|Aufstellung der Publikationen der Arbeitsgruppe 2005|
Aufgeführt sind Arbeiten, die im Jahr 2005 publiziert
und zur Veröffentlichung eingereicht oder angenommen worden sind.
Sonderdrucke oder Preprints können von den Autoren oder im Sekretariat
des Lehrstuhls angefordert werden (Postanschrift: Humboldt-Universität
zu Berlin, Institut für Physik, Physik der Grenzflächen und
dünnen Schichten, Newtonstraße 15, 12489 Berlin, Tel.: (+49
30) 20 93 - 78 91, Fax: - 78 99).
1. M. Gierlings, M. Gruyters, D. Riegel, M. J. Prandolini,
T. Funk, and W. D. Brewer
We report the first observation of the effects of exchange bias on the nuclear spin polarization and induced magnetic moments at the magnetic/nonmagnetic interfaces in Co/Au(x)/CoO trilayer systems using low temperature nuclear orientation (LTNO). This technique allows us to determine simultaneously the average alignment of the nuclear moments for the two radioactive probe isotopes 198Au and 60Co with respect to an external magnetic field axis. The total average Au g-ray anisotropy measured was found (i) to decrease with increasing Au thickness, indicating that large hyperfine fields are restricted to the interfacial Au layers and (ii) to be canted away from the applied field axis even when the Co layers are magnetically saturated. This canting was found to originate at the CoO/Au interface as could be shown from comparative measurments on CoO/Au/CoO trilayers containing two AFM CoO/Au interfaces and on a Co/Au/Co trilayer with two FM Co/Au interfaces. In the case of CoO/Au/CoO, the observed canting was found to be dependent on the Au layer thickness.
2. Z. L. Fang, T. Bernhard, and H. Winter
Electron emission for impact of 25 keV H+ and He+ ions on a Cu(001) surface under a grazing angle of incidence is studied using a LEED setup with respect to energy of emitted electrons and crystallographic orientation of the target surface. The data can be related to the two-dimensional reciprocal lattice of the target surface and analyzed in terms of a modified Ewald construction. Our studies provide clear evidence for diffraction phenomena concerning ion induced emission of electrons and reveal new information on the excitation process.
The number of electrons emitted during grazing scattering of Ne atoms with keV energies from an Al(111) surface is recorded in coincidence with the energy loss of scattered projectiles. Irrespective of the total projectile energies used, we observe a pronounced increase of total electron emission yields, when the energy for motion normal to the surface exceeds about 25 eV. Based on energy loss spectra and classical computer simulations of projectile trajectories we attribute electron emission under these scattering conditions to a promotion mechanism in binary collisions between Ne and Al target atoms resulting in single and double excitations of projectiles.
Charge fractions after scattering of Ne+ ions, Ne0 atoms and Ar+ ions with keV energies under a grazing angle of incidence from an atomically clean and flat Al(111) surface are studied. For incoming Ne+ ions we observe defined ion fractions in the scattered beams, whereas for incident Ne0 atoms ion fractions are more than one order of magnitude smaller. This experimental result provides clear evidence for a survival of Ne+ ions over the whole scattering event. From the dependence of ion fractions on the perpendicular energy component we derive neutralization rates as function of distance from the surface. These rates compare well with recent theoretical calculations for the system He+-Al(111). For incident Ar+ ions no survival of ions is found and upper limits for the survival probability and lower limits for the neutralization rate are determined.
5. A. Schüller, S. Wethekam, A. Mertens, K.
Maass, H. Winter, and K. Gärtner
For grazing scattering of keV Ne and Ar atoms from a Ag(111) and a Cu(111) surface under axial surface channeling conditions we observe well defined peaks in the angular distributions for scattered projectiles. These peaks can be attributed to "rainbow-scattering" and are closely related to the geometry of potential energy surfaces which can be approxmated by the superposition of continuum potentials along strings of atoms in the surface plane. The dependence of rainbow angles on the scattering geometry provides stringent tests on the scattering potentials. From classical trajectory calculations based on universal (ZBL), adjusted Moliere (O'Connor and Biersack), and individual interatomic potentials we obtain corresponding rainbow angles for comparison with the experimental data. We find good overall agreement with the experiments for a description of trajectories based on adjusted Moliere and individual potentials, whereas the agreement is poorer for potentials with ZBL screening.
