Physicists in Germany have developed a new technique for making tiny magnets that involves picking up and placing individual iron atoms using a microscope tip. The nanometre-sized magnets can be made in a range of different shapes, while the same microscope tip can also be used to measure their magnetic properties. After comparing their results with elaborate computer simulations of the nanomagnets, the researchers found deviations that could point to hitherto unknown atomic-scale magnetism effects.
“The assembly technique we used is very similar to the children’s game LEGO,” explains team member Jens Wiebe of Hamburg University. “Our building blocks are iron atoms that are laid on a very clean copper surface, and each block behaves like a small compass needle that can point in one of two directions – up or down. This allows us to assemble magnets the constituent atoms of which can be arranged in a variety of different configurations.”
The researchers were led by Roland Wiesendanger at Hamburg University and included scientists from the Institute for Advanced Simulation in Jülich. They used the sharp tip of a spin-polarized scanning tunnelling microscope to build their nanomagnets. The tip can be positioned with high precision above the iron atoms and is able determine the locations of individual atoms on the copper surface. If the tip is brought close enough to an individual atom, it can be used to “pick” the atom up and move it to another position.
Chains, triplets and flowers
“We can build artificial magnets atom by atom that have a variety of different shapes – such as chains, triplets and ‘flowers’,” says Wiebe. “What is more, the tip of the microscope is coated with a magnetic material, which allows us to measure the magnetization curve of each of the constituent iron atoms within the magnet.”
The team compared its experimental results with theoretical calculations based on the Ising model of magnetism. The researchers found that at low applied magnetic fields, the magnetization curves of chains of assembled iron atoms differs from those predicted by theory. However, at higher fields, the theoretical and experimental magnetic curves agree remarkably well.
“Empirically speaking, the opposite sloping curves we saw for the low-field cases hint at an additional magnetic field acting opposite to the applied magnetic field (B), or the presence of an additional magnetic moment that is coupled antiferromagnetically to the end atoms of the chains,” says Wiebe. “Indeed, we are able to reproduce the low-field anomalies of some chains by considering an additional magnetic field in the Ising model that scales with –B (pointing opposite to B) or by including an additional magnetic moment of around 5 Bohr magnetons antiferromagnetically coupled to the chain ends with an ‘exchange constant’ of about –50 µeV.”
The origin of such an additional magnetic field or moment is currently unknown though, he adds, and only seems to affect linear chains and not more compact nanostructures such as the triplets or the flower shapes.
According to the team, the same technique, if applied to magnets consisting of a larger number of atoms, could help scientists tackle important fundamental questions in magnetism concerning “spin glasses” or “spin liquids”, which are the magnetic states of particular solid-state materials.
The researchers now hope to build novel hard nanomagnets using an appropriate combination of elements from the periodic table.
The research is described in Nature Physics.