Gruppenseminar "Spintransport und Ultrakurzzeitdynamik"

Group guests and research reports 

30. April, 2018

16:15, Alexander Paarmann, Fritz-Haber Institute, Berlin

7. Mai 2018

16:15, Dennis Nenno, TU Kaiserslautern

21. Mai, 2018

16:15, Raghavendra Palankar, Unimedizin Greifswald

Group research reports 

24. April, 2018

16:15, Jaroslaw Klos, Krupp Wissenschaftskolleg and Poznan University

"Spin wave pinning in CoFeB Stripes"

8. May, 2018

16:15, Justina Rychly, Poznan University

"Surface states in the sequence of magnetic stripes"

29. May, 2018

16:15, Neha Jah, Greifswald University

"OMTJs with PLY-Cu"

12. June, 2018

16:15, Christian Denker/ Yannik Junk, Greifswald University

"Skyrmioncs"

19. June, 2018

16:15, Yuta Sasaki, Tohoku University

"THz Emitter I"

26. June, 2018

16:15, Nina Meyer/Tobias Kleinke, Greifswald University

"Photocurrents in topological insulator nanowires"

3. July, 2018

16:15, Tristan Winkel/ Finn F. Lietzow, Greifswald University

"THz Emitter II"

10. July, 2018

16:15, Jakob Waloswki, Greifswald University

"MOKE"

17. July, 2018

16:15, Markus Münzenberg, Greifswald University

"Thin film magnetism research: Yesterday, today and the day after tomorrow"

Group research reports 

5. Dezember, 2017 

10:15, Christian Denker, Universität Greifswald

„Skyrmion Bubbles in Ta/CoFeB/MgO“

28. November, 2017

10:5, Aleksandr Makarov, Universität Wladiwostok, Universität Greifswald

„Modelling Spin Ice II“

14. November, 2017

16:15, Robin John, Universität Greifswald

“Perspective and future possibilities of all-optical switching and report on small angle X-ray scattering of laser induced magnetic vortices”

21. November, 2017 

16:15, Yuriy Shevchenko, Universität Wladiwostok, Universität Greifswald

„Modelling Spin Ice I“

7. November, 2017 

16:15, Jaroslaw Klos, Universität Poznan, Universität Greifswald und Alfried Krupp Wissenschaftskolleg

„Magnonics“

Abstract:

Magnetic topological defects, including vortices and Skyrmions, can be stabilized as equilibrium structures by tuning intrinsic magnetic interactions and stray field geometries. Here, employing rapid quench conditions, we report the observation of a light-induced metastable magnetic texture, which consists of a dense nanoscale network of vortices and antivortices and exhibits glass-like properties. Our results highlight the emergence of complex ordering regimes in optically driven magnetic systems, opening up new pathways to harness ultrafast far-from-equilibrium relaxation processes.

References: T. Eggebrecht, M. Möller, J. G. Gatzmann, N. Rubiano da Silva, A. Feist, U. Martens, H. Ulrichs, M. Münzenberg, C. Ropers, S. Schäfer, arXiv:1609.04000v1.

Zeit:  Montag, 14:15 - 15:30 Uhr, 24. Oktober 2016 ACHTUNG - TERMIN GEÄNDERT AUF 31. Oktober 2016. 

Ort: Besprechungsraum Institut für Physik (Videoübertragung)

Abstract:

This talk is about a well-defined large scale hybrid neural network on semicon-
ductor microtube arrays. A quantum well is embedded in the
multi-layered wall of the microtube as a special feature giving it optically
active characteristics. Furthermore, a novel method of optical detection of
action potentials via optically active microtubes is proposed and supported
with experiments and simulations on artificial axons.

Time: Thursday, October 13th, 10:15
Location: Besprechungsraum Institut für Physik

18. July, 16:15: S. Sharma, Max-Planck-Institut für Mikrostrukturphysik, Weinberg 2, D-06120 Halle, Germany.

Title: Laser induced ultrafast demagnetization in solids: a time-dependent density functional theory perspective

Abstract:
Ultrafast manipulation of spins in a controlled manner is a milestone of solid state physics. The motivation for this is to use electronic spin for storing binary data, which can then be optically manipulated using lasers. The advantage of such a technique would be reduction in the size and efficiency of the storage device by several orders of magnitude. Recent experiments have demonstrated that demagnetization or spin-reorientation processes can be induced by femtosecond laser pulses. However, we are still far from achieving optimally controlled manipulation of spins required for production of devices. One of the reasons behind this is the lack of full understanding of the phenomena leading to demagnetization.
Time-dependent density functional theory (TDDFT) is a formally exact method for describing the real-time dynamics of electrons under the influence of an external field -- for example vector potential of the intense laser pulse. We use spin-resolved TDDFT to study of the process of optical demagnetization. The advantage of such a technique is clear from the fact that it is fully ab-initio in nature.
Our analysis shows that the demagnetization is a highly system dependent phenomenum; in bulk of simple metals like Fe, Co and Ni demagnetisation occurs as a two step process where first the electrons make transitions to excited states, followed by spin-orbit-mediated spin-flip transitions which lead to a loss of moment. In the bulk of complex systems like Heuslers demagnetization can partially occur without spin-orbit interaction, just by transfer of moment between two different spin-lattices. In metallic interfaces this process of demagnetisation is, to a large degree, dominated by spin currents across the interface. These observations appear to preclude the development of a single simple model describing spin dynamics in materials on short time scales.

