  The free energy and the entropy of magnetic skyrmions with respect to the
collinear state are calculated for a (Pt$_{0.95}$Ir$_{0.05}$)/Fe bilayer on
Pd(111) via atomistic spin model simulations. The simulations are carried out
starting from very low temperatures where the skyrmion number is conserved up
to the range where skyrmions are constantly created and destroyed by thermal
fluctuations, highlighting their quasiparticle nature. The higher entropy of
the skyrmions at low temperature leads to a reduced free energy, such that the
skyrmions become energetically preferred over the collinear state due to
entropic stabilization as predicted by linear spin-wave theory. Going beyond
the linear spin-wave approximation, a sign change is shown to occur in the free
energy as well as the entropy at elevated temperature.
