Electronic and magnetic structure of epitaxial NiO/Fe3_3O4_4(001) heterostructures grown on MgO(001) and Nb-doped SrTiO3_3(001)

We study the underlying chemical, electronic and magnetic properties of a number of magnetite based thin films. The main focus is placed onto NiO/Fe$_3$O$_4$(001) bilayers grown on MgO(001) and Nb-SrTiO$_3$(001) substrates. We compare the results with those obtained on pure Fe$_3$O$_4$(001) thin films. It is found that the magnetite layers are oxidized and Fe$^{3+}$ dominates at the surfaces due to maghemite ($\gamma$-Fe$_2$O$_3$) formation, which decreases with increasing magnetite layer thickness. From a layer thickness of around 20 nm on the cationic distribution is close to that of stoichiometric Fe$_3$O$_4$. At the interface between NiO and Fe$_3$O$_4$ we find the Ni to be in a divalent valence state, with unambiguous spectral features in the Ni 2p core level x-ray photoelectron spectra typical for NiO. The formation of a significant NiFe$_2$O$_4$ interlayer can be excluded by means of XMCD. Magneto optical Kerr effect measurements reveal significant higher coercive fields compared to magnetite thin films grown on MgO(001), and a 45$^{\circ}$ rotated magnetic easy axis. We discuss the spin magnetic moments of the magnetite layers and find that the moment increases with increasing thin film thickness. At low thickness the NiO/Fe$_3$O$_4$ films grown on Nb-SrTiO$_3$ exhibits a significantly decreased spin magnetic moments. A thickness of 20 nm or above leads to spin magnetic moments close to that of bulk magnetite

From Fe3O4/NiO bilayers to NiFe2O4-like thin films through Ni interdiffusion

Ferrites with (inverse) spinel structure display a large variety of electronic and magnetic properties, making some of them interesting for potential applications in spintronics. We investigate the thermally induced interdiffusion of Ni$^{2+}$ ions out of NiO into Fe$_3$O$_4$ ultrathin films, resulting in off-stoichiometric nickel ferrite–like thin layers. We synthesized epitaxial Fe$_3$O$_4$ bilayers on Nb-doped SrTiO$_3$(001) substrates by means of reactive molecular beam epitaxy. Subsequently, we performed an annealing cycle comprising three steps at temperatures of 400$^\circ$C, 600$^\circ$C, and 800$^\circ$C under an oxygen background atmosphere. We studied the changes of the chemical and electronic properties as result of each annealing step with help of hard x-ray photoelectron spectroscopy and found a rather homogeneous distribution of Ni and Fe cations throughout the entire film after the overall annealing cycle. For one sample we observed a cationic distribution close to that of the spinel ferrite NiFe$_2$O$_4$. Further evidence comes from low-energy electron diffraction patterns indicating a spinel-type structure at the surface after annealing. Site- and element-specific hysteresis loops performed by x-ray magnetic circular dichroism uncovered the antiferrimagnetic alignment between the octahedral coordinated Ni$^{2+}$ and Fe$^{3+}$ ions and the Fe$^{3+}$ ion in tetrahedral coordination. We find a quite low coercive field of 0.02 T, indicating a rather low defect concentration within the thin ferrite films

