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There is a good chance that the data storage systems of the coming decades will be based on spintronics. Spintronics is very promising for faster and more energy-efficient information technology, and even for the next steps in miniaturization. An international research team led by Konstanz physicist Elke Scheer has now shown that the Blatter radical could be a likely candidate for spintronic technologies. But what's the story behind that?

From electronics to spintronics
In today's information technology, for example in computers and smartphones, electrons are used to store and control information. Typically, the charge of the electron carries the information: whether a capacitor is charged or not encodes the information ("zero or one"). One problem with our current technology becomes apparent when we want devices to become ever smaller: "Everything that has to do with charge transport generates heat. This prevents further miniaturization", says Elke Scheer. She points out clear physical limits: The smaller the systems become, the more sensitive they are to overheating, which means that not only can the stored information be lost, but the circuits, too, may be destroyed.

If you are now wondering if devices can get smaller at all, molecular spintronics has good news: In spintronics, the information carrier can be as tiny as a single molecule and store the information in the state of a single electron. At the same time, it is not the charge that has to be transported, but a different property of the electron: "Electrons not only have a charge, but, due to their intrinsic angular momentum, also a magnetic moment, known as 'spin'. This spin always has the same size, but can have different directions", Scheer explains.

In spintronics, not only the charge of the electron is used to encode and control information, but also the spin. The advantage: The problems of charge transport, such as heat generation, are avoided. The disadvantage: Spins are highly sensitive and you need suitable materials that can reliably store and reproduce the information content. Researchers are investigating various solid-state materials, and also molecules, to find out which materials are particularly suitable for spintronics applications.

A radical for spintronics
This is where the Blatter radical molecule comes into play. A radical is an atom or molecule with a free electron. It is precisely this free electron that is ideally suited as an information carrier for spintronics. The only problem is that radicals are often very reactive: In most cases, the free electron forms a bond in a fraction of a second, which means that the molecule is no longer a radical, and the information is lost. A radical that is as "stable" as possible and remains a radical even under unfavourable conditions is therefore wanted.

Elke Scheer's research team has now focused on the Blatter radical. This radical has been known since the late 1960s and is a popular model system in chemistry. Elke Scheer describes its advantages: "The Blatter radical is robust and yet versatile. It is the size of a typical molecule of a few nanometres, so it is suitable for miniaturization. Moreover, its reproducibility is high: If you can make one of them, you can make a lot of them".

Elke Scheer and her colleagues from Germany, Belgium, China and the US have now demonstrated in theory and practice that the Blatter radical is well suited for spintronics. They subjected the radical to a strict suitability test, so to speak, and proved that the magnetic information of the Blatter radical is easy to read and can also be controlled externally via a magnetic field. Moreover, it remains stable, does not decompose over time and retains its magnetic degree of freedom even under adverse conditions. "Even though we put it in solution, in single-molecule structure or in contact with metal – all the effects that you normally use to break a radical", says Scheer. The research team suggests the Blatter radical as a model system for further spintronics research and as a promising molecule for spintronics technologies, for example for photodetectors or thermoelectric applications.

Mystery of physics solved
The researchers not only identified the Blatter radical as a suitable material for spintronics, but, "in passing", also solved a puzzle of physics. In numerous experiments over the last few decades, researchers have discovered an unexpected physical effect in electrical circuits with radical molecules, namely very high negative magnetoresistance. Magnetoresistance means that the electrical resistance depends on the magnetic field. This effect was repeatedly observed in single molecule contacts from radicals, but there was no explanation for it.

The Blatter radical now provided the research team with the key to solving this puzzle: Magnetoresistance in these systems is a result of the Kondo effect, which occurs in different "variations". The Kondo effect is the interaction of conduction electrons with magnetic impurities, which can have various effects on charge transport. The "singlet" Kondo effect produces a characteristic current-voltage relationship. In the less common "triplet" variant, however, it can also lead to negative magnetoresistance, as the researchers have now shown.

What is so special in the Blatter radical contacts is that, unlike in most other contacts of the radical molecules investigated, both effects occur, and that they are two variations of the same phenomenon. "For me, this is the essential new insight: finally, we can explain experiments of the past that we did not understand before", concludes Elke Scheer. Close collaboration with colleagues in theoretical physics and chemistry was crucial for this discovery. Based on the new knowledge, the research team now wants to return to experiments from previous years in order to gain a more complete picture.

Key facts:

• Original publication: Gautam Mitra, Jueting Zheng, Karen Schaefer, Michael Deffner, Jonathan Z. Low, Luis M. Campos, Carmen Herrmann, Theo A. Costi and Elke Scheer, Conventional versus Singlet-Triplet Kondo Effect in Blatter Radical Molecular Junctions: Zero-bias Anomalies and Magnetoresistance, published in Chem (Cell Press). Read the article
• Participating institutions: University of Konstanz (Germany), Université Catholique de Louvain (Belgium), Xiamen University (China), University of Hamburg (Germany), Columbia University New York (USA), Forschungszentrum Jülich (Germany)
• Research as part of the Collaborative Research Centre SFB 767 "Controlled Nanosystems: Interaction and Interfacing to the Macroscale" (2008 to 2019)