Magnetic interactions between transition metal impurities and clusters mediated by low-dimensional metallic hosts: A first principles theoretical investigation

The magnetic properties and interactions between transition metal (TM) impurities and clusters in low-dimensional metallic hosts are studied using a first principles theoretical method. In the first part of this work, the effect of magnetic order in 3d-5d systems is addressed from the perspective of its influence on the enhancement of the magnetic anisotropy energy (MAE). In the second part, the possibility of using external electric fields (EFs) to control the magnetic properties and interactions between nanoparticles deposited at noble metal surfaces is investigated. The influence of 3d composition and magnetic order on the spin polarization of the substrate and its consequences on the MAE are analyzed for the case of 3d impurities in one- and two-dimensional polarizable hosts. It is shown that the MAE and easy- axis of monoatomic free standing 3d-Pt wires is mainly determined by the atomic spin-orbit (SO) coupling contributions. The competition between ferromagnetic (FM) and antiferromagnetic (AF) order in FePtn wires is studied in detail for n=1-4 as a function of the relative position between Fe atoms. Our results show an oscillatory behavior of the magnetic polarization of Pt atoms as a function of their distance from the magnetic impurities, which can be correlated to a long-ranged magnetic coupling of the Fe atoms. Exceptionally large variations of the induced spin and orbital moments at the Pt atoms are found as a function of concentration and magnetic order. Along with a violation of the third Hund’s rule at the Fe sites, these variations result in a non trivial behavior of the MAE. In the case of TM impurities and dimers at the Cu(111), the effects of surface charging and applied EFs on the magnetic properties and substrate-mediated magnetic interactions have been investigated. The modifications of the surface electronic structure, impurity local moments and magnetic exchange coupling as a result of the EF-induced metallic screening and charge rearrangements are analysed. In a first study, the properties of surface substitutional Co and Fe impurities are investigated as a function of the external charge per surface atom q. At large inter-impurity distances the effective magnetic exchange coupling ∆E between impurities shows RKKY-like oscillations as a function of the distance which are not significantly affected by the considered values of q. For distances r < 10 Å, important modifications in the magnitude of ∆E, involving changes from FM to AF coupling, are found depending non-monotonously on the value and polarity of q. The interaction energies are analysed from a local perspective. In a second study, the interplay between external EF effects, internal magnetic order and substrate-mediated magnetic coupling has been investigated for Mn dimers on Cu(111). Our calculations show that EF (∼ 1eV/Å) can induce a switching from AF to FM ground-state magnetic order within single Mn dimers. The relative coupling between a pair of dimers also shows RKKY-like oscillations as a function of the inter-dimer distance. Their effective magnetic exchange interaction is found to depend significantly on the magnetic order within the Mn dimers and on their relative orientation on the surface. The dependence of the substrate-mediated interaction on the magnetic state of the dimers is qualitatively explained in terms of the differences in the scattering of surface electrons. At short inter-dimer distances, the ground-state configuration is determined by an interplay between exchange interactions and EF effects. These results demonstrate that external surface charging and applied EFs offer remarkable possibilities of manipulating the sign and strength of the magnetic coupling of surface supported nanoparticles.

Physik der Elektronen in Nanostrukturen und niedrigdimensionalen Systemen - Studie zweier Beispiele

