From the earliest batteries through vacuum tubes, solid state, and integrated circuits, electronics has staved off stagnation. Engineers and scientists have remade it repeatedly, vaulting it over one hurdle after another to keep alive a record of innovation unmatched in industrial history.
It is a spectacular and diverse account through which runs a common theme. When a galvanic pile twitches a frog's leg, when a triode amplifies a signal, or when a microprocessor stores a bit in a random access memory, the same agent is at work: the movement of electric charge. Engineers are far from exhausting the possibilities of this magnificent mechanism. But even if a dead end is not yet visible, the foreseeable hurdles are high enough to set some searching for the physics that will carry electronics on to its next stage. In so doing, it could help up the ante in the semiconductor stakes, ushering in such marvels as nonvolatile memories with enormous capacity, ultrafast logic devices that can change function on the fly, and maybe even processors powerful enough to begin to rival biological brains.v A growing band of experimenters think they have seen the future of electronics, and it is spin. This fundamental yet elusive property of electrons and other subatomic particles underlies permanent magnetism, and is often regarded as a strange form of nano-world angular momentum.
Microelectronics researchers have been investigating spin for at least 20 years. Indeed, their discoveries revolutionized hard-disk drives, which since 1998 have used a spin-based phenomenon to cram more bits than ever on to their disks. Within three years, Motorola Inc. and IBM Corp. are expected to take the next step, introducing the first commercial semiconductor chips to exploit spin--a new form of random access memory called M (for magnetic) RAM. Fast, rugged, and nonvolatile, MRAMs are expected to carve out a niche from the US $10.6-billion-a-year flash memory market. If engineers can bring the costs down enough, MRAMs may eventually start digging into the $35 billion RAM market as well.
The sultans of spin say memory will be just the beginning. They have set their sights on logic, emboldened by experimental results over the past two or three years that have shown the budding technologies of spin to be surprisingly compatible with the materials and methods of plain old charge-based semiconductor electronics. In February 2000, the Defense Advance Research Projects Agency announced a $15-million-a-year, five-year program to focus on new kinds of semiconductor materials and devices that exploit spin. It was the same Arlington, Va., agency's largesse of $60 million or so over the past five years that helped move MRAMs from the blackboard to the verge of commercial production.
Now proponents envision an entirely new form of electronics, called spintronics. It would be based on devices that used the spin of electrons to control the movement of charge. Farther down the road (maybe a lot farther), researchers might even succeed in making devices that used spin itself to store and process data, without any need to move charge at all. Spintronics would use much less power than conventional electronics, because the energy needed to change a spin is a minute fraction of what is needed to push charge around.
Other advantages of spintronics include nonvolatility: spins don't change when the power is turned off. And the peculiar nature of spin--and the quantum theory that describes it--points to other weird, wonderful possibilities, such as: logic gates whose function--AND, OR, NOR, and so on--could be changed a billion times a second; electronic devices that would work directly with beams of polarized light as well as voltages; and memory elements that could be in two different states at the same time. "It offers completely different types of functionality" from today's electronics, said David D. Awschalom, who leads the Center for Spintronics and Quantum Computation at the University of California at Santa Barbara. "The most exciting possibilities are the ones we're not thinking about."
Much of the research is still preliminary, Awschalom cautions. A lot of experiments are still performed at cryogenic temperatures. And no one has even managed to demonstrate a useful semiconductor transistor or transistor-like device based on spin, let alone a complex logic circuit. Nevertheless, researchers at dozens of organizations are racing to make spin-based transistors and logic, and encouraging results from groups led by Awschalom and others have given ground for a sense that major breakthroughs are imminent.
"A year and a half ago, when I was giving a talk [and] said something about magnetic logic, before I went on with the rest of my talk I'd preface my statement with, '...and now, let's return to the planet Earth,'" said Samuel D. Bader, a group leader in the materials science division at Argonne National Laboratory, in Illinois. "I can drop that line now," he added.