Aufstellung der Publikationen der Arbeitsgruppe 2004

Aufgeführt sind Arbeiten, die im Jahr 2004 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. H. Winter, K. Maass, S. Lederer, HP. Winter, and F. Aumayr
"Kinetic electron emission for planar vs. axial surface channeling of He atoms and ions"
Phys. Rev. B 69 (2004) 054110

Fast He+ ions and He0 atoms are scattered under grazing incidence from a clean and flat Al(111) surface. The target current recorded as function of azimuthal rotation with respect to the surface normal is enhanced for scattering along low-index crystallographic directions in the surface plane. From coincident studies on the number of electrons emitted per ion impact we discovered the interaction mechanism. In contrast to earlier interpretations our data reveal that the increase of apparent total yield for surface channeling along low-index directions is due to penetration of the surface layer by a small fraction of projectiles which produce a substantially larger number of electrons.

 

 2. T. Bernhard, R. Pfandzelter, and H. Winter
"Triangulation of a Cu(001) surface and an ultrathin Mn film via grazing ion surface scattering"
Nucl. Instr. Meth. B 219-220 (2004) 567

The structure of crystal surfaces and ultra-thin films can be studied via ion beam triangulation. The method is based on the enhanced kinetic emission of electrons for grazing angle scattering of fast ions from a mono-crystalline surface, whenever the azimuthal orientation of the target surface with respect to the projectile beam coincides with low index directions. In studies with 25 keV protons scattered from a clean Cu(0 0 1) surface as well as from the surface of an ultra-thin Mn film deposited on the Cu(0 0 1) substrate we have explored features concerning the angular width of the signals obtained in the application of this method.

 

 3. T. Bernhard, Z. L. Fang, and H. Winter
"Ion-induced low-energy electron diffraction"
Phys. Rev. A 69 (2004) 060901(R)

H+ and He+ ions with an energy of 25 keV are scattered under a grazing angle of incidence from a clean and flat Cu(001) surface. For specific azimuthal orientations of the crystal surface with respect to low index directions in the surface plane we observe the ion induced emission of electrons with a conventional LEED (Low Energy Electron Diffraction) setup. By operating the instrument in an energy dispersive mode we find intensity distributions of emitted electrons which can unequivocally be ascribed to diffraction effects at the target surface.

 

 4. M. Gruyters
"Anisotropic magnetoresistance in CoO/Co and CoO/Fe bilayers in the biased and unbiased state"
J. Appl. Phys. 95 (2004) 2587

Magnetoresistance and magnetic hysteresis loop measurements have been used to investigate the strong interfacial coupling in cobalt oxide/ferromagnet bilayers at low temperatures. For the biased state, an asymmetry in the magnetization reversal processes has been found. It is characterized by domain wall motion as the dominant mechanism for the decreasing field branch and an appreciable amount of magnetization rotation for the increasing field branch. The observed behavior is different from other exchange bias systems.

 

5. M. Gierlings, M. Gruyters, D. Riegel, M. J. Prandolini, T. Funk, and W. D. Brewer
"A study of the induced magnetism in the Au spacer layer of Co/Au/CoO Exchange Bias trilayers and related systems"
Phys. Rev. B, zur Veröffentlichung eingereicht

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.

 

 6. R. Kirsch, M. J. Prandolini, M. Gierlings, M. Gruyters, W. D. Brewer, D. Riegel
"Implantation of Single-Impurity Fe and its Magnetic Coupling in Er Studied by TDPAD"
J. Magn. Magn. Mater. 272-276 (2004) 760

Single Fe impurities were implanted in an Er single crystal and found to occupy both substitutional and interstitial sites, below a temperature of 200 K. The local susceptibility of Fe on both sites follows a Curie-Weiss law and exhibits a positive local Curie constant, indicating an antiferromagnetic coupling between the Fe and the surrounding Er moments. The corresponding nuclear spin relaxation rates follow a Korringa law as a function of temperature, confirming the dominance of local magnetism and the formation of local moments on each of the sites occupied by Fe.

 

7. Z. L. Fang, T. Bernhard, and H. Winter
"Diffraction Phenomena for Electrons Induced by Grazing Impact of Fast Ions on a Cu(001) Surface"
Phys. Rev. B, zur Veröffentlichung eingereicht

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.

 

 8. A. Schüller, G. Adamov, S. Wethekam, K. Maass, A. Mertens, and H. Winter
"Dynamic dependence of interaction potentials for keV atoms at metal surfaces"
Phys. Rev. A 69 (2004) 050901(R)

He and N atoms are scattered with keV energies under a grazing angle of incidence from clean and flat Ag(111) and Al(111) surfaces. For incidence along low index crystallographic directions in the surface plane, atomic projectiles are steered by rows of atoms ("axial surface channeling") giving rise to characteristic rainbows in their angular distribution. From the analysis of this effect we derive effective scattering potentials which reveal pronounced dynamical effects. We attribute our observation to the embedding energy for penetration of atoms in the electron gas of a metal.