Di 12.7. 16:00, J. Malindretos, IV. Phys. Institut, Universität Göttingen

Title „Spin injection in epitaxial MnGa/GaN”

Abstract:
The efficient realization of spin-injection and spin-detection is a prerequisite for many new device concepts in the field of semiconductor spintronics. However the conductivity mismatch in hybrid metal/semiconductor systems generally limits the spin-injection efficiency in the diffusive transport regime. Still, tunnel barriers might be employed in order to overcome this problem.
In this seminar, an experimental study of spin injection through reverse biased MnGa/GaN Schottky diodes is presented. Ferromagnetic MnGa(111) layers with high crystal quality were grown on GaN(0001) by molecular beam epitaxy. The doping of the structure was optimized to achieve single step tunneling across the metal/semiconductor junction. Spin injection experiments were conducted in a vertical LED device in Faraday geometry, where the degree of circular polarization of the emitted light is directly connected to the electron spin polarization in the active region. Interface mixing and spin polarization losses during the electrical transport in the GaN layer towards the quantum well detector are discussed in relation with the low value of the detected spin polarization.

8. July, 16:15: Vasily Temnov,IMMM CNRS 6283 (Le Mans, France) and Fritz-Haber Institute of the Max-Planck Society (Berlin, Germany)

Title "Ultrafast magneto-elastic interactions"

Abstract:
In nearly all experiments where ultrashort optical pulses are partially absorbed by an optically opaque solid sample, a rapid heating results in the generation of thermo-elastic stress, which launches ultrafast acoustic transients propagating inside the sample. While studying the ultrafast dynamics in magnetic materials  excited by femtosecond laser pulses, most efforts were previously focused on the understanding of ultrafastmdemagnetization whereas the magneto-acoustic interactions were largely neglected. The purpose of this talk is to fill this gap and review some recent progress in ultrafast magneto-elastics, where I will discuss two following examples.
First, femtosecond transient grating experiments are used to quantify the thermo-elastic generation of different surface acoustic modes observed through the resonant magneto-elastic excitation of ferromagnetic resonance (FMR) precession. Signatures of the nonlinear magneto-elastic interactions are evidenced by the experimental observation of the magneto-elastic sum frequency generation in FMR precession, in a good agreement with the solutions of elastically driven Landau-Lifshitz-Gilbert (LLG) equations.
Second, interaction of ultrashort (picosecond)  acoustic pulses with ferromagnetic thin films does not only lead to a well-known phenomenon of excitation and coherent control of FMR precession, but can also result in the excitation of high-order exchange coupled magnons. The preliminary theoretical interpretation of this recent experimental observation will be discussed.

17. Juni: Dr. Amilcar Bedoya-Pinto, Max-Planck Institute of Microstructure, Halle
Ort: 16:15 Grosser Seminarraum

Title “Spintronics with molecules:  Spin-Valves, Spinterfaces and Beyond”
 
Abstract:
In this talk, I will briefly review the advances and development of molecular spintronics over the last years, and, in particular, present results of molecular spin-valve devices showing functional properties beyond magnetoresistance [1]. Furthermore, I will address novel effects that emerge at certain metal-molecule interfaces, such as spin-dependent hybridization (Spinterface) [2], or the formation of magnetically active interfaces under strong metal-molecule interaction [3].  On the other hand, I will drive the attention to the special care that should be taken by the fabrication of vertical molecular-based devices, in view of the plethora of results found in literature.  In this respect, the routes to distinguish a genuine charge and spin transport across molecular states [4] will be presented and exemplified.  Finally, a grasp of the new trends and perspectives in molecular spintronics will be addressed, motivated by the recent observation of Spin-crossover or Spin-Hall effect in molecular materials.

[1]  X.Sun, A. Bedoya-Pinto, et.al. Active Morphology Control for Concomitant Long Distance Spin Transport and Photoresponse in a Single Organic Device. Advanced Materials28, 13, 2609 (2016)
[2] Sanvito, S.  Molecular spintronics: the rise of spinterface science.  Nature Physics 6, 562–564 (2010).
[3] A. Bedoya-Pinto, et.al. Rare-earth quinoline molecules on Cu surfaces: A chemically driven magnetic interface (submitted).
[4]A. Bedoya-Pinto, et.al. Spin-polarized hopping transport in magnetically tunable rare-earth quinolines. Advanced Electronic Materials 28, 13, 2609 (2015)

9. Mai: Torsten Huebner,Thin films and physics of nanostructures, Bielefeld University
Ort: 16:15 Besprechungsraum

Title "Comparison of the laser induced and intrinsic tunnel magneto-Seebeck effect"

Abstract:
We present a comparison of the tunnel magneto-Seebeck effect for laser induced and intrinsic heating. Therefore, CoFeB/MgAl2O4 and CoFeB/MgO magnetic tunnel junctions have been prepared. We find results using the intrinsic method that differ in sign and magnitude in comparison to the results of the laser heating. The intrinsic contributions can alternatively be explained by the Brinkman model and the given junction properties. Thus, we conclude that the symmetry analysis used for the intrinsic method is not suitable to unambiguously identify an intrinsic tunnel magneto-Seebeck effect.
 
[1] Time-resolved measurement of the tunnel magneto-Seebeck effect in a single magnetic tunnel junction, A. Boehnke et al. Rev. Sci. Instrum. 84, 063905 (2013).
[2] Comparison of laser induced and intrinsic tunnel magneto-Seebeck effect in CoFeB/MgAl2O4 and CoFeB/MgO magnetic tunnel junctions, Torsten Huebner, et al. arXiv:1604.07569
 

Monday 01.02: Prof. Dr. V. Cros, Paris Unité Mixte de Physique CNRS/Thaleshttp://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, 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 the photoelectron 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.

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)