May 7, 2004  




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.”



The Stanford Linear Accelerator Center is managed by Stanford University for the US Department of Energy

Last update Tuesday May 04, 2004 by Emily Ball