Magnetic nanoworld

Where does the seemingly magical power that holds a magnet on the refrigerator come from? To answer this question, one has to look deep inside matter. "Imagine an atom with the electrons orbiting the nucleus," says Frithjof Nolting, head of the PSI Laboratory for Condensed Matter and professor at the University of Basel. "This classic picture is wrong from a strictly scientific point of view, but extremely helpful as an aid to thinking about it." That's because it is the electrons that produce a magnetic field. The electrically charged particles move in their orbit around the atomic nucleus and also have their own angular momentum, called spin. These two components generate a magnetic moment that can, in simplified terms, be thought of as a small bar magnet.

"For a material to become externally magnetic, the magnetic moments of the individual atoms inside it must all orient themselves in the same direction – a very complicated mechanism," Nolting explains. If the magnetic moments are all aligned in the same way, this is called ferromagnetism. If neighbouring magnetic moments point in opposite directions, the material is antiferromagnetic and non-magnetic towards the outside. Areas in which the magnetic moments of all atoms align themselves in the same way are called domains; in between there is a domain wall.

With magnetic data storage on hard drives, the information bits are stored in the form of domains in thin layers. Bit by bit, an electromagnet serving as a write head changes the direction of magnetisation. Nolting and his research group found out how you could do without this magnetic field for writing: The direction of magnetisation of small structures can also be switched selectively using a pulse of laser light. This process would be much faster than using an electromagnet and would also consume less energy – an exciting field for future studies at SwissFEL. The physicist warns that applications of this new method are still a long way off. He is particularly interested in the fundamental knowledge that can be gained when researchers penetrate into the nanoworld, into dimensions of a millionth of a millimetre. "Exotic phenomena occur there that lead to the funny properties of these nanomagnets."

Frustrated magnets

One example is the frustrated magnets. If your first thought is that ascribing this human frame of mind to the physical world indicates an inability to achieve a desired state, you're not far off the mark.  But what the experts really mean by this is not easy to explain. Frithjof Nolting uses his hands to help and points up with his right index finger and down with his left. "Assuming the magnetic moments in a material should be aligned antiparallel," he explains, "it works with two elements, as you can see here. But if a third is added in the middle, it doesn't know where to point. It's frustrated."

Frustrated magnets are the specialty of Oksana Zaharko, head of the Solid-State Structures Group at PSI. She picks up Nolting's comparison with the three elements: "We have a system in which two strong forces fight each other. And when two big ones wage war, the little third one profits. This is exactly what happens in frustrated systems." Zaharko's system is a tiny crystal – a pretty, metallic octahedron that can barely be seen by the naked eye, formed from the elements manganese, scandium, and sulphur. Because in this antiferromagnetic crystal the alignments of the magnetic moments conflict with one another, a directional dependency arises, a so-called anisotropy. "That is our laughing third party," Zaharko explains. The anisotropy causes tiny eddies to form in the alignment of the magnetic moments.

Tiny vortices

These wondrous nanostructures are the result of frustration and have a correspondingly exotic name: skyrmions. "In science, some ideas spread like a virus – albeit a good-natured one," says the physicist. "This is the case with the skyrmions. There is intensive research in this field worldwide, and I too was infected by the enthusiasm for these objects with their interesting properties." With their crystal, Zaharko and her group succeeded in 2020 for the first time in creating and demonstrating antiferromagnetic skyrmions – an important step for possible future applications of these nano-vortices in information technology.

Skyrmions are seen as promising units for a new type of data storage. They are significantly smaller than the domains that serve as bits in conventional storage media. This would allow the data to be packed more tightly and to be written and read out more quickly. "Our skyrmions are tiny and meet this requirement particularly well," says Zaharko. In addition, the newly discovered nano-vortices – like the crystal itself – are antiferromagnetic. This means that neighbouring magnetic moments are aligned in such a way that one points upwards and the next points downwards, whereas the moments in previously known skyrmions are parallel. "Antiferromagnetic skyrmions are easier to control because, when electricity is applied, they are less likely to be diverted from their straight path than ferromagnetic skyrmions," the researcher explains. "This is very useful if you want to manufacture a product with them."

Still, this is a long way off. For the tiny eddies to arise, the researchers have to cool their crystal to almost absolute zero and place it in a very strong magnetic field. "We have a millimetre-sized sample, and around it a huge apparatus with a magnet weighing tonnes – an unbelievable contrast," says Zaharko, describing the measuring station at the large-scale research facility SINQ. Here neutrons are shot at the crystal and scattered. From the data obtained in this way, algorithms calculate what it looks like inside the material. "This is how we can detect the skyrmions," says the researcher. "We cannot see them directly."

High-resolution images and movies

Making skyrmions directly visible is something researchers at the Swiss Light Source SLS are able to do. "The focus of our work at SLS is on X-ray microscopy with spatial resolution of up to 20 nanometres," says Jörg Raabe, head of the Microspectroscopy Group. His team produces not only high-resolution images, but also movies: "In some experiments we also work with time resolutions in the range of 100 picoseconds, that is, 100 trillionths of a second," the physicist says. The results reveal how magnetic skyrmions are created and how they move. This is of interest to teams from the UK, Germany, Korea, China, Russia, and the USA. In their experiments at PSI, the researchers have been able to show, among other things, that skyrmions in a material composed of several layers of iridium-cobalt-platinum are stable even without an external magnetic field, which is important for potential applications.

Researchers are also dealing with frustrated magnetism at PSI, investigating materials with the help of the Swiss Muon Source SμS. Muons are unstable elementary particles that are similar to electrons, but are a good 200 times as heavy. If you shoot them into a material, they can act as local probes to explore their magnetic environment. "This method is more sensitive than other techniques by a factor of a hundred to a thousand," says Hubertus Luetkens, group leader in the Laboratory for Muon Spin Spectroscopy. He is convinced: "At PSI we have excellent research opportunities, some of which are unique in the world." The researchers used SμS to examine a crystal, made from the elements cobalt, tin, and sulphur, that exhibits strange magnetic behaviour. At low temperatures the cobalt atoms are oriented ferromagnetically; at higher temperatures an antiferromagnetic orientation becomes more prevalent. The rival magnetic orders influence the electronic behaviour of the materials, and they can be controlled by means of chemical composition, pressure, and external magnetic fields.

Such a material could also prove suitable one day for new types of electronic components. But there is still a long way to go. "In terms of physics, all of these studies are very exciting," says Frithjof Nolting. "In practice, however, this would mean a complete change in technology – a gigantic engineering effort and an extremely high hurdle. But who knows? We've already cleared a lot of hurdles, and sometimes it went faster than expected."

Text: Barbara Vonarburg