Magnetic overthrow: physicists expose a hidden facet of a familiar phenomenon.
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 National Institute of Standards and Technology, governmental agency within the U.S. Dept. of Commerce with the mission of "working with industry to develop and apply technology, measurements, and standards" in the national interest. (NIST (National Institute of Standards & Technology, Washington, DC, www.nist.gov) The standards-defining agency of the U.S. government, formerly the National Bureau of Standards. It is one of three agencies that fall under the Technology Administration (www.technology. ) 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 wobble /wob·ble/ (wob´'l) to move unsteadily or unsurely back and forth or from side to side. See under hypothesis.
1. 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 tor·ques
A band of feathers, hair, or coloration around the neck.
[Latin torqu , 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 subatomic /sub·atom·ic/ (-ah-tom´ik) of or pertaining to the constituent parts of an atom.
1. Of or relating to the constituents of the atom.
2. torque roughly a century ago, notes Mark Covington of Seagate Research in Pittsburgh.
In 1996, practical-minded theorists John C. Slonczewski of IBM (International Business Machines Corporation, Armonk, NY, www.ibm.com) The world's largest computer company. IBM's product lines include the S/390 mainframes (zSeries), AS/400 midrange business systems (iSeries), RS/6000 workstations and servers (pSeries), Intel-based servers (xSeries) T.J. Watson Research Center in Yorktown Heights, N.Y., and Luc Berger of Carnegie Mellon University Carnegie Mellon University, at Pittsburgh, Pa.; est. 1967 through the merger of the Carnegie Institute of Technology (founded 1900, opened 1905) and the Mellon Institute of Industrial Research (founded 1913). 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 Re`o´ri`ent
a. 1. Rising again.
The life reorient out of dust.
Verb 1. the magnetization of that magnet. The theorists had in mind ultrathin ul·tra·thin
Very thin. 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 A one-way direction of a signal or the molecules within a material pointing in one direction. 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 star·tle
v. star·tled, star·tling, star·tles
1. To cause to make a quick involuntary movement or start.
2. To alarm, frighten, or surprise suddenly. See Synonyms at frighten. . Further observations reported in 1998 by Maxim Tsoi, now of the University of Texas at Austin “University of Texas” redirects here. For other system schools, see University of Texas System.
The University of Texas at Austin (often referred to as The University of Texas, UT Austin, UT, or Texas , 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 What's Going On is a record by American soul singer Marvin Gaye. Released on May 21, 1971 (see 1971 in music), What's Going On reflected the beginning of a new trend in soul music. ," 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 magnetic fields,
n.pl the spaces in which magnetic forces are detectable; created by magnetostrictive ultrasonic scalers to cause the tips of instruments such as ultrasonic scalers to vibrate. generated by other magnets, notes Chia-Ling Chien of Johns Hopkins University Johns Hopkins University, mainly at Baltimore, Md. Johns Hopkins in 1867 had a group of his associates incorporated as the trustees of a university and a hospital, endowing each with $3.5 million. Daniel C. in Baltimore.
Creating these magnetic fields requires bulky components--a disadvantage in computer and other technologies that continue to miniaturize min·i·a·tur·ize
tr.v. min·i·a·tur·ized, min·i·a·tur·iz·ing, min·i·a·tur·iz·es
To plan or make on a greatly reduced scale.
min . What's more, because magnetic fields spread unevenly through space, they're tricky to employ, adds Rippard. The use of polarized currents, instead of magnetic fields, keeps the process neatly within the data bits, notes Chien.
This approach probably won't be applicable to most magnetic technologies because it only works at the nanoscale. That's because the intensity of the required currents is extremely high. Flipping one bit, for instance, requires the equivalent of 5 million amperes of current per square centimeter. By contrast, ordinary household wiring carries current densities that reach only a few hundred amperes per square centimeter.
The high-intensity bit-flipping currents generate much heat. That would be a problem for larger devices, but those spanning mere tens to hundreds of nanometers have high ratios of surface area to volume and so can dissipate heat effectively.
Among researchers quick to embrace the spin-torque approach are the developers of magnetic random access memory, or MRAM--which many data-storage specialists regard as the most promising memory chip of the future (SN: 12/18&25/04, p. 389). Spin torque offers a way to circumvent flaws of conventional MRAM (Magnetic RAM) A non-volatile, random access memory technology that is designed to initially replace flash memory and, potentially, DRAM memory. MRAM uses magnetic, thin film elements on a silicon substrate that can be built on the same chip with the logic circuits. designs, in which currents in pairs of crisscrossing wires generate magnetic fields to flip bits. But fields along single wires can make nearby bits unstable--a problem expected to get worse as memory bits are crammed closer together. On the other hand, spin-torque flipping is predicted to get easier as memory chips shrink.
At the International Electron Devices meeting The International Electron Devices Meeting is an annual conference held alternatively in San Francisco, California and Washington D.C. Established in 1954, IEDM is the world's main forum on advancement in semiconductor and electronic devices. last month in Washington, D.C., a team of researchers from Sony Corp. in Atsugi, Japan, reported the first prototype magnetic-memory chip composed of an array of spin-torque bits. Because spin-torque switching is more efficient than magnetic field methods, the 4-kilobit chip, dubbed the spin-RAM, uses only one-twentieth as much power to flip a bit as does conventional MRAM, the researchers claim.
"This may be a breakthrough in nonvolatile high-density memory for consumer applications," says Tom Bonifield of Texas Instruments in Dallas.
