K. Wildberger

Local moments of 3d, 4d, and 5d atoms at Cu and Ag (001) surfaces

Abstract We present ag-initio calculations for the electronic structure of 3d, 4d and 5d transition-metal impurities at the (001) surface of Cu and Ag and determine the surface enhancement of the local moments. For 3d impurities we find a sizable enhancement of the local moments, being most important for V and Cr. Large local moments are obtained for 4d and 5d impurities which are in general non-magnetic in the bulk. Some of the adatoms (Nb, Mo, Tc, W, Re) on Ag (001) have “giant” magnetic moments between 3 and 4 μ B .

Ab-initio calculations for 3d impurities on Fe(001) and Ni(001)

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 Greens 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.

Magnetism of free and supported vanadium clusters

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)-Greens 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.

Magnetic bistability of supported Mn clusters

Using the KKR Greens 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.

Metamagnetic states of 3d nanostructures on the Cu(001) surface

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 Greens 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.

Transition metal magnetic nanostructures on metal surfaces

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-Greens 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.

Metamagnetic states in metallic nanostructures

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 Greens function method to supported clusters. A detailed consideration of the Cu(001) surface with 3d supported metallic nanostructures is presented.

Interlayer exchange coupling and interface reflectivities in Fe/Cu, Co/Cu and Ni/Cu (0 0 1) layers

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-Greens 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.

Direct exchange and interaction of 3d impurities on the (0 0 1) surface of iron

We present a detailed ab-initio study of direct exchange and interaction processes of 3d atoms on the Fe(0 0 1) surface. The calculations are based on local density functional theory and apply a KKR-Greens function method for impurities on surfaces. For practically all 3d transition metal impurities on Fe(0 0 1) 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(0 0 1) by the NIST group and present valuable predictions for all 3d/Fe(0 0 1) systems.