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All-Optical Switching Promises THz Hard Drives

AMES, Iowa, April 5, 2013 — Magnetic memory technology could become at least 1000 times faster with an ultrashort laser pulse technique that switches magnetism — a method used to encode information in hard drives, magnetic random access memory and other computing devices. The discovery could pave the way for terahertz hard drives, an international study has found.

Physicist Jigang Wang of the US Department of Energy’s Ames Laboratory and colleagues from Iowa State University used short laser pulses to create ultrafast changes in the magnetic structure — within femtoseconds — from antiferromagnetic to ferromagnetic ordering in colossal magnetoresistive materials, which are promising for use in next-generation memory and logic devices. A group led by Ilias E. Perakis at the University of Crete in Greece developed the theory to explain the observation.

“The challenge facing magnetic writing, reading, storing and computing is speed, and we showed that we can meet the challenge to make the magnetic switches think ultrafast in the femtosecond range by using quantum ‘tricks’ with ultrashort laser pulses,” said Wang, an assistant professor of physics and astronomy at Iowa State.


Magnetic structure in a colossal magnetoresistive manganite is switched from antiferromagnetic to ferromagnetic ordering during about 100 fs laser pulse photoexcitation. With time so short and the laser pulses still interacting with magnetic moments, the magnetic switching is driven quantum-mechanically — not thermally. This could open the door to terahertz and faster memory writing/reading speeds. Images courtesy of the US Department of Energy’s Ames Laboratory.

Magnetic field or continuous laser light is used in current magnetic storage and magneto-optical recording technology; for example, photoexcitation causes atoms in ferromagnetic materials to heat up and vibrate, and the vibration, with the help of a magnetic field, causes magnetic flips. The flips are part of the process used to encode information.

“But the speed of such thermal magnetic switching is limited by how long it takes to vibrate the atoms and by how fast a magnetic field can reverse magnetic regions,” Wang said. “And it is very difficult to exceed the gigahertz switching speed limit of today’s magnetic writing/reading technology.”

So, some researchers have turned to colossal magnetoresistive (CMR) materials because they are highly responsive to the external magnetic fields used to write data into memory, yet do not require heat to trigger magnetic switching.

“Colossal magnetoresistive materials are very appealing for use in technologies, but we still need to understand more about how they work,” Wang said. “And, in particular, we must understand what happens during the very short periods of time when heating is not significant and the laser pulses are still interacting with magnetic moments in CMR materials. That means we must describe the process and control magnetism using quantum mechanics. We called this ‘quantum femto-magnetism.’ ”

The investigators used ultrafast spectroscopy, which Wang likens to high-speed strobe photography, because both use an external pump of energy to trigger a quick snapshot that can be replayed afterward.

“In one CMR manganite material, the magnetic order is switched during the 100-femtosecond-long laser pulse. This means that switching occurs by manipulating spin and charge quantum-mechanically,” he said. “In the experiments, the second laser pulse ‘saw’ a huge photoinduced magnetization with an excitation threshold behavior developing immediately after the first pump pulse.”


Jigang Wang (center) and his team, Tianqi Li (left) and Aaron Patz (right), specialize in ultrafast spectroscopy, which helps scientists understand changes in materials in very short time scales.

The fast switching speed and large magnetization observed in the experiment meet both requirements for applying CMR materials in ultrafast, terahertz magnetic memory and logic devices, the investigators say.

“Our strategy is to use all-optical quantum methods to achieve magnetic switching and control magnetism. This lays the groundwork for seeking the ultimate switching speed and capabilities of CMR materials, a question that underlies the entire field of spin-electronics,” Wang said. “And our hope is that this means someday we will be able to create devices that can read and write information faster than ever before, yet with less power consumed.”

The research, supported by the DOE’s Office of Science and by the National Science Foundation, appeared in Nature (doi: 10.1038/nature11934).

For more information, visit: www.ameslab.gov



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