“The assembly technique we used is very similar to the children’s game LEGO,” explained team member Jens Wiebe of Hamburg University. “Our building blocks are iron atoms that are laid down 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 whose constituent atoms can be arranged in a variety of different configurations.”

The researchers, led by Roland Wiesendanger, used the sharp tip of a spin-polarized scanning tunnelling microscope to build their nanomagnets. The tip can be positioned with high precision over the iron atoms. When held far from the atoms, it can sense the atoms and so locate where they are situated on the copper surface. And if the tip is brought closer to an individual atom, it can be used to “pick” the atom up and move it to another position.

“In this way, we can build artificial magnets atom by atom that have a variety of different shapes – such as chains, triplets and ‘flowers’,” Wiebe told nanotechweb.org. “What is more, in the microscope we employed, the tip is coated with a magnetic material, which allows us to measure the magnetization curve of each of the constituent iron atoms within the magnet.”

Theory and experiment

The team compared its experimental results directly to theoretical calculations (so-called Ising models) and found that at low applied magnetic fields, the magnetization curves of chains of assembled iron atoms differed from those predicted by theory. However, at higher fields, the theoretical and experimental magnetic curves agreed remarkably well, say the researchers.

“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,” said 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 around –50 microelectronvolts.”

The origin of such a 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.

Fundamental questions

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 Germany researchers now hope to build novel hard nanomagnets using an appropriate combination of elements from the periodic table.

The current work is detailed in Nature Physics.