Tuning of a skyrmion cluster in magnetoelectric Cu$_2$OSeO$_3$ by electric field

Chiral magnetic textures with non-trivial topology are known as skyrmions, and due to their unique properties they are promising in novel magnetic storage applications. While the electric manipulation of either isolated skyrmions or a whole skyrmion lattice have been intensively reported, the electric effects on skyrmion clusters remain scarce. In magnetoelectric compound Cu$_2$OSeO$_3$, a skyrmion cluster can be created near the helical-skyrmion phase boundary. Here, we report the in situ electric field writing/erasing of skyrmions in such a skyrmion cluster. Our real space/time image data obtained by Lorentz transmission electron microscopy and the quantitative analysis evidence the linear increase of the number of skyrmions in the cluster upon the application of a creating electric field. The energy needed to create a single skyrmion is estimated to be $\mathcal{E}=4.7 \times 10^{-24}$ J.