Prediction of maximally efficient spin switching of magnetic skyrmionic textures in spin-polarized STM from theoretical calculations

High-resolution tunneling electron spin transport properties (longitudinal spin current and spin transfer torque (STT) maps) of topologically distinct real-space magnetic skyrmionic textures are reported by employing a 3D-WKB combined scalar charge and vector spin transport theory in the framework of spin-polarized scanning tunneling microscopy (SP-STM). For our theoretical investigation metastable skyrmionic spin structures with various topological charges ($Q=-3,-2,-1,0,1,2$) in the (Pt$_{0.95}$Ir$_{0.05}$)/Fe/Pd(111) ultrathin magnetic film are considered. We find that the STT efficiency (torque/current) acting on the spins of the skyrmions can reach large values up to $\sim$25 meV/$\mu$A ($\sim$0.97 $h/e$), but they considerably vary between large and small values depending on the lateral position of the STM tip above the topological spin textures. A simple expression for the STT efficiency is introduced to explain its variation. Our results are expected to guide SP-STM experiments to select STM tip positions for an efficient switching of the skyrmionic structures by localized tunneling STT. Furthermore, our calculated spin transport vectors can be used for the investigation of tunneling-current-induced spin dynamics of topologically distinct surface magnetic skyrmionic textures.