How do Graphene Nanostructures Become Magnetic?


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Graphene Nanostructures (also called Nanographenes) can have very different properties, depending on the shape and orientation of the edges – for example, they can be electrically conductive, semiconductive or insulating. One property, however, has been virtually unattainable: Magnetism.

How is it possible for Graphene to acquire Magnetic Properties?

Empa researchers, together with colleagues from the Technical University of Dresden, Aalto University in Finland, the Max Planck Institute for Polymer Research in Mainz, and the University of Bern, have now succeeded in building a graphene nanostructure that has magnetic properties – and could even be a crucial component for spin-based electronics that work at room temperature.

Graphene is made of carbon atoms, but magnetism is a property rarely associated with carbon. So how is it possible for graphene to acquire magnetic properties? Understanding this requires a foray into the world of chemistry and atomic physics.

In graphene, the carbon atoms are arranged as in a honeycomb lattice. Each carbon atom forms either single or double bonds with its three neighbors. In the case of a single bond, one electron from each atom – a so-called valence electron – bonds with its neighbor; in the case of a double bond, two electrons from each atom bond with its neighbor.

The representation of organic compounds using alternating single and double bonds is known as the Kekulé structure. It is named after the German chemist August Kekulé, who first used this representation in 1865 for the simplest organic compound, benzene.

It follows from Wolfgang Pauli’s quantum mechanical exclusion principle that pairs of electrons in the same orbital must each differ in their direction of rotation – called spin.

“However, in certain structures built from hexagons, it is impossible to find an alternating sequence of single and double bonds that satisfies the bonding requirements of all carbon atoms. In these structures, one electron – or even several – is forced to remain outside, unable to form a bond,” explains Shantanu Mishra, who conducts research on novel nanographenes in Empa’s nanotech@surfaces research group led by Roman Fasel. This phenomenon of involuntarily unpaired electrons is called “topological frustration.”

But what does this have to do with magnetism? The answer lies in the “spins” of electrons. The rotation of an electron around its own axis causes a tiny magnetic field, a magnetic moment. If, as usual, two electrons with opposite spins are in one orbital of an atom, these magnetic fields cancel each other out.

If, on the other hand, an electron is alone in its orbital, the magnetic moment persists – and a measurable magnetic field is the result.
This alone is fascinating. But to use the spin of electrons as circuit elements, we need one more step.

One answer could lie in a structure that looks something like a fly under the scanning tunneling microscope.

Two frustrated Electrons in one Molecule

As early as the 1970s, the Czech chemist Erich Clar, a luminary in the field of so-called polycyclic aromatic hydrocarbons (nanographenes), predicted a special structure known as “Clar’s Goblet.” It consists of two symmetrical halves and is constructed in such a way that one electron in each of the halves must remain topologically frustrated.

However, since the two electrons are nevertheless connected via the molecular structure, they are antiferromagnetically coupled – that is, their spins necessarily point in opposite directions.

In its antiferromagnetic state, the goblet could act as a logic “NOT” gate, i.e., as an inverter: If the spin at the input is reversed, the output is forced to rotate as well.

However, it is also possible to bring the structure into a ferromagnetic state, with both spins in the same direction. To do this, the structure must be excited with a certain energy, called the exchange coupling energy, so that one of the electrons flips its spin.

However, for the gate to remain stable in its antiferromagnetic state, it must not spontaneously switch to the ferromagnetic state. For this to happen, the exchange coupling energy must be higher than the energy released when the gate is operated at room temperature. This is a key requirement for ensuring that a future spintronic circuit (see box) based on graphene nanostructures will also function faultlessly at room temperature.

From Theory to Reality

Until now, however, room-temperature-stable, antiferromagnetic carbon nanostructures had only been predicted in theory. Now, for the first time, researchers have succeeded in producing such a structure in practice and showing that the theory actually corresponds to reality.

“Realizing the structure is challenging because Clar’s Goblet is highly reactive on the one hand, and the synthesis is very complex on the other,” Mishra explains. From a precursor molecule, the researchers were able to realize Clar’s Goblet in ultra-high vacuum on a gold substrate. Using various experiments, the researchers were able to show that it has exactly the predicted properties.

They also found that the exchange coupling energy in this molecule is relatively high at 23 meV – and thus spin-based logic operations could be stable at room temperature. “This represents a small but important step towards spintronics,” says Roman Fasel. The study has just been published in the renowned journal Nature Nanotechnology.

Spintronika

Spintronics – composed of the words “spin” and “electronics” – is a field of research in nanotechnology. Its goal is to create electronics in which information is encoded not with the electrical charge of electrons, as has been the case up to now, but with their magnetic moment caused by the rotation of the electron (“spin”).

This electron spin is a quantum mechanical property – a single electron can therefore not only have a fixed state “spin-up” or “spin-down”, but a quantum mechanical superposition of these two states. Spintronics could thus not only enable further miniaturization of electronic circuits in the future, but could also make electrical switching elements with completely new, previously unknown properties a reality.