Magnetic antivortex-core reversal by circular-rotational spin currents

Topological singularities occur as antivortices in ferromagnetic thin-film microstructures. Antivortices behave as two-dimensional oscillators with a gyrotropic eigenmode which can be excited resonantly by spin currents and magnetic fields. We show that the two excitation types couple in an opposing sense of rotation in the case of resonant antivortex excitation with circular-rotational currents. If the sense of rotation of the current coincides with the intrinsic sense of gyration of the antivortex, the coupling to the Oersted fields is suppressed and only the spin-torque contribution locks into the gyrotropic eigenmode. We report on the experimental observation of purely spin-torque induced antivortex-core reversal. The dynamic response of an isolated antivortex is imaged by time-resolved scanning transmission x-ray microscopy on its genuine time and length scale

Vortex core switching by coherent excitation with single in-plane magnetic field pulses

The bistability of the core magnetization of nano-scaled magnets with a magnetic vortex configuration has great potential for data storage applications. To exploit this, reliable switching between the two possible states is needed. Time resolved x-ray microscopy was used to study the response of the vortex core to excitation pulses at sub-ns timescales and image the vortex core switching. A reliable switching process by coherent excitation with leading and trailing edges of in-plane magnetic field pulses was found and compared with micromagnetic simulations.Comment: 4 pages, 3 figure

Polarisation selective magnetic vortex dynamics and core reversal in rotating magnetic fields

A magnetic vortex occurs as an equilibrium configuration in thin ferromagnetic platelets of micron and sub-micron size and is characterised by an in-plane curling magnetisation. At the centre, a magnetic singularity is avoided by an out-of-plane magnetisation core. This core has a gyrotropic excitation mode, which corresponds to a circular motion of the vortex around its equilibrium position, where the rotation sense is determined by the direction of the vortex core magnetisation, its polarisation. Unlike linear fields or spin polarised currents, which excite both polarisation states, an in-plane rotating field can selectively excite one of the polarisation states. Here we report the observation of vortex dynamics in response to rotating magnetic fields, imaged with time-resolved scanning X-ray microscopy. We demonstrate that the rotating field only excites the gyrotropic mode if the rotation sense of the field coincides with the vortex gyration sense and that such a field can selectively reverse the vortex polarisation

The state of Fortran

A community of developers has formed to modernize the Fortran ecosystem. In this article, we describe the high-level features of Fortran that continue to make it a good choice for scientists and engineers in the 21st century. Ongoing efforts include the development of a Fortran standard library and package manager, the fostering of a friendly and welcoming online community, improved compiler support, and language feature development. The lessons learned are common across contemporary programming languages and help reduce the learning curve and increase adoption of Fortran

Polarization selective magnetic vortex core reversal in ferromagnetic nanostructures by rotating magnetic fields

Hauptziel der vorliegenden Arbeit war es, magnetische Vortizes magnetischer Landaustrukturen selektiv anzuregen und dies experimentell zu realisieren. Im Fokus stand dabei insbesondere der Wunsch, die Vortexkern-Polarisation dynamisch mit Frequenzen im sub-Gigahertz Bereich kontrolliert zu schalten. Im Gegensatz zur Anregung mit linearen Feldern, die unabhängig von der Polarisation den Vortexkern zur Gyration bringen, kann, wie mikromagnetische Simulationen gezeigt haben, mit Hilfe eines in der Ebene mitrotierenden Magnetfelds nur eine der beiden Orientierungen angeregt werden. Stimmen der natürliche Gyrationssinn und der Drehsinn des rotierenden Feldes überein, so erfolgt die Anregung in Resonanz und die Vortexkern-Polarisation kann selektiv geschaltet werden.The main objective of this thesis was to experimentally realize the selective excitation of magnetic vortices in Landau structures. The emphasis was especially on the controlled dynamical switching of the vortex core polarization with frequencies in the sub-gigahertz range. In contrast to the excitation with linear fields, which excites the vortex core gyration independent of its polarization, micromagnetic simulations have shown that the polarization of the vortex core can be selectively excited by using a co-rotating in-plane magnetic field. If the natural gyration sense matches with the direction of the rotating magnetic field, the excitation is in resonance and the vortex core polarization can be selectively switched

Validation of a Golden Angle Radial Sequence (GOLD) for Abdominal T1 Mapping During Free Breathing: Demonstrating Clinical Feasibility for Quantifying Gastric Secretion and Emptying

Purpose To validate a magnetic resonance imaging sequence suitable for quantitative assessment of acid suppression by a proton pump inhibitor (PPI) on gastric secretion and emptying in clinical practice. Methods A golden angle radial sequence (GOLD) was validated in a series of in vitro and in vivo experiments and clinical feasibility was shown in two studies. The impact of free breathing and image plane orientation on T1 values was evaluated in a controlled in vivo experiment. The free-breathing GOLD sequence was compared against a standard breath-hold gradient echo sequence for gastric half emptying time in 23 subjects during a gastric emptying study. Pilot data from five subjects assessed the sensitivity of the GOLD sequence to detect changes in acid secretion volume produced by PPI treatment. Results The coronal free-breathing GOLD sequence and the axial breath-hold standard gradient echo sequence showed good agreement of the gastric half emptying time (6 ± 3 min, P = 0.053). The GOLD sequence demonstrated sensitivity to reduction of gastric secretion volumes induced by PPI treatment (55 ± 5 mL, P < 0.001). Conclusion The GOLD sequence allowed for free breathing, multislice, combined imaging and T1 mapping of the stomach content. GOLD presents a promising multipurpose, noninvasive imaging tool for monitoring gastric function in clinical studies