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RTM Untersuchungen an zweidimensionalen Nanostrukturen am Beispiel der reinen Au(100)-Oberfläche und des selbstassemblierten Systems Sauerstoff auf Cu(110)
No full textIn this STM study, nanostructures on metallic single crystals are observed under UHV conditions. The aim of the work at the Au(100)-surface is to evaluate an energetic quantity, namely the step free energy, from a topographic structure information. Adatom and vacancy islands are produced on the tempered reconstructed Au(100)-surface by evaporation of submonolayer quantities of gold and sputtering with Argon ions, respectively. As noted before, these islands have a rectangular shape with the long side oriented along the parallel reconstruction lines. The initial (aspect) ratio of the long to the short side is about 10. The decay of these islands is correlated with a decrease of the aspect ratio to approximately 1. After that the aspect ratio varies merely stochastically around this mean value. We could observe adatom islands at T = 353 K, which neither grow nor decay over a long period of time and whose aspect ratio varies stochastically around the mean value of 1. In conclusion, we assume that these islands are in a thermodynamic equilibrium situation or at least close to it. By averaging over these island shapes, we obtain the equilibrium shape for this temperature. From island shape fluctuations we make a quantitative determination of the mean step free energy to 170 meV per atom. In completely separated experiments from the described ones above, the formation of the so-called (2x1) oxygen stripe phase on Cu(110) is imaged in situ, and simultaneously a quantitative determination of the induced surface stress is accomplished. The bending bar technique is utilized with the bending measured by the STM. Due to the anisotropy of the Cu(110) surface, two experiments, one with the [1(-1)0]- and a second one with the [001]-direction along the bar have to be carried out for a quantitative analysis of the induced surface stress. From the two experiments we calculate the induced stresses in both directions. We find that the difference of the surface stresses of the (2x1)O phase on Cu(110) and the clean Cu(110) surface is compressive for both directions, in accordance with the adsorption of oxygen on other surfaces. The stress depends significantly on the coverage. The data can be fitted with an exponential function. Furthermore the fit for the stress difference orthogonal to the stripes is used for a quantitative analysis of the periodicity of the oxygen stripes
Wachstum und Charakterisierung des Tunnelmagnetowiderstand-Mehrschichtsystems Co-Al 2 O 3 -CoAl und das Studium spinfrustrierter Vortices in Ringstrukturen
No full textThis work discusses two major topics: The study of a Co-Al2O3-CoAl(100) TMR model system and the spin frustration in magnetic rings. CoAl(100) crystallizes in a CsCl-type structure. The scanning tunneling microscopy (STM) image displays flat and large terraces with a width of 40-100 nm separated by steps with a height of 0.28 nm. This agrees with the lattice constant of CoAl and represents a double atomic step of the CsCl structure. The ABAB stacking sequence is disturbed at the surface. The disturbance is caused by antisite atom segregation of Co atoms to the nominal Al sublattice in the topmost surface plane. Adsorption of O2 to the intermetallic alloy CoAl(100) allows the preparation of thin alumina films that can serve as a tunnel barrier in the tunneling magnetoresistance (TMR) model system. Exposure to oxygen at 300 K leads to the growth of amorphous oxide. The electron energy loss (EEL) spectra exhibit two losses at 640 und 890 cm-1. The self-limiting thickness of the amorphous alumina at saturation amounts to 0.7 nm. For this film a band gap of 3.7 eV was found. A new phenomenon shows up, namely the segregation-induced magnetism of CoAl(100). During the oxidation process Al segregates to the oxide-gas phase interface. The relative enrichment of Co in the crystal is sufficient to produce ferromagnetism of CoAl at 300 K. Oxidation at elevated temperatures leads to the growth of well-ordered alumina. At first, we study the oxidation at 800 K. The oxide is characterized by Fuchs-Kliewer modes at around 430, 630, 780 and 920 cm-1, which is indicative for the growth of a well-ordered oxide. LEED shows a two-domain (2x1) structure. We conclude that a well-ordered teta-Al2O3 grows on CoAl(100). Due to Al missing chains in the topmost layer STM exhibits streaks that are separated by 0.564 nm, which agrees with the lattice constant of the oxide. The thickness of the oxide film amounts to around 1 nm, and the band gap amounts to 4.4 eV. A thermal stability of the oxide film up to 1300 K was found. Co exhibits Volmer-Weber-type growth on both amorphous and teta-Al2O3. This is a consequence of the huge difference in the surface free energy of Co and that of the oxide. On