Magnetic switch: Terahertz controller

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  • Published: Aug 15, 2013
  • Author: David Bradley
  • Channels: Atomic
thumbnail image: Magnetic switch: Terahertz controller

PSI laser

Christoph Hauri - Photo: Scanderbeg Sauer Photography

A terahertz laser developed at the Paul Scherrer Institute in Switzerland could be used to control the magnetization in a material on a timescale of just picoseconds. The extremely short pulses of electromagnetic radiation allow the magnetization to be changed with only a tiny delay opening up a new route to on-off switching in such materials for microelectronics and computing applications.

"Controlling magnetization dynamics with a femtosecond laser is attracting interest both in fundamental science and in industry because of the potential to achieve magnetic switching at ever faster speeds," says Christoph Hauri, Professor at the Swiss Federal Institute of Technology in Lausanne and head of the PSI laser group, in the current issue of Nature Photonics. The team shone extremely short light pulses from the laser on to a magnetic material, where the magnetic moments - "elementary magnets" - were all aligned in parallel. The light pulse's magnetic field was able to deflect the magnetic moments from their idle state in such a way that they exactly followed the change of the laser's magnetic field with only a minor delay. Of particular relevance is that the laser is phase-stable, which means precise changes within the electrical and magnetic field in each pulse can be defined reliably.

Magnetic memory

While solid state and optical storage media are becoming more and more widespread in computing, improvements to magnetic media could far outstrip those technologies given the right impetus. The PSI team working with colleagues from Université Pierre et Marie Curie in Paris, France, have exploited a powerful laser to control magnetization on the picoseconds timescale. The terahertz radiation does not cause heating in the material being probed by the laser, a major limitation for some uses of infrared, visible light and ultraviolet lasers.

The terahertz laser itself was developed by Hauri's group within the SwissFEL project at the PSI. Until only recently there were few strong terahertz lasers available to researchers. This latest powerful device uses an organic crystal to reduce the frequency of the laser light, according to Hauri. "If we shine on to the crystal using a strong laser with a high frequency, it emits radiation on a terahertz scale," he explains. The technology was developed in partnership with Swiss company Rainbow Photonics AG.

Memories made of this

At the moment, the laser flash is not quite intense enough to cause a full flip of the magnetization, only the dynamics of the process can be observed. However, the team suggests that the work is important milestone for demonstrating the concept of the ultrafast and exact manipulation of magnetism with a laser. Hauri is confident that a complete flip-over of the magnetism can be achieved. "There are tricks to enhance the fields of a weak laser to such an extent that they could switch the magnetization," he suggests. This would also involve selecting a special pulse form and generating a pulse where the magnetic field initially points in one direction weakly, then strongly in the other, then point back in the original direction weakly again. If only the middle strong part of the pulse is strong enough to flip over the magnetization, one could use such pulses to re-magnetize materials. PSI now has the capacity to generate such precisely defined pulses.

The researchers have achieved "a coherent, phase-locked coupling between a high-field single-cycle terahertz transient and the magnetization of ferromagnetic cobalt films, they say. Moreover, the "The magnetic response occurs on the timescale of the stimulus and is thus two orders of magnitude faster than the Larmor precession response."

"Our ultimate goal is to achieve switching of a magnetic domain on a femtosecond timescale by means of the optical magnetic field only," Hauri revealed to SpectroscopyNOW. "By imprinting the laser field properties on to the magnetization dynamics we have now found  a way to control the magnetization domain orientation on an ultrafast timescale. For the ultimate flipping, i.e. for a change of the magnetic domain orientation of 180 degree,  the current laser field was a little bit too low but novel technology will allow this soon."

Related Links

Nature Photonics 2013, online: "Off-resonant magnetization dynamics phase-locked to an intense phase-stable terahertz transient"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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