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@PHDTHESIS{Sun:844515,
      author       = {Sun, Xiao},
      title        = {{M}agnetic {P}roperties of {S}elf-assembled {M}anganese
                      {O}xide and {I}ron {O}xide {N}anoparticles - {S}pin
                      {S}tructure and {C}omposition},
      volume       = {176},
      school       = {RWTH Aachen},
      type         = {Dr.},
      address      = {Jülich},
      publisher    = {Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag},
      reportid     = {FZJ-2018-01925},
      isbn         = {978-3-95806-345-7},
      series       = {Schriften des Forschungszentrums Jülich. Reihe
                      Schlüsseltechnologien / Key Technologies},
      pages        = {II, 178 S.},
      year         = {2018},
      note         = {RWTH Aachen, Diss., 2018},
      abstract     = {Magnetic nanoparticles (NPs) have attracted much interest
                      for decades due to their potential applications in high
                      density data storage, spintronic devices and nanomedicine.
                      In analogy to atoms, they can represent building blocks for
                      NP superstructures. Belowa critical size, NPs prefer to be
                      in a magnetic single domain state. In this case, each NP can
                      be considered as possessing one large magnetic moment called
                      $\superspin".$ The superspins in self-assembled NP
                      superstructures interact with each other usually via dipolar
                      interactions. With the interplay of individual and
                      collective behaviour of the NPs, novel materials with
                      appealing magnetic and electronic properties may be
                      fabricated. In this thesis, the magnetic properties of two
                      different transition metal oxide NP and bulk systems,
                      manganese oxide and iron oxide, were studied. For manganese
                      oxides, the spin structure inside the NPs is of interest.
                      Zero field cooled (ZFC) magnetization curves of MnO NPs
                      usually show a low temperature peak at $\thicksim$ 25 K. No
                      feature indicating the antiferromagnetic
                      (AF)-to-paramagnetic (PM) phase transition of MnO was found
                      near its bulk Néel temperature at 118 K. However, polarized
                      neutron scattering shows the expected vanishing of the AF
                      order parameter of MnO near 118 K. This contradiction
                      between magnetometry and neutron scattering results can be
                      explained assuming an AF-superparamagnetic (SPM) core with a
                      ferrimagnetic (FiM) Mn$_{3}$O$_{4}$ shell. In order to study
                      the magnetization dynamics of the AF-SPM core,
                      AC-susceptibility measurements were performed. A low
                      temperature peak is observed similar to that found in the
                      ZFC curves. This peak shows a weak frequency dependence,
                      which is expected for an AF system. To further investigate
                      the origin of the low temperature peak, bulk MnO was studied
                      as a reference. Magnetometry and polarized neutron
                      scattering experiments were performed on MnO powder and a
                      single crystal to be compared with the results of NPs. The
                      phase of manganese oxide was tuned by annealing the
                      as-prepared samples in various environments. Their magnetic
                      properties were compared to that of the as-prepared ones.
                      Moreover, Monte Carlo simulations were performed for MnO NPs
                      and \bulk". In order to study the magnetic properties of
                      self-organized NP superstructures, spherical iron oxide NPs
                      were used. 2D ordered NP films were produced using various
                      methods. The order of the NP superstructures was
                      characterized using scanning electron microscopy (SEM) and
                      grazing incidence small angle x-ray scattering (GISAXS). A
                      weak feature near the Verwey transition of magnetite is
                      found in ZFC/FC magnetization curves of NPs with 15 and 20nm
                      diameters. Above T$_{N}$ of wüstite, the overall magnetic
                      moment increases in the ZFC curves. This is due to the
                      coupling between AF wüstite and FiM magnetite or maghemite.
                      Moreover, a peak indicating the crossover between the
                      blocked and the unblocked SPM or superspin glass (SSG)
                      states of the NP superspins is observed in the ZFC curves.
                      This peak temperature shows a strong field dependence as
                      usually found for SPM and SSG systems. Moreover, the peak
                      temperature shows a decrease as the NP size decreases due to
                      the smaller energy barriers of smaller particles. In the
                      hysteresis loops of 11 - 20nm NPs, an Exchange Bias (EB)
                      effect and a hardening effect are observed. The iron oxide
                      phases of the NPs were also tuned using various annealing
                      procedures. By annealing the particles in vacuum at
                      318$^{\circ}$C for several hours, the ratio of wüstite and
                      magnetite is increased. After the particles were annealed in
                      air, they tend to be fully oxidized to maghemite.},
      cin          = {JCNS-2 / PGI-4 / JARA-FIT / JCNS-FRM-II},
      cid          = {I:(DE-Juel1)JCNS-2-20110106 / I:(DE-Juel1)PGI-4-20110106 /
                      $I:(DE-82)080009_20140620$ /
                      I:(DE-Juel1)JCNS-FRM-II-20110218},
      pnm          = {144 - Controlling Collective States (POF3-144) / 524 -
                      Controlling Collective States (POF3-524) / 6212 - Quantum
                      Condensed Matter: Magnetism, Superconductivity (POF3-621) /
                      6213 - Materials and Processes for Energy and Transport
                      Technologies (POF3-621) / 6G4 - Jülich Centre for Neutron
                      Research (JCNS) (POF3-623)},
      pid          = {G:(DE-HGF)POF3-144 / G:(DE-HGF)POF3-524 /
                      G:(DE-HGF)POF3-6212 / G:(DE-HGF)POF3-6213 /
                      G:(DE-HGF)POF3-6G4},
      experiment   = {EXP:(DE-MLZ)DNS-20140101},
      typ          = {PUB:(DE-HGF)3 / PUB:(DE-HGF)11},
      url          = {https://juser.fz-juelich.de/record/844515},
}