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Abstract: The passive film stability on stainless steel can be affected by hydrogen absorption and lead to microstructure embrittlement. This work shows that the absorption of hydrogen results in surface degradation due to oxide reduction and ionic defect generation within the passive film, which decomposes and eventually vanishes. The passive film provides a barrier to entering hydrogen, but when hydrogen is formed, atomic hydrogen infuses into the lattices of the austenite and ferrite phases, causing strain evolution, as shown by synchrotron x-ray diffraction data. The vacancy concentration and hence the strains increase with increasing electrochemical cathodic polarization. Under cathodic polarization, the surface oxides are thermodynamically unstable, but the complete reduction is kinetically restrained. As a result, surface oxides remain present under excessive cathodic polarization, contesting the classical assumption that oxides are easily removed. Density-functional theory calculations have shown that the degradation of the passive film is a reduction sequence of iron and chromium oxide, which causes thinning and change of the semiconductor properties of the passive film from n-type to p-type. As a result, the surface loses its passivity after long cathodic polarization and becomes only a weak barrier to hydrogen absorption and hence hydrogen embrittlement.

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Abstract: W-type hexaferrites with varied Co/Zn ratios were synthesized and the magnetic order was investigated using neutron powder diffraction. In SrCo2Fe16O27 and SrCoZnFe16O27 a planar (Cm′cm′) magnetic ordering was found, rather than the uniaxial ordering (P63/mm′c′) found in SrZn2Fe16O27 which is common in most W-type hexaferrites. In all three studied samples, non-collinear terms were present in the magnetic ordering. One of the non-collinear terms is common to the planar ordering in SrCoZnFe16O27 and uniaxial ordering in SrZn2Fe16O27, which could be a sign of an imminent transition in the magnetic structure. The thermomagnetic measurements revealed magnetic transitions at 520 and 360 K for SrCo2Fe16O27 and SrCoZnFe16O27, and Curie temperatures of 780 and 680 K, respectively, while SrZn2Fe16O27 showed no transition but a Curie temperature at 590 K. This leads to the conclusion that the magnetic transition can be adjusted by fine-tuning the Co/Zn stoichiometry in the sample.

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Abstract: The ability to control the structural properties of molecular layers is a key for the design and preparation of organic electronic devices. While microscopic growth studies of planar, rigid and symmetric π-conjugated molecules have been performed to a larger extent, this is less the case for elongated donor-acceptor molecules with flexible functional groups, which are particularly interesting due to their high dipole moments. Prototypical molecules of this type are merocyanines (MCs), which have been widely studied for the use as efficient absorbers in organic photodetectors. For maximized light absorption and optimized electronic properties the molecular arrangement which is affected by the initial assembly of the films at the supporting substrate interface is decisive. The situation deserves special attention, when the surface nucleation leads to so far not known and bulk-unlike aggregates. Here, we report on the growth of a typical MC (HB238) on the Ag(100) surface, serving as the substrate. In the energetically preferred phase, the molecules adsorb in a face-on geometry and organize in tetramers with a circular dipole arrangement. The tetramers further self-order in large, enantiopure domains with a periodicity that is commensurate to the Ag(100) surface, likely due to a specific bonding of the thiophene and thiazol rings to the Ag surface. Using scanning tunneling microscopy (STM) in combination with low energy electron diffraction we derive the detailed structure of the tetramers. The center of the tetramer, which is most prominent in STM images, consists of four upward pointing tert-butyl groups from four molecules. It is encircled by a ring of four hydrogen bonds between terminal CN-groups and thiophene rings on neighboring molecules. In parallel, the surface interaction modifies the intramolecular dipole, which is revealed from photoemission spectroscopy. Hence, this example shows how the surface template effect leads to an unforeseen molecular organization which is considerably more complex to that in the bulk phases of HB238, which feature paired dipoles.

