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@PHDTHESIS{Caspers:845780,
      author       = {Caspers, Christian},
      title        = {{M}agnetic oxide heterostructures : {E}u{O} on cubic oxides
                      and on silicon},
      volume       = {69},
      school       = {Universität Duisburg},
      type         = {Dr},
      address      = {Jülich},
      publisher    = {Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag},
      reportid     = {FZJ-2018-02989},
      isbn         = {978-3-89336-891-4},
      series       = {Schriften des Forschungszentrums Jülich. Reihe
                      Schlüsseltechnologien / Key Technologies},
      pages        = {XIII, 152 S.},
      year         = {2013},
      note         = {Universität Duisburg, Diss., 2013},
      abstract     = {In the thesis at hand, we explore fundamental properties of
                      ultrathin europium oxide (EuO) films. EuO is a model system
                      of a localized 4$\textit{f}$ Heisenberg ferromagnet, in
                      which the ferromagnetic coupling– provided a high
                      crystalline quality – can be tuned by biaxial lattice
                      strain. Moreover, the magnetic oxide EuO is perfectly suited
                      as a spin-functional tunnel contact for silicon spintronics.
                      However, upto now a challenging bulk and interface chemistry
                      of EuO and Si has hampered a seamless integrationinto
                      functional silicon heterostructures.In order to investigate
                      fundamental aspects of the magnetic and electronic structure
                      of ultrathin EuO,in the first part of this thesis, we
                      synthesize EuO thin films on conductive YSZ substrates from
                      bulk-like thicknesses down to one nanometer by oxide
                      molecular beam epitaxy (MBE). The EuO thin films are of
                      textbook-like single-crystalline quality, and show bulk-like
                      magnetic properties. We control the stoichiometry of buried
                      EuO thin films by hard X-ray photoemission spectroscopy
                      (HAXPES); even a 1 nm ultrathin EuO film exhibits no valence
                      change or interface shifts. Furthermore, we conduct an
                      advanced magnetic characterization by the magnetic circular
                      dichroism (MCD) of Eu core-levels in photoemission, this
                      gives us insight into the intra-atomic exchange coupling of
                      EuO thin films. The MCD reveals large asymmetries of up to
                      49\% in the well-resolved Eu $\textit{4d}$ photoemission
                      multiplet. Thus, ultrathin EuO coherently grown on
                      conductive YSZ allows us to explore fundamental magnetic and
                      electronic properties of a $\textit{4f}$ magnetic oxide.
                      Biaxial lateral strain applied to single-crystalline EuO is
                      of fundamental interest, since it alters the electronic
                      structure and magnetic coupling in a controlled way. We
                      apply +4.2\% tensile biaxial strain to EuO by epitaxial
                      EuO/LaAlO$_{3}$ (100) heterostructures. EuO seamlessly
                      adapts the lateral lattice parameter of LaAlO$_{3}$, while
                      the perpendicular parameter of EuO is the unchanged EuO bulk
                      value, thus the strained EuO thin film shows a Poisson ratio
                      of ν$_{EuO}$ ≈ 0. The tensile strain reduces the Curie
                      temperature significantly by 12.3 K. The MCD effect provides
                      an advanced magnetic characterization:theMCDasymmetries in
                      Eu core-level photoemission reveal a larger reduction due to
                      the tensile strain than obtained from bulk-averaging SQUID
                      measurements. Thus, the mechanism of tensile strain on
                      intra-atomic exchange (indicated by MCD) is significantly
                      different than on the spin order of the 4$\textit{f}^{7}$
                      shell (indicated by SQUID). Experiments on EuO by MCD,
                      thereby, reveal exciting perspectives for studying
                      fundamental magnetic properties of EuO. In the second part
                      of this thesis, we explore how to integrate EuO directly
                      with Si (001). We focus on interface engineering of
                      structural and chemical properties of the EuO/Si (001)
                      spin-functional hetero interface. In response to the
                      extremely high chemical reactivity and pronounced surface
                      kinetics ofEu, EuO, and Si during EuO synthesis at elevated
                      temperatures, we initially conduct a thermodynamic analysis
                      of the EuO/Si interface. In this way, we decide to
                      investigate three in situ passivation techniques for the Si
                      (001) surface, in order to prevent metallic and oxide
                      contaminations at the EuO/Si interface – both being main
                      antagonists for spin-selective tunneling. We conduct a
                      comprehensive optimization study of the EuO/Si
                      heterointerface by tuning the passivation parameters of the
                      Si (001)surface and the growth parameters of EuO. Using
                      HAXPES, we evaluate Si and Eu core-level spectra and
                      determine the minimum of interface contaminants as
                      d$_{opt}$(SiO$_{x}$) = 0.69 nm concomitant with
                      d$_{opt}$(EuSi$_{2}$) = 0.20 nm, both of which are clearly
                      in the subnanometer regime. In conclusion, our ultrathin
                      EuO/Si (001) heterostructures reveal a high chemical quality
                      of the spinfunctionalinterface, combined with magnetic
                      properties of the EuO layer akin to bulk. By selected
                      interface passivation methods, we achieve a reduction of
                      residual contaminations to clearly below a closed interface
                      coverage. Thus, we could confirm a heteroepitaxial
                      integration of EuO on Si (001), which is the experimental
                      basis for possible band-matched coherent tunneling. This is
                      the first time that a direct integration of high quality EuO
                      on silicon was experimentally realized – without insertion
                      of additional oxide buffer layers. Such optimized EuO/Si
                      (001) heterointerfaces are paving the pathway for
                      near-future spin-functional devices using EuO tunnel
                      contacts.},
      keywords     = {Europiumoxide (gnd) / Dünne Schicht (gnd) / Silicium
                      (gnd)},
      cin          = {PGI-6},
      ddc          = {500},
      cid          = {I:(DE-Juel1)PGI-6-20110106},
      pnm          = {899 - ohne Topic (POF3-899)},
      pid          = {G:(DE-HGF)POF3-899},
      typ          = {PUB:(DE-HGF)3 / PUB:(DE-HGF)11},
      url          = {https://juser.fz-juelich.de/record/845780},
}