What is magnetoresistance? It’s one more thing graphene does differently Premium

Graphene, a single-atom-thick layer of carbon atoms bonded in a honeycomb pattern, already has many unusual properties.

An artist’s representation of a graphene crystal, showing the arrangement of carbon atoms in a hexagonal pattern. AlexanderAIUS, CC BY-SA 3.0 | Photo Credit: AlexanderAIUS, CC BY-SA 3.0

Researchers in the UK, led by Nobel laureate Andre Geim, have discovered another property of graphene – a single-atom-thick layer of carbon atoms bonded in a honeycomb pattern – that further distinguishes this ‘wonder’ material.

Dr. Geim & co. found that graphene displays an anomalous giant magnetoresistance (GMR) at room temperature.

GMR is the result of the electrical resistance of a conductor being affected by magnetic fields in adjacent materials. It is used in harddisk drives and magnetoresistive RAM in computers, biosensors, automotive sensors, microelectromechanical systems, and medical imagers.

GMR-based devices are particularly used to sense magnetic fields. The new study has found that a graphene-based device, unlike conventional counterparts, wouldn’t need to be cooled to a very low temperature to sense these fields. The finding was published in Nature on April 12.

What is GMR?

An illustration of the circumstance in which GMR appears. The big arrows indicate the direction of the magnetic field. ‘FM’ stands for ferromagnetic material and ‘NM’ for non-magnetic material. | Photo Credit: Guillom, CC BY-SA 3.0

Say a conductor is sandwiched between two ferromagnetic materials (commonly, metals attracted to magnets, like iron). When the materials are magnetised in the same direction, the electrical resistance in the conductor is low. When the directions are opposite each other, the resistance increases. This is GMR.

The magnetoresistance observed in the graphene-based device was “almost 100-times higher than that observed in other known semimetals in this magnetic field range,” Alexey Berdyugin, assistant professor of physics at the National University of Singapore and the paper’s coauthor, told The Hindu by email.

The effect is due to the way electrons in the conductor scatter off electrons in the ferromagnets depending on the orientation of the latter’s spin, which is affected by the direction of the magnetic field.

Conventional GMR devices are cooled to low temperatures to suppress the kinetic energy of their constituent particles, keeping them from deflecting the electrons moving past them. In graphene, the researchers found this suppression unnecessary.

What did the study find?

In their study, the magnetoresistance in monolayer graphene at 27º C held between two layers of boron nitride increased by 110% under a field of 0.1 tesla. To compare, the magnetoresistance in these conditions increases by less than 1% in normal metals.

The magnetoresistivity of monolayer graphene used in the study (solid red line) versus the magnetic field strength. | Photo Credit: https://doi.org/10.1038/s41586-023-05807-0

The team attributed this to the presence of a ‘neutral’ plasma and the electrons’ mobility.

Plasma is usually a gas of charged particles. But in the experiment, the “plasma consists of equal numbers of thermally excited electrons and holes,” Dr. Berdyugin said.

A ‘hole’ is a site where an electron is supposed to be but isn’t, thus behaving as if it is positively charged. The researchers had ‘tuned’ the graphene to have as many electrons as holes. “As a result, the total charge of this plasma is zero” – which is desirable because it stifles an effect that comes in the way of GMR.

Second, the researchers used an “extremely clean” setup and graphene without “any defects”. The electrons in the neutral plasma weren’t scattered by vibrations in the atomic lattice either. Together, the electrons in the material had “anomalously high” mobility at room temperature.

A schematic diagram showing the placement of monolayer graphene and the hexagonal boron nitride layers. | Photo Credit: https://doi.org/10.1038/s41586-023-05807-0

This said, a graphene-based GMR device can’t replace existing devices because the latter have other properties that the former doesn’t. For example, as magnetic fields are applied and removed, the conductor’s resistivity in the two types of devices evolves differently.

“Our device is more robust at high temperatures,” Dr. Berdyugin said. “So it’s possible that [it] will be used in novel applications that require magnetic-field sensing in extreme conditions.”

“People working on graphene like myself always felt that this gold mine of physics should have been exhausted long ago,” Dr. Geim, who along with Konstantin Novoselov was awarded the Nobel Prize for physics in 2010, for their work on graphene, said in a statement. “The material continuously proves us wrong, finding yet another incarnation.”

• GMR is the result of the electrical resistance of a conductor being affected by magnetic fields in adjacent materials.
• GMR-based devices are particularly used to sense magnetic fields.
• In their study, the magnetoresistance in monolayer graphene at 27º C held between two layers of boron nitride increased by 110% under a field of 0.1 tesla.