6. HP. Winter, S. Lederer, H. Winter, C. Lemell, and I. Burgdörfer
Near threshold behavior of electron emission for noble gas atoms impinging with keV energies on an Al(111) surface under a grazing angle of incidence is studied by coincident time-of-flight and electron number spectroscopy. From our data we conclude that other possible mechanisms for electron ejection than kinetic electron emission via binary encounter between atomic projectiles and conduction electrons can be excluded. With increasing atomic number of the noble gas atoms studied, we observe small but defined fractions of emitted electrons below the corresponding threshold for the latter process. This finding is interpreted in terms of Auger deexcitation of two spatially correlated hot electrons excited in collisions with single atomic projectiles.
The growth and chemical composition of Co on Cu(100) has been studied by He ion scattering and Auger electron spectroscopy (AES) at different temperatures (T=130 K, T=300 K and T=410 K). Based on different information depths for electron-induced Cu-MNN and Cu-LMM Auger signals and inner shell production by grazingly scattered protons restricted to the topmost layer, we have modelled the Auger data in order to obtain quantitative information on the chemical composition of ultrathin intermixed Co-Cu films. We find that intermixing and Cu diffusion is not only important for film growth at elevated temperatures, but also for heterogeneous Co/Cu epitaxy at room temperature.
The magnetic ordering in ultrathin Co films on Cu(001) has been investigated via the polarization of light emitted after capture of spin-polarized electrons into excited atomic terms during grazing He scattering. A strong increase in the Curie temperature and the remanent spin polarization has been observed at a critical Co thickness tCocr » 1.7 ML. In agreement with a recent study on the same heteroepitaxial system, this behavior is attributed to the percolation of Co islands. For growth at an elevated temperature of 410 K, hysteresis loops reveal a strong increase in the irreversibility field at which the loop closes. This behavior is explained by an inhomogeneous distribution of growth-induced local magnetic anisotropies which make a complete reversal of the magnetization more difficult.
The energy loss of He atoms with energies E £ 28 keV scattered from Al(111) under a grazing angle of incidence is recorded in coincidence with the number of electrons emitted in the collision. From our data we deduce the mean energies for emission of a specific number of electrons during impact of the atomic projectiles. We observe that only a fraction of some percent of the energie transferred to metal electrons results in their emission. The mean energy for emission of electrons in head-on binary collisions of He atoms with conduction electrons is estimated from a simple classical model which describes the data fairly well. For higher energies of the projectile motion with respect to the surface normal, i.e. collisions with surface atoms under smaller impact parameters, the mean energy transfer to emitted electrons is enhanced which is attributed to an electron promotion mechanism.
For scattering of fast atoms from metal surfaces under axial channeling conditions we observe pronounced peaks in the angular distributions of scattered projectiles which can be interpreted in terms of rainbow scattering. These structures are closely related to te interaction potentials which can be deduced from the data with good precision. We have scattered He, Ne, and N atoms with keV energies from Al(111), Cu(111), and Ag(111) surfaces and compare the observed rainbow structures using different approaches for the interatomic pair potentials. From our analysis we find experimental evidence for an additional contribution to the overall interaction potential for atoms at metal surfaces caused by screening effects in the electron gas in front of the surface. As an example for the relevance of our finding for ion/atom surface collisions, we discuss the widths of angular distributions for projectiles scattered under glancing angles. Incorpration of this contribution to the scattering potential allows us to explain the different behaviour of angular distributions for noble gas and reactive atoms with scattering conditions.
The magnetism of a ultrathin Co film deposited on a Cu(001) substrate is studied via capture of spin-polarized electrons into the He I 3p 3P term after grazing scattering of 25 keV He+ ions. For films of 5 ML we explore the effect of ion beam irradiation on the data and investigate the removal of film atoms by incident ions via Auger electron spectroscopy. From our work we deduce sputtering yields for bombardment of the film surface with He+ and Ar+ ions and determine the regime of ion fluences where the method of electron capture is applicable for studies on surface magnetism of ultrathin films.
