  The atomic-scale spin structure of individual isolated skyrmions in an
ultrathin film is investigated in real space by spin-polarized scanning
tunneling microscopy. Their axial symmetry as well as their unique rotational
sense is revealed by using both out-of-plane and in-plane sensitive tips. The
size and shape of skyrmions change as a function of magnetic field. An
analytical expression for the description of skyrmions is proposed and applied
to connect the experimental data to the original theoretical model describing
chiral skyrmions. Thereby, the relevant material parameters responsible for
skyrmion formation can be obtained.
