| Date | 11th, Apr 2022 |
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Microscopic arrays of magnets with strange and unusual properties can order themselves by increasing entropy or the tendency of physical systems to disorder. This behaviour contradicts standard thermodynamics—but doesn’t.
“Paradoxically, the system orders because it wants to be more disordered,” said Cristiano Nisoli, a physicist at Los Alamos and coauthor of a paper about the research published in Nature Physics. “Our research demonstrates entropy-driven order in a structured system of magnets at equilibrium.”
Nanomagnet arrays, like tetris spin ice, show promise as circuits of logic gates in neuromorphic computing, a leading-edge computing architecture that closely mimics how the brain works. They also have possible applications in a number of high-frequency devices using “magnonics” that exploit the dynamics of magnetism on the nanoscale. Image credit: Yale University/Credit: University of Illinois at Urbana-Champaign
The system examined in this work, known as Tetris spin ice, was studied as part of a long-standing collaboration between Nisoli and Peter Schiffer at Yale University, with theoretical analysis and simulations led at Los Alamos and experimental work conducted at Yale. The research team includes scientists from several universities and academic institutions.
Like Tetris spin ice, Nanomagnet arrays show promise as circuits of logic gates in neuromorphic computing, a leading-edge computing architecture that closely mimics how the brain works. They also have possible applications in several high-frequency devices using “magnonics” that exploit the dynamics of magnetism on the nanoscale.
Entropy measures a physical system’s state of disorder, randomness, or uncertainty. A liquid, for instance, has high entropy because at warm temperatures—high energy— its molecules are free to move around in a random, disordered way.
But when liquids are cooled to form solids, the molecules calm down and order themselves through interactions to optimize their energy. They can arrange themselves in a crystal lattice in only a limited number of configurations. This lowers their entropy: they are highly ordered.
Some systems, however, are not so simple. Parts of the system settle orderly, but others don’t. These “frustrated” systems retain disorder.
Tetris spin ice, composed of a 2D array of tiny magnets that interact but are frustrated, is a strange mix of the two cases. The magnetic pole orientations are frustrated so that the system retains some order while remaining disordered. At low temperatures, it decomposes into alternating ordered and disordered stripes.
The apparent paradox of increasing entropy with increasing order is resolved by the entropic interaction between the alternating layers. The system increases the disorder in the other stripes by the mutual ordering of the ordered lines. Thus, order happens without any decrease in energy but via an increase in entropy.
Schiffer observed and recognised the nanomagnets’ notable changes by applying modern imaging methods.
“Modern microscopy lets us use x-rays to take a picture of how each of the hundreds of nanomagnets has its north and south poles pointing, and we can “watch” the poles flip back and forth as the temperature changes,” Schiffer said. “This allows us to understand the system’s entropy in detail and unusual.”
“No law of thermodynamics is truly broken,” Nisoli said. “The concept that systems order by reducing entropy applies to most systems, but, as we show, not to all. Our system is exotic and behaves counterintuitively, with increased entropy, a measure of disorder, the driver of visible order.”
Source: Yale University
