Long-range ordering is typically associated with a decrease in disorder, or entropy. Yet, it can also be driven by increasing entropy in certain special cases. In a recent DOE-funded study, researchers demonstrated that certain artificial spin-ice arrays—nanomagnets lithographically patterned to form Tetris-like shapes—can produce such entropy-driven order.
“Sometimes when things are complicated and messy, they force other things to be simpler and more orderly,” said Cristiano Nisoli, a theoretical physicist at Los Alamos National Laboratory and co-author of the study. “With this simple system, we can explore precisely how disorder in one area can lead to order in another, and then we can apply the lessons to more complex systems.”
“Tetris ice” is a frustrated system—that is, competing magnetic interactions prevent the system from settling into a single low-energy configuration. It is conceptually related to water ice, which retains this kind of intrinsic randomness (residual entropy) even at absolute zero. The tetris ice was designed by researchers at Los Alamos and fabricated by a group at Yale, in collaboration with groups at the University of Minnesota and Liverpool University.
To probe the unusual effects of tetris ice, the researchers used photoemission electron microscopy (PEEM) with x-ray magnetic circular dichroism (XMCD) contrast at Beamline 220.127.116.11 of the Advanced Light Source. Combined with computer simulations, the results revealed that, at its lowest energy, the system consists of two subsystems of alternating stripes—backbones and staircases.
“It turns out that disorder in the staircases can drive order between backbones” said Peter Schiffer, a professor of physics at Yale and co-author of the study. “Related phenomena have been observed in very different systems, but the observation in artificial spin ice opens a new door to studying this apparently paradoxical idea.”
Artificial spin ice structures have been proposed as platforms for information storage and computers that mimic how the human brain works. More broadly, natural frustrated magnets with similar unusual behavior can be hard to synthesize in pure form, and they demonstrate their exotic properties at narrow (often inconvenient) ranges of temperature or applied field. The ability to create magnets with interesting peculiar behaviors enables the exploration of fundamental concepts in physics with potentially broad significance.
H. Saglam, A. Duzgun, A. Kargioti, N. Harle, X. Zhang, N.S. Bingham, Y. Lao, I. Gilbert, J. Sklenar, J.D. Watts, J. Ramberger, D. Bromley, R.V. Chopdekar, L. O’Brien, C. Leighton, C. Nisoli, and P. Schiffer, “Entropy-driven order in an array of nanomagnets,” Nat. Phys. 18, 706 (2022), doi:10.1038/s41567-022-01555-6 (2022).