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Researchers switch magnetic domains from hard to soft

Posted: 01 Oct 2007 ?? ?Print Version ?Bookmark and Share

Keywords:magnetic domain? magnetic field? hard-disk media? ferromagnetic material?

Perpendicular magnetic recordings can now be switched from hard (permanent) to soft (erasable) by way of a discovery recently made at the University College London. Using a designer material with strong anisotropy, the researchers demonstrated how to switch magnetic domains from permanent to erasable by applying a longitudinal magnetic field to modulate the domain walls' strength.

"Our idea for changing a magnet from hard to soft involves applying a small magnetic field orthogonally to the preferred direction for the magnetization," said Gabriel Aeppli, director of the University's London Centre for Nanotechnology. "The magnetic field can be used to modulate the strength of the domain wall pinning, which is what determines whether we have a hard or a soft magnet."

Using an orthogonal magnet to switch a perpendicular domain wall from hard/permanent to soft/erasable can only be performed with certain magnetic formulae with strong anisotropy, such as those used for perpendicular hard-disk media, Aeppli said. "In a disk drive, we talk about 'perpendicular' recording media, where the magnetization is perpendicular to the plane of the disk," he noted.

The London Centre for Nanotechnology currently is protecting its intellectual property rights regarding perpendicular recording in disk-drive media, and consequently, it is keeping the details of Aeppli's discovery as it applies to hard-disk media a trade secret. But Aeppli said engineers will find many other uses for switching magnetics' domain-wall strength. "Our discovery will enable transformer cores with tunable inductances, and permanent magnets whose ability to hold objects can be switched on and off," he said.

Domain wall pattern for a ferromagnet with an average 100nm distance between walls.

Aeppli's research group released photographs of ferromagnet domain-wall patterns spaced just 100nm apart, showing how the walls could be moved by changing the force with which they are pinned. First, the magnetic domain was that of a hard, permanent magnet with strong pinning. After switching, domain walls exhibited the soft pinning of an erasable magnetic domain

Aeppli led an international team of scientists, including U.S. members at the University of Chicago, who together contributed to the discovery that magnetic domains can be switched by subjecting them to an orthogonal magnetic field. The scientists demonstrated that magnetic domains could be tuned through a continuous rangefrom completely hard to completely softat low temperatures. Now, the group is attempting to extend its work by demonstrating the same tunability at higher temperatures.

The group is also experimenting with doping ferromagnetic materials with impurities to alter their magnetic properties, similar to the way semiconductors are doped to tune their electrical properties. Thus far, even trace amounts of impurities have been demonstrated to have a huge effect on the magnetic properties of ferromagnetic materials with strong anisotropy.

- R. Colin Johnson
EE Times

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