The largest magnetic anisotropy energy ever has been measured by a team of physicists from Switzerland, Italy, France and Germany. Pietro Gambardella and Harald Brune from the EPFL in Lausanne and colleagues found that cobalt atoms on a platinum substrate can have a magnetic anisotropy energy (MAE) of over 9 milli-electron-volts. The result will lead to a deeper understanding of nanomagnetism and will help in the design of new magnetic materials for use in information storage (P Gambardella et al. 2003 Science 300 1130).
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.
