Bandstruktur-Untersuchungen wohl definierter, multiferroischer Schichtstrukturen
Zusammenfassung der Projektergebnisse
The central objective of the submitted research project has been to gain deeper insights in the interfacial coupling mechanisms in multiferroic heterostructures by electronic band structure analysis. Systematic optimization of the pulsed laser deposition of thin BaTiO3 (BTO) films on top of magnetic La0.67Sr0.33MnO3 (LSMO) electrodes enabled heterostructures with well-defined and single-terminated interfaces. There, the stacking of the polar planes of LSMO defines the ferroelectric polarization direction in the adjacent BTO layer. These highly crystalline, welldefined heterostructures have been a prerequisite for further investigation of the electronic band structure by momentum microscopy (k-PEEM). After sample transfer under high vacuum to avoid unwanted adsorbates, the Fermi surfaces of a pure LSMO layer and a BTO/LSMO heterostructure were mapped and compared. One clearly observes a significant influence of the BTO layer on the LSMO band structure. The objective of future beamtimes is to reveal the particular influence of the ferroelectric polarization direction. Due to the superior ferroelectric properties of strained BiFeO3 (BFO) thin films compared to BTO, layered heterostructures of BFO on top of LSMO electrodes have been elaborated in parallel. There, it has been possible to stabilize a novel structural phase (S-phase) in the complete BFO film. So far, this phase was only known in nanoscale mixed-phase regions. Detailed structural and functional analysis revealed its piezoelectric and also ferroelectric properties which are enhanced compared to bulk material. Future research on device-like structures has to demonstrate how these functionalities can be harnessed and if they offer an advantage compared to the BFO-phases already used in devices. The second part of my research activity has been dedicated to heterostructures of ferroelectric BFO and Mott insulating Ce-doped CaMnO3 (CCMO). Firstly, the structural and functional properties of CCMO films of different thickness and different doping levels were thoroughly analyzed. In detail, X-ray diffraction, transmission electron microscope, superconducting quantum interference device, magneto-optical Kerr effect and transport measurements were performed. The electronic band structure was accessed by soft X-ray angle resolved photoemission spectroscopy (ARPES) and a clear influence of the electron doping due to the chemical substitution of Ca2+ by Ce4+ is observed and effects mainly the Mn3d (eg) states. The Fermi surface of CCMO consists of stripe features which are crossing in the Γ-point but also spots at half distance between Γ and M. These half order spots could be explained by the antiferromagnetic order of the material or the orthorhombic distortion of the unit cell. This detailed characterization of the CCMO structural and functional properties is the basis for future ARPES experiments to probe the influence of an adjacent ferroelectric film. Besides, magnetotransport measurements of CCMO films on YAO show, in addition to the expected normal and anomalous Hall effect, a signal strongly resembling the topological Hall effect (THE) observed in skyrmion systems. This is possibly the first time that THE in a perovskite thin film is observed. The confirmation that the THE signal is due to the presence of a skyrmion lattice would be a major breakthrough for the field of oxide electronics and spintronics and we are intensively working on delivering a proof. To conclude, my work has been focused on thin film heterostructures consisting of a ferroelectric and a magnetic or a Mott insulating layer, respectively. Regarding the proposed research project we succeeded to realize high quality heterostructures and to access their band structure, but for a deeper understanding of the detailed interfacial coupling mechanisms more experimental and theoretical investigations have to be done. The research fellowship allowed me to learn various oxide thin film deposition and characterization techniques. The ensemble of my competences in surface science and in the field of oxide heterostructures is a promising basis for future, also interdisciplinary research projects dedicated to interface or surface effects where a proper control and knowledge of the interface is needed.
Projektbezogene Publikationen (Auswahl)
- “Manipulation of a Mott insulator by a ferroelectric in oxide heterostructures” Workshop on Oxide Electronics, 8. Oktober 2015, Paris
Anke Sander
- “Manipulation of a Mott insulator by a ferroelectric in oxide heterostructures” Workshop on Oxide Electronics, 8. Oktober 2015, Paris
Anke Sander
- “Epitaxial stabilization of pure S-phase BiFeO3” MRS Spring Meeting, 29. März 2016, Phoenix (AZ, USA)
Anke Sander
