Cycloidal versus skyrmionic states in mesoscopic chiral magnets

When subjected to the interfacially induced Dzyaloshinskii-Moriya interaction, the ground state in thin ferromagnetic films with high perpendicular anisotropy is cycloidal. The period of this cycloidal state depends on the strength of the Dzyaloshinskii-Moriya interaction. In this work, we have studied the effect of confinement on the magnetic ground state and excited states, and we determined the phase diagram of thin strips and thin square platelets by means of micromagnetic calculations. We show that multiple cycloidal states with different periods can be stable in laterally confined films, where the period of the cycloids does not depend solely on the Dzyaloshinskii-Moriya interaction strength but also on the dimensions of the film. The more complex states comprising skyrmions are also found to be stable, though with higher energy.Comment: 8 pages, 10 figure

Effects of spatially engineered Dzyaloshinskii-Moriya interaction in ferromagnetic films

The Dzyaloshinskii-Moriya interaction (DMI) is a chiral interaction that favors formation of domain walls. Recent experiments and ab initio calculations show that there are multiple ways to modify the strength of the interfacially induced DMI in thin ferromagnetic films with perpendicular magnetic anisotropy. In this paper we reveal theoretically the effects of spatially varied DMI on the magnetic state in thin films. In such heterochiral 2D structures we report several emergent phenomena, ranging from the equilibrium spin canting at the interface between regions with different DMI, over particularly strong confinement of domain walls and skyrmions within high-DMI tracks, to advanced applications such as domain tailoring nearly at will, design of magnonic waveguides, and much improved skyrmion racetrack memory

Tomorrow’s micromagnetic simulations

Micromagnetic simulations are a valuable tool to increase our understanding of nanomagnetic systems and to guide experiments through parameter spaces that would otherwise be difficult and expensive to navigate. To fulfill this task, simulations have always pushed the limits of what is possible in terms of software and hardware. In this perspective, we give an overview of the current state of the art in micromagnetic simulations of ferromagnetic materials followed by our opinion of what tomorrow's simulations will look like. Recently, the focus has shifted away from exclusively trying to achieve faster simulations, toward extending pure micromagnetic calculations to a multiphysics approach. We present an analysis of how the performance of the simulations is affected by the simulation details and hardware specifications (specific to the graphics processing unit-accelerated micromagnetic software package mumax3), which sheds light on how micromagnetic simulations can maximally exploit the available computational power. Finally, we discuss how micromagnetic simulations can benefit from new hardware paradigms like graphics cards aimed at machine learning

Paths to collapse for isolated skyrmions in few-monolayer ferromagnetic films

Magnetic skyrmions are topological spin configurations in materials with chiral Dzyaloshinskii-Moriya interaction (DMI), that are potentially useful for storing or processing information. To date, DMI has been found in few bulk materials, but can also be induced in atomically thin magnetic films in contact with surfaces with large spin-orbit interactions. Recent experiments have reported that isolated magnetic skyrmions can be stabilized even near room temperature in few-atom thick magnetic layers sandwiched between materials that provide asymmetric spin-orbit coupling. Here we present the minimum-energy path analysis of three distinct mechanisms for the skyrmion collapse, based on ab initio input and the performed atomic-spin simulations. We focus on the stability of a skyrmion in three atomic layers of Co, either epitaxial on the Pt(111) surface, or within a hybrid multilayer where DMI nontrivially varies per monolayer due to competition between different symmetry-breaking from two sides of the Co film. In laterally finite systems, their constrained geometry causes poor thermal stability of the skyrmion toward collapse at the boundary, which we show to be resolved by designing the high-DMI structure within an extended film with lower or no DMI

Adaptively time stepping the stochastic Landau-Lifshitz-Gilbert equation at nonzero temperature: implementation and validation in MuMax3

Thermal fluctuations play an increasingly important role in micromagnetic research relevant for various biomedical and other technological applications. Until now, it was deemed necessary to use a time stepping algorithm with a fixed time step in order to perform micromagnetic simulations at nonzero temperatures. However, Berkov and Gorn have shown that the drift term which generally appears when solving stochastic differential equations can only influence the length of the magnetization. This quantity is however fixed in the case of the stochastic Landau-Lifshitz-Gilbert equation. In this paper, we exploit this fact to straightforwardly extend existing high order solvers with an adaptive time stepping algorithm. We implemented the presented methods in the freely available GPU-accelerated micromagnetic software package MuMax3 and used it to extensively validate the presented methods. Next to the advantage of having control over the error tolerance, we report a twenty fold speedup without a loss of accuracy, when using the presented methods as compared to the hereto best practice of using Heun's solver with a small fixed time step.Comment: 9 pages, 9 figure

