Scientists at Brookhaven National Laboratory have accurately measured an important factor of electron interactions called non-adiabatic spin torque. This finding, essential to the future development of spintronic devices, defines the upper limit on processing speed that may underlie a spintronic revolution. This unprecedented precision also guides the reading and writing of digital information.
The advancement of digital electronics is a study in miniaturization. Every generation of circuitry uses less space and energy to perform the same tasks. Nevertheless, even as high-speed processors move into handheld smartphones, existing data storage technology has a functional limit.
Magnetically stored digital information becomes unstable when it is packed too tightly. The answer to keeping the dizzy pace of the ongoing computer revolution may be the denser, faster, and smarter technology of spintronics.
Measuring Spin Torque
Spintronics involves using electron spin, a delicate quantum characteristic, to write and read information. In order to marshal this emerging technology, scientists must comprehend exactly how to manipulate spin as a reliable carrier of computer code.
“In the past, no one was able to measure the spin torque accurately enough for detailed comparisons of experiment and mathematical models. 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,”
said Yimei Zhu, a physicist at Brookhaven Lab.
Non-adiabatic Spin Speed
Most current technology doesn’t take complete advantage of the electron, which features intrinsic quantum variables beyond the charge and flow driving electricity. One of these, a parameter known as spin direction, can be manipulated strategically to function as a high-density medium to store and transmit information in spintronics.
But as any computer scientist can attest, 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. 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,”
explains Shawn Pollard, a physics Ph.D. student and lead author of the paper.
Permalloy Magnetic Vortex
Imagine coffee being rapidly stirred in a cup. The motion of the spoon causes the liquid to spin, rising along the edges and spiraling low in the center.
Because the coffee can’t escape through the mug’s walls, the trapped energy generates the cone-like vortex in the center. A comparable phenomenon can be produced on magnetic materials to reveal fundamental quantum measurements.
The researchers 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 inside 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. 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,”
Using electricity in order to switch between magnetic polarity states that correspond to the “1” or “0” of binary computer code, modern high-speed, high-density hard drives in today’s computers write information into spinning disks of magnetic materials. There are, however, intrinsic problems that materialize with this method of data storage, conspicuously limits to speed due to the spinning disk, which is made less reliable by moving parts, significant heat generation, and the considerable energy needed to write and read information.
Furthermore, magnetic storage suffers from a 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.
Racetrack of Nanowires
According to Pollard,
“It takes less energy to manipulate spin torque parameters than magnetic fields. 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.”
One cutting-edge spintronic prototype is IBM’s Racetrack memory, 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.
The new measurement establishes a fundamental limit on data manipulation speeds, but the job of translating this work into practical limits on processor speed and hard drive space belongs to the scientists and engineers building the next generation of digital devices.