Graphene consists of an ultra-thin sheet of just one layer of carbon atoms. But the material, which was only discovered as recently as 2004, displays remarkable properties. Among them is its ability to conduct electricity extremely well, and that is precisely what international researchers from Germany, Poland, India, and the USA took advantage of.
They applied thousands of tiny, micrometer-sized graphene discs onto a small chip using established semiconductor techniques. This chip was then exposed to a particular type of radiation situated between the microwave and infrared range: short terahertz pulses.
To achieve the best possible conditions, the working group, led by the UDE, used a particular light source for the experiment: The FELBE free-electron laser at the HZDR can generate extremely intense terahertz pulses. The astonishing result: "The tiny graphene disks briefly turned into electromagnets," reports HZDR physicist Dr. Stephan Winnerl.
"We were able to generate magnetic fields in the range of 0.5 Tesla, which is roughly ten thousand times the Earth's magnetic field." These were short magnetic pulses, only about ten picoseconds or one-hundredth of a billionth of a second long.
Radiation pulses stir electrons
The prerequisite for success: The researchers had to polarize the terahertz flashes in a specific way. Specialized optics changed the direction of oscillation of the radiation so that it moved, figuratively speaking, helically through space.
When these circularly polarized flashes hit the micrometer-sized graphene discs, the decisive effect occurred: Stimulated by the radiation, the free electrons in the discs began to circle - just like water in a bucket stirred with a wooden spoon. And because, according to the basic laws of physics, a circulating current always generates a magnetic field, the graphene disks mutated into tiny electromagnets.
"The idea is actually quite simple," says Martin Mittendorff, professor at the University of Duisburg-Essen. "In hindsight, we are surprised nobody had done it before." Equally astonishing is the efficiency of the process: Compared to experiments irradiating nanoparticles of gold with light, the experiment at the HZDR was a million times more efficient - an impressive increase. The new phenomenon could initially be used for scientific experiments in which material samples are exposed to short but strong magnetic pulses to investigate certain material properties in more detail.
The advantage: "With our method, the magnetic field does not reverse polarity, as is the case with many other methods," explains Winnerl. "It, therefore, remains unipolar." In other words, during the ten picoseconds that the magnetic pulse from the graphene disks lasts, the north pole remains a north pole and the south pole a south pole - a potential advantage for certain series of experiments.
The dream of magnetic electronics
In the long run, those minuscule magnets might even be useful for certain future technologies: As ultra-short radiation flashes generate them, the graphene discs could carry out extremely fast and precise magnetic switching operations. This would be interesting for magnetic storage technology, for example, but also for so-called spintronics - a form of magnetic electronics.
Here, instead of electrical charges flowing in a processor, weak magnetic fields in the form of electron spins are passed on like tiny batons. This may, so it is hoped, significantly speed up the switching processes once again. Graphene disks could conceivably be used as switchable electromagnets to control future spintronic chips.
However, experts would have to invent very small, highly miniaturized terahertz sources for this purpose - certainly still a long way to go. "You cannot use a full-blown free-electron laser for this, like the one we used in our experiment," comments Stephan Winnerl. "Nevertheless, radiation sources fitting on a laboratory table should be sufficient for future scientific experiments." Such significantly more compact terahertz sources can already be found in some research facilities.
Research Report:Strong transient magnetic fields induced by THz-driven plasmons in graphene disks
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