Jan Honolka

Spin and Orbital Magnetic Moment Anisotropies of Monodispersed Bis(Phthalocyaninato)Terbium on a Copper Surface

The magnetic properties of isolated TbPc(2) molecules supported on a Cu(100) surface are investigated by X-ray magnetic circular dichroism at 8 K in magnetic fields up to 5 T. The crystal field and magnetic properties of single molecules are found to be robust upon adsorption on a metal substrate. The Tb magnetic moment has Ising-like magnetization; XMCD spectra combined with multiplet calculations show that the saturation orbital and spin magnetic moment values reach 3 and 6 mu(B), respectively.

In-plane magnetic anisotropy of Fe atoms on Bi2Se3(111).

The robustness of the gapless topological surface state hosted by a 3D topological insulator against perturbations of magnetic origin has been the focus of recent investigations. We present a comprehensive study of the magnetic properties of Fe impurities on the prototypical 3D topological insulator Bi(2)Se(3) using local low-temperature scanning tunneling spectroscopy and integral x-ray magnetic circular dichroism techniques. Single Fe adatoms on the Bi(2)Se(3) surface, in the coverage range ≈ 1% of a monolayer, are heavily relaxed into the surface and exhibit a magnetic easy axis within the surface plane, contrary to what was assumed in recent investigations on the supposed opening of a gap. Using ab initio approaches, we demonstrate that an in-plane easy axis arises from the combination of the crystal field and dynamic hybridization effects.

Oxygen Dissociation by Concerted Action of Di-Iron Centers in Metal-Organic Coordination Networks at Surfaces: Modeling Non-Heme Iron Enzymes

The high chemical reactivity of unsaturated metal sites is a key factor for the development of novel devices with applications in sensor engineering and catalysis. It is also central in the research for sustainable energy concepts, e.g., the efficient production and conversion of chemical fuels. Here, we study the process of oxygen dissociation by a surface-supported metal-organic network that displays close structural and functional analogies with the cofactors of non-heme enzymes. We synthesize a two-dimensional array of chemically active di-iron sites on a Cu(001) surface where molecular oxygen readily dissociates at room temperature. We provide an atomic-level structural and electronic characterization before and after reaction by combining scanning tunneling microscopy, X-ray absorption spectroscopy, and density functional theory. The latter identifies a novel mechanism for O2 dissociation controlled by the cooperative catalytic action of two Fe2+ ions. The high structural flexibility of the organic ligands, the mobility of the metal centers, and the hydrogen bonding formation are shown to be essential for the functionality of these active centers allowing to mimick biologically relevant reactions in a confined environment.

Magnetic surface nanostructures

Recent trends in the emerging field of surface-supported magnetic nanostructures are reviewed. Current strategies for nanostructure synthesis are summarized, followed by a predominantly theoretical description of magnetic phenomena in surface magnetic structures and a review of experimental research in this field. Emphasis is on Fe- or Co-based nanostructures in various low-dimensional geometries, which are studied as model systems to explore the effects of dimensionality, atomic coordination, chemical bonds, alloying and, most importantly, interactions with the supporting substrate on the magnetism. This review also includes a discussion of closely related systems, such as 3d element impurities integrated into organic networks, surface-supported Fe-based molecular magnets, Kondo systems or 4d element nanostructures that exhibit emergent magnetism, thereby bridging the traditional areas of surface science, molecular physics and nanomagnetism.

Substrate-controlled growth and magnetism of nanosize Fe clusters on Pt

The growth and magnetism of nanosize Fe clusters on Pt and other metal surfaces are investigated. Fe clusters have been fabricated directly on the substrates by buffer layer assisted growth under ultrahigh vacuum conditions. The mean cluster diameter and the average cluster spacing were controlled by the Fe coverage and the buffer layer thickness. The enhanced magnetic anisotropy of such clusters of diameters between 0.5 and 10nm with respect to bulk is discussed. Interface anisotropy contributions are compared with direct dipolar cluster-cluster interaction and indirect interactions mediated by the substrate, including preasymptotic ferromagnetic interaction. It is found that this preasymptotic exchange is rather strong in exchange-enhanced substrates, such as Pt, but it decreases rapidly with increasing distance between clusters and becomes negligible for the experimental cluster spacings in this work. Except for clusters that nearly touch each other, the leading interaction contributions are RKKY-type ...

Single 3d transition metal atoms on multi-layer graphene systems: electronic configurations, bonding mechanisms and role of the substrate

The electronic configurations of Fe, Co, Ni and Cu adatoms on graphene and graphite have been studied by x-ray magnetic circular dichroism and charge transfer multiplet theory. A delicate interplay between long-range interactions and local chemical bonding is found to influence the adatom equilibrium distance and magnetic moment. The results for Fe and Co are consistent with purely physisorbed species having, however, different 3d-shell occupations on graphene and graphite ( +

Magnetic anisotropy of deposited transition metal clusters

Abstract.We present results of magnetic torque calculations using the fully relativistic spin-polarized Korringa-Kohn-Rostoker approach applied to small Co and Fe clusters deposited on the Pt(111) surface. From the magnetic torque one can derive amongst others the magnetic anisotropy energy (MAE). It was found that this approach is numerically much more stable and also computationally less demanding than using the magnetic force theorem that allows to calculate the MAE directly. Although structural relaxation effects were not included our results correspond reasonably well to recent experimental data.

Absence of local magnetic moments in Ru and Rh impurities and clusters on Ag(100) and Pt(997)

6 pages, 5 figures.-- PACS nrs.: 75.20.Hr; 78.20.Ls; 78.70.Dm.-- ArXiv pre-print available at: http://arxiv.org/abs/0708.3975

Cobalt nanoclusters on metal-supported Xe monolayers: Influence of the substrate on cluster formation kinetics and magnetism

Department Chemie und Biochemie, Ludwig-Maximilians-Universit¨at Mu¨nchen, 81377 Mu¨nchen, Germany(Dated: September 1, 2009)The growth dynamics of submonolayer coverages of Cobalt during buffer layer assisted growthon Ag(111) and Pt(111) substrates is investigated by variable temperature scanning tunneling mi-croscopy in the temperature range between 80 and 150 Kelvin. It is found that attractive cluster-substrate interactions can govern the cluster formation on the Xe buffer layer, if the Xe layer issufficiently thin. The interpretation of the microscopy results are supported by x-ray magneticcircular dichroism which monitors the effect of cluster-substrate interactions on the formation ofmagnetic moments and magnetic anisotropy of Co nanocluster during the different stages of growth.Ab-initiocalculations show that the cluster magnetism is controlled by the interface anisotropy, lead-ing to perpendicular magnetization for Co on Pt(111). Limits of and new potential for nanoclusterfabrication by buffer layer assisted growth are discussed.

Complex domain-wall dynamics in compressively strained Ga 1-x Mn x As epilayers

The domain-wall-induced reversal dynamics in compressively strained Ga1�xMnxAs was studied employing the magneto-optical Kerr effect and Kerr microscopy. Due to the influence of a uniaxial part in the in-plane magnetic anisotropy 90° domain walls with considerably different dynamic behavior are observed. While the 90° + reversal is identified to be propagation dominated with a small number of domains, the case of 90° � reversal involves a larger number of nucleation centers. The domain-wall nucleation/propagation energies for both transitions are estimated using model calculations from which we conclude that single domain devices can be achievable using the 90° + mode.