by Staff Writers
Upton NY (SPX) Aug 30, 2012
The evolution of digital electronics is a story of miniaturization - each generation of circuitry requires less space and energy to perform the same tasks. But even as high-speed processors move into handheld smart phones, current data storage technology has a functional limit: magnetically stored digital information becomes unstable when too tightly packed. The answer to maintaining the breath-taking pace of our ongoing computer revolution may be the denser, faster, and smarter technology of spintronics.
Spintronic devices use electron spin, a subtle quantum characteristic, to write and read information. But to mobilize this emerging technology, scientists must understand exactly how to manipulate spin as a reliable carrier of computer code.
Now, scientists at the Department of Energy's (DOE) Brookhaven National Laboratory have precisely measured a key parameter of electron interactions called non-adiabatic spin torque that is essential to the future development of spintronic devices.
Not only does this unprecedented precision - the findings to be published in the journal Nature Communications on August 28 - guide the reading and writing of digital information, but it defines the upper limit on processing speed that may underlie a spintronic revolution.
"In the past, no one was able to measure the spin torque accurately enough for detailed comparisons of experiment and mathematical models," said Brookhaven Lab physicist Yimei Zhu.
"By precisely imaging the spin orbits with a dedicated transmission electron microscope at Brookhaven, we advanced a truly fundamental understanding that has immediate implications for electronic devices. So this is quite exciting."
"One of the big reasons that people want to understand this non-adiabatic spin torque term, which describes the ability to transfer spin via electrical currents, is that it basically determines how fast spintronic devices can be," said Shawn Pollard, a physics Ph.D. student at Brookhaven Lab and Stony Brook University and the lead author of the paper.
"The read and write speed for data is dictated by the size of this number we measured, called beta, which is actually very, very big. That means the technology is potentially very, very fast."
Building a Vortex
The Brookhaven physicists applied a range of high-frequency electric currents to a patterned film called permalloy, useful for its high magnetic permeability. This material, 50 nanometers (billionths of a meter) thick and composed of nickel and iron, was designed to strictly contain any generated magnetic field. Unable to escape, trapped electron spins combine and spiral within the permalloy, building into an observable and testable phenomenon called a magnetic vortex core.
"The vortex core motion is actually the cumulative effect of three distinct energies: the magnetic field induced by the current, and the adiabatic and non-adiabatic spin torques generated by electrons," Zhu said.
"By capturing images of this micrometer (millionth of a meter) effect, we can deduce the precise value of the non-adiabatic torque's contribution to the vortex, which plays out on the nanoscale. Other measurements had very high error, but our technique offered the spatial resolution necessary to move past the wide range of previous results."
Beyond that, magnetic storage suffers from a profound scaling issue. The magnetic fields in these devices exert influence on surrounding space, a so-called fringing field.
Without appropriate space between magnetic data bits, this field can corrupt neighboring bits of digital information by inadvertently flipping "1" into "0." This translates to an ultimate limit on scalability, as these data bits need too much room to allow endless increases in data density.
"It takes less energy to manipulate spin torque parameters than magnetic fields," said Pollard. "There's less crosstalk between databits, and less heat is generated as information is written and read in spin-based storage devices. We measured a major component critical to unlocking the potential of spintronic technology, and I hope our work offers deeper insight into the fundamental origin of this non-adiabatic term."
The new measurement pins down a fundamental limit on data manipulation speeds, but the task of translating this work into practical limits on processor speed and hard drive space will fall to the scientists and engineers building the next generation of digital devices.
Zhu and Pollard collaborated with two physicists specializing in nanomagnetism, Kristen Buchanan of Colorado State University and Dario Arena of Brookhaven's National Synchrotron Light Source (NSLS), to push the precision capabilities of the transmission electron microscope. This research was conducted at Brookhaven Lab's Department of Condensed Matter Physics and Materials Science, and funded by the U.S. Department of Energy's Office of Science.
Brookhaven National Laboratory
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