Discoveries from magnet and laser experiments could be a boon for energy-efficient data storage.
"We wanted to study the physics of optomagnetic interactions," said Rahul Jangid, who led the data analysis for the project while earning his Ph.D. in materials science and engineering under the direction of Roopali Kukreja, an associate professor at UC Davis. "What happens when you hit a magnetic domain with a very short laser pulse?"
A domain is a region within a magnet that flips from the north pole to the south pole. This property is used for data storage, such as in computer hard disk drives.
Jangid and his colleagues found that when a magnet is hit by a pulsed laser, the domain walls in the ferromagnetic layer move at about 66 kilometers per second, which is about 100 times faster than the previously thought speed limit.
Domain walls moving at such speeds could dramatically affect how data is stored and processed, providing faster, more stable memory and reducing the energy consumption of spintronics devices, such as hard disk drives, which use electron spins within multiple layers of magnetic metal to store, process or transmit information.
"No one thought these walls could move this fast because they were supposed to reach their limits," Jangid said. "It sounds absolutely bananas, but it's true." It's "bananas" because of the Walker breakdown phenomenon, which says that domain walls can only be pushed so far at a given speed before they effectively break down and stop moving. However, this study provides evidence that lasers can be used to drive domain walls at previously unknown speeds.
While most personal devices such as laptops and cell phones use faster flash drives, data centers use cheaper, slower hard drives. However, every time a bit of information is processed or flipped, the drives burn a lot of energy by using a magnetic field to conduct heat through the coils. If drives could use laser pulses on the magnetic layers, the devices would operate at lower voltages and the energy required for bit flipping would be greatly reduced.
Current projections suggest that ICTs will account for 21 percent of the world's energy demand by 2030, contributing to climate change, a finding highlighted by Jangid and co-authors in a paper titled "Extreme Domain Wall Velocities under Ultrafast Optical Excitation," which was published Dec. 19 in the journal Physical Review Letters. The discovery comes at a time when the search for energy-saving technologies is critical.
To conduct the experiment, Jangid and his collaborators, including researchers from the National Institute of Science and Technology; the University of California, San Diego; the University of Colorado, the University of Colorado Springs, and Stockholm University used the Multidisciplinary Research Facility for Free Electron Laser Radiation, a free-electron laser source located in Trieste, Italy.
"The Free Electron Laser is crazy facility," Jangid said. "It's a 2-mile-long vacuum tube where you take a handful of electrons, accelerate them to the speed of light, and finally swing them around to produce X-rays so bright that if you're not careful, your sample could be vaporized. Think of it as focusing all the sunlight that falls on Earth on a penny-that's how much photon flux we have at the free-electron laser.
At Fermi, the group used X-rays to measure what happens when nanoscale magnets with multiple layers of cobalt, iron, and nickel are excited by femtosecond pulses. A femtosecond is defined as 10 to the minus fifteenth of a second, or one millionth of a billionth of a second.
"There are more femtoseconds in a second than there are days in the age of the universe," Jangid said. "These are very small, extremely fast measurements, and it's hard to get your head around them."
Jangid is analyzing the data and has found that it is these ultrafast laser pulses that excite the ferromagnetic layer, causing the domain walls to move. Based on how fast these domain walls move, the study suggests that these ultrafast laser pulses could switch stored bits of information about 1,000 times faster than the magnetic field or spin-current based methods used today.
The technique is far from practical because current lasers consume a lot of power. However, Jangid says that processes similar to those used by compact disks to store information using lasers and CD players to play information using lasers could work in the future.
Next steps include further exploring the physical properties of the mechanism that allows ultrafast domain wall speeds above previously known limits, as well as imaging the domain wall motion. This research will continue at UC Davis under Kukreja's leadership.Jangid is now conducting similar research at the National Synchrotron Light Source 2 at Brookhaven National Laboratory.
"There are many aspects of ultrafast phenomena that we are just beginning to understand," Jangid said. "I'm eager to tackle some of the outstanding questions that could unlock transformative advances in the fields of low-power spintronics, data storage and information processing."
