Fig. 2: Skyrmion size dependence of the pinning.

a Skyrmion occurrences in all measurements at fields between −43 and −31 µT with an average skyrmion size between 1.27 and 1.68 µm with standard deviations of about 5%. Every detected skyrmion is depicted by a scatter plot point at the coordinate of observation. The colors represent the skyrmion size. The color intensity indicates the probability density of finding a skyrmion at a certain position. The top histogram shows the size distribution of all observed skyrmions. The colored circles on the bottom indicate schematically the size relation of the skyrmions among each other and the coordinate frame. b–e Histograms of the occurring skyrmion center positions in the same sample area at external fields of (b) −39 µT, (c) −37 µT, (d) −35 µT, and (e) −33 µT. The greyscale denotes the number of skyrmion center occurrences at a position. Even for small field changes of 2 µT corresponding to size changes of 5–8% in region depicted by (b–e), the skyrmion distribution varies drastically and skyrmions are pinned at different positions indicating size-dependent pinning sites. In (d), predominant pinning sites are highlighted by the dashed purple circles and labelled with integers 1–4.