teta-Al2O3, already after the deposition of 0.08 nm Co, clusters with a mean diameter of 2 nm and a mean height of 0.7 nm can be found. After deposition of 0.6 nm Co the surface is fully covered. Finally, after depositing 3 nm of Co, clusters between 1 and 4.5 nm show up. Due to the interaction with the underlying oxide these clusters are partly oriented in chains. The thermal stability of the Co/Al2O3/CoAl TMR model system is limited by diffusion of Co into the sample. The diffusion sets in at 500 K. Interacting rings in direct contact, arranged in a tri-ring structure, exhibit a magnetic reversal process that is strongly affected by the direction of the applied external field. At saturation, all three rings yield an onion state, where the magnetization follows the two halves of a ring. Reducing the field leads to nucleation of vortices and vortex cores. However, the formation of vortices is frustrated due to the interaction between the rings within the tri-ring structure. In particular, different intermediate magnetization states can be selected by changing the orientation of the applied field. While the preferred reversal mode of vortex nucleation is determined by the angle of the applied field with respect to the tri-ring orientation, the stability of the intermediate vortex states can be improved by decreasing the width of the rings
Bestimmung von absoluten Werten für Kinken- und Stufenenergien aus der Gleichgewichtsform von Inseln
No full textThe equilibrium shape of adatom and vacancy islands on Cu(100), Cu(111) and Ag(111)surfaces is investigated by scanning tunneling microscopy in the temperature range between290 - 450 K. From the equilibrium shape the relative free step energies are determined, makinguse of the inverse Wulff-construction. To obtain absolute values for kink and step energies,the Ising-model is used, yielding reasonable results for the (111) surfaces, but failing forthe (100) surface. To overcome this shortcoming, two novel methods are derived and appliedto the data: First, from the minimum curvature of the nearly straight step sections of the islands,the kink energies are determined. Second, by analysing the temperature dependence ofthe aspect ratio of the islands, the absolute values of the step energies are derived. Theeseresults are compared to values of the step free energies, obtained from an analysis of islandshape fluctuations and data, provided by theoretical calculations. Except for the Isingdescriptionof the (100) surface, all results agree well, confirming the introduced methods todetermine kink and step energies to be appropriate. It is discussed, why the Ising-model isnot an adequate model for the analysis of kink and step energies for (100) surfaces of fcc-metals
RTM Untersuchungen an zweidimensionalen Nanostrukturen am Beispiel der reinen Au(100)-Oberfläche und des selbstassemblierten Systems Sauerstoff auf Cu(110)
No full textIn this STM study, nanostructures on metallic single crystals are observed under UHV conditions. The aim of the work at the Au(100)-surface is to evaluate an energetic quantity, namely the step free energy, from a topographic structure information. Adatom and vacancy islands are produced on the tempered reconstructed Au(100)-surface by evaporation of submonolayer quantities of gold and sputtering with Argon ions, respectively. As noted before, these islands have a rectangular shape with the long side oriented along the parallel reconstruction lines. The initial (aspect) ratio of the long to the short side is about 10. The decay of these islands is correlated with a decrease of the aspect ratio to approximately 1. After that the aspect ratio varies merely stochastically around this mean value. We could observe adatom islands at T = 353 K, which neither grow nor decay over a long period of time and whose aspect ratio varies stochastically around the mean value of 1. In conclusion, we assume that these islands are in a thermodynamic equilibrium situation or at least close to it. By averaging over these island shapes, we obtain the equilibrium shape for this temperature. From island shape fluctuations we make a quantitative determination of the mean step free energy to 170 meV per atom. In completely separated experiments from the described ones above, the formation of the so-called (2x1) oxygen stripe phase on Cu(110) is imaged in situ, and simultaneously a quantitative determination of the induced surface stress is accomplished. The bending bar technique is utilized with the bending measured by the STM. Due to the anisotropy of the Cu(110) surface, two experiments, one with the [1(-1)0]- and a second one with the [001]-direction along the bar have to be carried out for a quantitative analysis of the induced surface stress. From the two experiments we calculate the induced stresses in both directions. We find that the difference of the surface stresses of the (2x1)O phase on Cu(110) and the clean Cu(110) surface is compressive for both directions, in accordance with the adsorption of oxygen on other surfaces. The stress depends significantly on the coverage. The data can be fitted with an exponential function. Furthermore the fit for the stress difference orthogonal to the stripes is used for a quantitative analysis of the periodicity of the oxygen stripes
