  The stability of magnetic skyrmions against thermal fluctuations and external
perturbations is investigated within the framework of harmonic transition state
theory for magnetic degrees of freedom. The influence of confined geometry and
atomic scale non-magnetic defects on the skyrmion lifetime is estimated. It is
shown that a skyrmion on a track has lower activation energy for annihilation
and higher energy for nucleation if the size of the skyrmion is comparable with
the width of the track. Two mechanisms of skyrmion annihilation are considered:
inside the track and escape through the boundary. For both mechanisms, the
dependence of activation energy on the track width is calculated. Non-magnetic
defects are found to localize skyrmions in their neighborhood and strongly
decrease the activation energy for creation and annihilation. This is in
agreement with experimental measurements that have found nucleation of
skyrmions in presence of spin-polarized current preferably occurring near
structural defects.
