Shock wave velocity and shock pressure for low density powders: A novel approach

A novel approach is presented to predict the shock wave velocity as well as the shock wave pressure in powder materials. It is shown that the influence of the specific volume behind the shock wave on shock wave velocity and shock pressure decreases with decreasing initial powder density. The new model is compared with experimental data of various materials: Fe, Cu, Al, C, UO2, Ce2O3, SiO2 (quartz), NaCl, and polystyrene. It is concluded that the model holds in particular for initial powder densities less than 50% and for flyer plate velocities up to 5 km/s.

Exchange-torque-induced excitation of perpendicular standing spin waves in nanometer-thick YIG films

Spin waves in ferrimagnetic yttrium iron garnet (YIG) films with ultralow magnetic damping are relevant for magnon-based spintronics and low-power wave-like computing. The excitation frequency of spin waves in YIG is rather low in weak external magnetic fields because of its small saturation magnetization, which limits the potential of YIG films for high-frequency applications. Here, we demonstrate how exchange-coupling to a CoFeB film enables efficient excitation of high-frequency perpendicular standing spin waves (PSSWs) in nanometer-thick (80 nm and 295 nm) YIG films using uniform microwave magnetic fields. In the 295-nm-thick YIG film, we measure intense PSSW modes up to 10th order. Strong hybridization between the PSSW modes and the ferromagnetic resonance mode of CoFeB leads to characteristic anti-crossing behavior in broadband spin-wave spectra. A dynamic exchange torque at the YIG/CoFeB interface explains the excitation of PSSWs. The localized torque originates from exchange coupling between two dissimilar magnetization precessions in the YIG and CoFeB layers. As a consequence, spin waves are emitted from the YIG/CoFeB interface and PSSWs form when their wave vector matches the perpendicular confinement condition. PSSWs are not excited when the exchange coupling between YIG and CoFeB is suppressed by a Ta spacer layer. Micromagnetic simulations confirm the exchange-torque mechanism.Comment: 9 pages, 6 figure

Steering effect on the shape of islands for homoepitaxial growth of Cu on Cu(100)

The steering effect on the growth of islands is investigated by combining molecular dynamics (MD) and kinetic Monte Carlo (KMC) simulations. Dynamics of depositing atoms and kinetics of atoms on a substrate are realized by MD and KMC, respectively. The reported experimental results on the asymmetric island growth [van Dijken {\it et al.}, Phys. Rev. Lett. {\bf 82}, 4038 (1999).] is well reproduced. A salient phenomenon, the reversal of the asymmetry, is found as the island size increases, and attributed to the asymmetric flux on the lower terrace of island.Comment: 5 figur

Multimodal imaging of brain tumors:treatment planning, prognostication and treatment follow-up with MRI and PET

Imaging with conventional MRI plays an essential role in the diagnosis, treatment planning and treatment follow-up of brain tumor patients. However, imaging with conventional MRI has several limitations challenging clinical decision making. This thesis explores the use of multimodal imaging with advanced methods to improve the clinical management of brain tumor patients. Part I discusses the treatment planning and prognostication of brain tumor patients. There is a special focus on the anatomical relationship of glioblastoma with the ventricles. Patients with ventricle-contacting glioblastoma have a poorer prognosis compared to patients with non-contacting tumors. This thesis finds that ventricle-contacting glioblastomas demonstrate higher peritumoral perfusion and proliferation rates as demonstrated by advanced imaging methods. These aggressive features possibly explain the survival difference between patients with ventricle contacting and non-contacting glioblastomas. Part II emphasizes on the treatment follow-up of glioblastoma patients. Due to the inevitable recurrence of glioblastomas, patients undergo frequent MRI scanning throughout treatment. However, treatment effects such as pseudoprogression can mimic tumor progression on conventional MRI. The inability to accurately differentiate pseudoprogression from tumor progression hinders reliable decision-making regarding continuation or discontinuation of treatment. This thesis demonstrates that multimodal imaging with advanced MRI and PET methods improves the treatment evaluation of glioblastoma patients. The current practice of standard scheduled MRI scans during treatment is also questioned. Pseudoprogression causes a considerable amount of uncertainty on scheduled scans and treatment decisions are often postponed. This thesis substantiates the value of multimodal imaging to aid in clinical decision making in brain tumor patients

Influence of magnetic field and ferromagnetic film thickness on domain pattern transfer in multiferroic heterostructures

Domains in BaTiO$_3$ induces a regular modulation of uniaxial magnetic anisotropy in CoFeB via an inverse magnetostriction effect. As a result, the domain structures of the CoFeB wedge film and BaTiO$_3$ substrate correlate fully and straight ferroelectric domain boundaries in BaTiO$_3$ pin magnetic domain walls in CoFeB. We use x-ray photoemission electron microscopy and magneto-optical Kerr effect microscopy to characterize the spin structure of the pinned domain walls. In a rotating magnetic field, abrupt and reversible transitions between two domain wall types occur, namely, narrow walls where the magnetization vectors align head-to-tail and much broader walls with alternating head-to-head and tail-to-tail magnetization configurations. We characterize variations of the domain wall spin structure as a function of magnetic field strength and CoFeB film thickness and compare the experimental results with micromagnetic simulations.Comment: 5 pages, 5 figure

Steering effects on growth instability during step-flow growth of Cu on Cu(1,1,17)

Kinetic Monte Carlo simulation in conjunction with molecular dynamics simulation is utilized to study the effect of the steered deposition on the growth of Cu on Cu(1,1,17). It is found that the deposition flux becomes inhomogeneous in step train direction and the inhomogeneity depends on the deposition angle, when the deposition is made along that direction. Steering effect is found to always increase the growth instability, with respect to the case of homogeneous deposition. Further, the growth instability depends on the deposition angle and direction, showing minimum at a certain deposition angle off-normal to (001) terrace, and shows a strong correlation with the inhomogeneous deposition flux. The increase of the growth instability is ascribed to the strengthened step Erlich Schwoebel barrier effects that is caused by the enhanced deposition flux near descending step edge due to the steering effect.Comment: 5 page

Field Tuning of Ferromagnetic Domain Walls on Elastically Coupled Ferroelectric Domain Boundaries

We report on the evolution of ferromagnetic domain walls during magnetization reversal in elastically coupled ferromagnetic-ferroelectric heterostructures. Using optical polarization microscopy and micromagnetic simulations, we demonstrate that the spin rotation and width of ferromagnetic domain walls can be accurately controlled by the strength of the applied magnetic field if the ferromagnetic walls are pinned onto 90 degrees ferroelectric domain boundaries. Moreover, reversible switching between magnetically charged and uncharged domain walls is initiated by magnetic field rotation. Switching between both wall types reverses the wall chirality and abruptly changes the width of the ferromagnetic domain walls by up to 1000%.Comment: 5 pages, 5 figure

Indoor environment in Dutch primary schools and health of the pupils

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