% IMPORTANT: The following is UTF-8 encoded.  This means that in the presence
% of non-ASCII characters, it will not work with BibTeX 0.99 or older.
% Instead, you should use an up-to-date BibTeX implementation like “bibtex8” or
% “biber”.

@PHDTHESIS{Tober:476345,
      author       = {Tober, Steffen},
      othercontributors = {Stierle, Andreas},
      title        = {{N}ear-surface {I}ron {C}ation {T}ransport in {M}agnetite},
      school       = {Universität Hamburg},
      type         = {Dissertation},
      address      = {Hamburg},
      publisher    = {Verlag Deutsches Elektronen-Synchrotron DESY},
      reportid     = {PUBDB-2022-01695, DESY-THESIS-2022-007},
      series       = {DESY-THESIS},
      pages        = {230},
      year         = {2022},
      note         = {Dissertation, Universität Hamburg, 2021},
      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.},
      cin          = {FS-NL},
      cid          = {I:(DE-H253)FS-NL-20120731},
      pnm          = {632 - Materials – Quantum, Complex and Functional
                      Materials (POF4-632) / DFG project 192346071 - SFB 986:
                      Maßgeschneiderte Multiskalige Materialsysteme - M3
                      (192346071) / SFB 986 A07 - Adsorption organischer Säuren
                      auf Oxidoberflächen und Nanostrukturen (A07) (318017425) /
                      PHGS, VH-GS-500 - PIER Helmholtz Graduate School
                      $(2015_IFV-VH-GS-500)$},
      pid          = {G:(DE-HGF)POF4-632 / G:(GEPRIS)192346071 /
                      G:(GEPRIS)318017425 / $G:(DE-HGF)2015_IFV-VH-GS-500$},
      experiment   = {EXP:(DE-H253)Nanolab-01-20150101 /
                      EXP:(DE-H253)Nanolab-03-20150101 /
                      EXP:(DE-H253)Nanolab-04-20150101 /
                      EXP:(DE-H253)Nanolab-02-20150101 /
                      EXP:(DE-H253)P-P01-20150101},
      typ          = {PUB:(DE-HGF)3 / PUB:(DE-HGF)11},
      doi          = {10.3204/PUBDB-2022-01695},
      url          = {https://bib-pubdb1.desy.de/record/476345},
}