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A five-layer graphene surprise

Unusual electronic behavior in a specific form of the material could help pack more data into magnetic memory devices.

January 4, 2024
a pattern of alternating pink and blue spheres
When stacked in five layers in a rhombohedral pattern, graphene takes on a rare “multiferroic” state, in which the material’s electrons (illustrated here as spheres) exhibit two preferred electronic states: an unconventional magnetism (represented as orbits around each electron), and “valley,” or a preference for one of two energy states (depicted in pink versus blue). The results could help advance more powerful magnetic memory devices.Sampson Wilcox, RLE

Graphene, a form of carbon consisting of a single layer of atoms linked in a hexagonal lattice, is a material with many unusual electronic behaviors. Now MIT physicists have discovered another: when stacked in five layers offset so that the atoms form a specific three-dimensional pattern, graphene becomes “multiferroic,” a rare state in which coordinated behavior is seen in more than one of a material’s electric, magnetic, or structural properties. 

A magnet is a common example of a ferroic material: its electrons can coordinate to spin in the same direction. But this form of graphene exhibits both unconventional magnetism—its electrons coordinate their orbital motion as opposed to their spin—and an exotic type of electronic behavior involving a preference for one of two “valleys,” the lowest energy state an electron can naturally settle in.

“We don’t see this property in one, two, three, or four layers,” says team leader Long Ju, an assistant professor of physics at MIT. But “in five layers, electrons happen to be in a lattice environment where they move very slowly, so they can interact with other electrons effectively,” he says. To get samples with the right configuration, the researchers started with a small block of graphite, from which they carefully peeled individual flakes of graphene. They used optical techniques to examine each one, looking specifically for five-layer flakes arranged naturally in a rhombohedral pattern.

The discovery could help engineers design ultra-low-power, high-capacity data storage devices for classical and quantum computers.

“Having multiferroic properties in one material means that if it could save energy and time to write a magnetic hard drive, you could also store double the amount of information compared to conventional devices,” Ju says. 

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