetism and catalysis of atomically engineered materials.
    Physical Review B 02/1997; 56(14). · 3.66 Impact Factor
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    ABSTRACT: Materials of mesoscopic dimensions can be produced by modern scanning microscopic and chemical techniques. This opens the possibility of creating new nanoscale magnets, which consist only of a few atoms. These systems may exhibit novel and unusual properties. We demonstrate, that metamagnetic states are possible in supported metallic nanostructures. This effect can lead to magnetic transitions between different magnetic states by a change of external parameters like temperature or applied fields. We apply the local approximation of the density functional theory and a recently developed KKR Green's function method to supported clusters. A detailed consideration of the Cu(001) surface with 3d supported metallic nanostructures is presented.
    Solid State Communications 02/1997; 101(8):559-562. · 1.53 Impact Factor
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    ABSTRACT: Different magnetic states for a given real structure are known for bulk metals and alloys and also for free transition metal clusters. We have calculated the magnetic properties of small 3d transition metal clusters on the Cu(001) surface by means of an ab initio KKR Green's function method. It is shown that multiple magnetic states exist in these nanostructures. High spin and low spin ferromagnetic states as well as antiferromagnetic states occur. The energy differences between the different states are calculated.
    Journal of Magnetism and Magnetic Materials 01/1997; · 2.00 Impact Factor
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    ABSTRACT: Using the KKR Green's function method and the local spin density approximation of the density functional theory, we have studied ultrasmall Mn clusters on an Ag(001) substrate. Our results show that supported Mn clusters not only possess a large magnetic moment per atom, but also exhibit magnetic bistability, making it possible to probe quantum tunneling in mesoscopic systems. We discuss our results in the light of recent experiments on supported clusters.
    Surface Science - SURFACE SCI. 01/1997; 384(1).
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    ABSTRACT: Ab initio calculations of the electronic and magnetic properties of metallic nanostructures on (001) surfaces of Cu and Ag are presented. The LDA approximation of the density functional theory and the KKR-Green's function method are used. It is shown that 3d, 4d and 5d small clusters can be magnetic on metal surfaces. The effect of intermixing with the substrate on the magnetic properties is investigated for Fe clusters on Cu(001). The influence of the geometry and the size of the clusters on the magnetic properties of 4d clusters is shown. Sizeable magnetic moments for 5d clusters are found on the Ag surface.
    Surface Science. 01/1997;
  • P.H. Dederichs, K. Wildberger, R. Zeller
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    ABSTRACT: We present ab inition calculations for the interlayer exchange coupling of FCC Fe, Co and Ni layers in Cu (0 0 1) which have been performed by a KKR-Green's function method for planar defects. The calculations show strong differences in the amplitudes for the short and long oscillation periods, which in a first approximation can be understood from the bulk band structures of Fe, Co and Ni. We discuss results for the quantum well states in the Cu spacer layers and their relation to interlayer coupling. We investigate in detail the effect of the finite thickness of the magnetic layers and present calculations for the reflectivity of a Cu Blochwave at a magnetic layer of finite thickness. We discuss the temperature dependence of the coupling and the transition from finite-to-infinite layer thickness.
    Physica B Condensed Matter 01/1997; · 1.28 Impact Factor
  • B. Nonas, K. Wildberger, R. Zeller, P.H. Dederichs
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    ABSTRACT: We present ab-initio calculations for the electronic structure of 3d impurities on Fe and Ni(001) surfaces. The calculations are based on local density functional theory and apply a KKR Green's function method for impurities on surfaces. We consider single 3d impurities as adatoms on hollow sites in the first ‘vacuum’ layer and as substitutional impurities in the surface layer. For Fe, Mn, and Cr impurities the moments are considerably enhanced compared to the bulk, but the moments of the adatom and the surface site differ surprisingly little in size. While a Mn adatom couples ferromagnetically to the bulk moments, we find antiferromagnetic coupling for the surface position. We also perform calculations for adatom dimers and determine their magnetic coupling. On the Ni(001) surface the 3d moments are more strongly enhanced than on the Fe surface and within the 3d series a broad two state region exists.
    Journal of Magnetism and Magnetic Materials 01/1997; · 2.00 Impact Factor
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    ABSTRACT: Ab initio calculations for fcc Fe, Co and Ni layers in Cu(100) have been performed using a KKR-Green's function method for planar defects. The exchange coupling energy of magnetic bilayers was calculated from the single-particle energies within the framework of the frozen potential approximation. The calculations revealed that for Ni the amplitude of the long and the short period is strongly reduced compared with Co and Fe. While the short period dominates the coupling for Ni and Co layers, the long period becomes important in the case of Fe. The asymptotic behavior for large spacer thicknesses and the amplitudes of the different oscillation periods are discussed with the help of spin- and q‖-resolved density of states and the corresponding bulk band structures of the magnetic layers. For the longer period the minority bands of Ni are sp-like and very similar to the Cu bands, while the corresponding Co bands contain a larger d-admixture. In contrast to this, in the case of Fe the Fermi level is situated in a band gap slightly below the Δ1-band.
    Journal of Magnetism and Magnetic Materials 01/1997; 165:454-457. · 2.00 Impact Factor

Publication Stats

552 Citations
89.17 Total Impact Points

Institutions

  • 1994–2012
    • Forschungszentrum Jülich
      • Institute for Advanced Simulation (IAS)
      Jülich, North Rhine-Westphalia, Germany
  • 1997
    • Martin Luther University of Halle-Wittenberg
      • Institut für Physik
      Halle, Saxony-Anhalt, Germany
    • Virginia Commonwealth University
      • Department of Physics
      Richmond, VA, United States