[tt] [Phys. Rev. Focus] Moving Walls with Current

[Christian Weisgerber]

[I'm forwarding this because somebody fed it to eager journalists
 and it is now making the rounds under vastly overhyped headlines
 like "hard disks that don't spin".  It's just a bit of basic
 research at this stage. --naddy]

Physical Review Focus
http://focus.aps.org/story/v19/st14

4 May 2007

Michael Schirber
Moving Walls with Current

   [IMAGE: Fast recall? In the "magnetic racetrack" concept, the bits are
   stored on a magnetic wire and pushed by electric currents across the
   magnetic sensor. New measurements show these bits could move at 110
   meters per second, 100 times faster than in previous experiments]

   Imagine a hard drive that doesn't spin. In one scheme for increasing
   computer data storage and speed, an electric current would push
   magnetic regions along a wire instead of the computer relying on the
   physical motion of a disk to read data. In the 4 May Physical Review
   Letters, a team demonstrates that they could push so-called magnetic
   domain walls at 110 meters per second--100 times faster than ever
   before--by using nanosecond pulses of electric current. But the bad
   news is that the walls sometimes move much slower--or not at all--as
   they become stuck on imperfections in the wire.

   The atoms of a magnet act like tiny bar magnets, each with its north
   pole pointing in the same direction. But a magnetic material can also
   have many regions, each with a different collective alignment of
   atoms. These magnetic "domains" are separated by domain walls, thin
   slices within which the atomic magnets change orientation. Computers
   store data on a hard disk by creating tiny domains magnetized to the
   left for a "one" and to the right for a "zero," for example. Reading
   the data requires spinning the disk underneath a magnetic sensor. But
   this retrieval is very slow compared with integrated circuits, where
   memory is stored electronically. On the other hand, electronic data
   disappears when switched off, while magnetic storage is more
   permanent--and about 100 times less expensive.

   One could have the best of both worlds if magnetic data could be moved
   across the magnetic sensor electronically, rather than mechanically.
   In 2004, Stuart Parkin of IBM Almaden Research Center in San Jose,
   California, patented the magnetic "racetrack" concept, in which the
   bits would slide sequentially down a thin magnetic wire that could be
   tightly coiled inside a chip. In Parkin's concept, the domain walls
   represent the bits, rather than the domains between them--"one" for a
   domain wall; "zero" for no domain wall. The domain wall motion could
   be driven by spin-polarized currents, in which the moving electrons
   have their spins aligned. These currents can rotate the atomic bar
   magnets in the domain wall, creating a sort of domino effect in which
   the wall tumbles forward. Theory predicts the rate at which domain
   walls move, but when real domain walls were subjected to pulses of
   polarized current, experimenters measured velocities 100 to 1000 times
   slower than expected.

   "My feeling is that previous experiments saw slower speeds because
   they measured for too long," says Guido Meier of the University of
   Hamburg. He and his colleagues shortened the length of the current
   pulses from microseconds to nanoseconds to reduce the chances that a
   wall would get stuck on imperfections in the crystalline structure
   during its brief motion.

   The team used a single, 3-micron-wide domain wall confined to a thin
   wire of permalloy, a magnetic material made of iron and nickel that is
   widely used for disk drives. The researchers tracked the location of
   the domain wall with 15 nanometer resolution using polarized x-ray
   images taken before and after the current pulse. They recorded speeds
   of up to 110 meters per second, just as theory suggested.

   However, many of the pulses gave smaller speeds, or no movement at
   all, when the domain walls hit crystal imperfections. The distance a
   domain wall could travel unabated was anywhere from zero to about 1
   micron.

   This is "mixed news for applications," says Mathias Kläui of the
   University of Konstanz in Germany. Ideas like the magnetic racetrack
   will benefit from such high speeds, but the random nature of the
   domain wall jumps "makes reliable switching a challenge," he says.
   Kläui hopes that changing the geometry of the wires in some way might
   give a more stable flow.

Related Information:

   Direct imaging of stochastic domain wall motion driven by nanosecond
   current pulses
   Guido Meier, Markus Bolte, René Eiselt, Benjamin Krüger, Dong-Hyun
   Kim, and Peter Fischer
   Phys. Rev. Lett. 98, 187202
   (issue of 4 May 2007)

-- 
Christian "naddy" Weisgerber                          naddy at mips.inka.de