Laser pulses could speed memory

By Kimberly Patch, Technology Research News

Researchers from the Research Institute for Materials in the Netherlands and Siemens AG in Germany have found a way to switch a magnetic bit more quickly. The potential payoff is faster computer memory.

The key is a method to precisely shape incredibly fast magnetic pulses.

In today's magnetic random access memory (MRAM) chips, electric pulses switch microscopic bits of magnetic material that represent the 1s and 0s of computer information. And it takes a certain amount of time -- the rise time -- for a given electric pulse to impart the intensity needed to switch a bit from the magnetic orientation that represents a 0 to the magnetic orientation that represents a 1. MRAM is a relatively new type of memory that retains data even when the power is turned off.

This rise time is like the upward curve of a graph. The steeper the curve, the faster the rise time; and the faster the rise time, the faster the memory.

The researchers' device produces magnetic pulses with curves so steep the pulses are essentially square, according to Thomas Gerrits, a researcher at the Research Institute for Materials in the Netherlands (NSRIM). The researchers make a square pulse by hitting the two tines of a fork-shaped piece of semiconductor material with ultrafast laser pulses timed slightly apart. The laser pulses produce electric pulses that overlap as the semiconductor tines come together. The overlapping electric pulses, in turn, produce the square magnetic pulse.

The laser pulses used to shape the switching pulse last only 150 femtoseconds, or million billionths of a second, and their paths are only a few microns, or thousands of a millimeter apart, according to Gerrits. The resulting switching pulses showed a rise time of about 10 picoseconds, or trillionths of a second, which means they are capable of switching a bit within 200 picoseconds, according to Gerrits. This is about seven times faster than today's commercial MRAM.

The switching rate of today's MRAM depends on accommodating the relaxation time of the system, or the time it takes to complete a pulse cycle including a longer a rise time, which is on the order of 1.5 nanoseconds, or billionths of a second, said Gerrits.

The researchers' method works because the shaped magnetic pulse matches the intrinsic properties of the magnetic elements, according to Gerrits. "This shaping method is used to control the precessional response of a magnetic system, which can only be achieved by short-rise-time magnetic field pulses," he said. The processional response is the way a bit's magnetic field rotates as it shifts from the 0 orientation to the 1 orientation and vice versa.

A major logistical problem remains in adapting the pulse-shaping method for practical use, said Gerrits. The pump-laser pulses, which last only 150 femtoseconds, are not easy to produce. "There are shoe-box size systems on the market," but the miniaturized versions required for a practical memory chip do not exist yet, he said. "A combination of these laser systems and semiconductors is up to industry," he said.

The researchers' experiment is well designed, said Caroline Ross, an associate professor of materials science and engineering at the Massachusetts Institute of Technology. "This is useful because it would ideally allow you to make a very fast MRAM with precise switching," she said.

The experiment has provided useful information about the way a small magnetic structure switches, said Ross. Using the method in a real device will be difficult, however, she said. "The problem with the method from a device standpoint is that you need to apply a very carefully shaped [magnetic] pulse."

The thin metal interconnects that connect components in real devices have higher resistance to electrical current than the researchers' experimental equipment, said Ross. "Making very sharp and fast current pulses to generate a very sharp field pulse," using standard equipment might be tricky, she said. If it can be done, however, it would significantly improve the future performance of devices like MRAMs, Ross said.

Developing a practical system and integrating it into a working computer would take somewhere between 2 and 10 years, said Gerrits.

Gerrits' research colleagues were Hugo A. M. van den Berg, Julius Hohlfeld, O. Gielkens and Theo Rasing from the Research Institute for Materials and Ludwig Bär from Siemens AG in Germany. They published the research in the August 1, 2002 issue of the journal Nature. The research was funded by the Netherlands Organization for Scientific Research (NWO), Philips Research, and the European Union.

Timeline:   < 10 years
Funding:   Corporate, Government
TRN Categories:  Data Storage Technology; Physics
Story Type:   News
Related Elements:  Technical paper, "Ultrafast Precessional Magnetization Reversal by Picosecond Magnetic Field Pulse Shaping," Nature, August 1, 2002.


December 11-25, 2002

Page One

DNA prefers diamond

Material soaks up the sun

Design links quantum bits

Microscopic mix strengthens magnet

Laser pulses could speed memory


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