For grazing collisions of atoms and ions with monocrystalline surfaces,
projectiles are scattered under well defined trajectories. Electron emission
proceeds under specific conditions that substantially differ from large-angle
impact on polycrystalline target surfaces. Total electron yields can be
reliably measured in coincidence with the energy loss of scattered projectiles
down to 10-4 electrons per projectile, which enables studies near the
threshold for kinetic emission.
He atoms and ions with keV energies are scattered under a grazing angle of incidence from a flat and clean Al(111) surface. From the analysis of angular distributions for scattered projectiles we reveal that the angular shifts between distributions for incident neutral atoms and positive ions show a reverseal in sign as function of distace of closest approach to the surface plane. This observation allows us to deduce the energy shift of the He groundstate as function of distance from the surface. Our work provides important information towards a fundamental microscopic understanding of atomic interactions close to metal surfaces.
H+, He+, and N+ ions with energy of 25 keV are scattered under a grazing angle of incidence from a clean and flat Cu(001) surface during deposition of ultrathin Co films. Making use of the ion induced emission of electrons allows us to monitor growth of thin films via simple measurements of target current or from energy spectra of emitted electrons. The method provides excellent signals and is also applicable in the regime of poor layer growth.
The magnetization reversal of ultrathin Co films on Cu(001) has been investigated by grazing ion scattering and magneto-optical Kerr effect. Differences in the behavior of surface and bulk magnetization are found and attributed to the reduced coordination and site symmetry at the surface. The reversal behavior of the surface magnetization depends on the chemical surface composition. For pure Co films, the reversal of the bulk magnetization is preceded by a complete reversal of the surface magnetization. A particular magnetic state of the surface is suggested as precursor for magnetization reversal.
We report on spin- and energy-resolved electron emission induced by grazingly scattered 100 keV protons from
17. M. Gierlings, M.J. Prandolini, H. Fritzsche, M. Gruyters and D. Riegel
We report on the observation of the effects of exchange bias on the magnetization reversal processes in a [Co/CoO/Au]20 system using polarized neutron reflectometry (PNR). The focus in this study is the investigation of the offspecular scattering of neutrons from magnetic domain structures during the magnetization reversal. In a previous PNR study on the same system, an asymmetry in magnetization reversal has been observed on opposite sides of the samehysteresis loop. For the decreasingfield branch, the reversal was found to be dominated by domain wall motion of domains directed parallel or antiparallel to the applied field. In contrast, the reversal on the increasingfield branch was characterized by rotation of magnetization. A significant loss of intensity was found for the specular reflected neutrons, while off-specular scatteringexperiments reveal that this magnetization reversal is not determined by coherent rotation but rather by a breakingup into smaller domains with different orientations.
The structure and magnetism of ultrathin Fe films expitaxially grown on a Cu(001) surface are investigated by grazing scattering of fast H and He atoms or ions. By making use of a new variant of ion beam triangulation based on the detection of the number of emittede electrons we obtain direct information on the structure of the film surface. We observe for room temperature growth a dominant and defined fcc-like structure. Complex surface reconstructions as reported in recent STM and LEED studies are only observed for cooling and H2 dosing of the film.
19. H. Winter
Electron emission from metal surfaces induced by grazing impact of fast atoms is studied via the coincident detection of energy loss off scattered projectiles with the number of electrons emitted during a scattering event. We will show that this method combined with the feature of well defined trajectories located in front of the surface allows one to reveal details on the electronic excitation and emission mechanisms. It turns out that binary type of collisons between atomic projectiles and conduction electrons play the dominant role for electron emission from metal surfaces. For scattering of keV He atoms from an Al(111) surface we derive information on the momenta of fermi electrons in the selvedge of the surface and on the effective electronic surface potential. Energy loss spectra can be described in term of collisions of low energy electrons with free atoms.
