  Magnetic skyrmions are key candidates for novel memory, logic, and
neuromorphic computing. An essential property is their topological protection
caused by the whirling spin texture as described by a robust integer winding
number. However, the realization on an atomic lattice leaves a loophole for
switching the winding number via concerted rotation of individual spins. Hence,
understanding the unwinding microscopically is key to enhance skyrmion
stability. Here, we use spin polarized scanning tunneling microscopy to probe
skyrmion annihilation by individual hot electrons and obtain maps of the
transition rate on the nanometer scale. By applying an in-plane magnetic field,
we tune the collapse rate by up to four orders of magnitude. In comparison with
first-principles based atomistic spin simulations, the experiments demonstrate
a radial symmetric collapse at zero in-plane magnetic field and a transition to
the recently predicted chimera collapse at finite in-plane field. Our work
opens the route to design criteria for skyrmion switches and improved skyrmion
stability.
