Insights into the orbital magnetism of noncollinear magnetic systems

The orbital magnetic moment is usually associated with the relativistic spin-orbit interaction, but recently it has been shown that noncollinear magnetic structures can also be its driving force. This is important not only for magnetic skyrmions, but also for other noncollinear structures, either bulk-like or at the nanoscale, with consequences regarding their experimental detection. In this work we present a minimal model that contains the effects of both the relativistic spin-orbit interaction and of magnetic noncollinearity on the orbital magnetism. A hierarchy of models is discussed in a step-by-step fashion, highlighting the role of time-reversal symmetry breaking for translational and spin and orbital angular motions. Couplings of spin-orbit and orbit-orbit type are identified as arising from the magnetic noncollinearity. We recover the atomic contribution to the orbital magnetic moment, and a nonlocal one due to the presence of circulating bound currents, exploring different balances between the kinetic energy, the spin exchange interaction, and the relativistic spin-orbit interaction. The connection to the scalar spin chirality is examined. The orbital magnetism driven by magnetic noncollinearity is mostly unexplored, and the presented model contributes to laying its groundwork

A Pirâmide das RP: Os media sociais e o papel das Relações

This paper explores the relationship between social media as tools used by public relations professionals and as part of the daily lives of organizations’ stakeholders, identifying emergent practices in public relations and confronting new perspectives, both professional and academic, on public relations functions and on its role within organizational communication. Departing from the agreement shared by academics and professionals on a profound shift in public relations as a consequence of the increasingly widespread, intense and frequent use of social media, this paper intends to clarify the nature and terms of that shift. Two perspectives are confronted: one of them is focused on emergent professional practices and regards social media as tools at the disposal of the PR professional; the other is broader in scope and views social media as a contextual factor that influences both the stakeholders’ behavior patterns and PR practices, thus redefining the role of public relations within organizational communication. The paper presents results from an exploratory study whose goal was to identify a conceptual framework for understanding the impact of social media on public relations.A relevant case study was identified, presenting the solution found by TAP, the Portuguese airline company, to deal with communication crisis involving the social media and to successfully manage social media use as a complementary communication channel. TAP’s social media presence is managed through an articulation of public relations, marketing and customer support where public relations assume a pivotal role. Drawing on this case study, we propose the PR pyramid as a theoretical model that redefines the role of public relations as the orchestrator of the consistent, coherent and integrated communication that is demanded by the contemporary digital context

Nonlocal orbital magnetism of 3d adatoms deposited on the Pt(111) surface

The orbital magnetic moment is still surprisingly not well understood, in contrast to the spin part. Its description in finite systems, such as isolated atoms and molecules, is not problematic, but it was only recently that a rigorous picture was provided for extended systems. Here we focus on an intermediate class of systems: magnetic adatoms placed on a non-magnetic surface. We show that the essential quantity is the ground-state charge current density, in the presence of spin-orbit coupling, and set out its first-principles description. This is illustrated by studying the magnetism of the surface Pt electrons, induced by the presence of Cr, Mn, Fe, Co and Ni adatoms. A physically appealing partition of the charge current is introduced. This reveals that there is an important nonlocal contribution to the orbital moments of the Pt atoms, extending three times as far from each magnetic adatom as the induced spin and local orbital moments. We find that it is as sizable as the latter, and attribute its origin to a spin-orbital susceptibility of the Pt surface, different from the one responsible for the formation of the local orbital moments.Comment: 6 pages, 3 figures, submitte

The chiral biquadratic pair interaction

Magnetic interactions underpin a plethora of magnetic states of matter, hence playing a central role both in fundamental physics and for future spintronic and quantum computation devices. The Dzyaloshinskii-Moriya interaction, being chiral and driven by relativistic effects, leads to the stabilization of highly-noncollinear spin textures such as skyrmions, which thanks to their topological nature are promising building blocks for magnetic data storage and processing elements. Here, we reveal and study a new chiral pair interaction, which is the biquadratic equivalent of the Dzyaloshinskii-Moriya interaction. First, we derive this interaction and its guiding principles from a microscopic model. Second, we study its properties in the simplest prototypical systems, magnetic dimers deposited on various substrates, resorting to systematic first-principles calculations. Lastly, we discuss its importance and implications not only for magnetic dimers but also for extended systems, namely one-dimensional spin spirals and complex two-dimensional magnetic structures, such as a nanoskyrmion lattice