Als Beispiele für die vielfältigen Phänomene der Physik der Elektronen in niedrigdimensionalen Systemen wurden in dieser Arbeit das Cu(110)(2x1)O-Adsorbatsystem und die violette Li0.9Mo6O17-Bronze untersucht. Das Adsorbatsystem bildet selbstorganisierte quasi-eindimensionale Nanostrukturen auf einer Kupferoberfläche. Die Li-Bronze ist ein Material, das aufgrund seiner Kristallstruktur quasi-eindimensionale elektronische Eigenschaften im Volumen aufweist. Auf der Cu(110)(2x1)O-Oberfläche kann durch Variation der Sauerstoffbedeckung die Größe der streifenartigen CuO-Domänen geändert werden und damit der Übergang von zwei Dimensionen auf eine Dimension untersucht werden. Der Einfluss der Dimensionalität wurde anhand eines unbesetzten elektronischen Oberflächenzustandes studiert. Dessen Energieposition (untere Bandkante) verschiebt mit zunehmender Einschränkung (schmalere CuO-Streifen) zu größeren Energien hin. Dies ist ein bekannter quantenmechanischer Effekt und relativ gut verstanden. Zusätzlich wurde die Lebensdauer des Zustandes auf der voll bedeckten Oberfläche (zwei Dimensionen) ermittelt und deren Veränderung mit der Breite der CuO-Streifen untersucht. Es zeigt sich, dass die Lebensdauer auf schmaleren CuO-Streifen drastisch abnimmt. Dieses Ergebnis ist neu. Es kann im Rahmen eines Fabry-Perot-Modells als Streuung in Zustände außerhalb der CuO-Streifen verstanden werden. Außer den gerade beschriebenen Effekten war es möglich die Ladungsdichte des diskutierten Zustandes orts- und energieabhängig auf den CuO-Streifen zu studieren. Die Li0.9Mo6O17-Bronze wurde im Hinblick auf das Verhalten der elektronischen Zustandsdichte an der Fermikante untersucht. Diese Fragestellung ist besonders wegen der Quasieindimensionalität des Materials interessant. Die Messungen von STS-Spektren in der Nähe der Fermienergie zeigen, dass die Elektronen in der Li0.9Mo6O17-Bronze eine sogenannte Luttingerflüssigkeit ausbilden, die anstatt einer Fermiflüssigkeit in eindimensionalen elektronischen Systemen erwartet wird. Bisher wurde Luttingerflüssigkeitsverhalten erst bei wenigen Materialien und Systemen experimentell nachgewiesen, obschon die theoretischen Voraussagen mehr als 30 Jahre zurückliegen. Ein Charakteristikum einer Luttingerflüssigkeit ist die Abnahme der Zustandsdichte an der Fermienergie mit einem Potenzgesetz. Dieses Verhalten wurde in STS-Spektren dieser Arbeit beobachtet und quantitativ im Rahmen eines Luttingerflüssigkeitsmodells beschrieben. Auch die Temperaturabhängigkeit des Phänomens im Bereich von 5K bis 55K ist konsistent mit der Beschreibung durch eine Luttingerflüssigkeit. Generell zeigen die Untersuchungen dieser Arbeit, dass die Dimensionalität, insbesondere deren Einschränkung, einen deutlichen Einfluss auf die elektronischen Eigenschaften von Systemen und Materialien haben kann.

Beschreibung von Oberflächenphänomenen durch relativistische Clusterrechnungen unter Verwendung eines Einbettungsverfahrens

For the theoretical investigation of local phenomena (adsorption at surfaces, defects or impurities within a crystal, etc.) one can assume that the effects caused by the local disturbance are only limited to the neighbouring particles. With this model, that is well-known as cluster-approximation, an infinite system can be simulated by a much smaller segment of the surface (Cluster). The size of this segment varies strongly for different systems. Calculations to the convergence of bond distance and binding energy of an adsorbed aluminum atom on an Al(100)-surface showed that more than 100 atoms are necessary to get a sufficient description of surface properties. However with a full-quantummechanical approach these system sizes cannot be calculated because of the effort in computer memory and processor speed. Therefore we developed an embedding procedure for the simulation of surfaces and solids, where the whole system is partitioned in several parts which itsself are treated differently: the internal part (cluster), which is located near the place of the adsorbate, is calculated completely self-consistently and is embedded into an environment, whereas the influence of the environment on the cluster enters as an additional, external potential to the relativistic Kohn-Sham-equations. The basis of the procedure represents the density functional theory. However this means that the choice of the electronic density of the environment constitutes the quality of the embedding procedure. The environment density was modelled in three different ways: atomic densities; of a large prepended calculation without embedding transferred densities; bulk-densities (copied). The embedding procedure was tested on the atomic adsorptions of 'Al on Al(100) and Cu on Cu(100). The result was that if the environment is choices appropriately for the Al-system one needs only 9 embedded atoms to reproduce the results of exact slab-calculations. For the Cu-system first calculations without embedding procedures were accomplished, with the result that already 60 atoms are sufficient as a surface-cluster. Using the embedding procedure the same values with only 25 atoms were obtained. This means a substantial improvement if one takes into consideration that the calculation time increased cubically with the number of atoms. With the embedding method Infinite systems can be treated by molecular methods. Additionally the program code was extended by the possibility to make molecular-dynamic simulations. Now it is possible apart from the past calculations of fixed cores to investigate also structures of small clusters and surfaces. A first application we made with the adsorption of Cu on Cu(100). We calculated the relaxed positions of the atoms that were located close to the adsorption site and afterwards made the full-quantummechanical calculation of this system. We did that procedure for different distances to the surface. Thus a realistic adsorption process could be examined for the first time. It should be remarked that when doing the Cu reference-calculations (without embedding) we begun to parallelize the entire program code. Only because of this aspect the investigations for the 100 atomic Cu surface-clusters were possible. Due to the good efficiency of both the parallelization and the developed embedding procedure we will be able to apply the combination in future. This will help to work on more these areas it will be possible to bring in results of full-relativistic molecular calculations, what will be very interesting especially for the regime of heavy systems.

