Nanofiltration (NF) is an effective barrier for removing nanoplastics (NPs) from water. However, NPs can deposit on the membrane surface and remain even after backwash, altering surface properties and reducing filtration performance. In this study, laser-induced breakdown detection (LIBD) is coupled in-line with a bench-scale NF system to quantify the deposition and release of polystyrene (PS) particles and weathered NPs at environmentally relevant concentrations (1-500 µg/L; 107-109 particles/mL). Particle deposition during filtration and release during backwash were successfully determined in all experiments, supported by theoretical analysis of the interplay between hydrodynamic and intermolecular forces. At permeate fluxes higher than 100 L/m2.h, 50-100% of PS particles were deposited by the end of filtration experiments, forming cake layers up to 9 particle diameters thick. In contrast, weak permeate drag forces corresponding to fluxes below 50 L/m2.h (i.e., just beyond the critical flux) resulted in insignificant deposition. Backwash fluxes from 15 to 57 L/m2.h exhibited negligible differences in the release of deposited particles owing to only a little increase in backwash drag force. Two mechanisms were observed for the release of NPs during backwash: i) complete release in small circular areas (diameter ≤2 µm) for thin deposits and ii) fracturing of the cake layer for thick deposits. For weathered NPs, irreversible deposition on the membrane surface was observed, although potential particle aggregation and polydispersity must be accounted for to obtain truly quantitative results. The successful quantification of particle deposition and release showcases LIBD as an effective method for fundamental investigations on NP transport during membrane filtration.

Many bacteria share the fascinating ability to sense Earth's magnetic field-a process known as magnetotaxis. These bacteria synthesize magnetic nanoparticles, called magnetosomes, within their own cell body and arrange them to form a linear magnetic chain. The chain, which behaves like a compass needle, aligns the microorganisms with the geomagnetic field. Here, we measure the magnetic hysteresis of an individual bacterium of the species Magnetospirillum gryphiswaldense via ultrasensitive torque magnetometry. These measurements, in combination with transmission electron microscopy and micromagnetic simulations, reveal the magnetic configurations of the magnetosomes, their progression as a function of applied field, as well as the total remanent magnetic moment and effective magnetic anisotropy of a chain within a single bacterium. Knowledge of these properties is crucial both for understanding the mechanisms behind magnetotaxis and for the design of systems exploiting magnetotactic bacteria in biomedical applications.

Generation and characterisation of metal nanoparticles (NP) gained attention in recent years due to their significant potential in applications as diverse as catalysis, electronics or energy storage. Despite the high interest in NPs, their characterization is challenging and detailed quantitative information on size, number concentration and morphologies are key to understand their properties. In this study we generated NPs from four metals, Au, Pt, Cu and Ni, via spark ablation in the aerosol phase, which allows to produce NPs as small as 1 nm in high quantities and purity. Particles were characterised with transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDX) as well as online aerosol particle size distribution measurement techniques. Particle modes for the four metals ranged between 3 nm and 4 nm right after generation. The number and size of particles generated correlated with thermal properties of the metals such as thermal or electrical conductivity. The four metal NPs were also coagulated with larger TiO2 NPs of about 120 nm size and the metal surface coverage of the TiO2 particles was characterised with electron microscopy and EDX spectroscopy.

The microstructure, crystallographic phases, and mechanical properties of a newly developed Al2O3 – TiO2 – ZrO2 ternary ceramic coating were characterized. The coatings were produced by atmospheric plasma spraying as a preblended powder on Ti-6Al-4 V substrates using the new generation of the Debye-Larmor cascaded plasma torch. The 400 μm thick as-sprayed ternary ceramic coating is compact and neither delamination nor inter-/trans-granular cracks were found. The coating consists of single phase α-Al2O3, monoclinic m-ZrO2, and a nanocrystalline dual phase structure of α-Al2O3 and m-ZrO2. Ti is either present as ZrTiO4 or as solute in the dual phase. Cracking from the tip of the indent is rare and delamination was not observed after the progressive scratch test. The coating has potential in high wear applications for example in medical devices.

