Scientists generate novel form of magnetism from graphene

11/17/2020 / By Michael Alexander

A “eureka moment” is perhaps the best way to describe what a team of physicists from Stanford University went through after they discovered a property that, at first, only existed as a theory: orbital ferromagnetism.

Orbital ferromagnetism is a form of magnetism caused by the lining up of the electrons’ orbital motions.

According to the scientists led by David Goldhaber-Gordon, they were only trying to check if they could replicate and then build on the results of an earlier groundbreaking experiment conducted by Pablo Jarillo-Herrero of the Massachusetts Institute of Technology in 2018, which resulted in the discovery of superconductivity in graphene.

Graphene is a form of carbon that is considered by many to be a “wonder material” and “the future of technology.”

“We were not aiming for magnetism. We found what may be the most exciting thing in my career to date through partially targeted and partially accidental exploration,” Goldhaber-Gordon, a professor of physics at Stanford, said.

“Our discovery shows that the most interesting things turn out to be surprises sometimes,” he added.

Changes in the process led to the new discovery

As detailed in their study, which was published in the academic journal Science, Goldhaber-Gordon and his group introduced two seemingly unimportant changes during their take on the MIT experiment.

The first change was the inadvertent rotation of one of the protective hexagonal boron nitride lattice layers. This layer was accidentally twisted into near alignment with the main material for the experiment, which was twisted bilayer graphene.

The second change was more deliberate. The Stanford team intentionally overshot the angle of rotation between the two graphene sheets, with the team aiming for 1.17 degrees instead of the 1.1 degrees used by Jarillo-Herrero.

According to the team, they did this based on the fact that twisted graphene sheets tend to settle into smaller angles during the manufacturing process.

“We figured if we aim for 1.17 degrees, then it will go back toward 1.1 degrees, and we’ll be happy,” Goldhaber-Gordon said in a statement.

As a result of their changes, the graphene sheet ended up moving by 1.2 degrees. However, the effects that these changes had on the experiment only became apparent when they decided to test its conductive properties and how it changed as its flat band — a state where there is no electrical charge in the semiconductor — gets filled with or emptied of electrons.

As relayed in their study, first author Aaron Sharpe was pumping electrons into a graphene sample that had been cooled close to absolute zero when they detected the appearance of a large electrical voltage. This voltage was perpendicular to the flow of the current and only appeared when the flat band was three-quarters full.

This phenomenon, known as a Hall Effect, only occurs in the presence of an external magnetic field. However, the researchers noted that the voltage persisted even after the external magnetic field had been switched off.

This anomalous Hall Effect could only be explained if the graphene sample was generating its own internal magnetic field, with the property arising from the coordinated orbital motions of the electrons.

Orbital ferromagnetism in graphene could be used for quantum computing

This is the first known example of a material exhibiting orbital ferromagnetism as a property, the researchers said in their paper. (Related: Quantum computer allows you to see “multiple futures.”)

The Stanford researchers estimate that the magnetic field generated by their twisted graphene sample is a million times weaker than that of a conventional refrigerator magnet.

However, this weakness could be a strength in certain scenarios, such as the creation of memory for quantum computers. According to Goldhaber-Gordon, this is because magnetic bilayer graphene can be switched on with minimal power, meaning that it can be read electronically very easily.

Graphene, according to experts, has the following properties that make it perfect for quantum computing: It is 100 times stronger than the strongest steel. It conducts heat and electricity very efficiently and is nearly transparent.

“The fact that there’s not a large magnetic field extending outward from the material means you can pack magnetic bits very close together without worrying about interference,” Goldhaber-Gordon said.

For similar stories, head over to FutureScienceNews.com.

Sources include:

ScienceDaily.com

QuantaMagazine.org

DigitalTrends.com

Science.ScienceMag.org