Computers that are normally off

Most computer components need a constant power supply. That’s because the core processor that reads, writes, and changes information typically works with electrical charge, stored and manipulated with vast arrays of transistors and other electrical components. Turn off the power and you need to start all over again, except for “nonvolatile” memory that is not directly accessible by the processor. But if components based on magnetic spin could be devised to work closer to the core—say as the main random-access memory or even as on-chip cache memory—then a computer could presumably be “normally off” between operations, even between keystrokes. That approach could lead to mobile personal computers using solar cells or hand-cranked dynamos. Needed are switching speeds as fast as 10 ns, low-power switchability, and densities up to 1 Gbit per chip. Conventional magnetic storage—a hard drive—is not up to that task because of the relatively large and power-hungry electromagnetic coils. Enter spin-transfer torque (STT) technologies, in which an electrical current is spin polarized as it traverses a ferromagnetic tunneling junction, either inducing or detecting the spin orientation on one side of the tunnel barrier. When the current is turned off, the spins remain. Many groups are now exploring STT technologies for nonvolatile computer components. At last November’s 58th Conference on Magnetism and Magnetic Materials, Koji Ando of the National Institute of Advanced Industrial Science and Technology in Tsukuba, Japan, gave a concise summary of the field from his group’s perspective; that summary is now available in print. (K. Ando et al., J. Appl. Phys. 115, 172607, 2014, doi:10.1063/1.4869828 .)