Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

How to induce magnetism in graphene

10.12.2019

Graphene, a two-dimensional structure made of carbon, is a material with excellent mechanical, electronic and optical properties. However, it did not seem suitable for magnetic applications. Together with international partners, Empa researchers have now succeeded in synthesizing a unique nanographene predicted in the 1970s, which conclusively demonstrates that carbon in very specific forms has magnetic properties that could permit future spintronic applications. The results have just been published in the renowned journal Nature Nanotechnology.

Depending on the shape and orientation of their edges, graphene nanostructures (also known as nanographenes) can have very different properties – for example, they may exhibit conducting, semiconducting or insulating behavior.


3D-rendered high-resolution scanning tunneling micrograph of Clar’s goblet.

Empa

However, one property has so far been elusive: magnetism. Together with colleagues from the Technical University in Dresden, Aalto University in Finland, Max Planck Institute for Polymer Research in Mainz and University of Bern, Empa researchers have now succeeded in building a nanographene with magnetic properties that could be a decisive component for spinbased electronics functioning at room temperature.

Graphene consists only of carbon atoms, but magnetism is a property hardly associated with carbon. So how is it possible for carbon nanomaterials to exhibit magnetism? To understand this, we need to take a trip into the world of chemistry and atomic physics.
The carbon atoms in graphene are arranged in a honeycomb structure.

Each carbon atom has three neighbors, with which it forms alternating single or double bonds. In a single bond, one electron from each atom – a so-called valence electron – binds with its neighbor; while in a double bond, two electrons from each atom participate.

This alternating single and double bond representation of organic compounds is known as the Kekulé structure, named after the German chemist August Kekulé who first proposed this representation for one of the simplest organic compound, benzene.

The rule here is that electron pairs inhabiting the same orbital must differ in their direction of rotation – the so-called spin – a consequence of the quantum mechanical Pauli’s exclusion principle.
"However, in certain structures made of hexagons, one can never draw alternating single and double bond patterns that satisfy the bonding requirements of every carbon atom.

As a consequence, in such structures, one or more electrons are forced to remain unpaired and cannot form a bond," explains Shantanu Mishra, who is researching novel nanographenes in the Empa nanotech@surfaces laboratory headed by Roman Fasel. This phenomenon of involuntary unpairing of electrons is called "topological frustration" (Figure 1).

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

If, however, an electron is alone in its orbital, the magnetic moment remains – and a measurable magnetic field results.
This alone is fascinating. But in order to be able to use the spin of the electrons as circuit elements, one more step is needed. One answer could be a structure that looks like a bow tie under a scanning tunneling microscope.

Two frustrated electrons in one molecule
Back in the 1970s, the Czech chemist Erich Clar, a distinguished expert in the field of nanographene chemistry, predicted a bow tie-like structure known as "Clar's goblet" (Figure 1).

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 connected via the structure, they are antiferromagnetically coupled – that is, their spins necessarily orient in opposite directions.

In its antiferromagnetic state, Clar's goblet could act as a "NOT" logic gate: if the direction of the spin at the input is reversed, the output spin must also be forced to rotate.

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

In order for the gate to remain stable in its antiferromagnetic state, however, it must not spontaneously switch to the ferromagnetic state. For this to be possible, the exchange coupling energy must be higher than the energy dissipation when the gate is operated at room temperature.

This is a central prerequisite for ensuring that a future spintronic circuit based on nanographenes can function faultlessly at room temperature.

From theory to reality

So far, however, roomtemperature stable magnetic carbon nanostructures have only been theoretical constructs. For the first time, the researchers have now succeeded in producing such a structure in practice, and showed that the theory does correspond to reality.

"Realizing the structure is demanding, since Clar's goblet is highly reactive, and the synthesis is complex," explains Mishra. Starting from a precursor molecule, the researchers were able to realize Clar’s goblet in ultrahigh vacuum on a gold surface, and experimentally demonstrate that the molecule has exactly the predicted properties.

Importantly, they were able to show that the exchange coupling energy in Clar’s goblet is relatively high at 23 meV (Figure 2), implying that spinbased logic operations could therefore be stable at room temperature. "This is a small but important step toward spintronics," says Roman Fasel.

