Vst.-Nr. 5705009

Berichte aus laufenden Arbeiten zu den Themen und aktueller Forschung.

Programm WiSe 2016

Monday 01.02: Prof. Dr. V. Cros, Paris Unité Mixte de Physique CNRS/Thales www.trt.thalesgroup.com/ump-cnrs-thales/phonebook/cros.htm

Monday 1st February at 2.15 pm transmission Besprechungsraum

Title: “Origin and control of phase noise of spin transfer vortex oscillators: From mode coupling to mutual electrical synchronization”

26. January:Dr. Jaime Sánchez-Barriga, Abteilung für Magnetisierungsdynamik (M-A1)

Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Wilhelm-Conrad-Röntgen-Campus (Adlershof),

Elektronenspeicherring BESSY-II

"Spin-based phenomena in topological insulators: From the ground state to dynamics"

Abstract:

In this talk, I will provide an overview of our activity on spin-based phenomena in topological insulators primarily using time-, spin- and angle-resolved photoemission. In the first part of the talk, I will concentrate on the spin-dependent properties of topological surface states (TSSs) in equilibrium by highlighting our recent results using both synchrotron- and laser-based photoemission. This includes the observation of anisotropic lifetimes due to spin-dependent scattering in the presence of warping, circular-dichroic effects, the impact of magnetic impurities and the manipulation of thephotoelectron spin polarization of TSSs. In the second part of the talk, I will focus on dynamical aspects such as the observation of anisotropic coherent-phonon oscillations or the electron relaxation dynamics near the critical point of a trivial to topological quantum-phase transition. Finally, I will present our recent results on the observation of ultrafast spin-polarized electrical currents originating from Dirac fermions following optical excitation with circularly-polarized femtosecond infrared pulses.

1. December: Jarosław W. Kłos,Faculty of Physics, Adam Mickiewicz University in Poznań

"Spin waves in periodic nanostructures"

Abstract:

The dispersion of spin waves in periodic magnetic structures is characterized by the presence of frequency gaps and bands, similarly as for other kind of excitations in differed sorts of periodic media: photonic crystals or phononic crystals. However the description of the magnonic systems is much more complex that their counterparts. The main features making the magnonic system peculiar are: (i) spin wave frequency spectrum do not scale with the sizes of the system due to different range of exchange and dipolar interactions, (ii) dipolar interactions can be are anisotropic and non-reciprocal for confined (e.g. planar) geometries, (iii) the spin wave dynamics is not fully determined by the geometry but depends also on static magnetic configuration which is a function of external field.

The results of numerical investigations of spin waves dynamics in periodic and quasiperiodic planar magonic crystals will be presented for bi-component systems or antidotlattices. The simulation were performed both in real space and time domain (where the propagation of Gaussian beams were considered) and for reciprocal space and frequency domain (where the spin wave dispersion was investigated).  The following problems will be shortly discussed: (1) impact of the shape of inclusions on spin wave spectrum in planar magnonic crystals, (2) universal dependence of the frequencies of spin  wave modes on the ratio  of ‘separation between antidotes’ to ‘thickness’  for planar magnonic antidote lattice, (3) influence of structural changes and static external field of the spectrum of periodic (antidot based) magnonic waveguide, (4) non-reciprocal spin wave propagation in magnetic slab with corrugated top face, (5) spin wave localization in Fibonacci sequence of magnetic wires, (6) Goos‑Hӓnchen shift of the spin wave beams on the interface of two magnetic materials, (7) all angle collimation of the magnonic Gaussian beams on the interface homogeneous and patterned magnetic layer.

Programm SoSe 2015

12. May: Henning Ulrichs, I. Phys. Institute, Georg-August Universität Göttingen

"Simulation of pump-probe reflectivity experiments"

Abstract:

In this talk I will shed some light on the physics involved in ultrafast laser heating. In particular a numerical model will be explained, which allows to simulate pump-probe reflectivity experiments. If a fs laser pulse hits a surface, energy deposited in the vicinity of the surface leads to a local temperature increase, starting a thermal diffusion. Simultaneously the lattice expands, and the resulting thermal stress excites elastic dynamics. Experimental reflectivity spectra carry information from all these processes. By means of the numerical modelling the spectra can be interpreted. Some examples will be discussed, which illustrate the benefits and limitations of the numerical approach.

The work presented here is part of the SFB 1073 'Atomic scale control of energy conversion'.

2. June: Ulrich Parlitz, Max Planck Institute for Dynamics and Self-Organization

"Nonlinear dynamics of the heart"

Abstract:

Physiological and pathological states of the heart are governed by complex spatial-temporal dynamics. Therefore, concepts of the theory of nonlinear dynamical systems provide novel perspectives to enhance understanding of cardiac dynamics and arrhythmias, including experimental and theoretical approaches towards modeling, analysis and control of electrical forms of heart disease.

This approach will be discussed in the context of cardiac arrhythmias, a highly significant cause of mortality and morbidity worldwide. The term dynamical disease was coined for cardiac arrhythmias, suggesting that they can be best understood from the dynamical system’s perspective, integrating multidisciplinary research on all relevant spatial and temporal scales.

In the presentation we shall show how cardiac arrhythmias are a result of underlying complex spatial-temporal electrical excitation patterns following fast developing electro-mechanical instabilities. These dynamical states can be detected and classified using optical mapping and time series analysis. Furthermore, mathematical models of (collective) cell activities will be introduced and evaluated. Finally, a novel approach (LEAP) for terminating cardiac arrhythmias using low-energy pulses will presented.

