Tunable magnetic anisotropy and magnetotransport properties of epitaxial oxide ferromagnetic heterostructures

This dissertation discusses the magnetic and magnetotransport properties of different perovskite oxide thin films. The focus lies on the ferromagnets strontium ruthenate and ruthenium-substituted lanthanum strontium manganite. Both materials are promising candidates in the view of the creation of topologically non-trivial structures, such as magnetic skyrmions. This originates from the possibility to modify the magnetic properties, such as the magnetic anisotropy, for instance by layer thickness variations and interfacial engineering. After the first observation of hump-like features that resemble a topological Hall effect in SrRuO3-SrIrO3 bilayers, several studies aimed to unravel the origin of these anomalies. If skyrmions indeed form in such SrRuO3-SrIrO3 heterostructures, the magnetic coupling between the magnetic layers will be of particular relevance, since the coupling of the skyrmions across the multilayer stack would be desirable. This question was addressed in the framework of the thesis by the artificial design and investigation of SrRuO3-SrIrO3 heterostructures. For 2 MLs thick insulating spacer, only very weak coupling between the individual magnetic SrRuO3 layers was observed, whereas no coupling was observed for thicker SrIrO3 spacers. Such magnetic decoupling of the SrRuO3 is undesirable in the view of the coupling of skyrmions across the multilayer stack. Thus, alternative perovskite oxides should be considered as spacer materials in order to achieve ferromagnetic coupling of the SrRuO3 layers. Since enhanced ferromagnetic coupling was observed when the SrRuO3 layers were separated by metallic LaNiO3 spacers, a similar heterostructure design with strong spin-orbit coupled, but metallic spacers might be of future research interest. Due to the experimental challenges in the imaging of nanosized skyrmions, Hall effect measurements are frequently used to detect fingerprints of magnetic skyrmions. When conduction electrons get scattered by skyrmions, the topological Hall effect (THE) can be detected with technically simple experimental set-ups. However, it is problematic that also other phenomena, such as multiple, parallel (anomalous) Hall channels, can generate features that resemble a topological Hall effect. This issue was emphasized by different examples within this thesis. The magnetic force microscopy (MFM) study of an ultrathin SrRuO3-SrIrO3 bilayer, capped by SrZrO3, showed that peak-like features can be observed in Hall measurements without the existence of skyrmions. The MFM investigations revealed variations of the local layer thickness and corresponding differences of the switching fields in a bare SrRuO3 thin film. These thickness variations were also seen in the trilayer and expected to lead to band structure variations of the anomalous Hall constant. Within the model of multiple anomalous Hall channels, these local variations of switching field and AHE constant can explain the THE-like features. It was demonstrated in a second study that hump-like anomalies can be generated in the Hall loops in SrRuO3-based heterostructures, when the individual SrRuO3 layers possess distinct switching fields and anomalous Hall constants. For this purpose, heterostructures with two SrRuO3 layers of different thickness and therefore with different anomalous Hall constants and coercive fields were investigated. Here, the total Hall voltage can be written as the sum of the Hall voltages of the individual layers. This emphasizes that the conclusion about the presence of skyrmions based on transport measurements only, can be faulty. This further highlights the importance of techniques that are capable to proof the existence of skyrmions, such as real space imaging. In the second part of this dissertation, ruthenium-substituted lanthanum strontium manganite (La0.67Sr0.33Mn0.95Ru0.05O3) films, grown under moderate compressive strain on LSAT(100) substrates, were investigated. A nonmonotonic dependence of the magnetic anisotropy on the layer thickness was observed and attributed to the distinct temperature dependencies of the individual contributions of the magnetic anisotropy. Finally, strong in-plane anisotropic magnetoresistance was seen in a 42.5 nm La0.67Sr0.33Mn0.95Ru0.05O3 thin film deposited on LSAT(100). This anisotropic magnetoresistance, with mirrorlike features for the two orthogonal current directions, could be related with the magnetic anisotropy. The macroscopic magnetic behavior is in good agreement with the formation of parallel magnetic stripe domains, which were observed in a magnetic force microscopy study. The preferential alignment of the magnetic stripe domains furthermore explained the current-direction dependent contribution of magnetic domain wall resistance to the overall magnetoresistance. However, the understanding of the connection between the size and orientation of the magnetic stripe domains and the structural domains still needs to be improved. In contrast to the investigated SrRuO3 films which typically show strong perpendicular magnetic anisotropy, when it is interfaced with SrIrO3 or SrZrO3, the La0.67Sr0.33Mn0.95Ru0.05O3 films under study possessed a weak magnetic anisotropy with preferential magnetization alignment perpendicular to the thin film surface in a thickness-dependent temperature range. This is promising in the view of the tailoring of the magnetic state by small distortions of the (magnetic) energetic balance by modifications of the heterostructure design. The interfacing of Ru-substituted LSMO layers with a strong spin-orbit coupling material, such as SrIrO3, is therefore a possibility that might stabilize magnetically non-trivial structures and is considered as promising research topic in the future

