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| Book/Dissertation / PhD Thesis | PUBDB-2022-01695 |
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2022
Verlag Deutsches Elektronen-Synchrotron DESY
Hamburg
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Please use a persistent id in citations: doi:10.3204/PUBDB-2022-01695
Report No.: DESY-THESIS-2022-007
Abstract: Magnetite ($\text{Fe}_3\text{O}_4$), one of the first known magnetic materials, has multiple applications as for example as a catalyst or catalyst component. Magnetite nanoparticles are used as contrast agents in medicine or building blocks in novel hierarchical materials with outstanding mechanical properties. Magnetite thin-films are promising for the development of spintronic devices. For all these applications the surface structure and the processes in the near-surface region of magnetite are crucial.On the magnetite (001) surface a non-stoichiometric $(\sqrt{2} × \sqrt{2}) R45^\circ$ reconstruction is observed after preparation in ultra-high vacuum (UHV). The reconstruction’s structural motif was found to be the combination of a tetrahedrally coordinated interstitial cation and two octahedrally coordinated vacancies in the subjacent layer. Lifting of the reconstruction by adsorption of molecules or annealing therefore requires cation transport processes to fill the vacancies with cations from the bulk. During annealing to 900 K at $1.3 × 10^{−6}$ mbar oxygen an oxidative regrowth of new magnetite layers was observed on the (001) surface. The cations forming the new layers are supplied by the oxidation of magnetite to haematite deep in the bulk.Transport processes involved in the lifting of the reconstruction or the regrowth phenomenon were monitored under UHV conditions in the temperature range from 470-770 K relevant for catalysis applications and the manufacturing of magnetite-based materials and devices. Under these conditions, magnetite is thermodynamically unstable, but the phase transfer to thermodynamically stable haematite is kinetically hindered due to the energy needed to transform the cubic magnetite to the hexagonal haematite structure and the low energy gain of the phase transformation. Oxidation instead leads to the formation of a kinetically stabilised cubic maghemite phase.Cation transport was observed at the interface of isotopically labelled $^{57}\text{Fe}_3\text{O}_4$ thin-films and magnetite substrates by neutron reflectivity (NR), nuclear forward scattering (NFS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The growth, structure and surface morphology of the thin-films were studied by surface X-ray diffraction (SXRD), low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Magnetite thin-films were homoepitaxially grown by reactive molecular beam epitaxy. Chemical characterisation by AES and XPS showed that the prepared thin-films had a close to perfect magnetite stoichiometry. The thin-film structures were characterised by SXRD, LEED and XRR indicating a slightly reduced density of the thin-films and a minor contraction of their structure along the surface normal, otherwise being in good agreement with the structure of magnetite. X-ray growth intensity oscillations observed during the deposition indicated an ordered layer-by-layer growth of magnetite. Modelling the oscillations with the birth-death model of epitaxy indicated a slightly reduced order of the growth process for increasing growth rates. The modelled growth unit corresponds to 1/4 of the unit cell of magnetite. AFM images of the thin-films but also of substrates heated under growth conditions at 420 K in $8 × 10^{−7}$ mbar oxygen showed large flat islands covering the surface. The islands result most likely from the regrowth process described above. They seem to be formed in parallel to the homoepitaxial growth process and might partly explain the deviations of the thin-film and the bulk magnetite structure discussed above.Cation transport in the near-surface region was observed by monitoring the interface of a $^{57}\text{Fe}_3\text{O}_4$ thin-film and its magnetite substrate. A considerable intermixing at the isotopic interface resulting from the growth procedure was already observed before the actual transport experiment. Cation transport was induced by annealing the thin-films in UHV. Changes of the $^{57}\text{Fe}$ distribution were found by NR, NFS and ToF-SIMS starting at 470 K. Using NFS, the changes could be attributed predominantly to the octahedrally coordinated cations. Under the given slightly oxidising conditions, cation transport via the octahedral sublattice is also predicted by the point defect model developed for the cation transport in the volume of thermodynamically stable magnetite. The diffusion coefficients estimated from NR and NFS are, however, four to five orders of magnitude smaller than expected from the point defect model. The discrepancy might be explained partly by the defect structure and the complex surface morphology of the thin-films. As the experiments were carried out outside the thermodynamic stability range of magnetite side effects may have taken place slowing down the statistical transport of $^{57}\text{Fe}$ along the surface normal.The observation of cation migration in the near-surface region of magnetite at only 470 K clearly shows that growth and transport processes are non-negligible for the manufacturing process and the application of magnetite based structures.
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