  In ferromagnetic multilayers, a single laser pulse with a fluence above an
optical nucleation threshold can create magnetic skyrmions, which are randomly
distributed over the area of the laser spot. However, in order to study the
dynamics of skyrmions and for their application in future data technology, a
controllable localization of the skyrmion nucleation sites is crucial. Here, it
is demonstrated that patterned reflective masks behind a thin magnetic film can
be designed to locally tailor the optical excitation amplitudes reached,
leading to spatially controlled skyrmion nucleation on the nanometer scale.
Using x-ray microscopy, the influence of nanopatterned back-side aluminum masks
on the optical excitation is studied in two sample geometries with varying
layer sequence of substrate and magnetic Co/Pt multilayer. Surprisingly, the
masks' effect on suppressing or enhancing skymion nucleation reverses when
changing this sequence. Moreover, optical near-field enhancements additionally
affect the spatial arrangement of the nucleated skyrmions. Simulations of the
spatial modulation of the laser excitation, and the following heat transfer
across the interfaces in the two sample geometries are employed to explain
these observations. The results demonstrate a reliable approach to add
nanometer-scale spatial control to optically induced magnetization processes on
ultrafast timescales.
