Bloch point dynamics in exchange-spring heterostructures

Magnetization textures that are stabilized by topological constraints, such as skyrmions and chiral bobbers, as well as the emergent electrodynamics associated with their motion, provide a promising avenue toward novel energy-efficient nanomagnetic devices. Here, it is shown that exchange-spring-type heterostructures, where soft ferromagnets with azimuthal symmetry are exchange-coupled to a ferromagnetic layer with perpendicular magnetic anisotropy, can be used for the creation and control of skyrmion tubes and Bloch points during magnetization reversal of the soft ferromagnet, where the rapid motion of the Bloch points induces an emergent electric field with a magnitude of the order of megavolts per meter. The exchange coupling to the hard ferromagnet restores the system to its original configuration, making the process fully reversible and repeatable, and the duration of the magnetization processes and the motion of the Bloch points can be tuned by adjusting the size of the ferromagnet. Based on these numerical predictions, it is proposed that exchange-spring heterostructures could be used to generate picosecond electromagnetic pulses.