 

 9. H. Winter
"Coincident studies on electronic interaction mechanisms during scattering of fast atoms from a LiF(001) surface"
Correlation Spectroscopy of Surfaces, Thin Films, and Nanostructures,
Jamal Berakdar (Editor), Jürgen Kirschner (Editor)
Wiley 2004, p.144

The electronic interaction mechanisms during grazing scattering of hyper-thermal hydrogen atoms from a LiF(001) surface is studied via projectile-electron coincidences. This type of translation energy spectroscopy applied to atom-surface scattering allows one to investigate in detail the relevant electronic processes. It turns out that the transient formation of negative ions during the scattering process is the precursor for electronic excitations of the solid as well as for emission to vacuum. The specific nature of electron transfer in terms of a local capture event from the anion sites of the ionic crystal plays a decisive role in the efficient formation of negative ions. Our studies unravel the complete reaction paths during the scattering process and provide a consistent interpretation for the at first glance surprisingly large total electron emission yields for impact of atoms/ions on the surface of ionic crystals.

 

 10. M. Busch, R. Drozdowski, Th. Ludwig and G. von Oppen
"Anticrossing spectroscopy of He atoms excited by 30-300 keV He+ impact"
J. Phys. B: At. Mol. Opt. Phys. 37 (2004) 2903

By measuring the anticrossing spectra of the 1s4l and 1s5l He I levels, we investigate single-electron excitation in 30-300 keV He+-He collisions. The postcollisional He I states are highly coherent superpositions of spherical states with large electric dipole moments. These superposition states vary gradually with the impact
energy from purely parabolic states at 30 keV to superpositions of l$2 states at 300 keV. The results indicate that for the whole energy range single-electron excitation should be considered as a process mainly localized in the saddle region of the twocenter Coulomb potential of the (He+)2 molecular core. A theoretical model based on
the symmetry of the collision system is discussed.

 

11. Y. Matulevich, S. Lederer, and H. Winter
"Near threshold emission of electrons during grazing scattering of keV Ne atoms from Al(111) surface"
Phys. Rev. B, zur Veröffentlichung eingereicht

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.

 

12. S. Wethekam, G. Adamov, and H. Winter
"Neutralization of keV Ne+ and Ar+ ions during grazing scattering from an Al(111) surface"
Nucl. Instr. Meth. B, im Druck

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.

 

13. A. Schüller, S. Wethekam, A. Mertens, K. Maass, H. Winter, and K. Gärtner
"Interatomic potentials from rainbow scattering of keV noble gas atoms under axial surface channeling"
Nucl. Instr. Meth. B, zur Veröffentlichung eingereicht

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.

 

14. HP. Winter, S. Lederer, and H. Winter
"Evidence for kinetic electron emission via correlated decay of hot conduction electrons"
Phys. Rev. Lett., zur Veröffentlichung eingereicht

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.

 

15. T. Bernhard, R. Pfandzelter, M. Gruyters, and H. Winter
"Temperature-dependent intermixing of ultrathin Co films grown on Cu(100)"
Surface Science, im Druck

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.

 

16. M. Gruyters, T. Bernhard, and H. Winter
"Structural effects on magnetic behaviour of ultrathin Co films at the Tcjump"
J. Magn. Magn. Mater., im Druck

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.

 

 17. Helmut Winter and Sven Lederer
"Electron emission from surfaces. How do fast atomic projectiles extract electrons from solids?"
Humboldt-Spektrum, Heft 3-4/2004, Schwerpunktheft Physik

The emission of electrons from solid surfaces induced by atomic particles is studied via the coincident detection of the projectile energy loss and the number of emitted electrons. This new type of »translation energy spectroscopy« applied to atom-surface scattering allows one to investigate in detail the relevant electronic processes, i.e., to relate the dissipation of projectile energy to the emission of a specific number of emitted electrons. With this method we could clear up the different microscopic interaction mechanisms for the emission of electrons from insulator and metal surfaces and explain the at first sight surprising feature of a moreefficient emission of electrons tightly bound in crystals of insulators compared to clean metal surfaces.

 

18. HP. Winter, S. Lederer and H. Winter
"Electron emission for grazing impact of keV He atoms on an Al(111) surface"
Nucl. Instr. Meth. B, zur Veröffentlichung eingereicht

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.

 

19. H. Winter and A. Schüller
"Rainbow scattering under axial surface channeling"
Nucl. Instr. Meth. B, zur Veröffentlichung eingereicht

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.

 

20. T. Bernhard and H. Winter
"Effect of ion beam irradiation on studies of surface magnetism via capture of spin-polarized electrons from a thin Co film on Cu(001)"
Nucl. Instr. Meth. B, zur Veröffentlichung eingereicht

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.

 

21. HP. Winter, S. Lederer, F. Aumayr, and H. Winter
"Electron emission for grazing slow atom and ion surface impact"
Comments on Mod. Phys., zur Veröffentlichung eingereicht

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.
In the present comment we outline recent experimental progress for grazing scattering of slow (> 100 eV up to some keV per amu) atoms and multiply charged ions on metal (Au, Al) and insulator (LiF) surfaces. The observed kinetic and potential electron emission can be quantitatively explained in terms of simple model assumptions. Such experiments can be carried out in a non-destructive manner which offers intriguing new aspects for surface analysis.

 

22. S. Wethekam and H. Winter
"Energy shift of He groundstate close to an aluminium surface"
Phys. Rev. Lett., zur Veröffentlichung eingereicht

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.

 

23. T. Bernhard and H. Winter
"Monitoring growth of ultrathin films via ion induced electron emission"
Phys. Rev. Lett., zur Veröffentlichung eingereicht

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.

 

24. M. Gruyters, T. Bernhard, and H. Winter
"Evidence for Noncollinearity between surface and bulk magnetization in ultrathin Co films"
Phys. Rev. Lett., zur Veröffentlichung eingereicht

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.

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