TOPSY TURVY Flipping is not the only gymnastic trick that magnets can perform when hit by spin-polarized currents. They can also make their magnetization directions twirl like dervishes.
That twirling Twirling is any of several artforms, hobbies, or sport and recreational activities accomplished by spinning or rotating the twirled object either for exercise, or in a rhythmic, or otherwise artful manner. , known to physicists as precession, is just unconsummated flipping, says Rippard. It takes place when, in the presence of an external magnetic field, spin currents are not large enough to completely overthrow the previous magnetization. Instead they tip the magnetization direction, or arrow, only partway part·way
To a certain degree or distance; in part: partway to town; not even partway reasonable. toward a reversed configuration, pushing it out of alignment with the external field. That field then forces the arrow to wobble, or precess pre·cess
intr.v. pre·cessed, pre·cess·ing, pre·cess·es
To move in or be subjected to precession.
[Back-formation from precession.]
Verb 1. , as the shaft of an unsteady top does.
Changing the precession speed is as easy as turning a knob: The stronger the current, the faster the twirl. That tunability could prove handy for microwave generators and detectors, which are used in a range of applications, including collision-avoidance and other radar systems and wireless devices such as cell phones, says Frederick B. Mancoff of Freescale Semiconductor in Chandler, Ariz.
Another advantage of the new devices is how rapidly they can oscillate To swing back and forth between the minimum and maximum values. An oscillation is one cycle, typically one complete wave in an alternating frequency. . Already at 35 billion cycles per second, or gigahertz (GHz), the rates are expected to reach 100 GHz, says Mancoff. Higher-frequency signals carry information faster than lower-frequency ones do. Today's cell phones typically operate at 1 to 2 GHz.
Recent results suggest a way to overcome a potential problem with the oscillators. Alone nano-oscillator produces signal strengths of only trillionths to billionths of a watt. That "won't give you enough signal to be useful," notes Jordan A. Katine of Hitachi San Jose (Calif.) Research Center.
Two teams--one composed of Katine, Rippard, and other NIST researchers and the other including Mancoff and his Freescale colleagues--have tested pairs of spin-torque structures. The groups independently observed that oscillations oscillations See Cortical oscillations. of structures separated by only a few hundred nanometers became synchronized. This phase locking resembles the synchronization of such physical and biological systems as pendulums swaying, planets orbiting the sun, and insects signaling.
The experiments, described in two reports in the Sept. 15, 2005 Nature, showed that the joint output of a pair of synchronized oscillators is approximately four times as great as the power of a single oscillator oscillator
Mechanical or electronic device that produces a back-and-forth periodic motion. A pendulum is a simple mechanical oscillator that swings with a constant amplitude, requiring the addition of energy at each swing only to compensate for the energy lost because of air . For larger numbers of oscillators in close proximity, the power should rise as the square of their number, the NIST group predicts. In that case, an array of fewer than a dozen nano-oscillators, occupying only a few square micrometers of a chip, in total, could produce sufficiently strong signals for practical applications.
In work reported in the Aug. 5, 2005 Physical Review Letters Physical Review Letters is one of the most prestigious journals in physics. Since 1958, it has been published by the American Physical Society as an outgrowth of The Physical Review. , the NIST team demonstrated that nano-oscillators synchronize with incoming microwaves, suggesting that the devices might be suitable for making directional receivers and transmitters that can pick up or radiate microwave energy in chosen orientations. The technology "could be a way toward nanoscale wireless communications--for instance, from one chip to another chip in a computer" Rippard says. Chips or cards with fewer wires may be simpler to make, denser, and faster than conventional wired circuits, he adds.
COMING ATTRACTIONS Scientists are considering where, beyond MRAM and oscillators, this new form of electromagnetic muscle is leading.
One possibility is a more exotic form of spin-torque-assisted memory. In a U.S. patent issued in late 2004, MRAM pioneer Stuart S.P. Parkin parkin
Brit a moist spicy ginger cake usually containing oatmeal [origin unknown] of IBM Almaden Research Center The IBM Almaden Research Center, located near San Jose, California, is one of IBM's largest research centers, specializing in both basic research in material science and applied research in computer storage, where many refinements and improvements were made in hard disc drive in San Jose, Calif., suggests adorning semiconductor chip surfaces with vertical U-shaped wires, up to 20 micrometers tall, made of a magnetic material. Each wire could store data bits as neighboring blips of magnetization, arranged like beads on a necklace.
A spin-polarized current passing through the wire could shunt To divert, switch or bypass. the beadlike memory regions back and forth past a read-write head at the U's base. Such a system could achieve data-transfer and storage capabilities that might eventually challenge hard disks, Parkin proposes.
Spin torque might even have a mechanical realization, contends Pritiraj Mohanty of Boston University. He and his colleagues are building a nanoscale I-shaped mechanical balance expected to tilt in response to spin-polarized currents and give a polarized electrical response to being tilted.
Such a "spin battery" could prove very important, Mohanty claims, because a new, electron spin-based approach to information processing, called spintronics, lacks reliable ways to generate spin-polarized currents on demand.
In the Sept. 2, 2005 Physical Review Letters, theorists in Russia, China, and Sweden calculated how much spin current a nanoscale I-beam might detect and generate. "You can get a huge effect--big enough to think about practical devices;' Mohanty says.
Now that engineers of the nanoworld can make flows of torque move in and out of structures at will, many more unexpected twists on technology are bound to follow.