Hard disk drives work by storing data in small magnetic domains on a thin film but their capacity is limited by how small the domains can be made. Indeed, when domains become smaller than several tens of nanometres across, thermal fluctuations cause the direction of magnetization to change in a random fashion. The resulting “superparamagnetism” makes it impossible to store data.

Some physicists believe this “superparamagnetic limit” could be beaten by storing data on tiny nano-particles comprising as few as one magnetic atom surrounded by non-magnetic atoms. Now, Andreas Heinrich and colleagues at IBM’s Almaden Research Center in California have made an important step in this direction by placing a single atom on the surface of a copper-nitride film so that its magnetic moment points along one direction. According to Heinrich, the team are the first to see this magnetic anisotropy in just one atom.

By using the STM to study the atom and its surroundings, the researchers concluded that the anisotropy was caused by interactions between the magnetic atom and its neighbours.

Heinrich told physicsworld.com that the effect could someday be used to create a device with a storage density 1000 times greater than the hard drives of today.

However, he cautions that much more research will needed before such a device could be built. For example, team has yet to show that data could actually be stored and retrieved using a single atom. They are currently studying a number of different magnetic atoms and substrates to find a system in which data could be read and written using the tip of a STM.

Another problem is that the measurements were made at very low temperatures of about 0.5 K, which would be impractical for commercial data storage systems. Heinrich said that the team would continue to work at very low temperatures while perfecting their read and write techniques.

Molecular logic
In the same issue of Science another team at IBM report a breakthrough in the use of single molecules as logic devices, which could someday be used to make extremely small and powerful computer chips. Peter Liljeroth and colleagues at IBM’s Zurich Research Laboratory showed that a single molecule of naphthalocyanine can be switched between “on” and “off” states without affecting its shape. While researchers at IBM and elsewhere have already switched single molecules, those molecules changed their shape and would therefore be unsuitable for building logic gates for computer chips or memory elements. The team is now trying combine a number of the molecules to create a logic device (Science 317 1203).