  Magnetic skyrmions are chiral spin textures with a nontrivial topology that
offer a potential for future magnetic memory and storage devices. The
controlled formation and adjustment of size and density of magnetic skyrmions
in Ta/CoFeB/MgO trilayers is demonstrated. It is the ideal candidate for the
use as a bottom electrode integration into CoFeB/MgO/CoFeB magnetic tunnel
junctions. Varying the CoFeB thickness close to the out-of-plane to in-plane
magnetic phase transition, we find that subtle energy contributions enable the
skyrmion formation in a narrow thickness window, corresponding to only around
10 pm variation in CoFeB thickness. Using magneto-optical imaging with
quantitative image processing, variations in skyrmion diameter and distribution
below the Abbe limit can be analyzed. We demonstrate a high degree of diameter
and density control. Zero-field stable skyrmions can be set with proper
magnetic field initialization. This demonstrated tunability and degree of
comprehension of skyrmion formation, paves the way for future skyrmion based
magnetic memory. Moreover, we demonstrate a controlled merging of individual
skyrmions to complex topological objects. We compare our results with the
baby-Skyrme model, developed to describe the soliton nature, for any
topological charge n, and demonstrate the ability to form multi-skyrmion
objects. These objects will be interesting for fundamental mathematical studies
of the topological behavior of solitons in the future.
