Influence of growth conditions on magnetic, electronic and structural properties of ultrathin Co-based oxide films

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Titel: Influence of growth conditions on magnetic, electronic and structural properties of ultrathin Co-based oxide films
Autor(en): Ruwisch, Kevin
ORCID des Autors:
Erstgutachter: Prof. Dr. Joachim Wollschläger
Zweitgutachter: Dr. Karsten Küpper
Zusammenfassung: This work deals with the growth of ultrathin magnetite (Fe3O4), cobalt ferrite (CoFe2O4) and nickel cobaltite (NiCo2O4) films and their characterization in terms of magnetic, electronic, and structural properties. The films are grown on different substrates using reactive molecular beam epitaxy (RMBE). First, a Fe3O4/CoxFe3-xO4 (x=0.5 and x=1) bilayer film system is investigated in terms of interfacial magnetization. Here, the Fe3O4 layer thickness varies from 0nm to 13nm, while the CoxFe3-xO4 layer thickness is kept constant. The quality and stoichiometry of the films are checked in-situ using X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). X-ray reflectivity (XRR) measurements are used to determine the film thicknesses of the respective films. To determine the cationic distribution and magnetic moments, X-ray magnetic circular dichroism (XMCD) measurements are performed and afterwards multiplet calculations and sum rules are applied, resulting in increased magnetization at the interface of the bilayers. Finally, magnetic hysteresis measurements are carried out using a superconducting quantum interference device (SQUID) showing typical exchange-spring behavior. In the second step, the growth behavior of ultrathin CoFe2O4 films in the early stages is investigated using operando hard x-ray photoelectron spectroscopy (HAXPES). For this purpose, the CoFe2O4 films are grown on Nb doped SrTiO3(001) by RMBE and simultaneously HAXPES measurements are performed during deposition. The varying parameter is the oxygen partial pressure, which is changed from 5E-7 mbar to 5E-6 mbar. This results in a classification of the samples into metallic, light oxide and cobalt ferrite-like growth. Final X-ray diffraction measurements confirm the findings that emerge from the HAXPES measurements. Finally, the material system is changed and NiCo2O4, also an Co-based oxide, is investigated. In particular, the influence of the use of oxygen plasma on the growth and stability is investigated here. For this purpose, two series of NiCo2O4 films on the substrates MgO(001), SrTiO3(001) and MgAl2O4(001) are investigated. Molecular oxygen is used for the first series and oxygen plasma is used for the second series. The in-situ XPS and LEED measurements show that a rock salt structure forms when molecular oxygen is used and the expected spinel structure forms when oxygen plasma is used. After transport under ambient conditions, XRR measurements are performed in order to determine the film thicknesses. Moreover, XRD and HAXPES measurements for structural and electronic properties are performed. The results of XRD measurements show that no spinel structure is present, regardless of whether molecular oxygen or oxygen plasma is used. The HAXPES measurements also show no spinel structure anymore, not even in deeper layers.
Schlagworte: magnetic thin films; magnetite; cobalt ferrite; nickel cobaltite; synchrotron radiation; XPS; HAXPES; XAS; XMCD; XRD; SQUID
Erscheinungsdatum: 13-Mär-2023
Lizenzbezeichnung: Attribution-NonCommercial-NoDerivs 3.0 Germany
URL der Lizenz:
Publikationstyp: Dissertation oder Habilitation [doctoralThesis]
Enthalten in den Sammlungen:FB06 - E-Dissertationen

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