In this work we have used micromagnetic simulations to report four ways to build traps for magnetic skyrmions. Magnetic defects have been modeled as local variations in the material parameters, such as the exchange stiffness, saturation magnetization, magnetocrystalline anisotropy and Dzyaloshinskii-Moriya constant. We observe both pinning (potential well) and scattering (potential barrier) traps when tuning either a local increase or a local reduction for each one of these magnetic properties. It is found that the skyrmion-defect aspect ratio is a crucial parameter to build traps for skyrmions. In particular, the efficiency of the trap is compromised if the defect size is smaller than the skyrmion size, because they interact weakly. On the other hand, if the defect size is larger than the skyrmion diameter, the skyrmion-defect interaction becomes evident. Thus, the strength of the skyrmion-defect interaction can be tuned by the modification of the magnetic properties within a region with suitable size. Furthermore, the basic physics behind the mechanisms for pinning and for scattering is discussed. In particular, we discover that skyrmions move towards the magnetic region which tends to maximize its diameter; it enables the magnetic system to minimize its energy. Thus, we are able to explain why skyrmions are either attracted or repelled by a region with modified magnetic properties. Results here presented are of utmost significance for the development and realization of future spintronic devices, in which skyrmions will work as information carriers.
In this work, we used numerical simulations to study the effect of a ring of magnetic impurities on the vortex core dynamics in nanodisks of Permalloy. The presence of the ring not only allowed us to modulate the gyrotropic frequency but also provided us a way to confine the vortex core. We observed that the gyrotropic frequency depends on the ring parameters. Moreover, we have noticed that the switching of the vortex core polarity can be obtained from the vortex core-impurity interaction under peculiar conditions, in particular, when the ring works for pinning the vortex core.
The dynamical behavior of a magnetic nanoparticle contaminated by pointlike impurities is studied by using a spin dynamics numerical simulation. It was observed that the impurities can behave both as pinning (attractive) and as scattering (repulsive) sites. A Gaussian profile was observed for the interaction potential energy ranging up to two lattice parameters. Using the known values of the parameters for Permalloy-79 we have calculated the interaction energy of the vortex core with a single defect. We estimated the interaction range as approximately 10nm. Both results agree quite well with experimental measurements.
Micromagnetic simulations have been performed to investigate the suppression of the skyrmion Hall effect in nanotracks with their magnetic properties strategically modified. In particular, we study two categories of magnetically modified nanotracks. One of them, repulsive edges have been inserted in the nanotrack and, in the other, an attractive strip has been placed exactly on the longest axis of the nanotrack. Attractive and repulsive interactions can be generated from the engineering of magnetic properties. For instance, it is known that the skyrmion can be attracted to a region where the exchange stiffness constant is decreased. On the other hand, the skyrmion can be repelled from a region characterized by a local increase in the exchange stiffness constant. In order to provide a background for experimental studies, we vary not only the magnetic material parameters (exchange stiffness, perpendicular magnetocrystalline anisotropy and the Dzyaloshinskii-Moriya constant) but also the width of the region magnetically modified, containing either a local reduction or a local increase for each one of these magnetic properties. In the numerical simulations, the skyrmion motion was induced by a spin-polarized current and the found results indicate that it is possible to transport skyrmions around the longest axis of the nanotrack. In practice, the skyrmion Hall effect can be completely suppressed in magnetic nanotracks with strategically modified magnetic properties. Furthermore, we discuss in detail 6 ways to suppress the skyrmion Hall effect by the usage of nanotracks with repulsive edges and nanotracks with an attractive strip.
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