Two-dimensional materials are of intense interest because their electronic and magnetic properties have the potential to power future technologies. Scientists have traditionally treated these two behaviors as separate. Engineers at Illinois Grainger Engineering showed that they are connected by the same underlying mathematics.
In a study published in Physical Review Xresearchers at the Granger Institute of Technology at the University of Illinois at Urbana-Champaign have demonstrated how a specially designed two-dimensional magnetic system can follow the same equations that describe mobile electrons in graphene. This mathematical relationship has the potential to impact the design of high-frequency devices and may also provide researchers with powerful new ways to analyze and design these materials.
“While it is not at all obvious that there is an analogy between 2D electronics and 2D magnetic behavior, we are still surprised at how well this analogy works,” said Bobby Kaman, lead author of the study. “Thanks to the discovery of graphene, 2D electronics has been very well studied, but now we have found that a less-studied class of materials follows the same fundamental physics.”
Inspiration from metamaterials and graphene
The concept arose from Kaman’s research on metamaterials. These materials are designed in such a way that their large-scale structure produces behaviors that do not normally occur with the material’s natural atomic arrangement.
Kaman, a materials science and engineering graduate student in Professor Axel Hoffmann’s research group, noticed that both electrons and microscopic magnetic excitations in graphene behave like waves in so-called magnonic materials. This similarity raises an interesting possibility. Perhaps magnetic systems could be designed to behave mathematically similar to graphene.
“Graphene is unique in that its conduction electrons are organized into massless waves, so we were curious whether changing the physical shape of the magnon material to make it look like graphene would make it behave like graphene,” Kaman said. “We expected it to have some similar properties to graphene, but the similarities were much deeper and richer than we expected.”
Designing a magnetic system that mimics graphene
To explore this idea, the researchers modeled a thin magnetic film containing tiny holes arranged in a hexagonal pattern. Within this structure, microscopic magnetic moments known as “spins” interact to produce traveling disturbances called spin waves.
The researchers calculated the energy of these spin waves and found that their mathematical behavior closely matches the behavior of electrons moving through graphene.
The system turned out to be more complex than expected. The researchers identified nine distinct energy bands, rather than a simple one-to-one similarity. These bands allow multiple types of behavior to appear simultaneously. These include massless spin waves similar to electron waves in graphene, low-dispersion bands associated with localized states, and even topological effects that span multiple bands.
“What’s remarkable about Bobby’s work is that it directly connects engineered spin systems with fundamental physical models,” Hoffman said. “Magnonic crystals are notorious for giving rise to an overwhelming variety of structure- and shape-dependent phenomena, most of which have been cataloged without really being understood. The similarities of graphene in this system provide a clear explanation for the observed behavior.”
Possibility of smaller microwave devices
This research goes beyond its importance as basic physics and has the potential to be put to practical use. The research team believes the system could be useful for microwave technology used in wireless and cellular communications.
“One such device is a ‘microwave circulator,’ which allows microwave radio signals to propagate in only one direction,” Hoffman explained. “Microwave devices are usually bulky, but using the magnonic system we have studied, it is possible to miniaturize microwave devices down to the micrometer scale.”
Hoffman’s research group has already filed a patent application covering the microwave device concept.
Jinho Lim and Yingkai Liu also contributed to the research.
Support for this research was provided by the Illinois Center for Materials Research, Science and Engineering through the National Science Foundation.
Axel Hoffmann is a professor in the Department of Materials Science and Engineering at the Illinois Granger School of Engineering. He is also affiliated with the Institute of Materials Research and holds a Founding Professor appointment.