16. June: Stefan Heinze,Institut für Theoretische Physik und Astrophysik, Christian-Albrecht-Universität zu Kiel

"Emergence of magnetic skyrmions at transition-metal interfaces"

Magnetic skyrmions are localized, topologically protected spin structures. They offer attractive perspectives for future spintronic applications [1,2] since they can be manipulated at electric current densities which are by orders of magnitude lower than those required for domain wall motion [3,4]. They were first observed in bulk magnets with a particular chiral crystal symmetry limiting the number of available systems and the adjustability of their properties. Recently, it has been discovered that due to the broken inversion symmetry at surfaces magnetic skyrmions can also occur in ultra-thin transition metal films [5,6] which opened a new class of systems.

Here, I will discuss the emergence of skyrmion phases in such ultra-thin transition metal films on surfaces and in transition-metal multilayers. In these systems the interplay of the exchange and the Dzyaloshinskii-Moriya interaction controls skyrmion formation and skyrmion properties can be tailored by interface engineering [7,8]. I will explain the origin of the experimentally observed skyrmion phases in an Fe monolayer and in a PdFe bilayer film on the Ir(111) surface [5-7] based on first-principles electronic structure theory. More attractive in terms of spintronic applications are transition-metal multilayers. Based on our first-principles based approach, we predict the occurrence of skyrmions in these systems and study their properties [8].

[1] N. Nagaosa and Y. Tokura, Nature Nanotech. 8, 899 (2013).

[2] A. Fert et al., Nature Nanotech. 8, 152 (2013).

[3] F. Jonietz et al., Science 330, 1648 (2010).

[4] X. Z. Yu et al., Nature Comm. 3, 988 (2012).

[5] S. Heinze et al., Nature Phys. 7, 713 (2011).

[6] N. Romming et al., Science 341, 636 (2013).

[7] B. Dupé et al., Nature Comm. 5, 4030 (2014).

[8] B. Dupé et al., arXiv:1503.08098.

23. June: Karin A. Dahmen, University of Illinois at Urbana-Champaign

"Universal slip statistics: from nanopillars to earthquakes"

Abstract:

The deformation of many solid materials is not continuous, but discrete, with intermittent slips similar to earthquakes. Here, we suggest that the statistical distributions of the slips, such as the slip-size distributions, reflect tuned criticality, with approximately the same regular (power-law) functions, and the same tunable exponential cutoffs, for systems spanning 13 decades in length, from tens of nanometers to hundreds of kilometers; for compressed nano-crystals, to amorphous materials, ]to earthquakes. The similarities are explained by a simple analytic model, which suggests that results are transferable across scales. This study provides a unified understanding of fundamental properties of shear-induced deformation in a wide range of systems. It also provides many new predictions for future experiments and simulations. The studies draw on methods from the theory of phase transitions, the renormalization group, and numerical simulations. Connections to other systems with avalanches, such as magnets and neuron firing avalanches in the brain are also discussed.

25 June (Thursday! 14:30): Dr. Helmut Schultheiß, Helmholtz-Zentrum Dresden-Rossendorf, Institut of Ion Beam Physics and Material Research

"Channeling, steering and detection of spin waves for magnonic applications"

Abstract:

The coherent transport of spin information is one of the great challenges in condensed matter physics and is of fundamental importance for the development of spintronic devices. Similar to spin currents, spin waves carry angular momentum and can be utilized to transport spin information over distances much larger than the spin diffusion length in metals. Recent experiments showing that spin waves can be manipulated via spin currents and vice versa due to spin torque, spin pumping, spin Hall and spin Seebeck effects have drawn great attention to the transport properties of spin waves. In this talk I will discuss spin-wave propagation in microstructures with reduced dimensionality. The inherent anisotropy of the spin-wave dispersion in thin film magnetic elements does not only impose challenges on spin-wave transport but also brings about advantages for systematically manipulating their propagation on small length scales. In the first part I will discuss how local magnetic fields arising from charge currents can be used to alter the magnetization direction in microstructures and, therefore, allow for guiding spin waves in curved geometries [1]. This concept of manipulating spin waves by locally rotating the magnetization vector is then applied to actually switch the propagation path of spin waves in a multiplexer [2]. We use Brillouin light scattering microscopy to address these topics in micron-sized spin-wave conduits made from permalloy. Besides this optical approach, which allows us to map the spin- wave intensity in spin-wave conduits with sub-micrometer resolution, we recently discovered a thermo-electric method for the detection of spin waves [3]. All spin waves, independent from their wavelength, generate heat due to dissipation of energy. This heat drains into the substrate of the sample and creates a temperature gradient normal to the sample plane and this results in an electromotive force via the anomalous Nernst effect. I will present how this effect can be used to detect spin waves in microstructures with a wavelength much smaller than accessible in conventional light scattering experiments.

Financial support by the Deutsche Forschungsgemeinschaft within project SCHU2922/1-1 is gratefully acknowledged. The work at Argonne was supported by the U.S. Department of Energy, Office of Science, Materials Sciences and Engineering Division. Lithographic patterning was carried out at the Center for Nanoscale Materials, which is supported by DOE, Office of Science, BES (#DE-AC02-06CH11357). The Carl-Zeiss-Stiftung supported the work of K. Vogt.

[1] K. Vogt, et al., Appl. Phys. Lett. 101, 042410 (2012).

[2] K. Vogt, et al., Nature Commun. 5, 3727 (2014).

[3] H. Schultheiss, et al., Phys. Rev. Lett. 109, 237204 (2012)