Creating and deleting magnets with a laser

An international team of scientists have found a way to write and delete magnets in an alloy using a laser beam – opening up new possibilities in the fields of material processing, optical technology, and data storage.

The researchers from Germany and the US studied the effect in an alloy of iron and aluminum. It was considered a useful prototype material as subtle changes to its atomic arrangement can completely transform its magnetic behavior.

Rantej Bali, a physicist working on the project from the Helmholtz-Zentrum Dresden-Rossendorf said: “The alloy possesses a highly ordered structure, with layers of iron atoms that are separated by aluminum atomic layers. When a laser beam destroys this order, the iron atoms are brought closer together and start to behave like magnets.”

Bali and his team prepared a thin film of the alloy which a laser beam was irradiating the film. When they directed a well-focused laser beam with a pulse of 100 femtoseconds at the alloy, a ferromagnetic area was formed. Shooting laser pulses at the same area again – this time at reduced laser intensity – was then used to delete the magnet.

Working with colleagues from the University of Virginia in Charlottesville the team were able to clarify what happens in the alloy during this process. It was found that the ferromagnetic state is formed when the laser pulse heats melts the thin-film material. As the alloy cools down it enters a “supercooled liquid” state where it remains molten, despite the temperature having dropped below the melting point. This state is reached because of a lack of nucleation sites – microscopic locations where the atoms can begin to arrange into a lattice. The atoms in the supercooled state seek to form a solid, and the iron and aluminum atoms end up trapped in random positions within the lattice. It is this random arrangement of atoms that renders the material magnetic.

To delete this magnetization, the weaker laser shot melts only thin layers of the film, creating a molten pool sitting on the solid alloy. With the lattice already formed and the temperature still being high enough, the atoms possess sufficient energy to diffuse through the lattice and separate into layers of iron and aluminum.

The team say the work, published in ACS Applied Materials & Interfaces, shows remarkably strong changes to the material property and may well lead to some interesting applications.