Surface or interface magnetism

Time: Tuesday 10:30–13:00

Location: H48

O 28.1

Tue 10:30

H48

Electron pair emission from ferromagnetic surfaces — ∙Frank O. Schumann1, Carsten Winkler1, Jürgen Kirschner1, Franz Giebels2, Herbert Gollisch2_{, and Roland Feder}2 _— 1_Max-

Planck-Institut für Mikrostrukturphysik, 06120 Halle, Germany —

2_{Theoretische Festkörperphysik,Universität Duisburg-Essen, 47048}

Duisburg, Germany

We present a combined experimental and theoretical study of the elec- tron pair emission from a Fe(001) surface which is excited by a spin- polarized primary electron beam. The aim is to study the spin depen- dence of the exchange-correlation hole. The in-plane magnetization direction of the Fe sample can be controlled by an external magnetic field. A spin-polarized primary beam hits the Fe(001) surface along the surface normal and the spin polarization of the beam is either parallel or antiparallel to the magnetization direction. The emitted electron pairs are detected via a time-of-flight coincidence set-up. The data are grouped into two subsets for which the spin polarization of the major- ity electrons is either parallel or antiparallel to the primary beam. We find that the angular distributions of the coincidence intensity are spin dependent. Furthermore, we are able to identify favorable conditions, which ensure that the scattering partner of the primary electron has a high spin polarization. This allows us to separate the contribution of the exchange from the Coulomb correlation to the size of the exchange- correlation hole. We find that the exchange part has a larger extension than the Coulomb part confirming a suggestion by Slater more than 75 years ago.

O 28.2

Tue 10:45

H48

Spin Polarized PES on interface states of MgO/Fe/GaAs(100) — ∙Daniel Gottlob1,2, Lukasz Plucinski2, Carsten Westphal1, and Claus M. Schneider2 —1Experimentelle Physik 1 - Technische Universität Dortmund, Otto-Hahn-Str. 4, D-44221 Dortmund, Germany — 2_{Institurte of Solid State Research - IFF-9}

Electronic Properties - Research Center Jülich, D-52425 Jülich, Ger- many

Spintronics is an important field of current Solid State Research and Magnetic Tunnel Junctions (MTJ’s) now are within our grasp. In MTJ’s the nature of the electronic structure at the interface determins the tunneling process, and thereby the magnetoresistive potential of the MTJ.

Electronic interface states can have influence on the tunneling pro- cess in epitaxial MTJs especially for thinner tunnel barriers. At our ongoing research we will take a closer look at an off-normal surface state of Fe/GaAs(100) and see whether it still exists as an interface state if we cap the Fe by 1-3 monolayers of MgO. We will collect spin- polarized spectra to confirm the spin polarization in this band after the evaporation of MgO.

The measurements take place at Beamline 5 at DELTA, Dortmund, with a unique detector setup. We can acquire 2-dimensional angle resolved data for band mapping and spin-polarized one-dimensional data quasi-simultaneously. Our samples are prepared in-situ by e-beam evaporation and characterized by LEED and Auger spectroscopy.

O 28.3

Tue 11:00

H48

Range of spin-polarization on the (111) surface of platinum induced by the proximity of cobalt stripes — ∙Focko Meier, Lihui Zhou, Jens Wiebe, and Roland Wiesendanger — Institute of Applied Physics, Hamburg University, Germany

The properties of magnetic nanostructures supported by metallic sub- strates are strongly governed by the electronic interaction between nanostructure atoms and substrate atoms. For the model system of Co impurities dissolved in Pt it is well known that these interactions lead to induced magnetic moments in the neighbouring Pt atoms forming a so called giant moment systems [1]. Furthermore it has been revealed that Pt conduction electrons mediate an RKKY exchange interaction between localized magnetic moments which relies on a spatially os- cillating spin-polarization [2]. However it is yet unclear how these two effects interfere. By applying spin-resolved scanning tunnelling spectroscopy at 0.3 K we investigated how the polarization of Pt is influenced in the vicinity of Co stripes on a Pt(111) surface. Our re- sults show that the Pt density of states near the Fermi energy shows a response to the magnetic orientation of the Co stripe. Interestingly

this response can be observed for distances larger than 1 nm from the stripe where the RKKY interaction is already antiferromagnetic.

