A High-Speed Digital Storage Called Spintronics
August 29, 2012

A High-Speed Digital Storage Called Spintronics

April Flowers for redOrbit.com - Your Universe Online

The next generation of high-speed digital devices might be just a spin away.

Every generation of digital electronics is smaller than the one before. Miniaturization is the way of the world, with each successive round of circuitry requiring less space and energy to perform the same tasks. No matter how small or how fast they get, today's digital technology has a functional limit. Magnetically stored digital information becomes unstable when it is too tightly packed.

The answer to this limit may be the denser, faster, and smarter technology of spintronics. Spintronics, or spin transport electronics, is an emerging technology using solid-state devices and the intrinsic spin of the electron and its associated magnetic moment.

Spintronic devices use electronic spin to write and read information. To use this new technology to the utmost, though, scientists need to understand exactly how to manipulate spin as a reliable carrier of computer code.

In a paper published in the August 28 issue of Nature Communications, a research team from the Department of Energy's Brookhaven National Laboratory describes their efforts to precisely measure a key parameter of electron interactions called non-adiabatic spin torque that is essential to the future development of spintronic devices. This unprecedented precision guides the reading and writing of digital information and 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.”

The electron features intrinsic quantum variables beyond the charge and flow driving electricity, and most current technology fails to take advantage of this. One of these quantum variable parameters is known as spin direction. Spin direction can be strategically manipulated to function as a high-density medium to store and transmit information in spintronics. However, dense data can mean very little without enough speed to process it efficiently.

“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.”

An apt analogy for the experiment the team created is found in stirring a cup of coffee. The motion of the spoon causes the liquid to spin, rising along the edges and spiraling low in the center. The coffee can't escape the motion through the mug's walls, so the trapped energy generates a cone-like vortex in the center. A similar phenomenon can be produced on magnetic materials to reveal fundamental quantum measurements.

The research team used a patterned film called permalloy, useful for its high magnetic permeability, and applied a range of high-frequency electric currents to it. The permalloy, only 50 nanometers thick and composed of nickel and iron, was designed to strictly contain any generated magnetic field. Like the trapped coffee, the electrons are unable to escape the permalloy. The trapped electron spins combine and spiral within the permalloy, building into an observable an 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.”

Today's computers use high-speed, high-density drives that write information into spinning disks of magnetic materials, using electricity to toggle between magnetic polarity states that correspond to the "1" or "0" of binary computer code. Some intrinsic problems arise with this method of data storage, most notably limits to speed because of the spinning disk. The disk is made more unreliable by moving parts, significant heat generation, and the energy levels needed to write and read information.

Magnetic storage also 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, the fringing field can corrupt neighboring bits of digital information by inadvertently flipping "1" to "0."

IBM's Racetrack memory is a pioneering spintronic prototype which uses spin-coherent electric current to move magnetic domains, or discrete data bits, along a permalloy wire about 200 nanometers across and 100 nanometers thick. The spin of these magnetic domains is altered as they pass over a read/write head, forming new data patterns that travel back and forth along the nanowire racetrack. This process not only yields the prized stability of flash memory devices, but also offers speed and capacity exceeding disk drives.

“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.”

Though the new measurement pins down a fundamental limit on data manipulation speeds, the task of translating this work into practical limits on processor speeds and hard drive space will fall to those 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.