In this investigation, the cation distribution in ferrites of spinel-type structure is spectroscopically investigated with respect to the observed magnetic behavior. The ferrite thin films were fabricated by pulsed laser deposition and consequently annealed at different temperatures as well as atmospheres. Structural properties were determined with various methods and the crystalline quality was examined. The dielectric function line-shape was parametrized based on empirical evidence and was found to be dominated by electronic transitions between d orbitals of Fe2+ cations as well as transitions from O 2p to 3d and 4s orbitals of iron and zinc cations. The strongest magneto-optical response was observed for transitions involving cations, which correspond to lattice disorder and inversion within the normal spinel structure.
With the decrease in the substrate temperature during fabrication, a decrease in the magnetic response was observed. The diminishing ferrimagnetic order was directly correlated to the decrease in strength of the transitions, involving Fe3+ on tetrahedral lattice sites. After thermal treatment in argon atmosphere and at a temperature below the deposition temperature, the increase in the magnetic response was explained through the facilitation of oxygen vacancies. With the increase in treatment temperature, a decrease in ferrimagnetic order was related to the recrystallization of the disordered spinel structure toward a more stable normal configuration, evident in the dielectric function spectra.
The cationic configuration distribution in the surface as well as the bulk region, as a function of Zn concentration, was determined from approximation of the XPS and the dielectric function spectra, respectively. The difference in the cation configuration distribution, in films of predominantly inverse configuration, was related to the weak magnetic response, as opposed to films of predominantly normal spinel configuration. Our results demonstrate that a defect-rich surface region could serve as a possible explanation for the ferrimagnetic order in a nominally non-magnetic normal spinel ZnFe2O4. In combination with structural property determination, the net magnetic behavior is explained through the local cationic disorder, determined from the parametrization of the dielectric function spectra in a wide spectral range.:1 Introduction
2 Theoretical background and fundamental considerations
2.1 Spinel ferrite crystal structure
2.2 Crystal field stabilization energy
2.3 Band structure description
2.4 Verwey transition
2.5 Magnetic exchange interactions
3 Sample preparation and modification
3.1 Macroscopic spinel film formation
3.2 Pulsed laser deposition
3.3 Thermal treatment
3.4 Sample overview
4 Methods and general properties
4.1 Structure characterization techniques
4.1.1 X-ray diffraction
4.1.2 X-ray reflectivity
4.1.3 Energy dispersive X-ray spectroscopy
4.1.4 Focused ion beam and scanning electron microscopy
4.1.5 Raman spectroscopy
4.2 Surface properties
4.2.1 Atomic force microscopy
4.2.2 X-ray photoelectron spectroscopy
4.3 Dielectric tensor properties
4.4 Spectroscopic ellipsometry
4.5 Magneto-optical Kerr effect
4.6 Magneto-static properties
5 Results and discussion
5.1 Magnetic and optical properties of Fe3O4 thin film and single crystal
5.2 Magneto-optical properties of ZnxFe3−xO4 thin films
5.3 Fabrication temperature dependent ferrimagnetic order
5.4 Thermally induced structural stabilization
5.5 Cation configuration in dependence on the Zn concentration
5.5.1 Structural property determination
5.5.2 Composition characterization
5.5.3 Magneto-static behavior
5.5.4 Section summary and discussion
6 Summary and outlook
Bibliography
List of article contributions
Selbstständigkeitserklärung
Acknowledgments
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:35443 |
| Date | 20 September 2019 |
| Creators | Zviagin, Vitaly |
| Contributors | Grundmann, Marius, Zollner, Stefan, Universität Leipzig |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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