(a) Schematic of Resonant Ultrasound Spectroscopy setup. (b)A new regime – SKX creep phase is identified between jc and jc*.
( Communications Materials 1, 83 (2020). https://doi.org/10.1038/s43246-020-00083-1 )
Background
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 thinfilms 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.
What we discover?
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.
Why is this important?
This work contains at least three aspects related to importance and originality: (1) A new dynamic regime of SKX under the driving current, the creep motion, is identified experimentally in bulk skrymion materials. (2) Technically, we exploit RUS as a tool to detect SKX movement. The results indicate that this technique can also be applicable to detect pinning-depinning transition and creep in other kinds of materials. (3) The observed creep motion isdetrimental for race-track memories, therefore, it is crucial that any devices based on the control of skyrmion dynamics must be carefully engineered to avoid uncontrolled creep.
Who did the research?
Yongkang Luo1,2*, Shi-Zeng Lin1, Maxime Leroux1, Nicholas Wakeham1, David M. Fobes1, Eric D. Bauer1, Jonathan B. Betts1, Joe D. Thompson1, Albert Migliori1, Marc Janoschek1,3*, and Boris Maiorov1
1Los Alamos National Laboratory, Los Alamos, NM 87545, USA.
2Wuhan National High Magnetic Field Center and School of Physics, Huazhong University of Science and Technology, 430074 Wuhan, China.
3aboratory for Neutron and Muon Instrumentation, Paul Scherrer Institute (PSI), 5232 Villigen, Switzerland.
Acknowledgements
We thank F. F. Balakirev for technical support, and F. Ronning and M. Garst for insightful conversations. Sample synthesis by EDB and characterization by JDT was performed under the U.S. DOE, Office of Science, BES project “Quantum Fluctuations in Narrow Band Systems”. Research by Y.L., S.L., D.M.F., M.L., N.D., B.M., and M.J. was supported by LANL Directed Research and Development program project “A New Approach to Mesoscale Functionality: Emergent Tunable Superlattices (20150082DR)” [PI Janoschek].Work by J.B. and A.M. was part of the Materials Science of Actinides, an Energy Frontier Research Center funded by the U.S. DOE, Office of Science, BES under Award DE-SC0001089.
