  Competing interactions produce finite-size textures in myriad condensed
matter systems, typically forming elongated stripe or round bubble domains.
Transitions between stripe and bubble phases, driven by field or temperature,
are expected to be reversible in nature. Here we report on the distinct
character of the analogous transition for nanoscale spin textures in chiral
Co/Pt-based multilayer films, known to host N\'{e}el skyrmions, using
microscopy, magnetometry, and micromagnetic simulations. Upon increasing field,
individual stripes fission into multiple skyrmions, and this transition
exhibits a macroscopic signature of irreversibility. Crucially, upon field
reversal, the skyrmions do not fuse back into stripes, with many skyrmions
retaining their morphology down to zero field. Both the macroscopic
irreversibility and the microscopic zero-field skyrmion density are governed by
the thermodynamic material parameter determining chiral domain stability. These
results establish the thermodynamic and microscopic framework underlying
ambient skyrmion generation and stability in chiral multilayer films and
provide immediate directions for their functionalization in devices.
