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Friday, 28 May 2004
Speed Limit of Magnetic Recordingsummary written by Jo Stöhr
Two important goals of technology are: smaller and faster. In line with this
goal the 50 billion dollar per year magnetic recording industry has long
wondered where it would run into obstacles set by fundamental physical
principles. The basic questions concern the lateral sizes at which the magnetic
domains that define the "1" and "0" bits become unstable and whether there is a
speed limit for the writing process of the bits. While the size limit of the
bits is well understood because it can readily be tested, exploration of a
potential speed limit has been impeded by the unavailability of magnetic field
pulses that are both ultrashort and sufficiently strong. In today's computers
the switching time is about 1 nanosecond and in R&D laboratories switching
times of about 100 picoseconds (ps) have been achieved. At this speed things
still work fine.
As recently reported in Nature (428, 831-833 [22 April 2004]), an
experiment performed at the Stanford Linear Accelerator Center has now shown
the existence of a fundamental speed limit. The experiment made use of the
unique capabilities of SLAC's 2-mile-long linear accelerator (linac) to produce
ultrafast and strong magnetic field pulses, the fields that surround the
electron beam in the linac. The beam consists of individual electron bunches
that produce field pulses that are the world's shortest, down to about 100
femtoseconds and strongest, up to about 10 Tesla.
By recording the magnetic pattern written by the beam into a magnetic sample
the experiment showed that the switching becomes non-deterministic when the
switching time is shortened to about 5 ps. A new mechanism is operative that
leads to a fracture of the magnetization at very fast time scales and requires
a new theory. This sets the speed limit to only a factor of 20 shorter than
that available in today's most advanced R&D laboratories.
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