20. M. Busch, M.Gruyters, and H. Winter
The structure and magnetism of thin iron oxide layers formed on a Fe(110) single crystal surface are investigated by auger electron spectroscopy, ion beam triangulation, and spin polarized secondary electron emission. The formation of a FeO(111)-like film on top of the metal substrate is observed for oxidation at elevated temperatures. Additional oxygen exposure at room temperature is suggested to lead to a gradual conversion to Fe3O4.
21. M. Baron, T. Bernhard, M. Gruyters, and H. Winter
The structure of ultrathin epitaxial Fe films grown on Cu(001) at room temperature is investigated by grazing scattering of fast H and He atoms. By making use of ion beam triangulation, direct information on the atomic structure of the film and substrate surfaces are obtained. We discuss a new variant of this method based on the detection of the number of emitted electrons. The data is analyzed via computer simulations using classical mechanics which provides a quantitative analyses with respect to projectile trajectories.
22. T. Bernhard, M. Baron, M. Gruyters, and H. Winter
The structure and magnetism of thin epitaxial Fe layers grown on Cu(001) is investigated by grazing scattering of fast H and He atoms. Information on the atomic structure of the film and substrate surfaces is obtained by making use of ion beam triangulation. The magnetic behavior is studied via the polarization of light emitted after capture of spin-polarized electrons into excited atomic terms during He scattering. For the formation of bcc(110)-like Fe films at higher coverages, we detect differences in structural and magnetic properties for room and low temperature growth. We suggest that the crystalline structure depends on the film morphology and that Cu impurities affect the magnetic properties.
Kinetic emission (KE) of electrons near its threshold is studied for grazing impact of keV He atoms on flat and clean Al(111), Al(110), and Cu(111) surfaces. We observe a defined dependence of the KE threshold behaviour on the scattering conditions. The data are analyzed in the framework of a classical model for electron emission based on momentum transfer in binary encounters between atomic projectiles and conduction electrons of the metal target. From the impact velocity dependence of KE yields near threshold we obtain information on momenta of fermi electrons as function of distance from the surface and on the electronic surface potential.
24. S. Wethekam , H. R. Dunham, J. C. Lancaster, and F. B. Dunning
The ionization of xenon Rydberg atoms excited to the lowest states in the n =17 and n =20 Stark manifolds at a flat Au(111) surface is investigated. Despite the strong perturbations in the energies and structure of the atomic states that occur as the surface is approached, it is shown that, under appropriate conditions, each incident atom can be detected as an ion and that the experimental data can be well fit by assuming that the ionization rate on average increases exponentially as the surface is approached. The ionization rates are compared to theoretical predictions.
25. M. Busch, Th. Ludwig, R.Drozdowski and G. von Oppen
Singlet-triplet anticrossing spectra of the helium isotope 3He have been measured for the first time. These anticrossing spectra differ strongly from those of the isotope 4He due to the hyperfine interaction. We exploited this difference for distinguishing between the excitation of target atoms and the formation of excited projectile atoms by electron capture in experimental investigations on the symmetric collision system He+-He.
26. V. K. Valev, M. Gruyters, A. Kirilyuk und Th. Rasing
Magnetization-induced optical Second Harmonic Generation (MSHG) from an exchange-biased CoO/Fe multilayer produces an asymmetrical hysteresis loop that indicates different magnetization reversal behaviour between the interface and the bulk ferromagnet. A more careful analysis of the data demonstrates that this asymmetry is in fact due to a quadratic dependence on the magnetization of the MSHG intensity.
The magnetic behavior of CoO nanoparticles and layered CoO/ferromagnetic (FM) structures has been investigated by magnetization and hysteresis loop measurements. In the amorphous CoO, a large uncompensation of spins is found that is closely related to spin-glass-like behavior below a freezing temperature TF ≈ 215–220 K. The spin-glass-like phase may be described by the de Almeida–Thouless line for Ising spin systems. The exchange bias in the layered CoO/FM structures is explained by the spin-glass-like state in the nanoparticles constituting the CoO film.