Emergent dynamic chirality in a thermally driven artificial spin ratchet*

The study of emergent phenomena in two-dimensional artificial spin ices is presently the focus of intense research. Artificial spin ices are composed of geometrically frustrated arrangements of nanomagnets and have so far mainly been used to investigate fundamental aspects of the physics of frustration. Recently, it has become clear that artificial spin ice has the potential to become a class of functional material with technological applications. I will present a spin ice based active material – consisting in a repeating pattern of chiral units – in which energy is converted into unidirectional dynamics, thus functioning like a ratchet [1]. Measurements performed using x-ray photoemission electron microscopy show that following saturation by an external field, thermal relaxation proceeds through the rotation of the average magnetization in a unique sense. Micromagnetic simulations demonstrate that this emergent chiral behavior is driven by the topology of the magnetostatic field at the boundaries of the nanomagnet array, resulting in an asymmetric energy landscape. This opens the possibility of implementing a Brownian ratchet, which may find applications in nanomotors, actuators or memory cells. I will also discuss other routes towards applications based on spin wave manipulation [2] as well as perspectives for functionalizing artificial spin ices in three dimensions in light of recent experimental advances in magnetic nanotomography [3].
[1] S. Gliga, G. Hrkac, C. Donnelly, J. Büchi, A. Kleibert, J. Cui, A. Farhan, E. Kirk, R. V. Chopdekar, Y. Masaki, N. S. Bingham, A. Scholl, R. L. Stamps, L. J. Heyderman, Nat. Mater. 16, 1106 (2017)
[2] S. Gliga, A. Kákay, R. Hertel, O. G. Heinonen, PRL 110, 117205 (2013)
[3] C. Donnelly, M. Guizar-Sicairos, V. Scagnoli, S. Gliga, M. Holler, J. Raabe, L. J. Heyderman, Nature 547, 328 (2017)