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Abstract: We have investigated the electronic and magnetic structures of topological kagome Fe1−𝑥⁢Mn𝑥⁢Sn (0≤𝑥≤0.3) thin films via neutron diffraction, electronic transport measurements, and ab initio density functional theory (DFT) to understand the interplay between hole doping, magnetism, and the electronic structures. Temperature-dependent neutron diffraction measurements on parent FeSn reveal the Néel temperature to be 𝑇N∼355 K and the underlying A-type antiferromagnetic ordering is associated with a wave vector 𝒒=(001/2). Upon Mn doping to 𝑥=0.15, 𝑇N decreases slightly while the magnetic ordering vector remains the same. Resistivity measurements show metallic characteristics and in-plane anisotropy down to 10 K for all the investigated samples. The effects of hole doping are mapped in terms of electronic ground state calculations via DFT which show that the Dirac point is moved closer to the Fermi level (𝐸F) and the flat bands get pushed away from 𝐸F upon hole doping. However, a comparison between hole-doped Fe1−𝑥⁢Mn𝑥⁢Sn and electron-doped Fe1−𝑥⁢Co𝑥⁢Sn indicates that the Néel temperature does not scale with the position of 𝐸F relative to the flat band. Our results establish the antiferromagnetic state of FeSn and Fe1−𝑥⁢Mn𝑥⁢Sn films at room temperature, laying the groundwork for future studies of magnetism in kagome heterostructures.

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Abstract: Tin oxide (SnOx) by atomic-layer deposition (ALD), in combination with fullerene, is widely employed as an electron transport layer in p–i–n perovskite solar cells. This study investigates the direct deposition of ALD SnOx on top of formamidinium (FA)-based perovskites, as a step toward the elimination of the fullerene interlayer and its poor effect on solar cell’s long-term stability. The interfacial chemistry between FA-based perovskites (FAPbI3 and FAPbBr3) and ALD SnOx was studied using soft and hard X-ray photoelectron spectroscopy (SOXPES and HAXPES) with a focus on investigating the separate roles FA and different halides play during interface formation. FAPbI3 and FAPbBr3 solar cell structures solely containing ALD SnOx resulted in s-shaped current–voltage characteristics, indicating the formation of a transport barrier at the interface. Both SOXPES and HAXPES measurements revealed the emergence of additional nitrogen states at the interface during the ALD SnOx deposition on FAPbI3 and FAPbBr3, where these states are linked to the decomposition of FA+. The FAPbI3/ALD SnOx interface also showed the presence of lead iodide (PbI2) through additional lead states other than that from FAPbI3 by using SOXPES measurements. Concerning the FAPbBr3/ALD SnOx interface, no additional lead states were observed; however, measurements instead revealed the formation of Sn–Br bonds at the interface along with the migration of bromine ions into the bulk of the ALD SnOx. Thus, FAPbI3 and FAPbBr3 undergo distinct reaction pathways upon direct deposition of ALD SnOx on top of them. We reason that the decomposition of FA+ in both perovskites and the formation of PbI2 at the FAPbI3/ALD SnOx interface and the incorporation of Br in SnOx at the FAPbBr3/ALD SnOx interface prove detrimental toward device performance. Therefore, careful interfacial engineering that can mitigate the formation of these products should be utilized to enhance the performance of perovskite solar cells.

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Abstract: To commercialize lead halide perovskites as light-emitting diodes (LEDs), the operational device lifetime needs to be drastically improved. For this to be achieved, an understanding of degradation behavior under bias is crucial. Herein, we perform operando measurements of the structural, chemical, and electronic changes using synchrotron-based grazing-incidence wide-angle X-ray scattering and hard X-ray photoelectron spectroscopy on full-stack deep blue mixed bromide/chloride lead halide perovskite LEDs. While a clear drop in optoelectronic performance is recorded under electrical bias, the accompanying X-ray scattering data reveals only minor changes in structural properties. However, photoelectron spectroscopy reveals substantial chemical changes at the electron-injecting interface after bias is applied, including the formation of unwanted metallic lead and a new chlorine species that is not in the perovskite structure. These operando approaches give important structural and interfacial perspectives to reveal the degradation mechanisms in these LEDs and highlight the need to address the top electron-injecting interface to realize step-changes in operational stability.

