Magnetic skyrmions probed by SP-STM: topology imprinted on the charge current and spin transfer torque

The controlled creation/annihilation of individual magnetic skyrmions have been demonstrated by using spin-polarized scanning tunneling microscopy (SP-STM) [Science 341, 636], where the spin-polarized current exerts a torque on the spin moments of the sample. However, the detailed microscopic mechanism of this process is presently unknown. Our work contributes to this understanding by a theoretical investigation of the tunneling electron charge and spin transport probing magnetic skyrmions. The spin-polarized charge current (I) and tunneling spin transport vector quantities, the longitudinal spin current and the spin transfer torque (STT), are consistently calculated within a simple electron transport theory [PRB 94, 064434]. The electron tunneling model is extended to SP-STM in high spatial resolution, and applied to magnetic skyrmions [PRB 97, 174402; PRB 98, 094409]. Besides the vector spin transport characteristics, the relationships among conventional charge current SP-STM images [PRB 96, 024410], the magnitudes of the spin transport quantities [PRB 97, 174402], and the topology of various skyrmionic objects are analyzed [J. Magn. Magn. Mater. 519, 167440]. It is also shown that at specific SP-STM tip positions the STT efficiency (STT/I) can reach very large values ~h/e.