“The magnetic moment of the ferromagnetic state can be reversed using several experimental knobs, such as an in-plane or out-of-plane magnetic field, or an electric field. We can take advantage of such versatile control to realize a 2D version of magnetic memory.”Jia Li
Researchers at Brown University have discovered an unexpected new phenomenon in graphene when sheets of the carbon nanomaterial are stacked together. By inducing spin-orbit coupling into the stacks, “magic-angle graphene” can become a powerful ferromagnet.
Magic-angle graphene has become a hot topic in research, since when graphene sheets are stacked together electrons start to interact not just with other electrons within a sheet but also with those in adjacent sheets. Altering the angle of the sheets with respect to each other changes those interactions, giving rise to interesting quantum phenomena such as superconductivity.
However, since ferromagnetism and superconductivity tend to be at opposite ends of the spectrum in condensed matter physics, it is unusual for them to be present in the same material platform. As reported in the journal Science [Lin et al. Science (2022) DOI: 10.1126/science.abh2889], here the addition of spin-orbit coupling – electron behavior in certain materials where each electron’s spin becomes linked to its orbit around the atomic nucleus – through designed atomic interface suggests the development of a bridge linking the two quantum phenomena.
To introduce spin-orbit coupling, the team interfaced magic-angle graphene with a block of tungsten diselenide, which has strong spin-orbit coupling, before exploring the system with external electrical currents and magnetic fields. An electric current flowing in one direction across the material in the presence of an external magnetic field was found to create a voltage in the direction perpendicular to the current, known as the Hall effect, a sign of an intrinsic magnetic field in the material.
The unexpected element here was that the magnetic state could be controlled with an external magnetic field oriented either in the plane of the graphene or out-of-plane. This is in contrast with magnetic materials without spin-orbit coupling, where the intrinsic magnetism can be controlled only when the external magnetic field is aligned along the direction of the magnetism.
One possible application for the breakthrough is in computer memory, as controlling the magnetic properties of magic-angle graphene with both external magnetic fields and electric fields means this 2D system is a promising candidate for a magnetic memory device with flexible read/write options.
As senior author of the research, Jia Li, told Materials Today, “The magnetic moment of the ferromagnetic state can be reversed using several experimental knobs, such as an in-plane or out-of-plane magnetic field, or an electric field. We can take advantage of such versatile control to realize a 2D version of magnetic memory.”
The team now hope to further investigate the interplay between superconductivity and ferromagnetism by adjusting the strength of the spin-orbit coupling across the atomic interface.
Spin-orbit coupling in magic-angle graphene creates ferromagnetism