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New Speed Limit on Magnetic Switching
By Davide Castelvecchi
The speed of magnetic recording—a crucial factor in a computer’s power
and multimedia capabilities—depends on how fast one can switch a
magnet’s poles. Using SLAC’s linear accelerator, or linac, a team led by
Hans Christof Siegmann (ESRD) and Joachim Stöhr (SSRL) found the
ultimate speed of magnetic switching is at least 1,000 times slower than
previously expected. The collaboration included Ioan Tudosa and
Christian Stamm (both ESRD), Frank King (PE), Alexander Kashuba (Landau
Institute for Theoretical Physics, Moscow) and researchers from Seagate
Technology, the world’s largest manufacturer of hard drives.
“It is also a wonderful illustration of the value of very different
disciplines working together: scientists from a synchrotron light source
using a high energy physics linear accelerator to do an experiment on
magnetism,” said Ray Orbach, Director of the DOE Office of Science.
How Magnetic Field is Created
In a computer hard drive, the writing head hovers over a rapidly
spinning disk. An electric current in the head creates a magnetic field
which records data by magnetizing tiny areas of the disk’s surface. The
disk is coated with a special grainy material that allows only two,
opposite directions representing the 0 or 1 of a basic unit of data, or
bit. High recording speed requires the coating material to switch
magnetic poles quickly enough to reliably record each bit.
The idea came to Siegmann in the mid-1990’s, literally out of a
lightning bolt. He realized that the linac could magnetically record the
same way that lightning leaves a magnetic signature when it strikes a
rock. The experiment relied on the unique capabilities of the linac,
whose electron beam played the role of the electric current in the hard
drive’s writing head. The linac’s electron bunches create magnetic
pulses that are some of the world’s strongest—at up to 10 Tesla, or
200,000 times the strength of the Earth’s magnetic field—and the world’s
briefest, at 2 picoseconds (trillionths of a second).
Researchers shot up to seven electron bunches through samples of
magnetic recording media placed in the FFTB. In the photographs of the
results, the researchers had expected to see dark and light concentric
rings around the focus point of the beam, corresponding to the two
possible magnetizations of the grains. Instead, the pictures showed all
shades of grey, indicating that the grains responded in an apparently
chaotic, or random, way.
A chaotic response was only expected with pulses lasting one femtosecond,
or one thousand times shorter than a picosecond, according to Stöhr. The
team hopes to carry out more systematic experiments in the future. “We
are lucky we’ve gotten the support we had so far,” Stöhr said. “Now we
want to know more.”
SLAC’s Linac Coherent Light Source (LCLS), scheduled to start operating
in 2008, will help researchers gain a better understanding of the
magnetic properties of matter. The LCLS will produce x-ray pulses
lasting just one femtosecond, enabling researchers to take snapshots of
the magnetization process. “We will take images observing not only what
has happened,” says Stöhr, “we will be able to see those processes while
they happen.”
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