Light control and solar power conversion by organic optoelectronic assemblies

Organische optoelektronische Baugruppen erreichen bislang noch nicht überall die Leistungsfähigkeit bereits etablierter anorganischer Systeme. Optimierungen der Einzelkomponenten sowie der Zusammenstellung der Baugruppen sind daher das Mittel der Wahl um die Wirksamkeit zu erhöhen. Die damit verbundenen Herausforderungen, insbesondere, dass sich die Eigenschaften einer Baugruppe nicht anhand der Eigenschaften der Einzelkomponenten vorhersagen lassen, werden in den ersten Teilen dieser Arbeit anhand der Modellsysteme der photorefraktiven Polymerkomposite und der organischen Solarzellen aufgezeigt. Ferner wird auf die Herausforderungen bei der Fertigung in herkömmlicher Atmosphäre eingegangen. Im letzten Teil der Arbeit wird anhand von Azopolymerfilmen gezeigt, dass diese sich zur Erzeugung von Oberflächenreliefs mittels strukturierter Lichtfelder eignen. Aus diesen Reliefs abgeformte PDMS-Filme erlauben die Beeinflussung der Lichtpropagation. Letztere wird mittels FDTD-Simulationen analysiert und verschiedene Strukturen bezüglich ihres Einflusses auf das Licht bewertet.To date, organic optoelectronic assemblies are not always able to achieve the performance of their inorganic counterparts. Hence, optimization of the single components as well as the compositions of the assemblies are the typical methods of choice to improve the efficiency. Challenges connected to this procedure, especially the fact that properties of an assembly cannot be derived from the properties of the single components, are demonstrated in the first parts of this thesis. Photorefractive polymer composites and organic solar cells act as model systems for this purpose. Additionally, challenges occurring during fabrication under ambient conditions are discussed. In the last part of this thesis, the fabrication of surface-relief gratings in azopolymer films via structured light fields is demonstrated. PDMS films molded from the reliefs allow for manipulation of light propagation. The effect is analyzed via FDTD simulations and multiple patterns are evaluated regarding their influence on light

Real-time monitoring of the structure of ultrathin Fe3O4 films during growth on Nb doped SrTiO3(001)

In this work, thin magnetite films were deposited on SrTiO3 via reactive molecular beam epitaxy at different substrate temperatures. The growth process was monitored in-situ during deposition by means of x-ray diffraction. While the magnetite film grown at 400 C shows a fully relaxed vertical lattice constant already in the early growth stages, the film deposited at 270 C exhibits a strong vertical compressive strain and relaxes towards the bulk value with increasing film thickness. Furthermore, a lateral tensile strain was observed under these growth conditions although the inverse behavior is expected due to the lattice mismatch of 7.5%. Additionally, the occupancy of the A and B sublattices of magnetite with tetrahedral and octahedral sites was investigated showing a lower occupancy of the A sites compared to an ideal inverse spinel structure. The occupation of A sites decreases for a higher growth temperature. Thus, we assume a relocation of the iron ions from tetrahedral sites to octahedral vacancies forming a deficient rock salt lattice.The authors acknowledge the Deutsche Forschungsgemeinschaft (DFG) via Grant No. KU2321/2-1 for financial support. Further, we would like to thank the ESRF for provision of synchrotron radiation in using BM25

From Fe3 O4 /NiO bilayers to NiFe2 O4 -like thin films through Ni interdiffusion

Ferrites with (inverse) spinel structure display a large variety of electronic and magnetic properties, making some of them interesting for potential applications in spintronics. We investigate the thermally induced interdiffusion of Ni2+ ions out of NiO into Fe3O4 ultrathin films, resulting in off-stoichiometric nickel ferrite-like thin layers. We synthesized epitaxial Fe3O4/NiO bilayers on Nb-doped SrTiO3(001) substrates by means of reactive molecular beam epitaxy. Subsequently, we performed an annealing cycle comprising three steps at temperatures of 400 °C, 600 °C, and 800 °C under an oxygen background atmosphere. We studied the changes of the chemical and electronic properties as result of each annealing step with help of hard x-ray photoelectron spectroscopy and found a rather homogeneous distribution of Ni and Fe cations throughout the entire film after the overall annealing cycle. For one sample we observed a cationic distribution close to that of the spinel ferrite NiFe2O4. Further evidence comes from low-energy electron diffraction patterns indicating a spinel-type structure at the surface after annealing. Site- and element-specific hysteresis loops performed by x-ray magnetic circular dichroism uncovered the antiferrimagnetic alignment between the octahedral coordinated Ni2+ and Fe3+ ions and the Fe3+ ion in tetrahedral coordination. We find a quite low coercive field of 0.02 T, indicating a rather low defect concentration within the thin ferrite films