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Abstract: The development of analog resistive non-volatile memory elements is of great interest for future information storage technology. These devices can be used to emulate the electronic behaviour of biological synapses and neurons for neuromorphic computing or sensing applications. Hafnia is a promising material to use in resistive memory systems due to its high industrial compatibility. Both the migration of oxygen ions and ferroelectricity are two mechanisms actively being explored to drive resistance change in hafnia based devices. This thesis explores the interplay between interfacial redox chemistry and ferroelectricity and how these influence the resistance switching behaviour in epitaxial hafnia films. A device stack is engineered to elicit large resistance changes through both ferroelectricity and interfacial redox reactions. Ultra-thin epitaxial Y doped HfO2 (YHO) is grown with a controlled oxygen stoichiometry. To stabilise the polar phase in YHO and provide a resistance-tuneable bottom interface, YHO is grown on La0.66Sr0.33MnO3 (LSMO) buffered Nb doped SrTiO3 substrates. In-depth X-Ray diffraction based structural analysis identified the polar nature of the film and a strain induced rhombohedral distortion. Ferroelectricity in the film was confirmed by piezoresponse force microscopy. In device configuration, using an Au top electrode with an ultra-thin Ti adhesion layer, a resistance switching mode with large on-off ratio was observed. The pristine device was stabilised in an intermediate resistance state and did not need an electro-forming process. The mode reversibly switched between a purely capacitive (Schottky) and resistive (Ohmic) state, a Schottky-to-Ohmic transition (SOT). A series of voltage pulse trains were used to explore the intermediate resistance states of the system. Both analog and integration behaviour was demonstrated, depending on the pulse profile. The SOT may therefore find application as both a synapse and a neuron respectively in neuromorphic applications. Capacitance-voltage measurements were employed to correlate the ferroelectric polarisa- tion reversal with the SET process and not the RESET process. Therefore, ferroelectricity was indicated to only be partially responsible for the observed resistance change. Electrode area dependent current measurements suggested that in the low resistance state, current transport occurred through a localised conductive filament. A detailed analysis of the inter- face chemistry suggested the importance of precisely designed oxygen scavenging at both interfaces to stabilise the SOT. Furthermore, interface stoichiometry changes were observed during resistance switching by a combined hard photo-electron spectroscopy and electron energy loss spectroscopy study. Impedance spectroscopy measurements were used to correlate the stoichiometry changes at the LSMO|YHO interface to resistance changes during the SOT. The conductive pristine resistance state was shown to predominantly be limited by the LSMO|YHO interface and was proposed to occur due to oxygen scavenging at the top electrode, by-passing through-grain conduction. Analysis of the SOT analog switching behaviour showed that both the YHO layer and the LSMO|YHO interface switch separately during the RESET operation, but simultaneously during the SET process. Furthermore, stack modifications demonstrated the importance of using an ultra-thin LSMO to stabilise the SOT. It was hypothesised to occur due to interface strain enhanced defect formation, oxygen transfer or symmetry changes at the LSMO|YHO interface.

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Abstract: Cobalt ferrite is of great interest due to its promising properties, such as its semiconducting transport behavior and high Curie temperature of 793K. Consequently, cobalt ferrite is used in various applications, including magnetic sensors and magnetic tunnel junctions. Particularly, ultrathin cobalt ferrite films are considered interesting candidates for efficient spin filters in spintronics. This is a significant field in physics that utilizes spin-dependent charge transport in solids. Unlike conventional electronics, spintronics exploits not only charge but also electron spin for information transfer, enabling faster and more energy-efficient devices. The spin-filter effect relies on the exchange splitting of energy levels in the conduction band of a magnetic insulator. As a result, the tunnel barriers in spin filters differ for spin-up and spin-down electrons, leading to a higher tunneling probability for one type, thereby generating a spin-polarized current. In theory, spin filters can reach 100% efficiency, but cobalt ferrites have so far only achieved spin polarization of -8%, indicating an excess of spin-down electrons. Improving spin-filter efficiency requires a deep understanding of the electronic, optical, and transport properties of the material. Therefore, this work investigates the properties of ultrathin cobalt ferrite films. Magnesium oxide (MgO) is used as a substrate due to its minimal lattice mismatch with cobalt ferrite. Since cobalt ferrite’s properties strongly depend on stoichiometry, this study examines how its characteristics vary with cobalt content. Synchrotron radiation was employed for sample measurements, as it offers advantages over conventional laboratory-based X-ray sources. Its extremely high intensity, small beam size, and adjustable energy range provide enhanced capabilities for thin film characterization. Moreover, it provides the excitation energy required for HAXPES experiments. The measurements had already been conducted prior to this work, and this study focuses on analyzing the results. The theoretical foundations are explained in Chapter 2. Before discussing measurement results in Chapter 5, Chapter 3 introduces the material system, and Chapter 4 presents the experimental background. Finally, Chapter 6 provides a summary and an outlook.