Magnetic interlayer coupling between ferromagnetic SrRuO3_3 layers through a SrIrO3_3 spacer

A key element to tailor the properties of magnetic multilayers is the coupling between the individual magnetic layers. In case of skyrmion hosting multilayers, coupling of skyrmions across the magnetic layers is highly desirable. Here the magnetic interlayer coupling was studied in epitaxial all-oxide heterostructures of ferromagnetic perovskite SrRuO$_3$ layers separated by spacers of the strong spin-orbit coupling oxide SrIrO$_3$. This combination of oxide layers is being discussed as a potential candidate system to host N\'{e}el skyrmions. First order reversal curve (FORC) measurements were performed in order to distinguish between magnetic switching processes of the individual layers and to disentangle the signal of soft magnetic impurities from the samples$'$ signal. Additionally, FORC investigations enabled to determine whether the coupling between the magnetic layers is ferromagnetic or antiferromagnetic. The observed interlayer coupling strength was weak for all the heterostructures, with SrIrO$_3$ spacers between 2 monolayers and 12 monolayers thick.Comment: 22 page

Origin of the hump anomalies in the Hall resistance loops of ultrathin SrRuO3_3/SrIrO3_3 multilayers

The proposal that very small N\'eel skyrmions can form in SrRuO$_3$/SrIrO$_3$ epitaxial bilayers and that the electric field-effect can be used to manipulate these skyrmions in gated devices strongly stimulated the recent research of SrRuO$_3$ heterostructures. A strong interfacial Dzyaloshinskii-Moriya interaction, combined with the breaking of inversion symmetry, was considered as the driving force for the formation of skyrmions in SrRuO$_3$/SrIrO$_3$ bilayers. Here, we investigated nominally symmetric heterostructures in which an ultrathin ferromagnetic SrRuO$_3$ layer is sandwiched between large spin-orbit coupling SrIrO$_3$ layers, for which the conditions are not favorable for the emergence of a net interfacial Dzyaloshinskii-Moriya interaction. Previously the formation of skyrmions in the asymmetric SrRuO$_3$/SrIrO$_3$ bilayers was inferred from anomalous Hall resistance loops showing humplike features that resembled topological Hall effect contributions. Symmetric SrIrO$_3$/SrRuO$_3$/SrIrO$_3$ trilayers do not show hump anomalies in the Hall loops. However, the anomalous Hall resistance loops of symmetric multilayers, in which the trilayer is stacked several times, do exhibit the humplike structures, similar to the asymmetric SrRuO$_3$/SrIrO$_3$ bilayers. The origin of the Hall effect loop anomalies likely resides in unavoidable differences in the electronic and magnetic properties of the individual SrRuO$_3$ layers rather than in the formation of skyrmions

Magneto-optical study of metamagnetic transitions in the antiferromagnetic phase of α-RuCl3

alpha-RuCl3 is a promising candidate material to realize the so far elusive quantum spin liquid ground state. However, at low temperatures, the coexistence of different exchange interactions couple the effective pseudospins into an antiferromagnetically zigzag (ZZ) ordered state. The low-field evolution of spin structure is still a matter of debate and the magnetic anisotropy within the honeycomb planes is an open and challenging question. Here, we investigate the evolution of the ZZ order parameter by second-order magneto-optical effects, the magnetic linear dichroism and magnetic linear birefringence. Our results clarify the presence and nature of metamagnetic transitions in the ZZ phase of alpha-RuCl3. The experimental observations show the presence of initial magnetic domain repopulation followed by a spin-flop transition for small in-plane applied magnetic fields (approximate to 1.6 T) along specific crystallographic directions. In addition, using a magneto-optical approach, we detected the recently reported emergence of a field-induced intermediate phase before suppressing the ZZ order. The results disclose the details of various angle-dependent in-plane metamagnetic transitions quantifying the bond-anisotropic interactions present in alpha-RuCl3

Species identification in heat processed meat products

In this work, species-specific PCR techniques were developed and optimized to identify pork, poultry and cow species in Frankfurt sausages. For pork and poultry, which are the most frequently used meats in Frankfurters, reference binary samples (pork in poultry and poultry in pork) were prepared in the laboratory. In both cases, the established conditions allowed the detection of levels as low as 0.1% of added pork/poultry meat. Real-time PCR assays based on the measurement of fluorescence increments using SYBR Green I dye were used for quantitative purposes. Several samples were acquired in the retail market and tested for their label compliance. Results showed 3 mislabelled samples regarding cow species and 2 for poultry species. Although the qualitative results showed the undeclared presence of pork meat in 2 poultry sausages samples, quantitative results showed levels < 0.1%. These low levels are probably not related to fraudulent practices, but possibly due to cross-contaminations during industrial processing