Focused-electron-beam-induced deposition is a promising technique for patterning nanomagnets for spin qubit control in a single step. We fabricate cobalt nanomagnets in such a process, obtaining cobalt contents and saturation magnetizations comparable to or higher than those typically obtained using electron-beam lithography. We characterize the nanomagnets using transmission electron microscopy and image their stray magnetic field using scanning NV magnetometry, finding good agreement with micromagnetic simulations. The magnetometry reveals the presence of magnetic domains and halo side-deposits, which are common for this fabrication technique. Finally, we estimate dephasing times for electron spin qubits in the presence of disordered stray fields due to these side-deposits.

A spin-photon interface should operate with both coherent photons and a coherent spin to enable cluster-state generation and entanglement distribution. In high-quality devices, self-assembled GaAs quantum dots are near-perfect emitters of on-demand coherent photons. However, the spin rapidly decoheres via the magnetic noise arising from the host nuclei. Here, we address this drawback by implementing an all-optical nuclear-spin cooling scheme on a GaAs quantum dot. The electron-spin coherence time increases 156-fold from T_{2}^{*}=3.9  ns to 0.608  μs. The cooling scheme depends on a non-collinear term in the hyperfine interaction. The results show that such a term is present even though the strain is low and no external stress is applied. Our work highlights the potential of optically active GaAs quantum dots as fast, highly coherent spin-photon interfaces.

We present a study on the structural and magnetic properties of Lu(Fe0.2Mn0.2Co0.2Cr0.2Ni0.2)O-3 (Lu5BO) high-entropy oxide perovskite thin films. We use synchrotron-based x-ray absorption spectroscopy employing x-ray magnetic circular and linear dichroism (XMCD and XMLD) to perform an element-sensitive study on single-crystal epitaxial Lu5BO thin films. Together with XMCD magnetometry, the results reveal dominant antiferromagnetic order with a transition temperature around 100 K.

Scanning superconducting quantum interference device (SQUID) microscopy is a magnetic imaging technique combining high-field sensitivity with nanometer-scale spatial resolution. State-of-the-art SQUID-on-tip probes are now playing an important role in mapping correlation phenomena, such as superconductivity and magnetism, which have recently been observed in two-dimensional van der Waals materials. Here, we demonstrate a scanning probe that combines the magnetic and thermal imaging provided by an on-tip SQUID with the tip-sample distance control and topographic contrast of a non-contact atomic force microscope (AFM). We pattern the nanometer-scale SQUID, including its weak-link Josephson junctions, via focused ion beam milling at the apex of a cantilever coated with Nb, yielding a sensor with an effective diameter of 365 nm, field sensitivity of 9.5 $\text{nT}/\sqrt{\text{Hz}}$ and thermal sensitivity of 620 $\text{nK}/\sqrt{\text{Hz}}$, operating in magnetic fields up to 1.0 T. The resulting SQUID-on-lever is a robust AFM-like scanning probe that expands the reach of sensitive nanometer-scale magnetic and thermal imaging beyond what is currently possible.

Highly transparent superconducting contacts to a topological insulator (TI) remain a persistent challenge on the route to engineer topological superconductivity. Recently, the higher-order TI WTe_2 was shown to turn superconducting when placed on palladium (Pd) bottom contacts, demonstrating a promising material system in pursuing this goal. Here, we report the diffusion of Pd into WTe_2 and the formation of superconducting PdTe_x as the origin of observed superconductivity. We find an atomically sharp interface in vertical direction to the van der Waals layers between the diffusion crystal and its host crystal, forming state-of-the-art superconducting contacts to a TI. The diffusion is discovered to be non-uniform along the width of the WTe_2 crystal, with a greater extend along the edges compared to the bulk. The potential of this contacting method is highlighted in transport measurements on Josephson junctions by employing external superconducting leads.

Artificial spin ices are a class of metamaterials consisting of magnetostatically coupled nanomagnets. Their interactions give rise to emergent behavior, which has the potential to be harnessed for the creation of functional materials. Consequently, the ability to map the stray field of such systems can be decisive for gaining an understanding of their properties. Here, we use a scanning nanometer-scale superconducting quantum interference device (SQUID) to image the magnetic stray field distribution of an artificial spin ice system exhibiting structural chirality as a function of applied magnetic fields at 4.2 K. The images reveal that the magnetostatic interaction gives rise to a measurable bending of the magnetization at the edges of the nanomagnets. Micromagnetic simulations predict that, owing to the structural chirality of the system, this edge bending is asymmetric in the presence of an external field and gives rise to a preferred direction for the reversal of the magnetization. This effect is not captured by models assuming a uniform magnetization. Our technique thus provides a promising means for understanding the collective response of artificial spin ices and their interactions.

