Rocket-like dynamics of ferrimagnetic domain walls in graded materials

Abstract: Domain wall motion underpins emerging spintronic technologies, such as high-speed racetrack devices and THz logic. Spatially non-uniform magnetic exchange and anisotropy in ferromagnets can pin or accelerate domain walls. In ferrimagnets, where Walker breakdown is suppressed, walls can approach the magnon speed. Here, we show that in non-uniform ferrimagnets such gradients not only exert a net force on the wall, but also modify its effective mass, enabling an entirely new acceleration mechanism. As a wall traverses regions of varying exchange or anisotropy, it can shed or gain mass leading to a "rocket effect" as in variable-mass systems. This phenomenon becomes increasingly pronounced as the wall approaches the magnon velocity, providing a natural route to ultrafast domain wall propulsion. The findings establish variable-mass domain walls as a new paradigm for efficient, high-velocity spintronics and THz-frequency magnetic technologies.