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@PHDTHESIS{He:1048648,
      author       = {He, Suqin},
      title        = {{T}opotactic phase transition in {L}a0.6{S}r0.4{C}o{O}3-δ
                      thin films: oxygen content, dynamics and reversibility},
      volume       = {116},
      school       = {RWTH Aachen University},
      type         = {Dissertation},
      address      = {Jülich},
      publisher    = {Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag},
      reportid     = {FZJ-2025-04778},
      isbn         = {978-3-95806-868-1},
      series       = {Schriften des Forschungszentrums Jülich Reihe Information
                      / Information},
      pages        = {ix, 137},
      year         = {2025},
      note         = {Dissertation, RWTH Aachen University, 2025},
      abstract     = {Topotactic phase transitions induced by changes in oxygen
                      vacancy concentration can significantly alter the physical
                      properties of complex oxides, including electronic and
                      magnetic properties. Such tunable properties are critical
                      for developing novel electronic and spintronic devices,
                      where control of magnetic and electronic functionalities is
                      essential. This thesis investigates an
                      oxygen-vacancy-induced topotactic phase transition from
                      perovskite (PV) to brownmillerite (BM) in epitaxial
                      La0.6Sr0.4CoO3−δ (LSCO) thin films. Depth-sensitive
                      polarized neutron reflectometry (PNR) enable quantitative
                      analysis of magnetization and oxygen content, revealing a
                      continuous transition from La0.6Sr0.4CoO2.97 to
                      La0.6Sr0.4CoO2.5. BM formation occurs at an oxygen content
                      of 2.67, while the electronic metal-to-insulator transition
                      (MIT) and magnetic ferromagnet-to-non-ferromagnet
                      (FM-to-non-FM) transition occur above an oxygen content of
                      2.77, without a BM signature. These findings demonstrate
                      that the MIT, FM-to-non-FM, and PV-to-BM transitions are
                      interrelated but distinct processes. To further understand
                      the phase transition, the reversibility between PV and BM
                      was studied over 20 cycles. XRD showed that, although the PV
                      and BM structures were maintained, the intensities of both
                      peaks decreased by half, suggesting lattice incoherence or
                      decomposition. Magnetometry indicated no change in magnetic
                      properties, while electronic transport measurements showed
                      partially reversible behaviour. X-ray Photoelectron
                      Spectroscopy (XPS) showed that the Co core-level spectra
                      remained unchanged for both the as-grown and redox-treated
                      PV phases. However, the BM-to-PV transition introduced new
                      features in the A-site core-level spectrum, indicating
                      surface chemical changes during this process. The activation
                      energy for the phase transition was found to be between 0.72
                      and 0.9 eV by in-situ XRD measurements, which is consistent
                      with that of oxygen surface exchange in LSCO. Using platinum
                      to accelerate the surface exchange process further enhanced
                      the phase transition. These suggest that surface exchange is
                      likely the rate-limiting step. Finally, the study was
                      extended to a free-standing LSCO + SrTiO3 membrane. It
                      exhibited similar structural and magnetic transitions from
                      the PV to BM phase and from a ferromagnetic to
                      non-ferromagnetic state, indicating that the membrane has
                      similar functionalities as the thin film. These findings
                      highlight the potential of oxygen defect engineering to
                      enable control of topotactic phase transitions, paving the
                      way for perovskite-based devices with tailored
                      functionalities.},
      cin          = {PGI-7},
      cid          = {I:(DE-Juel1)PGI-7-20110106},
      pnm          = {5233 - Memristive Materials and Devices (POF4-523)},
      pid          = {G:(DE-HGF)POF4-5233},
      typ          = {PUB:(DE-HGF)3 / PUB:(DE-HGF)11},
      doi          = {10.34734/FZJ-2025-04778},
      url          = {https://juser.fz-juelich.de/record/1048648},
}