First-principles investigation of spin wave dispersions in surface-reconstructed Co thin films on W(110)

We computed spin wave dispersions of surface-reconstructed Co films on the W(110) surface in the adiabatic approximation. The magnetic exchange interactions are obtained via first-principles electronic structure calculations using the Korringa-Kohn-Rostoker Green function method. We analyze the strength and oscillatory behavior of the intralayer and interlayer magnetic interactions and investigate the resulting spin wave dispersions as a function of the thickness of Co films. In particular, we highlight and explain the strong impact of hybridization of the electronic states at the Co-W interface on the magnetic exchange interactions and on the spin wave dispersions. We compare our results to recent measurements based on electron energy loss spectroscopy [E. Michel, H. Ibach, and C.M. Schneider, Phys. Rev. B 92, 024407 (2015)]. Good overall agreement with experimental findings can be obtained by considering the possible overestimation of the spin splitting, stemming from the local spin density approximation, and adopting an appropriate correction.Comment: 14 pages, 14 figure

Relativistic dynamical spin excitations of magnetic adatoms

We present a first-principles theory of dynamical spin excitations in the presence of spin-orbit coupling. The broken global spin rotational invariance leads to a new sum rule. We explore the competition between the magnetic anisotropy energy and the external magnetic field, as well as the role of electron-hole excitations, through calculations for 3$d$-metal adatoms on the Cu(111) surface. The spin excitation resonance energy and lifetime display non-trivial behavior, establishing the strong impact of relativistic effects. We legitimate the use of the Landau-Lifshitz-Gilbert equation down to the atomic limit, but with parameters that differ from a stationary theory.Comment: 8 pages, 3 figures, accepted in PR

Modelling spin waves in noncollinear antiferromagnets: spin-flop states, spin spirals, skyrmions and antiskyrmions

Spin waves in antiferromagnetic materials have great potential for next-generation magnonic technologies. However, their properties and their dependence on the type of ground-state antiferromagnetic structure are still open questions. Here, we investigate theoretically spin waves in one- and two-dimensional model systems with a focus on noncollinear antiferromagnetic textures such as spin spirals and skyrmions of opposite topological charges. We address in particular the nonreciprocal spin excitations recently measured in bulk antiferromagnet $\alpha$--$\text{Cu}_2\text{V}_2\text{O}_7$ utilizing inelastic neutron scattering experiments [Phys.\ Rev.\ Lett.\ \textbf{119}, 047201 (2017)], where we help to characterize the nature of the detected spin-wave modes. Furthermore, we discuss how the Dzyaloshinskii-Moriya interaction can lift the degeneracy of the spin-wave modes in antiferromagnets, resembling the electronic Rashba splitting. We consider the spin-wave excitations in antiferromagnetic spin-spiral and skyrmion systems and discuss the features of their inelastic scattering spectra. We demonstrate that antiskyrmions can be obtained with an isotropic Dzyaloshinskii-Moriya interaction in certain antiferromagnets.Comment: 26 pages, 9 figure

INTERNAL ORGANIZATION OF A F&B COMPANY IN THE BOP CONTEXT

Base of Pyramid (BoP) is nowadays one of global management’s top issues. Although several studies were focused on developing economies and their markets behaviour, very few have worked on the internal organization dramaBoP, developing economies, strategy, value chain

