Magnetic skyrmions are topologically-protected spin textures with attractive properties suitable for future spintronic device applications. In this talk, our recent experimental observations of skyrmions by utilizing the state-of-the-art X-ray transmission microscopy will be presented.
The presentation will first cover the observation of tunable nanosecond dynamics of skyrmions. The finding demonstrated that distinct dynamic states of magnetic skyrmions, triggered by current-induced spin-orbit torques, could be reliably tuned by changing the magnitude of spin orbit torques.  Second, I will demonstrate the experimental observation of antiferromagnetically-coupled skyrmions and their current-driven dynamics in a new material system: ferrimagnetic GdFeCo multilayer films. In the work, we confirmed that ferrimagnetic skyrmions can also move at reasonably high-velocity, ~50 m/s, with significantly reduced skyrmion Hall angle, θSkHE ~ 20 degrees. This observation highlights the possibility to build more reliable skyrmionic devices using ferrimagnetic and antiferromagnetic materials. 
Lastly, the deterministic writing and deleting of a single magnetic skyrmions will be presented. In this work, the stroboscopic pump-probe X-ray measurement serves as a key technique to reveal the deterministic and completely reproducible nature of the observation. We experimentally present that an engineered current pulses can efficiently create and annihilate a single skyrmion in ferrimagnetic materials, GdFeCo, in nanosecond time scale. Micromagnetic simulations reveal the microscopic origin behind the observed topological fluctuation with great qualitative and quantitative agreement. 
 S. Woo et al., Nat. Commun. 8, 15573 (2017)
 S. Woo et al., Nat. Commun. in press (2018)
 S. Woo et al., arXiv:1706.06726