Ultrafast laser pulses could reduce power requirements for data storage

A discovery from an experiment with magnets and lasers could be a boon for energy-efficient data storage.

“We wanted to study the physics of light-magnet interaction,” said Rahul Jangid, who led the project’s data analysis while earning his doctorate. in materials science and engineering at UC Davis under the supervision of Associate Professor Roopali Kukreja. “What happens when you hit a magnetic domain with very short pulses of laser light?”

Domains are areas within a magnet that reverse from the north poles to the south poles. This property is used for data storage, for example on computer hard drives.

Jangid and his colleagues found that when a magnet is struck by a pulsed laser, the domain walls in the ferromagnetic layers move at a speed of about 66 km/s, or about 100 times faster than the speed limit previously planned.

Domain walls moving at this speed could significantly affect how data is stored and processed, providing a faster, more stable means of memory and reducing power consumption in spintronic devices such as hard drives that use the spin of electrons in magnetic metal multilayers to store. , process or transmit information.

“No one thought it was possible to move these walls so quickly because they would have to reach their limit,” Jangid said. “It sounds absolutely trite, but it’s true.”

These are “bananas”, because of Walker’s breaking phenomenon, which says that domain walls can only be pushed so far at a given speed before breaking and stopping moving. This research, however, proves that domain walls can be driven at previously unknown speeds using lasers.

While most personal devices like laptops and cell phones use faster flash drives, data centers use cheaper, slower hard drives. However, each time information is processed or returned, the reader uses a magnetic field to conduct heat through a coil of wire, burning a lot of energy. If a reader could instead use laser pulses on the magnetic layers, the device would operate at a lower voltage and processing bit flips would require much less energy.

Current projections indicate that by 2030, information and communications technologies will account for 21% of global energy demand, exacerbating climate change. This finding, which was highlighted in a paper by Jangid and co-authors titled “Extreme Domain Wall Speeds under Ultrafast Optical Excitation” in the journal Physical Examination Letters December 19 comes at a time when the search for energy-saving technologies is essential.

When the laser meets the magnet

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, Colorado Springs and Stockholm University used the Free Electron Laser Radiation for Multigraduate Investigations (FERMI) Facility, a free electron laser source based in Trieste, Italy.

“Free electron lasers are crazy setups,” Jangid said. “It’s a 3 km long vacuum tube, and you take a small number of electrons, accelerate them to the speed of light, and at the end you stir them around to create X-rays if bright that if you’re not careful, your sample could be vaporized. Think of it like we took all the sunlight falling on Earth and focused it on a penny – that’s the amount of photon flux that we have in free electron lasers.

At FERMI, the group used X-rays to measure what happens when a nanoscale magnet with multiple layers of cobalt, iron and nickel is 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 era of the universe,” Jangid said. “These are extremely small, extremely fast measurements that are difficult to understand.”

Jangid, who analyzed the data, found that it was these ultrafast laser pulses that excited the ferromagnetic layers that drove the movement of the domain walls. Depending on how fast these domain walls were moving, the study posits that these ultrafast laser pulses can switch a stored bit of information about 1,000 times faster than methods based on magnetic field or spin current. currently used.

The future of ultrafast phenomena

The technology is far from being applied in practice, because current lasers consume a lot of energy. However, a process similar to how compact discs (CDs) use lasers to store information and CD players use lasers to read it could potentially work in the future, Jangid said.

Next steps include further exploring the physics of the mechanisms that enable ultrafast domain wall velocities greater than previously known limits, as well as imaging domain wall motion.

This research will continue at UC Davis under Kukreja’s direction. Jangid is currently pursuing similar research at Brookhaven National Laboratory’s National Synchrotron Light Source 2.

“There are so many aspects of the ultrafast phenomenon that we are just beginning to understand,” Jangid said. “I look forward to addressing open questions that could unlock transformative advances in the areas of low-power spintronics, data storage and information processing.”