A recent discovery from the University of California, Davis (UC Davis), could enable faster and more efficient magnetic hard drives by using ultrafast laser pulses to process data. The findings could significantly reduce energy consumption for data centers. The current hard drives used in data centers, while much cheaper than solid-state drives used by many commercial devices, are much slower and consume a lot of energy because they use a magnetic field to conduct heat through a wire coil each time an information bit is processed. The research work at UC Davis sought to test whether domain walls in certain multilayered ferromagnets could remain stable at speeds >10 km/s — a speed that scientists recently predicted was possible. The team investigated a multilayered, ferromagnetic metal containing striped and labyrinth-like domains — areas within a magnet that flip from north to south poles, often used for data storage. The researchers directed an ultrashort IR pulse at the metal, inducing domain-wall motion. After a few femtoseconds, they hit the metal with an ultrashort UV pulse that conveyed information about the material’s domain-wall dynamics to a detector. A representation of domain walls within a ferromagnetic layered material. New research shows that when these materials are hit with a free electron laser, magnetic domain walls move much faster than previously known. This opens new possibilities for energy-efficient data storage. Courtesy of Rahul Jangid, UC Davis. By comparing the detected signal to simulations, the researchers inferred the dynamics of the two domain types. They found that the walls of striped domains stayed roughly stationary, while those of the labyrinth-like domains moved at speeds of up to 66 km/s, without destabilizing. This peak speed is 10× the speed of sound in the material and significantly higher than the peak predicted by scientific theory, indicating that much more of the IR pulse’s energy was converted to domain-wall motion than expected. “No one thought it was possible to move these walls that fast because they should hit their limit,” said researcher Rahul Jangid, who led the data analysis for the project. “It sounds absolutely bananas, but it’s true.” 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 Multidisciplinary Investigations (FERMI), based in Trieste, Italy, for the experiments. At FERMI, the researchers used x-rays to measure the phenomena that occurred when a nanoscale magnet with multiple layers of cobalt, iron, and nickel was excited by femtosecond pulses. They observed ultrafast distortion of the diffraction pattern at markedly different timescales compared to the magnetization quenching. The diffraction pattern distortion showed a threshold dependence with laser fluence, not seen for magnetization quenching, consistent with a picture of domain-wall motion with pinning sites. Based on the movement of the domain walls caused by the ultrafast laser pulses exciting the ferromagnetic layers, the team postulated that a stored information bit could be switched ~1000× faster using ultrafast laser pulses than by using the existing data storage methods based on magnetic fields or spin currents. Before this technology can be practically applied, there are challenges to surmount, such as the large amount of power consumed by the lasers that are currently available. However, Jangid believes that a process comparable to how CDs use lasers to store information, and how CD players use lasers to play the information back, could potentially work for data center storage. The researchers plan to further explore the physics underlying the mechanisms that enable domain-wall velocities to exceed the previously known limits, as well as the imaging of the domain-wall motion. “We wanted to study the physics of light-magnet interaction,” Jangid said. “What happens when you hit a magnetic domain with very short pulses of laser light?" “There are so many aspects of ultrafast phenomenon that we are just starting to understand. I’m eager to tackle the open questions that could unlock transformative advancements in low power spintronics, data storage, and information processing.” The research was published in Physical Review Letters (www.doi.org/10.1103/PhysRevLett.131.256702).
