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Magnetism On The Move

Physicists have observed movements on subatomic scales in a crystal for the first time by watching magnetic domain walls move by as little as half an angstrom (10-10 m). This is 100 times better than the best spatial resolution achieved in previous experiments (K S Novoselov et al. 2003 Nature 426 812)

Many phenomena in condensed-matter physics are explained in terms of the "atomic washboard" energy potential that was introduced by Rudolph Peierls in 1940. This potential contains a series of peaks and troughs in three dimensions, with the peaks coinciding with the positions of the atomic planes in the crystal. However, the Peierls potential had never been probed directly until the latest experiments by Andre Geim and co-workers at the Centre for Mesoscience and Nanotechnology at Manchester University in the UK and the Institute for Microelectronics Technology in Chernogolokva, Russia.

The team studied the movement of the domain walls that separate regions of different magnetic polarization (e.g., north and south) in thin films of yttrium-iron garnet. These domain walls tend to be many atomic layers thick at room temperature. However, at the cryogenic temperatures used in the latest experiments they had thicknesses of only 11 nanometres - which is only about six times larger than the largest spacing between the planes in the yttrium-iron crystal.

Geim and colleagues used submicron Hall probes made from two-dimensional electron gases to follow the movement of the domain walls. These devices are extremely sensitive to small changes in magnetic flux, such as those caused by a small movement of a domain wall, and can be used to measure changes in the position of the wall. They found that the domain walls can become trapped between crystalline planes, and that they move through the crystal in a series of discrete jumps. The size of the smallest jump is equal to the magnetic period of the crystal in the direction of that the domain wall is moving in (about 1.75 nanometres).

However, theorists have predicted that the domain wall should also be able to move while it is trapped in a valley of the Peierls potential. By measuring the AC magnetic susceptibility of the system, the UK-Russia team found that it did indeed move by an average of 0.5 angstroms, although the details of this motion are not yet fully understood. The team has also detected atomic-sized "kinks" in the domain walls.

"The Peierls potential is a textbook phenomenon but it has eluded direct experimental detection for many decades until now," Geim told PhysicsWeb. "A few years ago no one, including myself, would have believed that this was possible," he says. In addition to opening up new avenues of fundamental research in condensed matter physics, the results could also lead to the development of new magnetic materials.

Peter Rodgers is Editor of Physics World