Magnon contribution to unidirectional spin Hall magnetoresistance

We develop a model for the magnonic contribution to the unidirectional spin Hall magnetoresistance (USMR) of heavy metal/ferromagnetic insulator bilayer films. We show that diffusive transport of Holstein-Primakoff magnons leads to an accumulation of spin near the bilayer interface, giving rise to a magnoresistance which is not invariant under inversion of the current direction. Unlike the electronic contribution described by Zhang and Vignale [Phys. Rev. B 94, 140411 (2016)], which requires an electrically conductive ferromagnet, the magnonic contribution can occur in ferromagnetic insulators such as yttrium iron garnet. We show that the magnonic USMR is, to leading order, cubic in the spin Hall angle of the heavy metal, as opposed to the linear relation found for the electronic contribution. We estimate that the maximal magnonic USMR in Pt|YIG bilayers is on the order of $10^{-8}$, but may reach values of up to $10^{-5}$ if the magnon gap is suppressed, and can thus become comparable to the electronic contribution in, e.g., Pt|Co. We show that the magnonic USMR at a finite magnon gap may be enhanced by an order of magnitude if the magnon diffusion length is decreased to a specific optimal value that depends on various system parameters.Comment: 9 pages, 7 figure

Microstructural topology effects on the onset of ductile failure in multi-phase materials - a systematic computational approach

Multi-phase materials are key for modern engineering applications. They are generally characterized by a high strength and ductility. Many of these materials fail by ductile fracture of the, generally softer, matrix phase. In this work we systematically study the influence of the arrangement of the phases by correlating the microstructure of a two-phase material to the onset of ductile failure. A single topological feature is identified in which critical levels of damage are consistently indicated. It consists of a small region of the matrix phase with particles of the hard phase on both sides in a direction that depends on the applied deformation. Due to this configuration, a large tensile hydrostatic stress and plastic strain is observed inside the matrix, indicating high damage. This topological feature has, to some extent, been recognized before for certain multi-phase materials. This study however provides insight in the mechanics involved, including the influence of the loading conditions and the arrangement of the phases in the material surrounding the feature. Furthermore, a parameter study is performed to explore the influence of volume fraction and hardness of the inclusion phase. For the same macroscopic hardening response, the ductility is predicted to increase if the volume fraction of the hard phase increases while at the same time its hardness decreases

On Micromechanical Parameter Identification With Integrated DIC and the Role of Accuracy in Kinematic Boundary Conditions

Integrated Digital Image Correlation (IDIC) is nowadays a well established full-field experimental procedure for reliable and accurate identification of material parameters. It is based on the correlation of a series of images captured during a mechanical experiment, that are matched by displacement fields derived from an underlying mechanical model. In recent studies, it has been shown that when the applied boundary conditions lie outside the employed field of view, IDIC suffers from inaccuracies. A typical example is a micromechanical parameter identification inside a Microstructural Volume Element (MVE), whereby images are usually obtained by electron microscopy or other microscopy techniques but the loads are applied at a much larger scale. For any IDIC model, MVE boundary conditions still need to be specified, and any deviation or fluctuation in these boundary conditions may significantly influence the quality of identification. Prescribing proper boundary conditions is generally a challenging task, because the MVE has no free boundary, and the boundary displacements are typically highly heterogeneous due to the underlying microstructure. The aim of this paper is therefore first to quantify the effects of errors in the prescribed boundary conditions on the accuracy of the identification in a systematic way. To this end, three kinds of mechanical tests, each for various levels of material contrast ratios and levels of image noise, are carried out by means of virtual experiments. For simplicity, an elastic compressible Neo-Hookean constitutive model under plane strain assumption is adopted. It is shown that a high level of detail is required in the applied boundary conditions. This motivates an improved boundary condition application approach, which considers constitutive material parameters as well as kinematic variables at the boundary of the entire MVE as degrees of freedom in...Comment: 37 pages, 25 figures, 2 tables, 2 algorithm

