Skyrmion generation engineered by ionic liquid gating in ultra-thin films

Magnetic skyrmions show topological spin textures with nanoscale size, which have great potential for spintronics applications. In this work, we investigated the skyrmion generation in MgO/Mn₂CoAl/Pd ultra-thin films with perpendicular magnetic anisotropy (PMA) by ionic liquid gating (ILG). For samples with a weak PMA, skyrmions can be generated directly by just applying a negative gate voltage, -2.5 V, which can be ascribed to the reduction of effective magnetic anisotropy into a spin-reorientation transition. In the case of samples with an as-grown strong PMA, skyrmions can also appear at -2.5 V after cycling positive and negative voltages which induces an accumulation of PdO_x layer after each cycling. These gating behaviours are non-volatile due to the oxygen ions diffuse into the Pd layer. On the other hand, by just applying a large gate voltage, -3 V, to etch away the nature oxide PdO_x layer, volatile skyrmions can be generated at a small positive gate voltage due to the electrostatic charge accumulation. Our results suggest that ILG is a useful method to engineer the magnetic properties of samples and generate skyrmions without designing samples in specific parameters, like thickness and weak PMA.