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Emergent dynamic chirality in a thermally driven artificial spin ratchet

Abstract

Modern nanofabrication techniques have opened the possibility to create novel functional materials, whose properties transcend those of their constituent elements. In particular, tuning the magnetostatic interactions in geometrically frustrated arrangements of nanoelements called artificial spin ice1,2 can lead to specific collective behaviour3, including emergent magnetic monopoles4,5, charge screening6,7 and transport8,9, as well as magnonic response10,11,12. Here, we demonstrate a spin-ice-based active material in which energy is converted into unidirectional dynamics. Using X-ray photoemission electron microscopy we show that the collective rotation of the average magnetization proceeds in a unique sense during thermal relaxation. Our simulations demonstrate that this emergent chiral behaviour is driven by the topology of the magnetostatic field at the edges of the nanomagnet array, resulting in an asymmetric energy landscape. In addition, a bias field can be used to modify the sense of rotation of the average magnetization. This opens the possibility of implementing a magnetic Brownian ratchet13,14, which may find applications in novel nanoscale devices, such as magnetic nanomotors, actuators, sensors or memory cells.

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Figure 1: Schematic representation of the chiral ice and evolution of the net magnetization at individual vertices within the array.
Figure 2: Measured clockwise evolution of the magnetization following saturation along the +y direction.
Figure 3: Simulated stray field structure and energy barriers for clockwise and counterclockwise rotations of the average magnetization.
Figure 4: Counterclockwise evolution of the system following saturation in the −y direction.

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Acknowledgements

The authors thank O. Sendetskyi, H. Arava, V. Guzenko, E. Deckardt and J. Bosgra for technical assistance. S.G. wishes to thank N. Leo and A. S. Arrott for helpful discussions as well as S. Arnold for advice on the graphics in the manuscript. R.L.S. thanks F. Nascimento for discussions. S.G. was funded by the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement no. 708674. The work of G.H. was supported by the EPSRC (grants EP/M015173/1 and EP/L019876/1), the Vienna Science and Technology Fund under WWTF Project MA14-44 and the Royal Society under Grant No. UF080837. The work of R.L.S. was supported by the EPSRC (grants EP/ L002922/1 and EP/M024423/1). This work was supported by JSPS Core-to-Core Program, A. Advanced Research Networks. A.F. was supported by the Swiss National Science Foundation. Part of this work was performed at the Surface/Interface: Microscopy (SIM) beamline of the Swiss Light Source, Paul Scherrer Institut, Villigen, Switzerland. This research used resources of the Advanced Light Source, which is a DOE Office of Science User Facility under contract no. DE-AC02-05CH11231. Use of the Center for Nanoscale Materials, an Office of Science user facility, was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

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R.L.S. and S.G. conceived the spin ice geometry and the experiment. S.G., A.F., C.D. and J.C. prepared the samples. S.G., C.D., J.C., J.B., A.K., A.F., R.V.C., E.K., A.S. and N.S.B. performed the experiments and analysed the experimental data. G.H., S.G. and J.B. performed and evaluated the micromagnetic simulations. S.G., G.H., R.L.S., J.B., C.D., A.K., Y.M. and L.J.H. interpreted the results. S.G. wrote the manuscript with input from all coauthors. All authors discussed the results and commented on the manuscript.

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Correspondence to Sebastian Gliga.

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The authors declare no competing financial interests.

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Gliga, S., Hrkac, G., Donnelly, C. et al. Emergent dynamic chirality in a thermally driven artificial spin ratchet. Nature Mater 16, 1106–1111 (2017). https://doi.org/10.1038/nmat5007

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