Spintronics: Storing Data in Atomic Nuclei Electronic devices use electrical current or electrons, which are negatively charged particles orbiting the nuclei or centers of atoms. Modern computers store data electronically: data are stored as binary “bits” in which zero is represented by “off,” or no electrical charge, and one is represented by “on” or the presence of electrical charge. In spintronics, data are stored by the spins of either electrons or, preferably, atomic nuclei. Spin often is compared with a tiny bar magnet like a compass needle, either pointing up or down – representing one or zero – in an electron or an atom’s nucleus. Nuclear spin orientations live longer, so are better for storing data. The 2010 study by Boehme and colleagues showed that nuclear spins of phosphorus in a silicon semiconductor could control electrical current, but at impractically low temperatures and strong magnetic fields. They had to use the magnetic fields to align spins of phosphorus electrons in the same direction, and then use intense light to transfer the same alignment to the spins of phosphorus nuclei. Then they bombarded the semiconductor with radio waves to reverse the nuclear spins and control the current. Boehme says scientists previously have claimed that current in plastic semiconductors – known formally as pi-conjugated polymers – can be controlled by the nuclear spins in hydrogen. Until the new study, “nobody has ever shown it directly” at room temperature by turning nuclear spins to change an electrical current, he adds.
The New Study In the new experiments, the physicists used magnetic resonance to reverse the nuclear spins in hydrogen isotopes embedded in the OLED, and then were able to detect how the reversed spins caused a change in the electrical current through the OLED. In the first two experiments, Boehme says, the physicists made nuclear spins in a proton and deuterium wiggle in characteristic ways, and were able to read corresponding wiggles in the resulting electrical current. In a third experiment, they flipped the spins back and forth at a rate they wanted instead of at the characteristic frequencies. “It worked,” Boehme says. “This shows you can turn a nuclear spin when you want, and only then the current turns around. We can control a current by controlling nuclear spins.” The researchers measured the current change directly, but not resulting changes in the OLED’s light output – changes so small they aren’t detectable with the naked eye. In both the 2010 and the new studies, the physicists did not read the spins of individual nuclei, but the collective spins of more than 1 million nuclei at a time. The ultimate goal is to be able to read the spins of nuclei individually. “If you want to store information, the highest storage density would be to store information in single nuclear spins,” Boehme says. Since the 2010 study, other physicists have achieved that in phosphorus nuclei, he adds.
Benefits of Spintronics By storing information using both spins and electrical charge, spintronic devices should have greater storage capacity and process data more quickly – although researchers still have years to go to figure out how to connect and process spintronically stored information in futuristic computers, conventional and quantum. “We don’t know if its five years, 50 years or never,” Boehme says. Yet he says spintronics already resulted in today’s terabyte-sized computer hard drives, which use spintronic “read heads” so small that data can be stored more densely. In 2012, Boehme and colleagues showed the same spintronic OLED in the new study works as a “dirt cheap” magnetic field sensor at room temperature without being compromised by degradation. Such sensors may enable more accurate spacecraft navigation systems, he says. Because nuclear spin-controlled electrical current regulates output of light by the OLED, it provides a way to study how to make OLEDs more efficient. OLEDs convert far more electricity into light than incandescent light bulbs, which turn most incoming electricity into heat. But there is much more room for improved efficiency. “Hopefully, OLEDs will become better – use less electricity and produce more light – because we learned here how nuclear spins’ orientation influences how well the OLED works,” Boehme says. “Any sort of efficiency limitation can only be overcome if the mechanism that imposes this limitation is understood.”
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CITATIONS
Science, Sept. 19, 2014; U.S. Department of Energy, DESC0000909; National Science Foundation DMR-1121252