  We present a study of the stability of room-temperature skyrmions in
[Ir/Fe/Co/Pt] thin film multilayers, using the First Order Reversal Curve
(FORC) technique and magnetic force microscopy (MFM). FORC diagrams reveal
irreversible changes in magnetization upon field reversals, which can be
correlated with the evolution of local magnetic textures probed by MFM. Using
this approach, we have identified two different mechanisms - (1) skyrmion
merger and (2) skyrmion nucleation followed by stripe propagation - which
facilitate magnetization reversal in a changing magnetic field. Analysing the
signatures of these mechanisms in the FORC diagram allows us to identify
magnetic "histories" - i.e. precursor field sweep protocols - capable of
enhancing the final zero-field skyrmion density. Our results indicate that FORC
measurements can play a useful role in characterizing spin topology in thin
film multilayers, and are particularly suitable for identifying samples in
which skyrmion populations can be stabilized at zero field.
