Halbach arrays at the nanoscale from chiral spin textures

Mallinson's idea that some spin textures in planar magnetic structures could produce an enhancement of the magnetic flux on one side of the plane at the expense of the other gave rise to permanent magnet configurations known as Halbach magnet arrays. Applications range from wiggler magnets in particle accelerators and free electron lasers, to motors, to magnetic levitation trains, but exploiting Halbach arrays in micro- or nanoscale spintronics devices requires solving the problem of fabrication and field metrology below 100 {\mu}m size. In this work we show that a Halbach configuration of moments can be obtained over areas as small as 1 x 1 {\mu}m^2 in sputtered thin films with N\'eel-type domain walls of unique domain wall chirality, and we measure their stray field at a controlled probe-sample distance of 12.0 x 0.5 nm. Because here chirality is determined by the interfacial Dyzaloshinkii-Moriya interaction the field attenuation and amplification is an intrinsic property of this film, allowing for flexibility of design based on an appropriate definition of magnetic domains. 100 nm-wide skyrmions illustrate the smallest kind of such structures, for which our measurement of stray magnetic fields and mapping of the spin structure shows they funnel the field toward one specific side of the film given by the sign of the Dyzaloshinkii-Moriya interaction parameter D.Comment: 12 pages, 4 figure

Multi-modal and quantitative magnetic force microscopy : application to thin film systems with interfacial dzyaloshinskii-moriya interaction

The applications of thin film magnetism play an important role in our everyday life. Magnetic data storage devices for example, rely on magnetic thin film structures for the magnetic recording media itself as well as for the read heads of those devices. Furthermore, different types of electronic sensors exploit the properties of magnetic thin film systems, engineered specifically for the desired function and often relying on processes taking place at the nanometer scale. Magnetic Force Microscopy (MFM) provides a tool for the microscopic magnetic characterization of these thin films and multilayers. Two novel methods for controlling the tip-sample distance for MFM based on bimodal cantilever excitation modes were developed in this thesis. With these, the topography and magnetic stray field emanating from the surface of a sample can be simultaneously measured in a single passage. Moreover these modes are compatible with operation in vacuum and prevent the use of typical lift-mode operation relying on intermittent contact mode for mapping the topography of the sample. The first method makes use of different decay lengths of magnetic and van der Waals forces acting between tip and sample. A stable MFM operation at tip sample distances of only a few nanometers becomes possible resulting in highest lateral magnetic resolution that can be better than 10 nm. However, this method is limited to small tip sample distances where the van der Waals forces become sufficiently large. This limitation is overcome by the second distance control method presented in the thesis. It uses the tip sample capacity as a proxy for the sample's topography and allows MFM-measurements performed with the tip tracing the local topography of the sample or with a tip sample distance that is kept constant in average. Both operation modes are suitable for measurements in externally applied B-fields, as long as possible field induced changes of the cantilever quality factor are considered and the corresponding feedback setpoints are adjusted. The operation at constant average height is preferred, when a later quantitative analysis of the MFM data is considered. The theoretical background of quantitative MFM, and the tip calibration procedures used here are reviewed. These data analysis and measurement techniques have been applied to study Pt/Co/Ir multi-layer ferromagnetic samples with perpendicular anisotropy and interfacially induced Dzyaloshinskii-Moriya (DM) interaction fabricated with a commercial ultra high vacuum, sput-ter deposition system. A novel method for the determination of the average DM interaction from the near-equilibrium domain structure was developed. The average DM interaction value of D=2.04 mJ/m² is higher than the D=1.6+- 0.2 mJ/m² found by Moreau-Luchaire et al. To date it remains unclear whether this higher values arise from a better interface quality of the samples that consist of only five repeats of Pt/Co/Ir compared to the ten repeats used in earlier studies by Moreau-Luchaire et al., or results from the improved analysis method developed here. MFM images of skyrmions performed after repeated saturation processes revealed that the skyrmions re-nucleate at a few specific locations of the film indicating that these may have distinct physical properties. High-resolution images of these skyrmions revealed that their MFM contrast varies substantially between 1.1 Hz and 2.8 Hz, and that some skyrmions have an elliptical shape. Local D-values could be obtained from fitting model calculations to the measured data. These D-values varied between D=3.06 mJ/m² and D=3.48 mJ/m² and are thus considerably larger than the average D=2.04mJ/m² determined from an analysis of the equilibrium do-main size. Apart from the skyrmions, a background contrast that remains constant in all applied fields was observed. Using quantitative MFM methods the spatial variation of the areal magnetic moment density could be determined

