Magnetic overthrow: physicists expose a hidden facet of a familiar phenomenon

Science News, Jan 7, 2006 by Peter Weiss

While conducting experiments for his physics Ph.D. in the early 1990s, Dan Ralph suddenly found himself in unfamiliar terrain without a compass. Examining nanoscale sandwiches of magnetic and nonmagnetic materials in a Cornell University lab, Ralph discovered that voltages caused by electric currents passing perpendicularly through these layers would sometimes increase abruptly for no apparent reason. "Something kind of drastic was going on,' he recalls.

Ralph wrote up the bizarre results as a small part of his doctoral thesis. "I speculated all sorts of things in my thesis. It turns out all of those were wrong" he says.

Although Ralph moved on to postdoctoral studies elsewhere, his graduate adviser wasn't about to let the matter rest. "I didn't have a good idea what was going on except that it was very interesting,' recalls Robert A. Buhrman. He urged other students to look at the nanostructures that Ralph had investigated, and he eventually also drew Ralph, now a Cornell physics professor, back into the studies.

Today, as a result of some clever theorizing and years of experimental work at many labs, physicists have a good idea of what was going on. They're now beginning to investigate under what conditions and for what applications magnets can respond to electricity in ways that no one recognized a dozen years ago.

"Ever since the discovery of magnetism, the only way known to change the direction of how a magnet points was to apply a magnetic field," notes William H. Rippard of the National Institute of Standards and Technology (NIST) in Boulder, Colo. Now, he says, research is exploring an "entirely new way" to influence magnetic behavior.

The approach relies on what physicists call the spin-torque, or spin-transfer, effect. In a nutshell, a swarm of electrons can make a magnet's polarity reverse or wobble because each electron has its own intrinsic magnetism, called its spin.

In the past few years, scientists have begun to demonstrate that the new effect could have big commercial payoffs. Some researchers have already harnessed the effect in a prototype magnetic digital memory, which may someday be a contender against, for instance, the flash memory in digital cameras and other electronics. Others have made tiny microwave beacons that can coordinate their signals in a manner reminiscent of crickets and fireflies synchronizing nightly chirrups and blinks. These developments may lead to smaller, faster, and more energy-thrifty devices for data storage, wireless communications, and information processing.

FLIP ANSWER An electron can be thought of as a tiny bar magnet whose north pole can point in any direction. Familiar magnets are objects in which multitudes of electron spins line up in one direction.

Electron spins can exert rotational forces, or torques, on each other, much as arm wrestlers create torques as they push against each other. Basic theoretical and experimental research by several scientists, including Albert Einstein, led physicists to recognize subatomic torque roughly a century ago, notes Mark Covington of Seagate Research in Pittsburgh.

In 1996, practical-minded theorists John C. Slonczewski of IBM T.J. Watson Research Center in Yorktown Heights, N.Y., and Luc Berger of Carnegie Mellon University in Pittsburgh independently proposed a novel twist on the phenomenon: If electron spins are aligned within an electric current flowing through a magnet, they might exert torques large enough to reorient the magnetization of that magnet. The theorists had in mind ultrathin sandwiches of magnetic and nonmagnetic metals similar to the oddly behaving structures that Ralph and other scientists had studied.

Physicists already knew that the spins of electrons in an electric current initially point in random directions, but as the current passes through a magnet, the spins take on the magnet's orientation. Slonczewski and Berger now proposed the reverse effect: A polarized current could force its orientation onto a magnet.

The notion that the electrons in the current might take the leading role in this wrestling match was startling. Further observations reported in 1998 by Maxim Tsoi, now of the University of Texas at Austin, and other physicists in France, Russia, and the United States seemed to bear out the theorists' proposals. "People started to say, 'Hey, maybe that's what's going on," Buhrman recalls.

By 1999, experimenters had confirmed the theorists' prediction. Their tests demonstrated that a polarized current that has passed through one magnetized layer and then flows through another can coerce the magnetization of the second layer to swing around to the same direction as the first. Once this alignment occurs, moreover, the polarized current flows more easily through the second layer than it had previously. Similar effects had produced the peculiar voltage hops observed by Ralph in his experiments of years earlier.

In data-storage devices, the magnetization of a layer represents a bit of digital information--a zero or a one. Flipping the magnetization therefore changes the bit's value. In the write heads of hard disk drives and in some other established technologies, bit flipping relies on magnetic fields generated by other magnets, notes Chia-Ling Chien of Johns Hopkins University in Baltimore.