Self organized formation of Ge nanocrystals in multilayers

The aim of this work is to create a process which allows the tailored growth of Ge nanocrystals for use in photovoltic applications. The multilayer systems used here provide a reliable method to control the Ge nanocrystal size after phase separation. In this thesis, the deposition of GeOx/SiO2 and Ge:SiOx~ 2/SiO2 multilayers via reactive dc magnetron sputtering and the self-ordered Ge nanocrystal formation within the GeOx and Ge:SiOx~ 2 sublayers during subsequent annealing is investigated. Mostly the focus of this work is on the determination of the proper deposition conditions for tuning the composition of the systems investigated. For the GeOx/SiO2 multilayers this involves changing the GeOx composition between elemental Ge (x = 0) and GeO2 (x = 2), whereas for the Ge:SiOx~ 2/SiO2 multilayers this involves changing the stoichiometry of the Ge:SiOx~ 2 sublayers in the vicinity of stochiometric silica (x = 2). The deposition conditions are controlled by the variation of the deposition rate, the deposition temperature and the oxygen partial pressure. A convenient process window has been found which allows the sequential deposition of GeOx/SiO2 or Ge:SiOx ~2/SiO2 without changing the oxygen partial pressure during deposition. For stoichiometry determination Rutherford back-scattering spectrometry has been applied extensively. The phase separation in the spatially confined GeOx and Ge:SiOx ~2 sublayers was investigated by X-ray absorption spectroscopy at the Ge K-edge. The Ge sub-oxides content of the as-deposited multilayers diminishes with increasing annealing temperature, showing complete phase separation at approximately 450° C for both systems (using inert N2 at ambient pressure). With the use of chemical reducing H2 in the annealing atmosphere, the temperature regime where the GeOx phase separation occurs is lowered by approximately 100 °C. At temperatures above 400° C the sublayer composition, and thus the density of the Ge nanocrystals, can be altered by making use of the reduction of GeO2 by H2. The Ge nanocrystal formation after subsequent annealing was investigated with X-ray scattering, Raman spectroscopy and electron microscopy. By these methods the existence of 2 - 5 nm Ge nanocrystals at annealing temperatures of 550 (GeOx) - 700° C (Ge:SiOx ~2) has been confirmed which is within the multilayer stability range. The technique used allows the production of extended multilayer stacks (50 periods ~ 300 nm) with very smooth interfaces (roughness ~ 0.5 nm). Thus it was possible to produce Ge nanocrystal layers with ultra-thin SiO2 separation layers (thickness ~ 1 nm) which offers interesting possibilities for charge transport via direct tunneling.:Contents 1 Introduction and motivation 1 2 Basic aspects 6 2.1 Microstructure of sub-stoichiometric oxides (SiOx, GeOx) 6 2.2 Phase transformations 9 2.3 Quantum confinement effect in nanocrystals 12 2.4 Applications of nanostructures in 3rd generation photovoltaics 17 3 Experimental setup 21 3.1 The magnetron deposition chamber 21 3.2 (Reactive) dc sputtering 22 3.3 Annealing processing 26 3.4 X-ray facilities 26 4 Analytical methods 30 4.1 Rutherford backscattering spectrometry (RBS) 30 4.2 Raman scattering 33 4.3 (Grazing incidence) X-ray diffraction (GIXRD) 35 4.4 X-ray reflectivity (XRR) 39 4.5 X-ray absorption near edge structure (XANES) 41 4.6 Transmission electron microscopy (TEM) 42 5 Properties of reactive dc magnetron sputtered Si-Ge-O (multi)layers 44 5.1 Deposition rate and film stoichiometry investigations 44 5.2 Stoichiometry dependent properties of GeOx/SiO2 multilayers 47 5.3 Lateral intercluster distance of the Ge nanocrystals in multilayers 51 6 Confined Ge nanocrystal growth in GeOx/SiO2 multilayers 54 6.1 Phase separation in GeOx single layers and GeOx/SiO2 multilayers 54 6.2 Crystallization in GeOx single layers and GeOx/SiO2 multilayers 58 6.3 Multilayer stability and smallest possible Ge nanocrystal size 60 6.4 Stacked Ge NC films with ultra thin SiO2 separation layers 66 7 Confined Ge nanocrystal growth in Ge:SiOx/SiO2 multilayers 71 7.1 Phase separation in Ge:SiOx/SiO2 multilayers 72 7.2 Crystallisation in Ge:SiOx/SiO2 multilayers 76 8 Summary and conclusions 79 List of Figures 83 List of Tables 85 Bibliography 8