  We examine the substructures of magnetic domain walls (DWs) in
[Pt/(Co/Ni)$_M$/Ir]$_N$ multi-layers using a combination of micromagnetic
theory and Lorentz transmission electron microscopy (LTEM). Thermal stability
calculations of Q=$\pm$1 substructures (2-$\pi$ vertical Bloch lines (VBLs) and
DW skyrmions) were performed using a geodesic nudged elastic band (GNEB) model,
which supports their metastability at room temperature. Experimental variation
in strength of the interfacial Dzyaloshinskii-Moriya interaction (DMI) and film
thickness reveals conditions under which these substructures are present and
enables the formation of a magnetic phase diagram. Reduced thickness is found
to favor Q=$\pm$1 substructures likely due to the suppression of hybrid DWs.
The results from this study provide an important framework for examining 1-D DW
substructures in chiral magnetic materials.
