  Skyrmions are chiral swirling magnetization structures with nanoscale size.
These structures have attracted considerable attention due to their topological
stability and promising applicability in nanodevices, since they can be
displaced with spin-polarized currents. However, for the comprehensive
implementation of skyrmions in devices, it is imperative to also attain control
over their geometrical position. Here we show that, through thickness
modulations introduced in the host material, it is possible to constrain
three-dimensional skyrmions to desired regions. We investigate skyrmion
structures in rectangular FeGe platelets with micromagnetic finite element
element simulations. First, we establish a phase diagram of the minimum-energy
magnetic state as a function of the external magnetic field strength and the
film thickness. Using this understanding, we generate preferential sites for
skyrmions in the material by introducing dot-like "pockets" of reduced film
thickness. We show that these pockets can serve as pinning centers for the
skyrmions, thus making it possible to obtain a geometric control of the
skyrmion position. This control allows stabilizing skyrmions at positions and
in configurations that they would otherwise not attain. Our findings may have
implications for technological applications in which skyrmions are used as
units of information that are displaced along racetrack-type shift register
devices.
