Magnetic skyrmions are topologically nontrivial spin textures with envisioned applications in energy-efficient magnetic information storage 1,2. Toggling the presence of magnetic skyrmions via writing/deleting processes is essential for spintronics applications, which usually requires a magnetic field, current injection, electric field, laser pulse, or thermal excitation 3,4. A key challenge is to directly image such ultrasmall spin textures in order to confirm their topological character.

Using spin-polarized low-energy electron microscopy (SPLEEM), we have demonstrated a new, field-free method to write/delete magnetic skyrmions at room temperature, via hydrogen chemisorption/desorption cycles on Ni/Co/Pd/W(110) 5. The spin structures of the written skyrmions are resolved to be hedgehog Néel-type via magnetization vector mapping using SPLEEM (Fig. 1). Supported by Monte-Carlo simulations, the skyrmion creation/annihilation is attributed to the hydrogen-induced magnetic anisotropy change on ferromagnetic surfaces. The roles of hydrogen and oxygen on magnetic anisotropy and skyrmion deletion on other magnetic surfaces are also demonstrated.

In another study, we focus on the interfacial Dzyaloshinskii-Moriya interaction (DMI), which stabilizes magnetic chirality with preferred handedness. Experimentally, controlling the handedness is crucial to tune the efficiency of current-induced manipulation of spin texture. This is conventionally achieved by stacking asymmetric multilayers where the thickness of each layer is at least a few monolayers. We observed an ultrasensitive chirality switching in (Ni/Co)n multilayer induced by capping only 0.22 monolayer of Pd 6. Using SPLEEM, we monitor the gradual evolution of domain walls from left-handed to right-handed Néel walls and quantify the DMI induced by the Pd capping layer (Fig. 2). We also observe the chiral evolution of a skyrmion during the DMI switching, where no significant topological protection is found as the skyrmion winding number varies. This corresponds to a minimum energy cost of < 1 attojoule during the skyrmion chirality switching.

These results open up new opportunities for designing energy-efficient skyrmionic and magneto-ionic devices. They also illustrate the effectiveness of SPLEEM in resolving magnetization vector in chiral spin textures with high spatial resolution.

This work has been supported in part by the NSF (DMR-2005108), SRC/NIST SMART Center and US DOE.

References 1 R. Wiesendanger, Nat. Rev. Mater. 1, 16044 (2016). 2 A. Fert, et al. Nat. Rev. Mater. 2, 17031 (2017). 3 W. Jiang, et al. Phys. Rep. 704, 1-49 (2017). 4 X. Zhang, et al. J. Phys.: Condens. Matter 32, 143001 (2020). 5 G. Chen, et al. Nat. Commun. 13, 1350 (2022). 6 G. Chen, et al. Nano Lett., in press; arXiv: 2206.12069.