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|X-Ray absorption spectroscopy of Fe complexes on surfaces|
|Haupttitel||X-Ray absorption spectroscopy of Fe complexes on surfaces|
|Titelzusatz||Electronic interactions and tailoring of the magnetic coupling|
|Titelvariante||Röntgenabsorptionsspektroskopie an Eisen-Komplexen auf Oberflächen:|
|Zusatz zur Titelvariante||Elektronische Wechselwirkungen und Maßschneidern der magnetischen Kopplung|
|Gutachter||Prof. Dr. Wolfgang Kuch|
|weitere Gutachter||Prof. Dr. Martin Weinelt,
Prof. Dr. Hans-Peter Steinrück
|Freie Schlagwörter||surface; organic; molecule; magnetic; monolayer; XAS; XMCD; NEXAFS; UHV; SCO; spin crossover; X ray; porphyrin; spin; coupling|
|Zusammenfassung||Within this thesis, two classes of transition metal complexes are studied on surfaces. Firstly, monomolecular layers of Fe(II) spin-crossover (SCO) complexes, prepared by in-situ sublimation onto Au(111) substrates and by self-assembly on Au(111)/mica, are investigated by means of X-ray absorption spectroscopy (XAS). For a multilayer of Fe(phen)2(NCS)2 and phenanthroline molecules on Au(111), a partly reversible SCO transition is demonstrated. It is found that the transition is suppressed if the Fe(phen)2(NCS)2 molecules are in direct contact with the Au(111) surface, possibly due to a chemical reaction of their ligands with the gold substrate. The intact deposition of Fe(bp)2 is demonstrated, being relatively unperturbed even when in direct contact with the Au(111) surface. The self-assembly of Fe(bppmc)2, which results in a monomolecular layer with both linker groups bound to the gold surface, is demonstrated by S 2p X-ray photoelectron spectroscopy. In both cases a high-spin state of the Fe centers is observed, implying that the strength of the ligand field would need to be increased to realize an SCO transition of the molecules on the surface.
The second class of transition metal complexes are quasi-planar Fe and Co octaethylporphyrin (OEP) molecules. Their magnetic properties on non-magnetic and ferromagnetic (FM) surfaces are analyzed in the submonolayer regime by X-ray magnetic circular dichroism (XMCD). The angle-dependent electronic structure at the metal center of Fe and Co OEP molecules on non-magnetic Cu(100) and oxygen-covered O/Cu(100) substrates, is determined by means of XAS. Measurements of the magnetic properties are carried out in a magnetic field of B = 5 T at T = 8 K. For Fe OEP on O/Cu, a very strong magnetic anisotropy is found, owing to the interaction with the oxygen and resulting in a factor of five between the in and out-of-plane XMCD signal. The magnetism of Co OEP on O/Cu(100) is dominated by the contributions of the dz2 orbital. This results in a characteristic angular dependence of the XMCD signal, due to the anisotropy of the spin-density.
By means of XMCD measurements it is shown that the magnetic moment of the Fe centers of Fe OEP molecules can be aligned at room temperature if they are deposited onto FM Ni and Co substrates. A simple theoretical model is utilized to determine the magnetic coupling energies from temperature-dependent measurements of the Fe and substrate magnetizations. A much stronger coupling is found for Fe OEP on Co than on Ni substrates. Tailoring of the magnetic coupling is achieved by placing atomic oxygen between the molecules and the FM Ni and Co substrates. For the first time, an antiferromagnetic coupling of Fe porphyrin molecules to FM substrates is realized here, as evidenced by the opposite sign of the Fe and substrate XMCD signals.
PDF-Datei von FUDISS_thesis_000000019678
|Seitenzahl||VI, 136 S.|
|Tag der Disputation||28.10.2009|
|Erstellt am||04.11.2010 - 07:09:29|
|Letzte Änderung||05.11.2010 - 13:06:24|