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Abstract: In this thesis, the static and dynamic properties of magnetic multilayer samples were studied using a variety of experimental techniques, based both at Exeter and the Diamond Light Source (DLS) and Advanced Light Source (ALS) synchrotron facilities. The exchange interaction, which acts to align spins, is a fundamental part in these magnetic multilayer samples. First, the glancing angle deposition (GLAD) technique was investigated as a tool for creating magnetic multilayers with exciting new exchange interactions. For this, Co thin films were grown by DC magnetron sputtering to tailor the magnetic anisotropy of the samples. These Co samples were structurally characterized using x-ray diffraction (XRD) and transmission electron microscopy (TEM). Vibrating sample magnetometry (VSM) was then performed to investigate the magnetic properties of the thin films as a result of the GLAD technique. From this, the necessary conditions for effective anisotropy control using the GLAD technique were identified. Synchrotron x-ray measurements, such as x-ray magnetic circular / linear dichroism (XMCD/XMLD) for static measurements, are vital for investigating magnetic multilayer samples with elemental resolution. To add depth-sensitivity to the synchrotron measurements, the idea of an ultra-thin Mn “spy layer” was investigated by inserting different thicknesses (tMn) of Mn into the NiFe layer in a FePt / NiFe bilayer. The effect on the static magnetic properties was studied using VSM and XMCD hysteresis loops before structural information was obtained using scanning transmission electron microscopy (STEM). The magnetization dynamics were probed using vector network analyzer ferromagnetic resonance (VNA-FMR) and element resolved x-ray ferromagnetic resonance (XFMR) measurements. From this, the ideal “spy layer” thickness of Mn was found to lie in the region 0Å < tMn < 5Å . Spin currents are a dynamic process found in magnetic multilayers and are driven by the exchange interaction. The measurement of a transverse charge current generated via the inverse spin Hall effect (ISHE) has become the principal technique for observing spin currents. During ISHE measurements, parasitic microwave effects were observed and a method to separate out the inverse spin Hall effect was identified. This method was then tested for a reference YIG / Pt bilayer. A more complex NiFe / NiO / Pd / FeCo sample was then studied using this procedure and the ISHE voltage was identified, despite the presence of additional parasitic effects. In addition to the DC spin current component, there is an AC spin current contribution. The AC spin current component was also investigated for the NiFe / NiO / Pd / FeCo sample series using XMCD, XMLD and XFMR measurements. The XMCD and XMLD data revealed the Ni and (Fe)Co spins possess perpendicular in-plane coupling relative to the magnetic moments within the NiO layer. To understand the magnetization dynamics in these samples, an evanescent spin wave model was invoked. This provides crucial insights for interpreting spin current propagation through NiO. Through the combination of work described above, new avenues for the fabrication of magnetic multilayers and the measurement of the magnetization dynamics in such systems are presented to yield a more complete understanding of the crucial role of the exchange interaction in the magnetization dynamics of magnetic multilayers.

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Abstract: In recent years, chromium sulphur bromide (CrSBr) has emerged as a promising highly- anisotropic semiconducting two-dimensional (2D) magnetic material to explore spintronics and quantum transport due to its strongly correlated quasiparticle interactions [1]. CrSBr is an A-type layered antiferromagnet; in the bulk material, above the Néel temperature (TN = 132K) it transitions to an intermediate ferromagnetic phase before becoming paramagnetic at high temperature. Experimental work on its fascinating optoelectronic properties has been heavily supported by electronic structure calculations using a variety of methods [2,3], but direct band structure measurements to test these predictions are still lacking. Recent angle- resolved photoemission microscopy (ARPES) measurements of bulk CrSBr were unable to measure below TN due to charging effects [4]. Here, we overcome this limitation through exfoliation of CrSBr flakes onto a template-stripped gold surface (Figure 1a) [5]. Using the nanoARPES endstation of the i05 beamline at Diamond Light Source, ARPES was acquired without charging from thin flakes (~10 nm thick) at temperatures down to < 40 K. Photon energy, and polarisation, dependent measurements confirm a strongly 2D dispersion and link the band dispersions to different atomic orbitals. Temperature-dependent measurements highlight electronic structure changes through the magnetic phase transitions, including shifts of the low energy valence bands and band splitting suggestive of spin-ordering (Figure 1b,c). These results also demonstrate a simple approach for the measurement of the low- temperature band structure of insulating layered materials.