Observation of Relativistic Domain Wall Motion in Amorphous Ferrimagnets

Domain walls in ferrimagnets and antiferromagnets behave as relativistic sine-Gordon solitons with the spin-wave group velocity setting the ultimate velocity of domain walls and speed of magnetic devices. While this relativistic regime has been achieved in crystalline ferrimagnets, they cannot be routinely integrated in devices. To enable technological breakthroughs, relativistic dynamics must be demonstrated in easy-to-integrate ferrimagnets such as rare-earth – transition-metal alloys. However, this scenario remains elusive due to the inherent magnetic disorder of these materials, their complex spin-wave spectra, and challenges in modeling their ultrafast dynamics. Here, relativistic domain wall motion is demonstrated in amorphous ferrimagnetic GdFeCo devices operated in the proximity of the angular momentum compensation point. The current-induced domain wall velocity saturates within 10% of the spin-wave speed of 2 kms−1, a behavior consistent with relativistic model of domain wall motion. The observation of relativistic dynamics in technologically relevant ferrimagnets opens the way to magnetic devices operating at the ultimate speed limit.