[1] Graham and Schreiber, PRL 17, 650 (1966) [2] Meier et al., Sci- ence 320, 82 (2008)

O 28.4

Tue 11:15

H48

The effect of postgrowth oxygen exposure on the magnetic properties of Ni on the Cu-CuO stripe phase — ∙Mariella Denk, Richard Denk, Michael Hohage, Lidong Sun, and Pe- ter Zeppenfeld — Institut für Experimentalphysik, Johannes Kepler Universität Linz, A-4040 Linz, Austria

The magnetism and morphology of thin Ni films deposited on clean and oxygen covered Cu(110) has been studied. Scanning Tunneling Mi- croscopy (STM), as well as Reflectance Difference Spectroscopy (RDS) are used to characterise the sample properties. The sensitivity of the RDS to the polar Magneto-Optic Kerr Effect has been exploited (RD- MOKE). Contrary to growth on pristine Cu(110), thin Ni films on oxygen covered Cu(110)-(2x1)O show a spin reorientation transition from in-plane to out-of-plane magnetisation at 9 ML Ni coverage [1], [2]. For Ni films evaporated on the Cu-CuO stripe phase, which con- sists of a periodic array of (2x1) reconstructed CuO stripes separated by bare Cu, the magnetic easy axis lies completely in-plane up to a coverage of 22.5 ML. Exceeding this coverage, a small remanent mag- netisation component pointing out-of-plane evolves. Upon postgrowth oxygen exposure the Ni film becomes completely out-of-plane magne- tised. STM images show a fully (2x1)O reconstructed surface after the oxygen exposure, but no morphological changes of the Ni film. We thus conclude that the oxygen strongly modifies the surface magnetic anisotropy.

[1] Th. Herrmann et al., Phys. Rev. B 73, 134408 (2006) [2] R. Denk et al., Phys. Rev. B 79, 073407 (2009)

O 28.5

Tue 11:30

H48

Nanoscale Iron with Extraordinary Magnetic Anisotropy — ∙Thomas Lödding1_,

Christian Praetorius1, Greg A. Ballentine1,4, Kai Fauth1,3, Armin Kleibert2,5, Norman Wilken2, Andris Voitkans2, and Karl-Heinz Meiwes-Broer2—

1_{Physikalisches Institut, Universität Würzburg, Am Hubland, 97074}

Würzburg, Germany — 2_{Institute of Physics, Rostock University,}

18051 Rostock, Germany —3MPI for Metals Research, Heisenbergstr. 3, 70569 Stuttgart, Germany — 4_{Physics Dpt., Brandon University,}

270 18th street, Brandon, Canada R7A 6A9 —5_{Swiss Light Source,}

Paul Scherrer Institute, 5232 Villigen, Switzerland

We shall show that iron nanocluster ensembles, prepared by deposition of preformed particles onto Cu(111), possess unusual and unexpected features in their magnetic response. We have studied the magnetic moments and response to applied fields using X-ray magnetic circu- lar dichroism as a sensitive element specific probe of the nanocluster magetism. We show that clusters with average diameters of 4 nm and 6 nm not only possess an enhanced magnetization at low temperature but also display an enhanced magnetic anisotropy energy density.

Overmore, a small but non-negligible fraction (≈ 10 %) of the deposited clusters possess extraordinarily large switching fields, i. e. 𝜇0𝐻𝑆𝑊> 0.5 T at 𝑇 = 12 K for particles with 6 nm diameter. It

is evident from our experiments that these large switching fields corre- spond to intrinsic properties of individual deposited nanoclusters. We shall discuss possible origins of this behavior.

O 28.6

Tue 11:45

H48

Correlation between stress, intermixing and magnetic anisotropy of Pt/Co/Pt (111) monolayers — ∙Anita Dhaka, Safia Ouazi, Zhen Tian, Dirk Sander, and Jürgen Kirschner — Max-Planck-Institut für Mikrostrukturphysik, Weinberg 2, 061120 Halle (Germany)

We present the results of a combined stress, low energy electron diffrac- tion (LEED) and magneto-optical kerr-effect (MOKE) study on the spin reorientation transition (SRT) of Co monolayers (ML) on Pt(111). We observe a SRT from out-of-plane to in-plane with increasing Co thickness at 3 ML at 300 K. The deposition of one layer Pt on top of 5 ML Co induces a SRT from in-plane to out-of plane. Our stress and LEED measurements identify intermixing at the Co/Pt(111) in- terface [1].Stress measurements reveal a stronger intermixing at 370

K, as compared to growth at 300 K. Co growth at 370 K leads to a shift of the SRT from out-of-plane to in-plane to a larger thickness of 5 ML. Our results suggest that an intermixed Pt-Co interface favors an out-of-plane easy magnetisation direction, whereas a Co-vacuum interface favors an in-plane easy magnetization direction.

