Revealing Creep Motion of a Skyrmion Lattice at Ultra-Low Current Densities

Magnetic skyrmions are well-suited for encoding information because they are nano-sized, topologically stable, and only require ultra-low critical current densities j_c to depin from the underlying atomic lattice. Above j_c, skyrmions exhibit well-controlled motion, making them prime candidates for race-track memories. In thin films thermally-activated creep motion of isolated skyrmions was observed below j_c as predicted by theory. Creep is an uncontrolled form of motion that for skyrmions is determined via the underlying pinning landscape due to defects and impurities. This uncontrolled skyrmions motion is detrimental for race-track memories and is not fully understood. Notably, the creep of skyrmion lattices in bulk materials remains to be explored. Here we show using resonant ultrasound spectroscopy—a probe highly sensitive to the coupling between skyrmion and atomic lattices—that in the prototypical skyrmion lattice material MnSi depinning occurs at j_c^* that is only 4 percent of j_c. Our experiments are in excellent agreement with Anderson-Kimtheory for creep and allow us to reveal a new dynamic regime at ultra-low current densitiescharacterized by thermally-activated skyrmion-lattice-creep with important consequences forapplications.