Shell-model interactions from chiral effective field theory

We construct valence-space Hamiltonians for use in shell-model calculations, where the residual two-body interaction is based on symmetry principles and the low-momentum expansion from chiral effective field theory. In addition to the usual free-space contact interactions, we also include novel center-of-mass--dependent operators that arise due to the Galilean invariance breaking by in-medium effects. We fitted the low-energy constants to 441 ground- and excited-state energies in the sd shell and obtained a root-mean-square derivation of 1.8 MeV at leading order and of 0.5 MeV at next-to-leading order, with natural low-energy constants in all cases. The developed chiral shell-model interactions enable order-by-order uncertainty estimates and show promising predictions for neutron-rich isotopes beyond the fitted data set.Comment: 19 pages, 12 figures, published versio

Smart Charging of Electric Vehicles with Cloud-based Optimization and a Lightweight User Interface – A Real-World Application in the Energy Lab 2.0: Poster

Smart Charging (SC) of Electric Vehicles (EVs) integrates them into the power system to support grid stability by power management. Large-scale adoption of SC requires a high level of EV user acceptance. Therefore, it is imperative to make the underlying charging scheme tangible for the user. We propose a web app for the user to start, adjust and monitor the charging process via a User Interface (UI). We outline the integration of this web app into an Internet of Things (IoT) architecture to establish communication with the charging station. Two scenarios demonstrate the operation of the system. Future field studies on SC should involve the EV user due to individual preferences and responses to incentive schemes. Therefore, we propose the Smart Charging Wizard with a customizable UI and optimization module for future research and collaborative development

Spontaneous Dissociation of Co2(CO)8 and Autocatalytic growth of Co on SiO2 : A Combined Experimental and Theoretical Investigation

We present experimental results and theoretical simulations of the adsorption behavior of the metal-organic precursor Co2(CO)8 on SiO2 surfaces after application of two different pre-treatment steps, namely by air plasma cleaning or a focused electron beam pre-irradiation. We observe a spontaneous dissociation of the precursor molecules as well as auto-deposition of cobalt on the pre-treated SiO2 surfaces. We also find that the differences in metal content and relative stability of these deposits depend on the pre-treatment conditions of the substrate. Transport measurements of these deposits are also presented. We are led to assume that the degree of passivation of the SiO2 surface by hydroxyl groups is an important controlling factor in the dissociation process. Our calculations of various slab settings using dispersion corrected density functional theory support this assumption. We observe physisorption of the precursor molecule on a fully hydroxylated SiO2 surface (untreated surface) and chemisorption on a partially hydroxylated SiO2 surface (pre-treated surface) with a spontaneous dissociation of the precursor molecule. In view of these calculations, we discuss the origin of this dissociation and the subsequent autocatalysis.Comment: 22 pages, 8 Figures, In Press Article, Beilstein Journal of Nanotechnology, 201

Raccoon Attack: Finding and Exploiting Most-Significant-Bit-Oracles in TLS-DH(E)

Diffie-Hellman key exchange (DHKE) is a widely adopted method for exchanging cryptographic key material in realworld protocols like TLS-DH(E). Past attacks on TLS-DH(E) focused on weak parameter choices or missing parameter validation. The confidentiality of the computed DH share, the premaster secret, was never questioned; DHKE is used as a generic method to avoid the security pitfalls of TLS-RSA. We show that due to a subtle issue in the key derivation of all TLS-DH(E) cipher suites in versions up to TLS 1.2, the premaster secret of a TLS-DH(E) session may, under certain circumstances, be leaked to an adversary. Our main result is a novel side-channel attack, named Raccoon attack, which exploits a timing vulnerability in TLS-DH(E), leaking the most significant bits of the shared Diffie-Hellman secret. The root cause for this side channel is that the TLS standard encourages non-constant-time processing of the DH secret. If the server reuses ephemeral keys, this side channel may allow an attacker to recover the premaster secret by solving an instance of the Hidden Number Problem. The Raccoon attack takes advantage of uncommon DH modulus sizes, which depend on the properties of the used hash functions. We describe a fully feasible remote attack against an otherwisesecure TLS configuration: OpenSSL with a 1032-bit DH modulus. Fortunately, such moduli are not commonly used on the Internet. Furthermore, with our large-scale scans we have identified implementation-level issues in production-grade TLS implementations that allow for executing the same attack by directly observing the contents of server responses, without resorting to timing measurements

Haematopoietic stem cells in perisinusoidal niches are protected from ageing.