Harmonizing and Optimizing CT Perfusion Stroke Imaging

This thesis focuses on harmonizing and optimizing CT perfusion (CTP) imaging for stroke. CTP imaging can help select patients with ischemic stroke for thrombectomy. However, due to a lack of consensus on the CTP acquisition and processing protocols, widespread acceptance of image-based treatment criteria has not been achieved. The CLEOPATRA care assessment was set up to determine whether it is cost-effective to select patients with ischemic stroke for thrombectomy based on stroke imaging. We focused on harmonizing and optimizing the protocols for CTP stroke imaging. Chapter 1 introduces some concepts of CTP imaging. In CTP imaging, contrast agent is injected into the bloodstream. By measuring changes in contrast enhancement over time, perfusion maps can be created. These perfusion maps show the blood flow in brain tissue and are used to estimate ischemic (less perfused) brain regions. The estimated ischemic regions help clinical decision-making for ischemic stroke. Chapters 2 and 3 discuss the injection protocol, scan protocol, and processing protocol of CTP imaging. Using an anthropomorphic digital CTP phantom, we argued that the processing protocol has the greatest impact on CTP imaging results. Additionally, variations in the injection and scan protocols between centers can, in some cases, lead to different CTP imaging results. Chapter 4 describes our efforts to create a physical CTP phantom from the digital phantom in chapters 2 and 3. By scanning paper sheets printed with contrast agent, we demonstrated that simulating anthropomorphic brain tissue perfusion with a physical phantom is feasible. Chapters 5 and 6 cover the estimation of ischemic regions. A standardized method that we developed could harmonize and optimize CTP imaging results to some extent. Additionally, the estimations from this standardized method resembled both manual segmentations of CTP perfusion maps and acute MR diffusion images. Chapter 7 presents a new use of CTP imaging as a tool to locate vessel occlusions. Smaller and more distal occlusions are increasingly difficult to locate with CT angiography imaging. We demonstrated that the location of the total ischemic region, determined from CTP imaging, can help locate the vessel occlusion. Chapter 8 recommends establishing a standardized framework for estimating ischemic regions to reduce inconsistencies in clinically relevant information derived from CTP imaging. Based on our research, this framework should utilize all perfusion data, consider spatial information, and rely on manual segmentations as a reference point. In summary, we believe that CTP imaging remains important for helping patients with ischemic stroke. This thesis argues that current inconsistencies arise from misinterpretations of perfusion maps. We advocate for a shifted perspective in which CTP imaging is assessed on its intrinsic value and broader clinical applications are considered. Only then can the full potential of CTP imaging be realized

Fracture initiation in multi-phase materials: a systematic three-dimensional approach using a FFT-based solver

This paper studies a two-phase material with a microstructure composed of a hard brittle reinforcement phase embedded in a soft ductile matrix. It addresses the full three-dimensional nature of the microstructure and macroscopic deformation. A large ensemble of periodic microstructures is used, whereby the individual grains of the two phases are modeled using equi-sized cubes. A particular solution strategy relying on the Fast Fourier Transform is adopted, which has a high computational efficiency both in terms of speed and memory footprint, thus enabling a statistically meaningful analysis. This solution method naturally accompanies the regular microstructural model, as the Fast Fourier Transform relies on a regular grid. Using the many considered microstructures as an ensemble, the average arrangement of phases around fracture initiation sites is objectively identified by the correlation between microstructure and fracture initiation -- in three dimensions. The results show that fracture initiates where regions of the hard phase are interrupted by bands of the soft phase that are aligned with the direction of maximum shear. In such regions, the hard phase is arranged such that the area of the phase boundary perpendicular to the principal strain direction is maximum, leading to high hydrostatic tensile stresses, while not interrupting the shear bands that form in the soft phase. The local incompatibility that is present around the shear bands is responsible for a high plastic strain. By comparing the response to a two-dimensional microstructure it is observed that the response is qualitatively similar (both macroscopically and microscopically). One important difference is that the local strain partitioning between the two phases is over-predicted by the two-dimensional microstructure, leading to an overestimation of damage