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| Home > Publications database > Topotactic phase transition in La0.6Sr0.4CoO3-δ thin films: oxygen content, dynamics and reversibility |
| Book/Dissertation / PhD Thesis | FZJ-2025-04778 |
2025
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
ISBN: 978-3-95806-868-1
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Please use a persistent id in citations: doi:10.34734/FZJ-2025-04778
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.
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