Wachstum und Charakterisierung des Tunnelmagnetowiderstand-Mehrschichtsystems Co-Al 2 O 3 -CoAl und das Studium spinfrustrierter Vortices in Ringstrukturen
No full textThis work discusses two major topics: The study of a Co-Al2O3-CoAl(100) TMR model system and the spin frustration in magnetic rings. CoAl(100) crystallizes in a CsCl-type structure. The scanning tunneling microscopy (STM) image displays flat and large terraces with a width of 40-100 nm separated by steps with a height of 0.28 nm. This agrees with the lattice constant of CoAl and represents a double atomic step of the CsCl structure. The ABAB stacking sequence is disturbed at the surface. The disturbance is caused by antisite atom segregation of Co atoms to the nominal Al sublattice in the topmost surface plane. Adsorption of O2 to the intermetallic alloy CoAl(100) allows the preparation of thin alumina films that can serve as a tunnel barrier in the tunneling magnetoresistance (TMR) model system. Exposure to oxygen at 300 K leads to the growth of amorphous oxide. The electron energy loss (EEL) spectra exhibit two losses at 640 und 890 cm-1. The self-limiting thickness of the amorphous alumina at saturation amounts to 0.7 nm. For this film a band gap of 3.7 eV was found. A new phenomenon shows up, namely the segregation-induced magnetism of CoAl(100). During the oxidation process Al segregates to the oxide-gas phase interface. The relative enrichment of Co in the crystal is sufficient to produce ferromagnetism of CoAl at 300 K. Oxidation at elevated temperatures leads to the growth of well-ordered alumina. At first, we study the oxidation at 800 K. The oxide is characterized by Fuchs-Kliewer modes at around 430, 630, 780 and 920 cm-1, which is indicative for the growth of a well-ordered oxide. LEED shows a two-domain (2x1) structure. We conclude that a well-ordered teta-Al2O3 grows on CoAl(100). Due to Al missing chains in the topmost layer STM exhibits streaks that are separated by 0.564 nm, which agrees with the lattice constant of the oxide. The thickness of the oxide film amounts to around 1 nm, and the band gap amounts to 4.4 eV. A thermal stability of the oxide film up to 1300 K was found. Co exhibits Volmer-Weber-type growth on both amorphous and teta-Al2O3. This is a consequence of the huge difference in the surface free energy of Co and that of the oxide. On teta-Al2O3, already after the deposition of 0.08 nm Co, clusters with a mean diameter of 2 nm and a mean height of 0.7 nm can be found. After deposition of 0.6 nm Co the surface is fully covered. Finally, after depositing 3 nm of Co, clusters between 1 and 4.5 nm show up. Due to the interaction with the underlying oxide these clusters are partly oriented in chains. The thermal stability of the Co/Al2O3/CoAl TMR model system is limited by diffusion of Co into the sample. The diffusion sets in at 500 K. Interacting rings in direct contact, arranged in a tri-ring structure, exhibit a magnetic reversal process that is strongly affected by the direction of the applied external field. At saturation, all three rings yield an onion state, where the magnetization follows the two halves of a ring. Reducing the field leads to nucleation of vortices and vortex cores. However, the formation of vortices is frustrated due to the interaction between the rings within the tri-ring structure. In particular, different intermediate magnetization states can be selected by changing the orientation of the applied field. While the preferred reversal mode of vortex nucleation is determined by the angle of the applied field with respect to the tri-ring orientation, the stability of the intermediate vortex states can be improved by decreasing the width of the rings
Potential- und temperaturvariable Experimente zur atomaren Bewegung auf Ag- und Cu-Elektroden in wässrigen Elektrolyten: eine Rastertunnelmikroskopiestudie
No full textThis thesis deals with the study of the atomic mass transport on the surface of single crystalelectrodes in contact with aqueous electrolytes. The study was performed using timecorrelation functions as a result of a quantitative analysis of equilibrium step fluctuations. Inscanning tunneling microscope images equilibrium step fluctuations give rise to a frizzyappearance of the monoatomic step edges. The experiments discussed were performed onvicinal Ag(111) electrodes in sulfuric acid containing copper ions and on vicinal Cu(111) indiluted hydrochloric acid. The experimental setup and the modifications of the microscope fortemperature variable experiments are presented.Different possible mass transport mechanisms and the characteristic time