We use a scanning nanometer-scale superconducting quantum interference device (SQUID) to image individual vortices in amorphous superconducting MoSi thin films. Spatially resolved measurements of the magnetic field generated by both vortices and Meissner screening satisfy the Pearl model for vortices in thin films and yield values for the Pearl length and bulk penetration depth at 4.2 K. Flux pinning is observed and quantified through measurements of vortex motion driven by both applied currents and thermal activation. The effects of pinning are also observed in metastable vortex configurations, which form as the applied magnetic field is reduced and magnetic flux is expelled from the film. Understanding and controlling vortex dynamics in amorphous thin films is crucial for optimizing devices such as superconducting nanowire single photon detectors (SNSPDs), the most efficient of which are made from MoSi, WSi, and MoGe.

In the first part of this thesis, ferromagnetic nanotubes (FNTs) consisting of a non-magnetic GaAs core and a ferromagnetic shell of Py or CoFeB with a hexagonal cross-section are investigated using two different magnetic imaging techniques. These techniques allow the investigation of equilibrium magnetic configurations of FNTs depending on the length and diameter. First, x-ray magnetic circular dichroism photoemission electron microscopy (XMCD-PEEM) is used to image the local magnetization of the FNTs at their surface. For such three-dimensional structures, the technique also yields information about the average magnetization of the volume traversed by the x-rays. Second, a nanometer-scale superconducting quantum interference device sensor located at the end of a hollow quartz tip (SQUID-on-tip) is fabricated and used to image the FNTs’ magnetic stray field distribution. The obtained magnetic images of the FNTs from both magnetic imaging tools provide direct evidence for flux-closure configurations, including a global vortex state, in which the magnetization points circumferentially around the tube axis. Consistent with an analytical theory by Landeros et al. and our own numerical simulations, the FNT length-to-diameter ratio is found to play a crucial role in stabilizing the global vortex state. The XMCD-PEEM images of the equilibrium magnetization configurations show that the relative circulation sense of vortex ends in real FNTs does not always match the lowest energy configuration calculated in simulations. Short FNTs are found not only in remnant global vortex states, but also in opposing vortex states, which include a Neel wall between two opposing vortices. Additional simulations suggest that sample imperfections including variations in thickness and deviation from a perfect geometry are responsible for this discrepancy. Our results show the promise of using geometry to program both the overall equilibrium magnetization configurations and the reversal process in nanomagnets. In the second part, an experiment is performed using scanning SQUID-on-tip to image the stray field of an artificial spin ice system, which displays structural chirality. Experiments are carried out in series of magnetic fields at 4.2 K. The “chiral ice” is a two-dimensional arrangement of lithographically patterned Py nanomagnets. Each nanomagnet is much thinner than its in-plane dimensions, producing a strong shape anisotropy that favors a single-domain magnetization configuration. The measurements, backed by micromagnetic simulations, reveal that the magnetization in the nanomagnets is not uniform close to zero-field, displaying a bending at the edges of the nanostructures. The results show that the number of degrees of freedom in an artificial spin ice can be much larger than typically captured in dipolar models. These additional degrees of freedom contribute to the field-induced dynamics and may be used to create reprogrammable magnonic crystals. The final part of the thesis deals with a further development of the SQUID-on-tip technology: the realization of a SQUID at the tip of a conventional atomic force microscopy (AFM) cantilever. To realize such a probe, a focused ion beam (FIB) is used to mill the apex of the Si-cantilever into a suitable shape, which later serves as a template for the nanoSQUID. A SQUID-on-tip, located at the cantilever apex, is obtained through the directional evaporation of a thin Pb film. With this new technology we expect to retain the favorable properties of the SQUID-on-tip technique, while also adding sensitivity to tip-sample forces through standard non-contact AFM techniques. Such a hybrid system would allow the simultaneous imaging of topography, magnetic stray field, and temperature on the nanometer-scale. Thereby it would be possible to directly correlate these quantities, which is crucial for many applications in basic research.

