Beyond Fixed-Size Skyrmions in Nanodots: Switchable Multistability with Ferromagnetic Rings.

We demonstrate a novel approach to controlling and stabilizing magnetic skyrmions in ultrathin multilayer nanostructures through spatially engineered magnetostatic fields generated by ferromagnetic nanorings. Using analytical modeling and micromagnetic simulations, we show that the stray fields from a Co/Pd ferromagnetic ring with out-of-plane magnetic anisotropy significantly enhance the Néel-type skyrmion stability in an Ir/Co/Pt nanodot, even stabilizing the skyrmion in the absence of Dzyaloshinskii-Moriya interactions. We demonstrate precise control over the skyrmion size and stability. We observe a multistability phenomenon, where the skyrmion can be stabilized at two or more distinct equilibrium diameters depending on the ring's magnetization orientation. These stable states exhibit energy barriers that substantially exceed thermal fluctuations at room temperature, suggesting potential applications in robust multibit memory storage. Furthermore, we demonstrate that a skyrmion can be switched between two metastable states using a suitably designed nanosecond magnetic field pulse. Our findings pave the way for advanced spintronic nanodevices.