A new study may help engineers design more reliable materials for disk drives. Researchers have taken a closer look at how “magnetic avalanches” happen and why they don't run out of control, wiping disk drives clean. The key, researchers say, may lie in ‘damping.’
Performing even a simple function like correcting an e-mail type-o sets off an “avalanche” of activity. To change bits of information, a magnetic head grazes a tiny patch of disk drive, forcing its polarity, or "spin," to align up or down (and giving it the magnetic equivalent of a one or a zero).
The patch's polarity in many magnetic materials changes in a haphazard series of large and small jumps that physicists liken to an avalanche, though new research shows it often behaves more like an explosion or runaway fire.
"The big advance in this paper is that in previous models of avalanches, the spin just flips from up to down as soon as they apply a magnetic field, and they're done. But that's not the way spin behaves in the real world," Joshua Deutsch, professor of physics at the University of California Santa Cruz and an author of the study, said in a statement.
As the disk drive head nears, each pin wobbles in a widening circle—pointing neither up nor down but somewhere in between—before it settles on its new polarity.
It takes a few nanoseconds for precession to die down. During that brief time, each magnetic field contributes forces that affect the precession of neighboring fields. The combined effects can add up to a wave of energy that topples adjacent pins and spreads across the magnet's surface.
Deutsch and Berger suggested that one of the reasons that avalanches die down is because the magnetic material has an inherent ability to damp out the spin precession. The damping comes from the way the spins interact with their nonmagnetic surroundings like electrons.
Materials with poor damping are susceptible to long-running avalanches, making those with higher damping better candidates for use in disk drives.
"Obviously, disk drive makers have already learned by an enormous amount of ingenuity and trial and error what materials make good disks," Deutsch said. "But now we understand a lot better one of the reasons why: because the materials are good at damping, and we can quantify how damping will stop runaway avalanches. We still can’t calculate their damping, but at least we can measure it."
The research, by Deutsch and Andreas Berger of Hitachi Global Storage Technologies, appeared in the July 13 online edition of Physical Review Letters.