Periodic transitions of topological charge in skyrmions confined within FeGe and Co/Pt nanodisks

The ability to control the polarity and the topological charge of skyrmions is essential for their integration into emerging spintronic technologies. Previous studies have demonstrated that polarity and chirality inversion can be induced by pulsed magnetic fields, alternating fields, or spin-polarized currents. In this work, we focus on the periodic inversion of the topological charge, which results from the dynamic change of the skyrmion polarity and vorticity, in FeGe (a bulk chiral magnet) and Co/Pt (an interfacial system). By applying an oscillating magnetic field along the out-of-plane direction, we induce a repeating cycle in the spin texture, leading to a robust and periodic switching between topological states. At specific excitation frequencies, we observe a sequence of transitions among $Q=1$, $Q=0$ and $Q=-1$. In FeGe, a skyrmion with $Q=1$ periodically evolves into a skyrmionium state with $Q=0$ , and afterward into a skyrmion with inverted charge $Q=-1$. In Co/Pt, the skyrmion is periodically annihilated into a monodomain state and subsequently recreated with opposite topological charge. These findings highlight distinct dynamic mechanisms for periodically modulating the skyrmion charge, paving the way for skyrmion-based functional devices.