On quantifying the topological charge in micromagnetics using a lattice-based approach

An implementation of a lattice-based approach for computing the topological skyrmion charge is provided for the open source micromagnetics code MuMax3. Its accuracy with respect to an existing method based on finite difference derivatives is compared for three different test cases. The lattice-based approach is found to be more robust for finite-temperature dynamics and for nucleation and annihilation processes in extended systems.Comment: 9 pages, 5 figure

Deflection of (anti)ferromagnetic skyrmions at heterochiral interfaces

Devising magnetic nanostructures with spatially heterogeneous Dzyaloshinskii-Moriya interaction (DMI) is a promising pathway towards advanced confinement and control of magnetic skyrmions in potential devices. Here we discuss theoretically how a skyrmion interacts with a heterochiral interface using micromagnetic simulations and analytic arguments. We show that a heterochiral interface deflects the trajectory of ferromagnetic (FM) skyrmions, and that the extent of such deflection is tuned by the applied spin-polarized current and the difference in DMI across the interface. Further, we show that this deflection is characteristic for the FM skyrmion, and is completely absent in the antiferromagnetic (AFM) case. In turn, we reveal that the AFM skyrmion achieves much higher velocities than its FM counterpart, yet experiences far stronger confinement in nanoengineered heterochiral tracks, which reinforces AFM skyrmions as a favorable choice for skyrmion-based devices

Recurrent uterine artery pseudo-aneurysm requiring repeat embolization during pregnancy : a case report

Background: Uterine artery pseudo-aneurysms (UAP) mainly occur after traumatic delivery or traumatic pregnancy termination. A UAP can be either asymptomatic or present with symptoms such as vaginal bleeding, abdominal pain, hypovolemic shock, or fever when infected. We describe a patient diagnosed with a uterine artery pseudo-aneurysm in pregnancy that required repeat embolization. The patient had no risk factors in her recent medical history. She did, however, undergo in-vitro fertilization with oocyte retrieval performed under transvaginal ultrasonographic guidance. We suggest the hypothesis of formation of the pseudo-aneurysm secondary to vascular injury during oocyte retrieval. Case Report: A 35-year-old primigravida, who conceived by in-vitro fertilization, presenting with severe abdominal pain at 19 2/7 weeks of gestation. Ultrasound examination with color doppler imaging revealed a hypoechoic lesion with turbulent arterial flow pattern on the lower left side of the uterus. Selective catheterization and subtraction angiography permitted diagnosis of a large pseudo-aneurysm of the left uterine artery. A selective embolization was performed. Recanalization of the embolized artery was confirmed 11 weeks after initial presentation, requiring repeat embolization. A planned caesarean section was performed at 34 weeks of gestation and a healthy boy was born with a birth weight of 2065 g. Conclusion: Uterine artery pseudo-aneurysm is a rare but potentially life-threatening condition. It can be diagnosed using (doppler) ultrasound, revealing a hypoechoic mass with swirling blood flow. Angiography is the standard reference in diagnosing UAP and may provide definitive treatment. Management with selective unilateral uterine artery embolization appears to be safe in hemodynamically stable patients. It does not compromise uteroplacental circulation and may help to prolong the pregnancy, reducing morbidity associated with preterm birth

Fast micromagnetic simulations on GPU : recent advances made with mumax³

In the last twenty years, numerical modeling has become an indispensable part of magnetism research. It has become a standard tool for both the exploration of new systems and for the interpretation of experimental data. In the last five years, the capabilities of micromagnetic modeling have dramatically increased due to the deployment of graphical processing units (GPU), which have sped up calculations to a factor of 200. This has enabled many studies which were previously unfeasible. In this topical review, we give an overview of this modeling approach and show how it has contributed to the forefront of current magnetism research

Improving abstract interpretations by combining domains

This article considers static analysis based on abstract interpretation of logic programs over combined domains. It is known that analyses over combined domains provide more information potentially than obtained by the independent analyses. However, the construction of a combined analysis often requires redefining the basic operations for the combined domain. A practical approach to maintain precision in combined analyses of logic programs which reuses the individual analyses and does not redefine the basic operations is illustrated. The advantages of the approach are that proofs of correctness for the new domains are not required and implementations can be reused. The approach is demonstrated by showing that a combined sharing analysis — constructed from "old" proposals — compares well with other "new" proposals suggested in recent literature both from the point of view of efficiency and accuracy