UCSD engineers use focused light to rewrite magnetic memory

Magnetic data storage has long relied on using magnetic fields to flip tiny material regions between binary states. This method consumes energy and restricts switching speeds. Now, engineers at the University of California San Diego (UCSD) have demonstrated a new approach using specially engineered light to rewrite magnetic information.

Their experiments show that optical switching can work in a magnetic material far thicker than previously demonstrated, with helicity-independent magnetization reversal. This approach could eventually help make magnetic memory faster, denser, and more energy efficient.

Overcoming previous optical limitations

Making light switch magnetism has not been straightforward. Earlier experiments showed optical switching only in magnetic stacks containing no more than three layers. Making the material thicker suppressed the effect, limiting both the physical design of the memory and potentially its ability to retain information. Researchers also had to use a specific polarization of light.

Instead of searching for another material that could overcome these restrictions, the UC San Diego team changed the light itself. “We’ve optically engineered the light to change the physics that’s happening in the material at the micro- and nanoscale,” Muhammad Waleed Khalid, lead researcher and a PhD student at UCSD, said.

Reshaping laser beams for precision

The researchers used an ultrafast laser and engineered its beam so that its energy was concentrated into an extremely small region. When the focused spot was reduced below 1.36 micrometers, the switching became helicity-independent. By shaping the beam, they could alter both the local heating and optical torques acting on the material, changing how its magnetization responds to the light.

They tested the approach on a magnetic material made from nine alternating layers of platinum and cobalt. This was substantially thicker than the three-layer limit seen in earlier experiments. The researchers found that the material could undergo helicity-independent magnetization reversal, overcoming a major constraint of earlier approaches.

How the optical switch operates

The switching itself happens through a sequence of ultrafast laser pulses. The first pulses concentrate enough energy into a tiny spot to heat a small region and create a reversed magnetic area. Additional pulses then cause that switched region to expand until it reaches a stable state.

“Working with a specialized laser allowed us to optically engineer the beam to have a certain shape and size. That gave us the room to explore more fundamental physics, which cannot be done with conventional beam lasers,” Khalid added.

The researchers estimate that optical switching could ultimately be more than 1,000 times faster than methods that use external magnetic fields. Also, since the switched region is determined by where the tightly focused light deposits its energy, shrinking the beam could allow magnetic information to be packed more densely.

“The smaller the size of the beam, the smaller and more dense the optical memory,” Abdoulaye Ndao, senior researcher and a professor at UCSD Jacobs School of Engineering, said.

Verifying unusual experimental results

The unusual results were not immediately easy to establish. The team repeatedly tested the experiments because the effects appeared so different from what researchers had previously observed.

“The effects we were seeing were so new and unusual that we had a difficult time convincing others in the field that our discovery was not a one-time fluke. We spent a lot of time and effort repeating and verifying our experiments to substantiate our work to the optics community,” Khalid said.