dependence of thetime correlation function are briefly presented. The mobility at the electrode surface increaseswith temperature and may depend on the applied electrode potential in a liquid environment.An activation energy for the underlying dominant mass transport may be deduced from anArrhenius plot of the increase of the time correlation function, i.e. the increase of the mobilityof atoms on the surface. Given a strong monotonous potential dependence of the stepfluctuations, an activation energy for the dominant mass transport may be obtained from apotential dependent analysis of the time correlation functions at a given temperature.Measurements performed on Ag(111) in sulfuric acid yield a dominant diffusion mechanism,which is mediated by an exchange of silver-atoms with the electrolytic double layer at positivepotentials with respect to the saturated calomel electrode. With increasing electrode potentialan enhanced mobility of atoms manifests itself in a stronger step frizziness of the monoatomicsteps. From a temperature dependent analysis of the step frizziness an activation energy isobtained. This value is comparable to a result obtained from a potential dependent analysis ofthe increase of the time correlation function at increasingly positive potentials.In contrast to Ag(111), equilibrium step fluctuations on an unreconstructed vicinal Cu(111)electrode in diluted hydrochloric acid are independent of the electrode potential in a widepotential range. An increasing electrode potential leads eventually to the formation of an x - reconstruction of the surface. From the time dependence of the fluctuations aterrace limited diffusion mechanism could be deduced. This special kind of diffusionmechanism was observed experimentally for the first time within the framework of thepresented thesis. Using temperature variable measurements the activation energy for thediffusion mechanism was measured and yield slightly lower value than comparable studiesperfomed under ultrahigh vacuum conditions
Induzierte Spannungen an Einkristalloberflächen
No full textÄnderungen der Oberflächenspannung OT(s) können anhand der Verbiegung dünner Einkristallplatten studiert werden. Mit dieser sogenannten Biegebalkenmethode wird aus einer Plattenverbiegung unter Verwendung der Elastizitätstheorie die Oberflächenspannungsänderung OT(s) bestimmt. Für elastisch isotrope Substratmaterialien sind die zugehörigen Gleichungen bekannt, die hier auf elastisch anisotrope Kristalle mit mindestens einer C2,-Symmetrie erweitert werden . Zudem werden in dieser Arbeit Gleichgewichtsformen von Kristallplatten mit (100)-, (110)- oder (111)-Oberflächen numerisch berechnet, die entlang einer Kante eingespannt sind und mit einer homogenen aber reinen isotropen oder anisotropen Oberflächenspannung belastet sind. Durch die Einspannung verursachte Störungen werden mit dem Korrekturfaktor beschrieben der mit Dimensionalität bezeichnet wird. Mit den tabellierten Dimensionalitäten können die Biegebalkenexperimente genau ausgewertet werden. Diese Ergebnisse werden auf Biegebalkenmethoden zur Bestimmung von magnetoelastischen Spannungen in dünnen Schichten übertragen . Es wird gezeigt, welche magnetoelastischen Konstanten in Domänen geordneter Überstrukturen auf der (100)-, oder (111)-Oberfläche mit dieser Methode bestimmt werden können . Zudem wird der Einfluß von Domänenwänden abgeschätzt . Darüber hinaus werden Eigenformen und Eigenfrequenzen von einseitig eingespannten Platten numerisch berechnet . Daraus ergeben sich genaue Einsichten zur Bedeutung der elastischen Anisotropie . Auf der Molybdän (110)-Oberfläche wird die durch Adsorption von Sauerstoff induzierte Oberflächenspannung OT[ooi] und OT[iio] entlang der Hauptkristallrichtungen bestimmt. Es wird eine Spannungsanisotropie der p(2 x 2)-Rekonstruktion nachgewiesen . Lithium erzeugt auf der Molybdän (110)-Oberfläche einen Anstieg der Oberflächenspannung bis zu einer Bedeckung von O = 0, 3 ML. Bei höheren Bedeckungen bis zu einer Monolage fällt die induzierte Oberflächenspannung ab und erreicht bei einer Monolage einen Wert kleiner -1, 2N/m. Es wird gezeigt, daß die induzierte Spannung von Kobalt auf der Kupfer (100)-Oberfläche linear mit der Bedeckung ansteigt und eine Spannung von 2, 4GPa erzeugt. Durch lonenbeschuß induzierte Spannungen wurden an der Kupfer (100)-Oberfläche nachgewiesen . Die Edelgase Argon, Neon und Helium erzeugen in einem Energiebereich zwischen 800 - 2200 eV eine Druckspannung (OT(s) < 0) von einigen N/m. Die zeitliche Entwicklung der beschußinduzierten Spannung wird durch ein einfaches Modell beschriebe
Adsorbatschwingungen und Substratoberflächenphononen der wasserstoff- und sauerstoffbedeckten (110)-Oberfläche von Molybdän
