  We report on the effect of the lateral confinement and a perpendicular
magnetic field on isolated room-temperature magnetic skyrmions in sputtered
Pt/Co/MgO nanotracks and nanodots. We show that the skyrmions size can be
easily tuned by playing on the lateral dimensions of the nanostructures and by
using external magnetic field amplitudes of a few mT, which allow to reach
sub-100 nm diameters. Our XMCD-PEEM observations also highlight the important
role of the pinning on the skyrmions size and stability under an out-of-plane
magnetic field. Micromagnetic simulations reveal that the effect of local
pinning can be well accounted for by considering the thin film grain structure
with local anisotropy variations and reproduce well the dependence of the
skyrmion diameter on the magnetic field and the geometry.
