  Theoretical calculations of thermally activated decay of skyrmions in systems
comprising several magnetic monolayers are presented, with a special focus on
bilayer systems. Mechanisms of skyrmion collapse are identified and
corresponding energy barriers and thermal collapse rates are evaluated as
functions of the interlayer exchange coupling and mutual stacking of the
monolayers using transition state theory and an atomistic spin Hamiltonian. In
order to contrast the results to monolayer systems, the magnetic interactions
within each layer are chosen so as to mimic the well-established Pd/Fe/Ir(111)
system. Even bilayer systems demonstrate a rich diversity of skyrmion collapse
mechanisms that sometimes co-exist. For very weakly coupled layers, the
skyrmions in each layer decay successively via radially-symmetric shrinking.
Slightly larger coupling leads to an asymmetric chimera collapse stabilized by
interlayer exchange. When the interlayer exchange coupling reaches a certain
critical value, the skyrmions collapse simultaneously. Interestingly, the
overall energy barrier for the skyrmion collapse does not always converge to a
multiple of that for a monolayer system in the strongly coupled regime. For a
certain stacking of the magnetic layers, the energy barrier as a function of
the interlayer exchange coupling features a maximum and then decreases with the
coupling strength in the strong coupling regime. Calculated mechanisms of
skyrmion collapse are used to ultimately predict the skyrmion lifetime. Our
results reveal a comprehensive picture of thermal stability of skyrmions in
magnetic multilayers and provide a perspective for realizing skyrmions with
controlled properties.
