A newly discovered type of electronic behaviour could enhance data storage in magnetic memory devices.
When reduced to atom-thin layers, pencil lead, known as graphene, displays unique properties. Thinner than human hair, it resembles hexagonal carbon patterns under a microscope. Despite its thinness, graphene is exceptionally strong, showcasing surprising electronic behaviours when layered and twisted.
MIT researchers discovered that when graphene is arranged in five layers using a rhombohedral pattern, it takes on the unique characteristic of a rare “multiferroic” state. In this state, graphene displays unconventional magnetism and unusual electronic behaviour that the team has termed “ferro-valleytricity.”
A preference for order
Ferroic materials, like magnets, have coordinated electric, magnetic, or structural properties. Only a few, known as multiferroics, simultaneously exhibit multiple coordinated properties. These materials are appealing in electronics, potentially speeding up and reducing energy costs for hard drives. Hard drives use magnetic domains to store data, switched by a slow, energy-intensive electric current. They could be switched faster and more efficiently if made from multiferroic materials. Intrigued by this, the team explored graphene, a thin material with unique quantum interactions. Their initial studies suggest a five-layer graphene structure might display this rare multiferroic behaviour.
In their study of five-layer graphene, researchers identified flakes with a rhombohedral pattern and examined them near absolute zero to enhance electron interactions. Their measurements revealed two distinct ferroic behaviours. First, the electrons displayed unconventional magnetism, coordinating their orbital motions. Unlike typical magnets, where electrons synchronise their spins, they circled in unison here. Secondly, while most conductive materials have valleys representing the lowest energy state electrons can occupy, graphene possesses two. Typically, electrons display no valley preference, but they demonstrated a clear predilection for one valley in the five-layer structure. These combined behaviours confirmed the rare multiferroic state of the five-layer graphene.
The researchers demonstrated the control of ferroic properties in five-layer graphene using an electric field. Incorporating such multiferroic materials could double chip data storage compared to traditional multiferroics, offering a promising avenue for enhancing electronic devices.