[1] O. Robach, C. Quiros, P. Steadman, K. F. Peters, E. Lundgren, J.Alvarez, H.Isern, and S. Ferrer: Phys. Rev. B 65, 054423 (2002)

O 28.7

Tue 12:00

H48

Switching a single spin on metal surfaces: ab initio stud- ies — ∙Kun Tao1_{, Valeri.S Stepanyuk}1_{, Wolfram Hergert}2_,

Ivan Rungger3, Stefano Sanvito3, and Patrick Bruno4 —

1_{Max-Planck-Institute of Microstructure Physics, Halle, Germany}

—2Fachbereich Physik, Martin-Luther-University, Halle-Wittenberg, Friedemann-Bach-Platz 6, D-06099 Halle, Germany — 3_{School of}

Physics and CRANN, Trinity College, Dublin 2, Ireland —4_European

Synchrotron Radiation Facility, F-38043 Grenoble Cedex, France We perform ab initio calculations of the the exchange coupling be- tween single magnetic adatoms adsorbed on metal surfaces and a spin polarized STM tip. We apply density functional theory (DFT) based methods and perform calculations in the fully relaxed geometries for the tip and the substrate. We demonstrate that the spin direction of single adatoms can be controlled by varying the tip-substrate dis- tance. The sign of the exchange energy is determined by the com- petition of the direct and the indirect interactions between the tip and the adatom[1]. Based the nonequilibrium Green function method, we perform the spin-dependent transport calculations and find a large magnetoresistance of the junction at short tip-substrate distances.

[1] Kun Tao, V.S. Stepanyuk, W. Herget, I. Rungger, S. Sanvito, P. Bruno, Phys. Rev. Lett 103, 057202 (2009)

O 28.8

Tue 12:15

H48

Size-dependent spin structures in individual Fe nanoparti- cles observed by XPEEM — Arantxa Fraile Rodriguez1_,

Armin Kleibert1, ∙Joachim Bansmann2, Laura Heyderman1, and Frithjof Nolting1_—1_{Paul Scherrer Institut, Villigen PSI, CH-}

5232 Switzerland —2Institute of Surface Chemistry and Catalysis, Ulm University, D-89069 Ulm

By combining x-ray magnetic circular dichroism (XMCD) with pho- toemission electron microscopy (PEEM), we present in situ observa- tions of the magnetization orientation of individual Fe nanoparticles in a size range from 5 to 25 nm being in contact with a ferromag- netic Co support. Our results reveal that the magnetic moments of smaller particles (below 8 nm) are aligned parallel to the magnetic domains of the substrate while a non-collinear alignment between par- ticles and substrate is observed for larger sizes. Numerical model calcu- lations reproduce the experimental trend and reveal a transition from an exchange- to an anisotropy-dominated regime: the smaller particles are in a single-domain state collinearly aligned to the substrate while the larger particles exhibit a spiral-like magnetic structure determined by the magnetic anisotropy energy. These results demonstrate that the balance between the local particle-substrate interaction and the individual properties of the particles can lead to unexpected spin ar- rangements.

O 28.9

Tue 12:30

H48

Intercalation of iron underneath graphene on Ni(111): PES and XMCD study — ∙Martin Weser, Karsten Horn, and Yuriy Dedkov — Fritz-Haber Institut der Max-Planck Gesellschaft, Berlin, Germany

Magnetic thin films with out-of-plane or perpendicular magnetic anisotropy play an important role in nanotechnology. Such systems can be used as perpendicular recording media, which is predicted to allow information storage densities of up to 1 Tbit/in.2_{a quadrupling}

of today’s highest areal densities. Along with the widely used mate- rials with out-of-plane magnetic anisotropy such as CoPt or FePt al- loys, 𝑓 𝑐𝑐 Fe thin films also showing perpendicular magnetic anisotropy have recently attracted considerable interest as possible candidates for applications in novel magnetic data storage devices. Here Fe which originally has 𝑏𝑐𝑐 structure can be stabilized in 𝑓 𝑐𝑐 𝛾-phase at room temperature in thin epitaxial films grown on suitable 𝑓 𝑐𝑐 substrates. The aim of the present work is the preparation of a 𝑓 𝑐𝑐 Fe-based sys- tem with potential out-of-plane magnetic anisotropy which behaves inert against an aggressive environment. Here we demonstrate the possibility to prepare such a system via intercalation of a thin Fe film underneath a graphene layer formed on a Ni(111) substrate. Here, we demonstrate that graphene behaves like a passivation layer conserving the underlying epitaxial Fe film. Electronic and magnetic properties of this system were studied by means of angle-resolved PES and XMCD, respectively.

