Electronic and spin control in functional oxide heterostructures
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Date
2021
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Abstract
Transition metal oxides exhibit a variety of physical properties due to their high tunability
of spin, orbital, and charge degrees of freedom. This thesis studies two complex,
transition metal oxide thin film/substrate systems, the NiFe2O4/Nb:SrTiO3
(NFO/STO) and the La0.3Sr0.7MnO3/Nb:SrTiO3 (LSMO/ STO). NFO is a ferrimagnetic
insulator. LSMO is a ferromagnetic half metal and below a critical thickness
transitions to an insulator. However, the physical properties of those complex oxides
heterostructures are often directly linked to the quality of the thin oxide film and the
interactions between the oxide thin film and its substrate at the interface. In this
thesis the ultra thin NFO film growth via pulsed laser deposition is optimized. First
those optimized, smooth and single crystalline NFO films are studied with lab measurement
methods to ensure a high film quality. In a next step their stoichiometry
and crystalline occupancy is studied via hard X-ray photoemission (HAXPES) and
X-ray absorption spectroscopy, which were recorded at the large scale synchrotron
BESSY II, PETRA III and the DIAMOND Light source. The stoichiometry and occupancy
is found to match the theoretical expectations of the NFO oxide in the inverse
spinel structure. The magnetic response of the NFO/STO heterostructure is researched
with the element selective X-ray magnetic circular dichroism (XMCD).
Due to the proximity effect the Ti signal mimics the ferrimagnetic
response of the NFO thin film. The electronic properties of Au/NFO/STO
and Pt/LSMO/STO devices were investigated via in-operando HAXPES measurements.
In-operando HAXPES is recording HAXPES spectra during a voltage is applied
to the sample. The band alignment and band bending are investigated by
applying Kraut' s method to the HAXPES spectra.
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Keywords
NiFe2O4, SrTiO3, LSMO, Oxide, Heterostructure, Oxide thin films, PLD, XAS, XMCD, In-Operando HAXPES, Band alignment, Band bending, Interface magnetism