Nonvolatile and Volatile Skyrmion Generation Engineered by Ionic Liquid Gating in Ultrathin Films

Magnetic skyrmions are topological spin textures with nanoscale size, which have great potential for spintronics applications. However, they are very sensitive to film thicknesses and interfaces in ultrathin films or multilayer heterostructures, and methods to generate and tune skyrmions are needed in order to use them in real-world applications. Electric field gating has been shown to modify the magnetic characteristics of thin films; however, these changes are limited by the low electric fields achievable using solid gate electrodes. In this work, we use ionic liquid gating to modify the magnetic characteristics of perpendicularly magnetized $\mathrm{Mg}\mathrm{O}$/$\mathrm{Mn}$${}_{2}$$\mathrm{Co}\mathrm{Al}$/$\mathrm{Pd}$ ultrathin films, applying a range of voltage sequences to generate skyrmions through both nonvolatile and volatile changes to these films. We achieve a giant anisotropy field tunability of 109.8 mT ${\mathrm{V}}^{\ensuremath{-}1}$ that is nonreversible, which can be ascribed to magneto-ionic effects. Reversible changes to the anisotropy and volatile skyrmion formation are achieved via electrostatic charge accumulation, which could induce an in-plane Rashba field. Our results strongly demonstrate that ionic liquid gating is a versatile method to engineer both nonvolatile and volatile skyrmions by tuning the magnetic characteristics of films to the regimes where they can exist.