Evolution of zero-field skyrmionic states in exchange-coupled composite multilayer nanodots

Ambient magnetic skyrmions stabilized in multilayer nanostructures are of immense interest due to their relevance to magnetic tunnel junction (MTJ) devices for memory and unconventional computing applications. However, existing skyrmionic nanostructures built using conventional metallic or oxide multilayer nanodots are unable to concurrently fulfill the requirements of nanoscale skyrmion stability and all-electrical readout and manipulation. Here, we develop a few-repeat hybrid multilayer platform consisting of metallic $[\mathrm{Pt}/\mathrm{Co}B/\mathrm{Ir}{]}_3$ and oxide [$\mathrm{Pt}/\mathrm{Co}B/\mathrm{Mg}O$] components that are coupled to evolve together as a single, composite stack. Zero-field (ZF) skyrmions with sizes as small as 50 nm are stabilized in the hybrid multilayer nanodots, which are smoothly modulated by up to $2\times$ by varying $\mathrm{Co}B$ thickness and dot sizes. Meanwhile, skyrmion multiplets are also stabilized by small bias fields. Crucially, we observe higher-order "target" skyrmions with varying magnetization rotations in moderately sized, low-anisotropy nanodots. These results provide a viable route to realize robust skyrmionic MTJs and alternative possibilities for multistate skyrmionic device concepts.