  Skyrmions have attracted significant interest due to their topological spin
structures and fascinating physical features. The skyrmion phase arises in
materials with Dzyaloshinskii-Moriya (DM) interaction at interfaces or in
volume of non-centrosymmetric materials. However, although skyrmions were
generated experimentally, one critical intrinsic relationship between
fabrication, microstructures, magnetization and the existence of skyrmions
remains to be established. Here, two series of CoFeB ultrathin films with
controlled atomic scale structures are employed to reveal this relationship. By
inverting the growth order, the amount of defects can be artificially tuned,
and skyrmions are shown to be preferentially formed at defect sites. The stable
region and the density of the skyrmions can be efficiently controlled in the
return magnetization loops by utilizing first-order reversal curves to reach
various metastable states. These findings establish the general and intrinsic
relationship from sample preparation to skyrmion generation, offering an
universal method to control skyrmion density.
