Electric field control of moiré skyrmion phases in twisted multiferroic NiI$_2$ bilayers

Abstract: Twisted magnetic van der Waals materials provide a flexible platform to engineer new forms of unconventional magnetism. Here we demonstrate the emergence of electrically tunable topological moir\'e magnetism in twisted bilayers of the spin-spiral multiferroic NiI$_2$. We establish a rich phase diagram featuring uniform spiral phases, a variety of $k\pi$-skyrmion lattices, and nematic spin textures ordered at the moir\'e scale. The emergence of these phases is driven by the local stacking and the resulting modulated frustration in the spin spiral stemming from the moir\'e pattern. Notably, when the spin-spiral wavelength is commensurate with the moir\'e length scale by an integer $k$, multi-walled skyrmions become pinned to the moir\'e pattern. We show that the strong magnetoelectric coupling displayed by the moir\'e multiferroic allows the electric control of the $k\pi$-skyrmion lattices by an out-of-plane electric field, which couples to the moir\'e-induced electric polarization. While adiabatic changes in the electric field preserve the topology of the spin configurations, abrupt variations can trigger transitions between different skyrmion lattice ground states. Our results establish a highly tunable platform for skyrmionics based on twisted van der Waals multiferroics, potentially enabling a new generation of ultrathin topologically-protected spintronic devices.