((Spintronics))

Spintronics – composed of the words "spin" and "electronics" is a field of research in nanotechnology. The aim is to create electronics in which information is not coded with the electrical charge of electrons, as is the case in conventional semiconductor circuits, but with their magnetic moment caused by the rotation of the electron ("spin"). The electron spin is a quantum mechanical property – a single electron can have not only a fixed state "spin up" or "spin down", but a quantum mechanical superposition of these two states. In the future, spintronics could therefore not only enable further miniaturization of electronic circuits, but could also make electrical switching elements with completely new, previously unknown properties a reality.

Wissenschaftliche Ansprechpartner:

Shantanu Mishra
nanotech@surfaces
Phone +41 58 765 4839
shantanu.mishra@empa.ch

Prof. Dr. Roman FaseL
Head of nanotech@surfaces Laboratory
Phone +41 58 765 4348
roman.fasel@empa.ch

Originalpublikation:

S. Mishra, D. Beyer, K. Eimre, S. Kezilebieke, R. Berger, O. Gröning, C. A. Pignedoli, K. Müllen, P. Liljeroth, P. Ruffieux, X. Feng and R. Fasel, Topological frustration induces unconventional magnetism in a nanographene, Nat. Nanotechnol (2019).

Weitere Informationen:

https://www.empa.ch/web/s604/topological-frustration

Karin Weinmann | Empa - Eidgenössische Materialprüfungs- und Forschungsanstalt

More articles from Materials Sciences:

nachricht Miniature double glazing: Material developed which is heat-insulating and heat-conducting at the same time
17.01.2020 | Max-Planck-Institut für Polymerforschung

nachricht 3D Printing: New high-Tech Device for Bremen Material Scientists
16.01.2020 | Universität Bremen

All articles from Materials Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Miniature double glazing: Material developed which is heat-insulating and heat-conducting at the same time

Styrofoam or copper - both materials have very different properties with regard to their ability to conduct heat. Scientists at the Max Planck Institute for Polymer Research (MPI-P) in Mainz and the University of Bayreuth have now jointly developed and characterized a novel, extremely thin and transparent material that has different thermal conduction properties depending on the direction. While it can conduct heat extremely well in one direction, it shows good thermal insulation in the other direction.

Thermal insulation and thermal conduction play a crucial role in our everyday lives - from computer processors, where it is important to dissipate heat as...

Im Focus: Fraunhofer IAF establishes an application laboratory for quantum sensors

In order to advance the transfer of research developments from the field of quantum sensor technology into industrial applications, an application laboratory is being established at Fraunhofer IAF. This will enable interested companies and especially regional SMEs and start-ups to evaluate the innovation potential of quantum sensors for their specific requirements. Both the state of Baden-Württemberg and the Fraunhofer-Gesellschaft are supporting the four-year project with one million euros each.

The application laboratory is being set up as part of the Fraunhofer lighthouse project »QMag«, short for quantum magnetometry. In this project, researchers...

Im Focus: How Cells Assemble Their Skeleton

Researchers study the formation of microtubules

Microtubules, filamentous structures within the cell, are required for many important processes, including cell division and intracellular transport. A...

Im Focus: World Premiere in Zurich: Machine keeps human livers alive for one week outside of the body

Researchers from the University Hospital Zurich, ETH Zurich, Wyss Zurich and the University of Zurich have developed a machine that repairs injured human livers and keep them alive outside the body for one week. This breakthrough may increase the number of available organs for transplantation saving many lives of patients with severe liver diseases or cancer.

Until now, livers could be stored safely outside the body for only a few hours. With the novel perfusion technology, livers - and even injured livers - can now...

Im Focus: SuperTIGER on its second prowl -- 130,000 feet above Antarctica

A balloon-borne scientific instrument designed to study the origin of cosmic rays is taking its second turn high above the continent of Antarctica three and a half weeks after its launch.

SuperTIGER (Super Trans-Iron Galactic Element Recorder) is designed to measure the rare, heavy elements in cosmic rays that hold clues about their origins...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

11th Advanced Battery Power Conference, March 24-25, 2020 in Münster/Germany

16.01.2020 | Event News

Laser Colloquium Hydrogen LKH2: fast and reliable fuel cell manufacturing

15.01.2020 | Event News

„Advanced Battery Power“- Conference, Contributions are welcome!

07.01.2020 | Event News

 
Latest News

A new 'cool' blue

17.01.2020 | Life Sciences

EU-project SONAR: Better batteries for electricity from renewable energy sources

17.01.2020 | Power and Electrical Engineering

Neuromuscular organoid: It’s contracting!

17.01.2020 | Life Sciences

VideoLinks
Science & Research
Overview of more VideoLinks >>>