With ageing, intrinsic haematopoietic stem cell (HSC) activity decreases, resulting in impaired tissue homeostasis, reduced engraftment following transplantation and increased susceptibility to diseases. However, whether ageing also affects the HSC niche, and thereby impairs its capacity to support HSC function, is still widely debated. Here, by using in-vivo long-term label-retention assays we demonstrate that aged label-retaining HSCs, which are, in old mice, the most quiescent HSC subpopulation with the highest regenerative capacity and cellular polarity, reside predominantly in perisinusoidal niches. Furthermore, we demonstrate that sinusoidal niches are uniquely preserved in shape, morphology and number on ageing. Finally, we show that myeloablative chemotherapy can selectively disrupt aged sinusoidal niches in the long term, which is linked to the lack of recovery of endothelial Jag2 at sinusoids. Overall, our data characterize the functional alterations of the aged HSC niche and unveil that perisinusoidal niches are uniquely preserved and thereby protect HSCs from ageing

Identification of genetic variants associated with Huntington's disease progression: a genome-wide association study

Background Huntington's disease is caused by a CAG repeat expansion in the huntingtin gene, HTT. Age at onset has been used as a quantitative phenotype in genetic analysis looking for Huntington's disease modifiers, but is hard to define and not always available. Therefore, we aimed to generate a novel measure of disease progression and to identify genetic markers associated with this progression measure. Methods We generated a progression score on the basis of principal component analysis of prospectively acquired longitudinal changes in motor, cognitive, and imaging measures in the 218 indivduals in the TRACK-HD cohort of Huntington's disease gene mutation carriers (data collected 2008–11). We generated a parallel progression score using data from 1773 previously genotyped participants from the European Huntington's Disease Network REGISTRY study of Huntington's disease mutation carriers (data collected 2003–13). We did a genome-wide association analyses in terms of progression for 216 TRACK-HD participants and 1773 REGISTRY participants, then a meta-analysis of these results was undertaken. Findings Longitudinal motor, cognitive, and imaging scores were correlated with each other in TRACK-HD participants, justifying use of a single, cross-domain measure of disease progression in both studies. The TRACK-HD and REGISTRY progression measures were correlated with each other (r=0·674), and with age at onset (TRACK-HD, r=0·315; REGISTRY, r=0·234). The meta-analysis of progression in TRACK-HD and REGISTRY gave a genome-wide significant signal (p=1·12 × 10−10) on chromosome 5 spanning three genes: MSH3, DHFR, and MTRNR2L2. The genes in this locus were associated with progression in TRACK-HD (MSH3 p=2·94 × 10−8 DHFR p=8·37 × 10−7 MTRNR2L2 p=2·15 × 10−9) and to a lesser extent in REGISTRY (MSH3 p=9·36 × 10−4 DHFR p=8·45 × 10−4 MTRNR2L2 p=1·20 × 10−3). The lead single nucleotide polymorphism (SNP) in TRACK-HD (rs557874766) was genome-wide significant in the meta-analysis (p=1·58 × 10−8), and encodes an aminoacid change (Pro67Ala) in MSH3. In TRACK-HD, each copy of the minor allele at this SNP was associated with a 0·4 units per year (95% CI 0·16–0·66) reduction in the rate of change of the Unified Huntington's Disease Rating Scale (UHDRS) Total Motor Score, and a reduction of 0·12 units per year (95% CI 0·06–0·18) in the rate of change of UHDRS Total Functional Capacity score. These associations remained significant after adjusting for age of onset. Interpretation The multidomain progression measure in TRACK-HD was associated with a functional variant that was genome-wide significant in our meta-analysis. The association in only 216 participants implies that the progression measure is a sensitive reflection of disease burden, that the effect size at this locus is large, or both. Knockout of Msh3 reduces somatic expansion in Huntington's disease mouse models, suggesting this mechanism as an area for future therapeutic investigation