No full textThe two adsorbate systems H/Mo(110) and 0/Mo(110) have been investigated by means of electron energy loss spectroscopy mainly with respect to adsorbate induced anomalies in the dispersion of substrate surface phonons. The clean surface is characterized by an intense loss peak at 225 cm in specular scattering geometry which is due to a dipoleactive surface resonance. The atomically adsorbed hydrogen exhibits three ordered superstructures with increasing coverage at a sample temperature of 110 K: at 0.5 ML a (2x2)2H phase was observed which turns into a (2x2)3H phase at 0.75 ML (until now the former has been believed to be the unique (2x2) superstructure); finally at 1ML the saturated (1x1)H superstructure was observed. The adsorption site is the triply coordinated hollow site for all phases. The (1x1)H phase reveals a damped vibrational mode in the specular spectra: the parallel hydrogen vibration mode exhibits a Fano-like line shape which is caused by a nonadiabatic coupling between the discrete vibrational mode and the electronic continuum of the substrate. For all phases, i. e. clean, (2x2)211, (2x2)3H, and (I x1)H, the dispersion of the Rayleighwave and the longitudinal mode was investigated along the symmetry directions of the surface Brillouin zone. The most striking observation is an anomalous indentation of the frequencies of both the Rayleigh-wave and the longitudinal mode at wave vectors of 0.9 along [001] and of 1.2 along [12]. Concerning the Rayleigh-wave this result has already been demonstrated by means of inelastic helium atom scattering. As a result of the present investigation the suggested connection between the observed specular loss continuum on saturated (1x1)H/W(110) and the likewise occuring phonon anomalies could be ruled out. Thus the current interpretation of this phenomenon in terms of a giant Kohn anomaly due to Fermi-surface nesting between adsorbate induced surface states is sound. In the case of oxygen it could be shown in agreement with the literature that the initial atomic adsorption ( 0.25 ML) leads to more complex superstructures and to a change of the adsorption site from the long bridge towards the triply coordinated hollow site. Angle resolved photoelectron spectroscopy data allow for Fermi-surface nesting between surface states of p(2x2)O/Mo(110) but the dispersion curves of the Rayleigh-wave and the longitudinal mode do not reveal any anomalous behaviour. As a result one must conclude that the electronic instability in terms of Fermi-surface nesting is necessary but not sufficient for a giant Kohn anomaly. At least two additional conditions must be fulfilled, i. e. the density of surface states participating in the nesting must be high enough and the coupling between the electrons and the phonons must be sufficiently strong
Spinabhängige Phänomene bei Elektronenbeugung und Photoemission an unmagnetischen Kristallen
No full textVibrations in adsorbed layers : proceedings of an International Conference held on June 12 - 14, 1978
No full textFor decades chemists and physicochemists working in the field of heterogeneous catalytic reactions have been faced with the problem of understanding the nature of adsorbed molecules and reaction intermediates. A significant improvement in the understanding was achievedwhen infrared spectroscopy became applicable to supported catalysts in the late fifties. For the first time molecular vibrations of adsorbed species could be observed under reaction conditions. In the pursuit of this work it became evident, however, that the assignment of the observed vibrations to specific species is frequently difficult due to the unknown structure and composition of surfaces in finely devided materials. A very different approach to adsorption phenomena has been undertaken by physicists. Starting with well defined clean single crystal surfaces new techniques were developed to study increasingly more complicated adsorption systems. Here again vibration spectroscopy proved to be rather powerful. lt was therefore proposed that a conference bringing together scientists working in the field of vibration spectroscopy in adsorbed layers yet with different techniques and on differently structured surfaces was timely. The conference took place in the KFA Jülich, Germany, June 12-14, 1978. This volume collects contributed and invited papers of the conference. In order to keep a volume of the proceedings small we had to impose limits on the available space. In some cases where the authors felt that their results could not easily be compressed into the few pages the abstract only is included in this volume while complete papers are available from the authors upon request. Our sincere thanks are due to all authors for their cooperation in providing edited manuscripts. We are also indebted to the chairmen of the sessions for mastering the, at times rather engaged, discussions. Finally we would like to express our appreciation to Mrs. Bongartz, Miss Dohmen, Mrs. Pilwat and Mr. Renn for their help with organizational matters and also to the boardof directors of the KFA for the generous financial support of the conference
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