Magnetic anisotropy is one of the most important properties of a magnetic material, and the MAE controls the alignment of the atomic spins that give rise to magnetism in a material. The larger the MAE, the more stable the magnet.

Gambardella and co-workers deposited single cobalt atoms onto a platinum substrate using molecular beam epitaxy, applied a magnetic field of up to 7 tesla, and then measured the magnetisation of the cobalt atoms both parallel and perpendicular to the field. They calculated a MAE value of 9.3 +/- 1.6 meV per cobalt atom, which is about 200 times larger than that of cobalt atoms in a bulk crystal. In comparison, samarium cobalt, a widely used permanent magnet, has a MAE of just 1.8 meV per cobalt atom.

At present, over 100 000 atoms are needed to make a stable magnetic bit for use in a hard disk. As the MAE of cobalt is so high, only a few hundred atoms would be needed for one bit, the researchers say. This would allow the information storage density to be greatly increased.