Robustness of momentum-indirect interlayer excitons in MoS2/WSe2 heterostructure against charge carrier doping

Monolayer transition-metal dichalcogenide (TMD) semiconductors exhibit strong excitonic effects and hold promise for optical and optoelectronic applications. Yet, electron doping of TMDs leads to the conversion of neutral excitons into negative trions, which recombine predominantly non-radiatively at room temperature. As a result, the photoluminescence (PL) intensity is quenched. Here we study the optical and electronic properties of a MoS2/WSe2 heterostructure as a function of chemical doping by Cs atoms performed under ultra-high vacuum conditions. By PL measurements we identify two interlayer excitons and assign them to the momentum-indirect Q-Gamma and K-Gamma transitions. The energies of these excitons are in a very good agreement with ab initio calculations. We find that the Q-Gamma interlayer exciton is robust to the electron doping and is present at room temperature even at a high charge carrier concentration. Submicrometer angle-resolved photoemission spectroscopy (micro-ARPES) reveals charge transfer from deposited Cs adatoms to both the upper MoS2 and the lower WSe2 monolayer without changing the band alignment. This leads to a small (10 meV) energy shift of interlayer excitons. Robustness of the momentum-indirect interlayer exciton to charge doping opens up an opportunity of using TMD heterostructures in light-emitting devices that can work at room temperature at high densities of charge carriers

Validity of magnetotransport detection of skyrmions in epitaxial SrRuO3 heterostructures

A technically simple way of probing the formation of skyrmions is to measure the topological Hall resistivity that should occur in the presence of skyrmions as an additional contribution to the ordinary and anomalous Hall effect. This type of probing, lately intensively used for thin film samples, relies on the assumption that the topological Hall effect contribution can be extracted unambiguously from the measured total Hall resistivity. Ultrathin films and heterostructures of the 4d ferromagnet SrRuO3 have stirred up a lot of attention after the observation of anomalies in the Hall resistivity, which resembled a topological Hall effect contribution. These anomalies, first reported for bilayers in which the SrRuO3 was interfaced with the strong spin-orbit coupled oxide SrIrO3, were attributed to the formation of tiny Néel-type skyrmions. Here we present the investigation of heterostructures with two magnetically decoupled and electrically parallel connected SrRuO3 layers. The two SrRuO3 layers deliberately have different thicknesses, which affects the coercive field and ferromagnetic transition temperature of the two layers, and the magnitude and temperature dependence of their anomalous Hall constants. The SrRuO3 layers were separated by ultrathin layers of either the strong spin-orbit coupling oxide SrIrO3 or of the large band-gap insulator SrZrO3. Our magnetic and magnetotransport studies confirm the additivity of the anomalous Hall transverse voltages for the parallel conducting channels originating from the two ferromagnetic SrRuO3 layers as well as the possibility to tune the global anomalous Hall resistivity by magnetic field, temperature, or structural modifications at the epitaxial all-oxide interfaces. The Hall voltage loops of these two-layer heterostructures demonstrate the possibility to generate humplike structures in the Hall voltage loops of SrRuO3 heterostructures without the formation of skyrmions and emphasize that the detection of skyrmions only by Hall measurements can be misleading

Low Power Compact 3D-Constructed AlScN Piezoelectric MEMS Mirrors for Various Scanning Strategies

In this paper, the newly developed 3D-constructed AlScN piezoelectric MEMS mirror is presented. This paper describes the structure and driving mechanism of the proposed mirror device, covering its driving characteristics in both quasi-static and resonant scan modes. Particularly, this paper deals with various achievable scan patterns including 1D line scan and 2D area scan capabilities and driving methods to realize each scanning strategy. Bidirectional quasi-static actuation along horizontal, vertical, and diagonal scanning directions was experimentally characterized and even under a low voltage level of ±20 V, a total optical scan angle of 10.4° was achieved. In addition, 1D line scanning methods using both resonant and non-resonant frequencies were included and a total optical scan angle of 14° was obtained with 100 mVpp under out-of-phase actuation condition. Furthermore, 2D scan patterns including Lissajous, circular and spiral, and raster scans were realized. Diverse scan patterns were realized with the presented AlScN-based MEMS mirror device even under a low level of applied voltage. Further experiments using high voltage up to ±120 V to achieve an enhanced quasi-static scan angle of more than 20° are ongoing to ensure repeatability. This multi-functional MEMS mirror possesses the potential to implement multiple scanning strategies suitable for various application purposes