21.12.2021

Metal-free radicals show magnetic behaviour

Casu Lab and international partners demonstrate for the first time magnetism in thin and very light films

Scientists from Tübingen, Hamburg, Berlin, Nebraska und Italy have demonstrated magnetism in completely organic thin films for the first time. The Lab of Professor Benedetta Casu at the University of Tübingen and its international partners used a worldwide unique experimental setup to analyze the films, which are more than 1000 times thinner than a human hair, for their magnetic properties. The results show that so-called organic radicals are very promising materials for electronics and quantum technology. The study has been published in the journal Chem (Cell Press).

Magnetism is a property of matter that has been known to mankind for several thousand years, long before these properties could be described in a theory. In our everyday lives, we are used to "classical magnets" made of metals or rare earth alloys, "hard materials" like the magnets that decorate our fridge. 

But there is also a class of materials that have a magnetic moment in the complete absence of metal ions, being composed only of light elements, such as carbon, nitrogen, and oxygen. They are transparent, light, and flexible materials. And they promise lower costs of production and could be sustainably produced: the family of organic radicals. These radicals are organic molecules that carry an unpaired electron, resulting in a permanent magnetic moment: they are therefore materials with permanent magnetic properties, i.e., their magnetic moment is not due to the effect of an external magnetic field. 

To use these radicals in a device, they must be used as a film, i.e., the radical molecules cover a substrate such as SiO2, forming a coating. In the past, magnetism in purely organic radicals was known only for crystals and it was completely unknown in thin films: it was never studied before because this was a huge challenge.

The Casu Lab’s first idea to study magnetism in completely organic thin films, 1000 times thinner than a human hair, dated back to 2010/2011. At that time, the experiment was not yet technically possible. Colleagues at the University of Hamburg developed a high magnetic field machine for ultracold measurements down to 0.1 K, i.e., very close to the absolute zero, and much colder than the temperature of outer space. It was necessary to work using synchrotron light, in an advanced synchrotron such as PETRA III in Hamburg (Germany) where the energy of the light can be scanned very fast in the soft X-ray region during the measurements. 

The first experiment was performed in 2016, the first of a series of dedicated beamlines. The measurements were very time consuming, for example, it took several days to get below 1 K and 100/150 scans on the materials surface had to be measured, each at a new point on the sample, to get a single averaged spectrum (1 sample, 1 temperature, 1 field). The combination of the beamline at PETRA III in Hamburg and the used machine (see picture) is unique and the only facility in the world where it is possible to obtain this result. 

The team used X-ray magnetic circular dichroism (XMCD), an element-specific measurement technique that i.e. makes it possible to identify all elements that form the materials. It based on the absorption of circular polarized X-rays that leads to electronic transitions. It is a well-established method to investigate classical magnets and to identify their magnetic behaviour. It was never used before on purely organic materials. 

The scientists applied a strong magnetic field at very low temperature to align the single magnetic moments in each molecule of their radical films along the direction of the field. The absorption spectra were measured once with left circularly polarized light and once with right circularly polarized light.

“It took a long time to analyse and interpret the data, as this is the first experiment of its kind, but it was very rewarding”, Benedetta Casu says. “We have shown that completely organic, light and transparent radical thin films have long-range magnetism and that their magnetic behaviour can be changed depending on the film production, for example, changing the temperature of the coated substrates during coating. These results open the way to the advent of flexible, light and transparent magnets and, thus, to a variety of new futuristic applications.”

Publication:

T. Junghoefer, A. Calzolari, I. Baev, M. Glaser, F. Ciccullo, E. Giangristomi, R. Ovsyannikov, F. Kielgast, M. Nissen, J. Schwarz, N. M. Gallagher, A. Rajca, M. Martins, M. B. Casu: “Magnetic behavior in metal-free radical thin films”. Chem (Cell Press) https://doi.org/10.1016/j.chempr.2021.11.021

Contact: 

Prof. Dr. Benedetta Casu
University of Tübingen /Casu Lab
+49 7071 29-76252
benedetta.casuspam prevention@uni-tuebingen.de