  Magnetic multilayers are a promising platform for storage and logic devices
based on skyrmion spin textures, due to the large materials phase space for
tuning properties. Epitaxial superlattice structures of [Pt/Co/Cu]n thin films
were grown by molecular beam epitaxy at room temperature. Spin-polarized
scanning tunneling microscopy (SP-STM) of these samples was used to probe the
connection between surface structure and skyrmion morphology with nanoscale
spatial resolution. Irregular-shaped skyrmion bubbles were observed, with
effective diameters from 20-200 nm that are much larger than the nanoscale
grain structure of the surface topography. Nucleation, annihilation, and motion
of skyrmion bubbles could be driven using the stray field of the ferromagnetic
tip in repeated imaging, and spin-polarized current/voltage pulses. Our
detailed comparison of STM topography and differential conductance images shows
that there are no surface defects or inhomogeneities at length scales that
could account for the range in skyrmion bubble size or shape observed in the
measurements.
