Disk data loss caused by magnetic avalanches

Atom spin direction changes can affect neighbouring atoms and alter bit value

Scientists have discovered that "avalanche" effects in spinning magnetic fields can cause data loss in disk drives - and are working on changes to recording layer chemistry that should give us more reliable storage.

Spinning magnetic fields can cause avalanches or ripples across an area of the disk's surface, according to two physicists, Joshua Deutsch, a physics professor at the Santa Cruz campus of the University of California; and Andreas Berger of Hitachi Global Storage Technologies, who reported the effect in the US Physical Review Letter on July 13th.

When a disk read/write head is above an area of disk and writes a one or zero, it alters the magnetic polarity of the atoms in the bit underneath the head. This polarity is actually a spinning direction of the atoms.

According to them, the spin change is not a simple switch-over, as was previously thought. Instead each atom wobbbles as it spins, much like the Earth wobbles on its axis as it spins throughout the year, a phenomenon known as precession, and one that causes the seasons.

The atoms undergo spin precession, which only lasts a few nanoseconds and is then stopped by the atoms settling into their new spin direction. But while exhibiting spin procession they can exert enough force on neighboring atoms to change their spin direction also.

Thus an avalanche-like propagation of spin precession occurs before it is damped out by the magnetic recording material, as the distance from the write head increases.

The researchers suggest that recording material with a better-formulated chemistry could increase the damping effect and reduce or stop the spin precession avalanche effect. This would reduce disk drive data loss from the effect.

Such data loss is not a common occurrence but the probability of it happening increases as disk drive capacity increases, meaning a greater areal density and more closely-packed and smaller magnetic areas (bits) on the disk surface.

The scientists hope to ensure more reliable disk drives, by measuring the damping ability of potential materials for the magnetic recording layer, as well as their other magnetic properties.

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Chris Mellor

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