« Previously: Light pulse flips magnetic memory cell at record speed  

Inspired by the 2007 discovery by Dutch and Japanese scientists showing that the magnetisation of an alloy of a rare earth element, called gadolinium (Gd), with iron (Fe) and cobalt (Co) could be switched using light pulses, University of Minnesota researchers used the alloy to replace the upper magnetic layer of a conventional magnetic tunnel junction. Another modification they made to the device was to use a transparent electrical material called indium tin oxide for the electrode to allow light to pass through it. These layers are stacked into a pillar with a diameter of 10µm, which is only one-tenth the diameter of a typical human hair.

To test their work, researchers sent laser pulses to the modified device using a low-cost laser based on optical fibres that emits ultrashort pulses of infrared light. The pulses are sent one in every microsecond (one millionth of a second), but each pulse is shorter than one trillionth of a second. Every time a pulse hit the magnetic tunnel junction pillar, the scientists observed a jump in the voltage on the device. The change in voltage confirms that the resistance of the magnetic tunnel junction “sandwich” changes each time the magnetisation of the GdFeCo layer is switched. Because each laser pulse lasts less than 1 picosecond, the device is capable of receiving data at a rate of 1 terabit per second.

Mo Li, an associate professor in the University of Minnesota Department of Electrical and Computer Engineering who led the research, said the research holds exciting prospects. “Our result establishes a new means of communication between fibre optics and magnetic devices. While fibre optics afford ultra-high data rate, magnetic devices can store data in a non-volatile way with high density,” he said.

Professor Jian-Ping Wang, director of the Centre for Spintronic Materials, Interfaces and Novel Structures (C-SPIN) based at the University of Minnesota and co-author of the study, also sees great promise. “The results offer a path toward a new category of optical spintronic devices that have the potential to address future challenges for developing future intelligent systems.

“These systems could use spin devices as neurons and synapses to perform computing and storage functions just like the brain, while using light to communicate the information,” Wang said.

The ultimate goal for the research team is to shrink the size of the magnetic tunnel junction to less than 100nm and reduce the required optical energy. To this end, the team is continuing its research, and is currently engaged in optimising the material and structure of the device, and working on integrating it with nanophotonics.

In addition to Li and Wang, postdoctoral associate Junyang Chen and graduate student Li He are lead authors of the paper.

This work was supported by C-SPIN, one of six centres of STARnet, a Semiconductor Research Corporation programme, sponsored by MARCO and the Defence Advanced Research Projects Agency (DARPA) of the U.S. Department of Defence.

« Previously: Light pulse flips magnetic memory cell at record speed