The magnetic exchange field (MEF) is a quantum mechanical effect that comes about as a result of the Pauli exclusion principle. It describes the effective interaction between the spins of neighbouring electrons whose electronic wavefunctions overlap. The MEF shows up as the energy splitting between electrons with an “up” spin and those with a “down” spin, and this splitting is called the exchange energy. In many bulk ferromagnetic materials, such exchange coupling reaches hundreds of Tesla.

Now, a team of researchers led by Ching-Tzu Chen at the IBM TJ Watson Research Center in New York has found that the exchange field between the magnetic insulator europium sulphide (EuS) and graphene (a sheet of carbon just one atom thick) can be higher than 10 Tesla. Such a high MEF could be used to efficiently control the spin of electrons in devices made of the 2D materials without compromising their delicate material structures.

MEF might be boosted by another order of magnitude

“Researchers have long speculated that this interfacial exchange field might be sufficiently strong for spin control in devices made from graphene and other 2D materials, but a quantitative justification had been lacking until now,” says Chen. “Our experiments show that the induced MEF easily exceeds 10 Tesla, even for a low Curie-temperature magnetic insulator such as EuS. Such a high field is larger than the range any electromagnet can provide, and which usually requires bulky superconducting magnets.”

The hope is that with further research in materials engineering, this MEF might be boosted by another order of magnitude for room-temperature applications, she tells nanotechweb.org.

The researchers obtained their result by measuring the magnetic enhancement in the “Zeeman spin Hall” signal in graphene when it was coupled to EuS. The Zeeman spin Hall effect (ZSHE) occurs when spin current is generated in the presence of an applied magnetic field (the Zeeman field). “Since the total Zeeman field experienced by the graphene electrons is the sum of the external applied magnetic field and the exchange field exerted by a magnetic layer nearby (in our case by EuS), the orders of magnitude enhancement in the ZSHE in the presence of EuS thus reflect a large induced exchange field, and we succeeded in measuring this,” explains Chen.

Quantum information processing applications

And that is not all: the researchers say that the strong magnetic exchange field they observed in these heterostructures might help them reach an unusual quantum regime in which the effect of the Zeeman field considerably exceeds that of the orbital field. “This would result in zero-energy spin-polarized chiral edge modes that are interesting for quantum information processing,” says Chen. “Indeed, we believe that other interesting quantum phases might emerge in heterostructures made from 2D materials and magnetic thin films if we were to engineer interfacial exchange coupling and combine it with spin-orbit coupling.”

As well as studying MEF, the IBM Watson Lab team says that it is also now busy looking into strong spin-orbit coupling materials and how they could be used in spin-based memory and logic. “The next stage in our study is to try and control these two types of magnetic interactions, not only for potential device applications, but also for fundamental physics research,” says Chen.

The team, which includes researchers from the Massachusetts Institute of Technology, Columbia University and Northeastern University in Boston, details its present work in Nature Materials doi:10.1038/nmat4603.