Thinner than a single strand of DNA yet 200 times stronger than steel, graphene is an excellent conductor of electricity and heat and it can conform to any number of shapes, from an ultrathin 2D sheet to an electronic circuit.
Researchers previously developed a multitasking graphene device that switches from a superconductor that efficiently conducts electricity, to an insulator that resists the flow of electric current, and back again to a superconductor. The current device provides three properties: superconducting, insulating, and a type of magnetism called ferromagnetism. So far, materials simultaneously showing all three properties have been very rare since it is difficult to induce magnetism in graphene because it is typically not magnetic.
The magnetic materials used in electronics today are made of ferromagnetic metals such as iron or cobalt alloys. Ferromagnetic materials, like the common bar magnet, have a north and a south pole. When ferromagnetic materials are used to store data on a computer’s hard disk, these poles point either up or down, representing zeros and ones — called bits. Graphene, however, is not made of a magnetic metal — it’s made of carbon.
The researchers engineered an ultrathin device, just 1 nanometer in thickness, featuring three layers of atomically thin graphene. When sandwiched between 2D layers of boron nitride, the graphene layers — trilayer graphene — form a repeating pattern called a moiré superlattice. By applying electrical voltages through the graphene device’s gates, the force from the electricity prodded electrons in the device to circle in the same direction, like tiny cars racing around a track. This generated a forceful momentum that transformed the graphene device into a ferromagnetic system.
The graphene system’s interior not only becomes magnetic but also insulating, and despite the magnetism, its outer edges morphed into channels of electronic current that move without resistance. Such properties characterize a rare class of insulators known as Chern insulators. The graphene device has not just one, but two conductive edges, making it a “high-order Chern insulator,” a consequence of the strong electron-electron interactions in the trilayer graphene.