Spin-polarized scanning tunneling microscopy characteristics of skyrmionic spin structures exhibiting various topologies

TL;DR

This study uses spin-polarized scanning tunneling microscopy calculations to identify and characterize various topological skyrmionic spin structures in ultrathin magnetic films, revealing how contrast patterns relate to their topologies.

Contribution

It establishes methods to determine skyrmion topological charge and sign from SP-STM images, enhancing experimental identification of complex magnetic structures.

Findings

Distinct SP-STM contrasts correspond to skyrmion symmetries.

Single in-plane SP-STM images can determine skyrmion charge magnitude.

In-plane tip rotation reveals skyrmion charge sign.

Abstract

The correct identification of topological magnetic objects in experiments is an important issue. In the present work we report on the characterization of metastable skyrmionic spin structures with various topological charges ($Q=-3,-2,-1,0,+1,+2$) in the (Pt$_{1-x}$Ir$_{x}$)Fe/Pd(111) ultrathin magnetic film by performing spin-polarized scanning tunneling microscopy (SP-STM) calculations. We find that an out-of-plane magnetized tip already results in distinguished SP-STM contrasts for the different skyrmionic structures corresponding to their symmetries. Our work also establishes an understanding of the relationship between in-plane SP-STM contrasts and skyrmionic topologies through an investigation of the variation of the in-plane angle between the spins along the perimeter of the structures, which can be characterized by the local vorticity or linear density of the winding number. For spin structures exhibiting a uniform sign of the local vorticity throughout the whole skyrmionic area, we demonstrate that (i) $|Q|$ can be determined from a single SP-STM image taken by any in-plane magnetized tip, and (ii) an in-plane tip magnetization rotation provides the sign of $Q$ independently of the sign of the effective spin polarization in the tunnel junction. We also discuss cases where the local vorticity is changing sign. Finally, by increasing the Ir content of the PtIr overlayer, we find an appearing secondary outer ring in-plane SP-STM contrast that is indicative of attractive skyrmions or antiskyrmions.