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@PHDTHESIS{Hoppe:283571,
      author       = {Hoppe, Michael},
      title        = {{M}agnetic, structural, and electronic properties of
                      {N}i{F}e$_{2}${O}$_{4}$ ultrathin films},
      volume       = {118},
      school       = {Universität Duisburg},
      type         = {Dr.},
      address      = {Jülich},
      publisher    = {Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag},
      reportid     = {FZJ-2016-01885},
      isbn         = {978-3-95806-122-4},
      series       = {Schriften des Forschungszentrums Jülich. Reihe
                      Schlüsseltechnologien / Key Technologies},
      pages        = {VII, 118 S.},
      year         = {2016},
      note         = {Universität Duisburg, Diss., 2015},
      abstract     = {The physical properties of transition-metal oxides are
                      strongly determined by the competition of charge, spin, and
                      orbital degrees of freedom. Continuing progress in the
                      deposition techniques of oxides nowadays allows to grow thin
                      film heterostructures with atomically sharp interfaces.
                      Tailoring the interface between oxides opens up the
                      possibility to explore novel nanoelectronic functionalities
                      and even to discover new phenomena only existing at the
                      interface. In this framework, oxides featuring
                      simultaneously magnetic and insulating properties offer a
                      promising approach for the optimized performance of
                      spintronic devices. They can realize a highly effective
                      spin-filter effect, where spin-polarized electron currents
                      are generated by a spindependent tunneling process. For this
                      purpose, the spinel ferrite NiFe$_{2}$O$_{4}$ is a very
                      auspicious material since it possesses both features even at
                      room temperature. In this thesis, the sensitive interplay
                      between magnetic, electronic and structural properties in
                      the ferrimagnetic oxide NiFe$_{2}$O$_{4}$ is investigated in
                      detail. Therefore, NiFe$_{2}$O$_{4}$ thin films are
                      deposited on Nb-doped SrTiO$_{3}$ (001) substrates via
                      pulsed laser deposition (PLD) and the growth conditions of
                      the deposition process are carefully evaluated. Based upon
                      this, a procedure is deduced, that allows the reproducible
                      growth of high-quality, epitaxial, and single-crystalline
                      NiFe$_{2}$O$_{4}$ thin films. With the aim towards
                      fabricating tunnel barriers, special emphasis is placed on
                      the impact of reduced dimensionality in the crossover from
                      bulk-like to ultrathin NiFe$_{2}$O$_{4}$ films. Here, an
                      enhanced saturation magnetization M$_{S}$ for ultrathin
                      NiFe$_{2}$O$_{4}$ films ($\textit{d}$ < 4nm) that coincides
                      with a reduced out-of-plane lattice constant under
                      compressive in-plane epitaxial strain is observed. The films
                      are investigated by complementing bulk- and
                      surface-sensitive analyses using HAXPES, XANES and XMCD
                      spectroscopy techniques. Hereby, a bulk-like cationic
                      coordination of the inverse spinel lattice independent of
                      the NiFe$_{2}$O$_{4}$ film thickness is found – thus
                      ruling out a cationic inversion that nominally could account
                      for an enhanced M$_{S}$. The spin and orbital contribution
                      to the net magnetization are investigated element-specific
                      by recording high-quality low noise XMCD spectra and
                      evaluating them using the sum rules. The resulting moments
                      agree with the magnetic structure of an inverse spinel.
                      However, they give no explanation for the observed enhanced
                      MS. Instead, a novel magnetism at the interface between the
                      NiFe$_{2}$O$_{4}$ films and SrTiO$_{3}$ substrates is
                      discovered, which originates from a ferromagnetic ordering
                      of the Ti electrons. The underlying mechanism is explained
                      by superexchange interaction across the interface which
                      imposes the ferromagnetic order of the electron in
                      NiFe$_{2}$O$_{4}$ onto the Ti electrons. The given results
                      open the path for a future integration of NiFe$_{2}$O$_{4}$
                      into spin filter tunnel junctions. Additionally, the
                      observed interfacial Ti ferromagnetism renders
                      NiFe$_{2}$O$_{4}$/SrTiO$_{3}$ heterostructures as a
                      intriguing system for exploring the interplay between the
                      various degrees of freedom in transition metal oxides.},
      cin          = {PGI-6},
      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/283571},
}