Examining Domain Wall Skyrmions and Domain Wall Substructures in a Pt/Co/Ni/Ir Multilayer System*

Topologically protected spin textures stabilized by the Dzyaloshinskii-Moriya interaction (DMI) have great potential for nonvolatile memory and neuromorphic computing. Towards this end, chiral domain walls (DW) and 2D skyrmions have been extensively characterized. Magnetic DW skyrmions are a markedly different spin texture that have been theoretically predicted. These DW substructures are 360° transitions of a chiral DW's internal magnetization and are analogous to vertical Bloch lines (VBL). Here, we performed a systematic study of a [Pt/(Co/Ni)_M/Ir]_N multilayer system using Lorentz TEM where M modulates the DMI strength and N primarily modulates the total thickness of the sample. We find that the formation of hybrid DWs in thicker films inhibit VBLs and DW skyrmions in contrast to the uniform magnetization expected through ultrathin films while DMI strength is found to primarily dictate the DW character. With these observations, we qualitatively formulated a magnetic phase diagram of the domain wall character with respect to DMI strength and film thickness.