Abstract
Magnetic skyrmions are well-suited for encoding information because they are nano-sized, topologically stable, and only require ultra-low critical current densities jc to depin from the underlying atomic lattice. Above jc 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 jc as predicted by theory. Uncontrolled skyrmion 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 jc. Our experiments are in excellent agreement with Anderson-Kim theory for creep and allow us to reveal a new dynamic regime at ultra-low current densities characterized by thermally-activated skyrmion-lattice-creep with important consequences for applications.
Highlights
Magnetic skyrmions are well-suited for encoding information because they are nano-sized, topologically stable, and only require ultra-low critical current densities jc to depin from the underlying atomic lattice
We show that our experimental results are in excellent agreement with Anderson–Kim theory for creep[19], and connect the creep motion of skyrmion lattices in bulk materials with the previously known creep dynamics in thin films
The SKX phase appears in a narrow range of temperatures and field within the conically (CO) ordered magnetic phase
Summary
Magnetic skyrmions are well-suited for encoding information because they are nano-sized, topologically stable, and only require ultra-low critical current densities jc to depin from the underlying atomic lattice. In bulk materials, where skyrmions typically form a hexagonal skyrmion lattice (SKX) oriented in a plane perpendicular to an external magnetic field (H) [see Fig. 2a], creep remains elusive. This raises the question whether the pinning mechanisms in thin films and bulk materials are fundamentally different, or if this merely due to differences in pinning landscape. For SKX thermal fluctuations may depin small fractions of the lattice [shown in light yellow in Fig. 1e], where each fraction encounters a distinct local pinning landscape, in turn, resulting in incoherent movement characteristic of local creep. Decreasing ΔU, the fraction of the SKX that may be depinned by fluctuation becomes larger
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