O 28.10

Tue 12:45

H48

Hard x-ray standing-wave excited photoemission experi- ments on the MgO/Fe interface — ∙Sven Döring1_{, Frank}

Schönbohm1, Ulf Berges1, Charles S. Fadley2,3, Daniel E. Bürgler4, Mihaela Gorgoi5, Walter Braun5, Claus M. Schneider4, and Carsten Westphal1—1DELTA/Experimentelle Physik I, TU Dortmund, Otto-Hahn-Str. 4, 44221 Dortmund, Ger- many — 2_{Materials and Sciences Division, Lawrence Berkeley Na-}

tional Laboratory, Berkeley, CA 94720, USA —3_{University of Cal-}

ifornia, Davis, CA 95616, USA —4_{Institut für Festkörperforschung,}

IFF-9, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany —

5_{Helmholtz-Zentrum Berlin - BESSY, Albert-Einstein-Str. 15, 12489}

Berlin, Germany

For layer systems, photoemission experiments with high and reliable depth resolution are essential in order to distinguish between interface effects and bulk photoemission signals. Here we extend the soft x-ray standing-wave photoemission method into the hard x-ray regime to achieve the required high depth resolution and high probing depth. An Fe wedge was grown on top of a MoSi2/Si multilayer with a pe-

riod of about 40 Å and subsequently covered by a thin MgO film. By moving the wedge structure with respect to the x-ray beam we were able to observe several cycles of the x-ray standing wave field moving through the sample layers. The analysis of the photoemission intensity modulations results in a well-resolved depth-profile of the sample. At the interface the intermixing of Fe and MgO can be studied due to the enhanced interface sensitivity provided by the x-ray standing field.

O 29: [CPP] Organic Electronics and Photovoltaics I (Joint Session DS/CPP/HL/O)

Time: Tuesday 9:30–12:45

Location: H37

O 29.1

Tue 9:30

H37

Side chain variations on dicyanovinyl-oligothiophenes stud- ied by photoinduced absorption spectroscopy: consequences for small molecule organic solar cells — ∙Hannah Ziehlke1_,

Roland Fitzner2, Christian Koerner1, Egon Reinold2, Peter Baeuerle2, Karl Leo1_{, and Moritz Riede}1_—1_{Institut für Ange-}

wandte Photophysik, Technische Universität Dresden —2_{Institut für}

Organische Chemie II und Neue Materialien, Universität Ulm Oligothiophenes capped with electron-withdrawing dicyanovinyl groups (DCVnT) are promising candidates for applications in small molecule organic solar cells. These oligothiophene derivatives can act as electron donor in blend layers with C60. Single heterojunction de-

vices reach efficiencies above 4 %. The introduction of side chains as intermolecular spacer has little effect on the energetics and mainly in- fluences the morphology of the evaporated thin film. In addition, the

distance between donor and acceptor molecules and thus the energy transfer and charge separation at the interface can be controlled. The energetic and dynamic properties of the long lived photoexcitations in pristine and blend layers can be obtained by photoinduced absorp- tion spectroscopy (PIA). We here characterize of DCV3T with zero, two, and four alkyl chains (Methyl and Butyl) via PIA spectroscopy complemented with morphological studies. Our results indicate that the energy transfer process at the heterojunction is more efficient for smaller spatial distances between donor and acceptor molecules.

O 29.2

Tue 9:45

H37

New multiwall molecular organic nanotubes — ∙Maren Rastedt1, Frauke Kutscher1, Okko Frey2, Rüdiger Beckhaus2, Christian Maibohm3_{, and Katharina Al-Shamery}1

— 1_{University of Oldenburg, Physical Chemistry 1, Germany}

3_{NanoSYD, Sonderborg, Denmark}

Nanotubes can be described as the newest star in the evening sky of nanotechnology. Since the discovery of carbon nanotubes , this nano- structure has grasped the mind of many. This structure has many application possibilities ranging from liquid and gas storage to mi- croelectronics and sub wavelength optical components and fibers. In our presentation we will give an introduction to multiwall nanotubes made from Tetrabenzofluorene(Tbf)-derivates. These Tbf-nanotubes have attracted interest because of their many properties, e.g. crystal- lization and waveguiding. Our nanotubes are prepared by the elegant and simple process of template assisted assembly. The assembly pro- cess will be presented together with specific properties of different Tbf- nanotubes. By changing growth parameters in the assembly process we have a method for probing fundamental questions and properties of our nanotubes. One future application of our nanotubes could be as an active element in a photovoltaic device.

