Miniaturised electromagnets made of ultra-thin carbon

A research team from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) has fired short terahertz pulses at micrometre-sized discs of graphene, thereby briefly turning these miniscule objects into strong magnets. This discovery could prove useful for developing future magnetic switches and storage devices. The working group presented its study in the scientific online journal Nature Communications.

Graphene consists of an ultra-thin sheet of one layer of carbon atoms. But the material displays remarkable properties, including an ability to conduct electricity extremely well. The researchers applied thousands of tiny, micrometre-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 researchers used a particular light source for the experiment: the FELBE free-electron laser that generates intense terahertz pulses. As a result, the tiny graphene discs briefly turned into electromagnets.

“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,” HZDR Physicist Stephan Winnerl said. These were short magnetic pulses, only about 10 picoseconds or one-hundredth of a billionth of a second long.

The researchers polarised the terahertz flashes in a specific way. Specialised optics changed the direction of oscillation of the radiation so that it moved helically through space. When these circularly polarised flashes hit the micrometre-sized graphene discs, the decisive effect occurred: stimulated by the radiation, the free electrons in the discs began to circle, like water in a bucket stirred with a wooden spoon. Because a circulating current always generates a magnetic field, the graphene discs mutated into tiny electromagnets.

Compared to experiments irradiating nanoparticles of gold with light, the experiment at the HZDR was reportedly more efficient. 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. “With our method, the magnetic field does not reverse polarity, as is the case with many other methods. It, therefore, remains unipolar,” Winnerl said.

In other words, during the 10 picoseconds that the magnetic pulse from the graphene discs lasts, the north pole remains a north pole and the south pole a south pole — a potential advantage for certain series of experiments. These miniscule magnets might be useful for certain future technologies: as ultra-short radiation flashes generate them, the graphene discs could carry out fast and precise magnetic switching operations, which could be interesting for magnetic storage technology and 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 speed up the switching processes once again. Graphene discs could conceivably be used as switchable electromagnets to control future spintronic chips.

However, experts would have to invent very small, highly miniaturised terahertz sources for this purpose. “You cannot use a full-blown free-electron laser for this, like the one we used in our experiment. Nevertheless, radiation sources fitting on a laboratory table should be sufficient for future scientific experiments,” Winnerl said.

Image caption: When a circularly polarised light pulse (red) hits a micrometre-sized graphene disc (grey), a magnetic field is created for a fraction of an instant (black lines). Image credit: Lucchesi, Uta (HZDR).