Hedgehog Skyrmion Bubbles in Ultrathin Films with Interfacial   Dzyaloshinskii-Moriya Interactions
  Collective spin ensembles known as skyrmion bubbles can exist in ultrathin magnetic film heterostructures and promise miniscule diameters and low-power all-electrical manipulation. The nucleation, identification and manipulation of skyrmions is of great interest, in part, because of their non-trivial topology. Here we ascertain the topology of 100 nm isolated hedgehog skyrmion bubbles and probe their response to external fields at room temperature in [ Pt \ Co (0.8 nm) \ Ir ] films. We find that structural disorder can stabilize skyrmion bubble states which exhibit creep-like domain wall motion. This makes the extraction of DMI from observations difficult and could explain the larger than predicted bubble sizes observed in interfacial DMI films. Intriguingly, we report on a critical size for hedgehog skyrmions, above which the bubbles lose circular symmetry and take up an irregular shape. Combining Lorentz TEM and fully three dimensional multi-layered simulations, we demonstrate the unique behavior of these topologically non-trivial spin textures. 
Javier F. Pulecio, Ale\v{s} Hrabec, Katharina Zeissler, Ryan M. White,   Yimei Zhu, and Christopher H. Marrows
cond-mat.mtrl-sci