We use a scanning nanometer-scale superconducting quantum interference device to map the stray magnetic field produced by individual ferromagnetic nanotubes (FNTs) as a function of applied magnetic field. The images are taken as each FNT is led through magnetic reversal and are compared with micromagnetic simulations, which correspond to specific magnetization configurations. In magnetic fields applied perpendicular to the FNT long axis, their magnetization appears to reverse through vortex states, that is, configurations with vortex end domains or in the case of a sufficiently short FNT with a single global vortex. Geometrical imperfections in the samples and the resulting distortion of idealized magnetization configurations influence the measured stray-field patterns.

The reversal of uniform axial magnetization in a ferromagnetic nanotube (FNT) has been predicted to occur through the nucleation and propagation of vortex domains forming at the ends. We provide experimental evidence for this behavior through dynamic cantilever magnetometry measurements of individual FNTs. In particular, we identify the nucleation of the vortex end domains as a function of applied magnetic field and show that they mark the onset of magnetization reversal. We find that the nucleation field depends sensitively on the angle between the end surface of the FNT and the applied field. Micromagnetic simulations substantiate the experimental results and highlight the importance of the ends in determining the reversal process. The control over end-vortex nucleation enabled by our findings is promising for the production of FNTs with tailored reversal properties.

We image the remnant magnetization configurations of CoFeB and permalloy nanotubes (NTs) using x-ray magnetic circular dichroism photoemission electron microscopy. The images provide direct evidence for flux-closure configurations, including a global vortex state, in which magnetization points circumferentially around the NT axis. Furthermore, micromagnetic simulations predict and measurements confirm that vortex states can be programmed as the equilibrium remnant magnetization configurations by reducing the ratio of the NT's length and diameter.

The reversal of a uniform axial magnetization in a ferromagnetic nanotube (FNT) has been predicted to nucleate and propagate through vortex domains forming at the ends. In dynamic cantilever magnetometry measurements of individual FNTs, we identify the entry of these vortices as a function of applied magnetic field and show that they mark the nucleation of magnetization reversal. We find that the entry field depends sensitively on the angle between the end surface of the FNT and the applied field. Micromagnetic simulations substantiate the experimental results and highlight the importance of the ends in determining the reversal process. The control over end vortex formation enabled by our findings is promising for the production of FNTs with tailored reversal properties.

We investigate the magnetization reversal mechanism in an individual permalloy (Py) nanotube (NT) using a hybrid magnetometer consisting of a nanometer-scale SQUID (nanoSQUID) and a cantilever torque sensor. The Py NT is affixed to the tip of a Si cantilever and positioned in order to optimally couple its stray flux into a Nb nanoSQUID. We are thus able to measure both the NT's volume magnetization by dynamic cantilever magnetometry and its stray flux using the nanoSQUID. We observe a training effect and a temperature dependence in the magnetic hysteresis, suggesting an exchange bias. We find a low blocking temperature T-B = 18 +/- 2 K, indicating the presence of a thin antiferromagnetic native oxide, as confirmed by x-ray absorption spectroscopy on similar samples. Furthermore, we measure changes in the shape of the magnetic hysteresis as a function of temperature and increased training. These observations show that the presence of a thin exchange-coupled native oxide modifies the magnetization reversal process at low temperatures. Complementary information obtained via cantilever and nanoSQUID magnetometry allows us to conclude that, in the absence of exchange coupling, this reversal process is nucleated at the NT's ends and propagates along its length as predicted by theory.

We have performed a study of thermally driven magnetic relaxation in building blocks of artificial kagome spin ice. For room-temperature measurements, we observe that low-energy states are accessed with high efficiency, particularly in structures with strong dipolar coupling and with low thicknesses. With carefully tuned heating experiments, we demonstrate how thermally active artificial spin ice systems relax magnetically from higher-energy states and eventually fall into low-energy states. The methods applied in our work offer the possibility to observe the thermodynamics of artificial spin ice systems in real space and time, and provide a way to directly investigate the nature of complex stochastic processes.