Although IBM, one of the three major contractors of the original MRAM program, had already been in the spintronics-based data storage business by the mid-1990s with its spin-valve technology, the company continued to push forward storage technology based on other spintronics developments, such as the invention of TMR, the phenomenon that would set Everspin on a pathway to success. Here, a thin insulator replaces the semiconductor between the magnetic layers in a traditional GMR device. This results in a magnetoresistance signal that is 100 times stronger than that of a spin valve. An article in The New York Times on Sept. 11, 2007, credited GMR technology up to that time – more than a decade ago – with making “consumer audio and video iPods, as well as Google-style data centers, a reality.”
“DARPA was largely responsible for launching the fields of semiconductor spintronics and quantum spintronics, the latter of which has also helped drive the emerging area of quantum computing,” he said. “This was high-risk science and, at the time, not obvious that the underlying physics and engineering would work out so well.”
Every new technology harbors the seeds of its own obsolescence. Even during DARPA’s early spintronics programs, researchers knew that in time they would eke out as much technological capability from the spintronic materials and structures they were developing as was physically and practically possible. They knew they would be able to engineer finer and finer magnetic bits of memory in GMR structures, but they also knew that as they did so, those bits would become ever more vulnerable to random switching effects due to thermal fluctuations. The size at which this bit-scrambling limitation would thwart further advances in MRAM technology was at about 50 nanometers (nm), or about twice the diameter of a polio virus. For ambitious memory technologists, even that is too big.
This is where the experimental demonstration in the early 2000s by a DARPA-supported group at Cornell University led by Robert Buhrman of yet another spintronic phenomenon, spin torque transfer, came into the picture. Predicted theoretically by other researchers at IBM and Carnegie Mellon, this phenomenon provided the pathway for overcoming what otherwise could have been an impasse to higher storage densities. If MRAM bit sizes were to shrink beyond sub-viral dimensions, Wolf said, “people knew the switching would have to move from magnetic-field switching to spin-torque switching,” which is based on electric current conditioned so that the spins of its constituent electrons align. That alignment or lack thereof – states that can be controlled – enables the electric current to mimic the effect of a magnetic field, including the ability to exert mechanical torque. And that meant the current could serve as the switching trigger in GMR structures. This opened up clever pathways around an obstacle that was confronting MRAM designers. As magnetic bits shrank, they had to be made of materials that were harder to switch because of the thermal jumbling problem. By turning to the spin torque phenomenon, engineers would be able to apply sufficient switching forces to flip the smaller but tough-to-switch magnetic bits.
Everspin, the company that made the world’s first commercial MRAM chips based in part on technology that emerged from the MRAM program, is now making spin torque transfer MRAM. “So it took several years for MRAM to run out of steam, but then we had a solution to scale beyond that,” said Wolf. “This was a major discovery within our spintronics programs.” Buhrmans’ group also pioneered work in spin torque oscillation, a subtle phenomenon that engineers have yet to exploit but, said Wolf, “may also lead to a revolution in signal processing and computation.” The same “nano-oscillations” could find applications in radiofrequency (RF) filters, minuscule RF sources, and a novel pattern-recognition technique.
Lukaszew envisions that skyrmion-based memory could, in her words, “offer the possibility of 100 times more storage density at 10 times less power, while being equally stable and rad-hard as today’s technology” compared to other spintronics-based memory.
Today, the quest to unveil yet more technological magic in electronic spins continues at DARPA. In 2017, Program Manager Rosa Alejandra “Ale” Lukaszew launched her Topological Excitations in Electronics (TEE) program, where the goal is to find yet new approaches to shrink electron-spin-based magnetic domains while maintaining their resistance to random switching due to thermal fluctuations. Lukaszew has challenged researchers (DARPA calls them “performers”) in the program to focus on skyrmions, which are vortex-shaped multi-electron structures that flip states only as a unit. Those traits bestow the skyrmions with more stability compared to simpler electron organizations. Lukaszew envisions that skyrmion-based memory could, in her words, “offer the possibility of 100 times more storage density at 10 times less power, while being equally stable and rad-hard as today’s technology” compared to other spintronics-based memory. That is the sort of technological leap that could put far more processing power into a soldier’s hands without asking him or her to carry more battery weight. She’s talking SWaP here. Said Lukaszew, “I want to create a community of experts that find materials that can do this.”
Making leaps, rather than taking steps, has always been the DARPA ideal. “DARPA was a huge accelerator” for spintronics, Slaughter said. “It came in just at the right time, just when the scientific developments made it sensible” for well-placed technology developers in the semiconductor sector to move forward on new products like MRAM. The ripples of DARPA’s investment and community building in spintronic continue to expand. “Most of the potential has yet to be realized,” Slaughter said, noting as an example that the world’s top semiconductor foundries are planning on embedding MRAM directly onto processor chips to create “systems on a chip.” Said Slaughter, “MRAM’s best days are to come.”