O 29.3

Tue 10:00

H37

Radical Molecular Wires — ∙Georg Heimel1, Egbert Zojer2, Lorenz Romaner3, Jean-Luc Brédas4, and Francesco Stellacci5 —1Institut für Physik, Humboldt-Universität zu Berlin, Germany — 2_{Institut of Solid State Physics, Graz University of}

Technology, Austria —3Chair of Atomistic Modeling and Design of Materials, University of Leoben, Austria — 4_{School of Chemistry}

and Biochemistry, Georgia Institute of Technology, Atlanta, USA —

5_{Department of Materials Science and Engineering, Massachusetts In-}

stitute of Technology, Cambridge, USA

The technological relevance and the functionality of semiconducting materials originate in the fact that their conductivity and the band alignment at the interfaces with other materials can be controlled through doping. Here, we consider the ultimate miniaturization of functional devices by computationally investigating the doping of molecular wires suspended between two metal electrodes.1 For repre- sentative systems we find that, upon doping, the molecular conductiv- ity is enhanced by more than two orders of magnitude. We elucidate the microscopic origin of this dramatic effect and present an intuitive picture, which rationalizes our observations in terms of Fermi-level pinning. Our results shed new light on recent experimental findings and, most importantly, in-depth understanding of the doping mecha- nism enables the targeted development of new functional components for sensing and switching at the single-molecule scale.

[1] G. Heimel et al., Nano Lett. 9, 2559 (2009).

O 29.4

Tue 10:15

H37

Structure property relationship in aza-bodipy absorber ma- terials for organic photovoltaics — ∙Roland Gresser, Toni Mueller, Moritz Philipp Hein, Karl Leo, and Moritz Riede — Institute of Applied Photophysics, Dresden University of Technol- ogy, Germany

In this joint experimental and theoretical study, we focus on the struc- ture property relationship of aza-bodipy dyes as active donor materials in vacuum deposited small molecule solar cells.

The position of the materials HOMO can be intentionally varied by the choice of the functional group attached to the molecule. The absorption spectra show a red shift of the maximum with increasing donor strength of the substituents due to the increasing HOMO energy and decreasing band gap.

Based on crystal structure data, the charge carrier mobility deter- mining parameters like reorganization energies and transfer integrals are calculated. The results show an increasing molecular orbital over- lap and significant higher transfer integrals upon planarization and rigidification of the molecule. With this information, the observed charge carrier mobility differences from experiment can be explained. In addition to the electronic properties a high thermal and photo stability is essential. From combined thermogravimetric analysis and mass spectroscopy we could determine the degradation process of the material and were able to increase the thermal stability by substitution of the involved species.

O 29.5

Tue 10:30

H37

Dicyanovinyl sexithiophenes: self-organization and photo- voltaic properties — ∙Marieta Levichkova1_{, David Wynands}1_,

Alexandr Levin1, Karl Leo1, Karsten Walzer2, Dirk Hildebrandt2, Peter Bäuerle3_{, Rosina Rentenberger}4_{, and}

Moritz Riede1—1_{Institut für Angewandte Photophysik, TU Dres-}

den, Germany —2_{Heliatek GmbH, Dresden, Germany —}3_{Institut für}

Organische Chemie II und Neue Materialien, Universität Ulm, Ger- many —4_{Institut für Physik, TU Ilmenau, Germany}

Recently, vacuum deposited films consisting of conjugated dicyanovinyl-capped (DCV) oligothiophenes have shown significant potential as photoactive layers in small molecule solar cells [1]. Here, we study the structural and optical properties of films of two DCV- derivatives both comprising six thiophene rings (DCV6Ts) but having different side groups. For both derivatives, neat DCV6T and mixed DCV6T:C60 films are compared using UV-VIS absorption and photo- luminescence spectroscopy, X-ray diffraction (XRD), and Atomic Force Microscopy. It is shown that the modification of the molecular struc- ture results in a structured and red shifted absorption band, which indicates better molecular arrangement in the solid state. The im- proved self-organization at room temperature deposition is confirmed

In document Surface Science Division Fachverband Oberflächenphysik (O) (Page 41-45)