Exploring the properties of magnetic nanomosaics

Magnetic skyrmions – particle-like, stable magnetic vortices that can form in certain materials and have fascinating properties – have been the focus of research for about ten years: Easily electrically controllable and only a few nanometers in size, they are future-proof applications in spin electronics, quantum computers or neuromorphic chips.

These magnetic vortices were initially found in regular grids, so-called skyrmion grids, later individual skyrmions were also observed at the University of Hamburg. Researchers at Kiel University and the University of Hamburg have now discovered a new class of spontaneously generated magnetic lattices.

They are related to skyrmion lattices, but their nanometer-scale “atomic bar magnets” are oriented differently. A fundamental understanding of how such complex spin structures arise, how they are arranged and remain stable is also required for future applications. The results are published in the current issue of nature communication.

Quantum Mechanical Interactions

Attaching magnets to a refrigerator or reading data from a hard drive is only possible through a quantum mechanical exchange interaction between the atomic bar magnets on a microscopic scale. This interaction, discovered by Werner Heisenberg in 1926, not only explains the parallel alignment of atomic bar magnets in ferromagnets, but also the occurrence of other magnetic configurations, such as antiferromagnets.

Many other magnetic interactions are known today, which have led to a large number of possible magnetic states and new research questions. This is also important for skyrmion lattices. Here the atomic bar magnets point in all spatial directions, which is only possible through the competition of different interactions.

“During our measurements, we found a hexagonal arrangement of magnetic contrasts and initially thought that this was also a skyrmion lattice. Only later did it become clear that it could be a nanoscale magnetic mosaic,” says PD Dr. Kirsten von Bergman.

With her team from the University of Hamburg, she experimentally examined thin metallic films made of iron and rhodium using spin-polarized scanning tunneling microscopy. This allows magnetic structures to be imaged down to the atomic scale. The observed magnetic lattices formed spontaneously as with a ferromagnet, ie without an applied magnetic field.

“We can reverse the mosaic lattice with a magnetic field because the opposite spins only partially compensate each other,” explains Dr. André Kubetzka, also from the University of Hamburg.

Surprising: magnetically different alignment

Based on these measurements, Prof. Dr. Stefan Heinze (University of Kiel) carried out quantum mechanical calculations on the high-performance computers of the North German High-Performance Computing Network (HLRN). They show that in the iron films examined, the tilting of the atomic bar magnets in a lattice of magnetic vortices, i.e. in all spatial directions, is very unfavorable. Instead, a nearly parallel or antiparallel orientation of neighboring atomic bar magnets is favored.

“This result completely surprised us. A lattice of skyrmions was therefore no longer an option to explain the experimental observations,” says Mara Gutzeit, doctoral student and first author of the study.

The development of an atomistic spin model made it clear that it must be a novel class of magnetic lattices, which the researchers called “mosaic lattices”. “We found that these mosaic-like magnetic structures are caused by higher-order exchange terms that were only predicted a few years ago,” says Dr. Soumyajyoti Haldar from the Kiel group.

“The study impressively shows how diverse spin structures can be and that close cooperation between experimental and theoretical research groups can be very helpful for their understanding. A few more surprises can be expected in this area in the future,” says Professor Stefan Heinze.