Electronic origin of bond softening and hardening in femtosecond-laser-excited magnesium

Many ultrafast structural phenomena in solids at high fluences are related to the hardening or softening of particular lattice vibrations at lower fluences. In this paper we relate femtosecond-laser-induced phonon frequency changes to changes in the electronic density of states, which need to be evaluated only in the electronic ground state, following phonon displacement patterns. We illustrate this relationship for a particular lattice vibration of magnesium, for which we—surprisingly—find that there is both softening and hardening as a function of the femtosecond-laser fluence. Using our theory, we explain these behaviours as arising from Van Hove singularities: We show that at low excitation densities Van Hove singularities near the Fermi level dominate the change of the phonon frequency while at higher excitations Van Hove singularities that are further away in energy also become important. We expect that our theory can as well shed light on the effects of laser excitation of other materials.

First-principles electronic theory of non-collinear magnetic order in transition-metal nanowires

The structural, electronic and magnetic properties of one-dimensional 3d transition-metal (TM) monoatomic chains having linear, zigzag and ladder geometries are investigated in the frame-work of first-principles density-functional theory. The stability of long-range magnetic order along the nanowires is determined by computing the corresponding frozen-magnon dispersion relations as a function of the 'spin-wave' vector q. First, we show that the ground-state magnetic orders of V, Mn and Fe linear chains at the equilibrium interatomic distances are non-collinear (NC) spin-density waves (SDWs) with characteristic equilibrium wave vectors q that depend on the composition and interatomic distance. The electronic and magnetic properties of these novel spin-spiral structures are discussed from a local perspective by analyzing the spin-polarized electronic densities of states, the local magnetic moments and the spin-density distributions for representative values q. Second, we investigate the stability of NC spin arrangements in Fe zigzag chains and ladders. We find that the non-collinear SDWs are remarkably stable in the biatomic chains (square ladder), whereas ferromagnetic order (q =0) dominates in zigzag chains (triangular ladders). The different magnetic structures are interpreted in terms of the corresponding effective exchange interactions J(ij) between the local magnetic moments μ(i) and μ(j) at atoms i and j. The effective couplings are derived by fitting a classical Heisenberg model to the ab initio magnon dispersion relations. In addition they are analyzed in the framework of general magnetic phase diagrams having arbitrary first, second, and third nearest-neighbor (NN) interactions J(ij). The effect of external electric fields (EFs) on the stability of NC magnetic order has been quantified for representative monoatomic free-standing and deposited chains. We find that an external EF, which is applied perpendicular to the chains, favors non-collinear order in V chains, whereas it stabilizes the ferromagnetic (FM) order in Fe chains. Moreover, our calculations reveal a change in the magnetic order of V chains deposited on the Cu(110) surface in the presence of external EFs. In this case the NC spiral order, which was unstable in the absence of EF, becomes the most favorable one when perpendicular fields of the order of 0.1 V/Å are applied. As a final application of the theory we study the magnetic interactions within monoatomic TM chains deposited on graphene sheets. One observes that even weak chain substrate hybridizations can modify the magnetic order. Mn and Fe chains show incommensurable NC spin configurations. Remarkably, V chains show a transition from a spiral magnetic order in the freestanding geometry to FM order when they are deposited on a graphene sheet. Some TM-terminated zigzag graphene-nanoribbons, for example V and Fe terminated nanoribbons, also show NC spin configurations. Finally, the magnetic anisotropy energies (MAEs) of TM chains on graphene are investigated. It is shown that Co and Fe chains exhibit significant MAEs and orbital magnetic moments with in-plane easy magnetization axis. The remarkable changes in the magnetic properties of chains on graphene are correlated to charge transfers from the TMs to NN carbon atoms. Goals and limitations of this study and the resulting perspectives of future investigations are discussed.