Unlocking the secrets of spin with high harmonic probes

Deep within each piece of magnetic material, electrons dance to the invisible rhythm of quantum mechanics. Their spins, similar to tiny atomic vertices, dictate the magnetic behavior of the material they inhabit. This microscopic ballet is the cornerstone of magnetic phenomena, and it is these spins that a team of JILA researchers, led by JILA fellows and professors Margaret Murnane and Henry Kapteyn of the University of Colorado at Boulder, have learned to be controlled with remarkable precision, potentially redefining the future of electronics and data storage.

In a Scientists progress publication, the JILA team, along with collaborators from universities in Sweden, Greece and Germany, probed the spin dynamics within a special material known as the Heusler compound: a mixture of metals which behaves like a single magnetic material.

For this study, the researchers used a compound of cobalt, manganese, and gallium, which behaved as a conductor for electrons with spins aligned upwards and as an insulator for electrons with spins aligned downwards.

Using a form of light called extreme ultraviolet harmonic generation (EUV HHG) as a probe, the researchers were able to track the spin reorientations inside the compound after exciting it with a femtosecond laser, causing the sample to modify its magnetic field. properties. The key to accurately interpreting the spin reorientations was the ability to tune the color of the light from the EUV HHG probe.

“In the past, people didn’t do this color tuning of HHG,” explained Sinéad Ryan, co-first author and JILA graduate student. “Usually, scientists only measured the signal with a few different colors, maybe one or two per magnetic element at most.” In a monumental first, the JILA team tuned its EUV HHG light probe to the magnetic resonances of each element in the compound to track spin changes with femtosecond (one quadrillionth of a second) precision.

“On top of that, we also changed the excitation fluence of the laser, so we changed the amount of power used to manipulate the spins,” explained Ryan, noting that this step was also an experimental first for this type of research.

Alongside their new approach, the researchers collaborated with theorist and co-first author Mohamed Elhanoty of Uppsala University, who visited JILA, to compare theoretical models of spin changes to their experimental data. Their results showed a strong match between data and theory. “We felt like we were setting a new standard through the agreement between theory and experiment,” Ryan added.

Optimization of light energy

To delve into the spin dynamics of their Heusler compound, the researchers proposed an innovative tool: high harmonic probes in the extreme ultraviolet. To produce the probes, the researchers focused 800-nanometer laser light into a tube filled with neon gas, where the laser’s electric field pulled electrons away from their atoms and then pushed them back.

As the electrons flew back, they acted like rubber bands released after being stretched, creating bursts of violet light at a higher frequency (and energy) than the laser that had expelled them. Ryan tuned these bursts to resonate with the energies of the cobalt and manganese present in the sample, measuring element-specific spin dynamics and magnetic behaviors within the material that the team could further manipulate.

A spin competition

From their experiment, the researchers found that by adjusting the power of the excitation laser and the color (or photon energy) of their HHG probe, they could determine which spin effects were dominant at different times at within their compound. They compared their measurements to a complex computer model called time-dependent density functional theory (TD-DFT). This model predicts how a cloud of electrons in a material will evolve from moment to moment when exposed to various inputs.

Using the TD-DFT framework, Elhanoty found agreement between the model and experimental data due to three competing spin effects within the Heusler compound.

“What he found in the theory was that spin flips were quite dominant on early time scales, and then spin transfers became more dominant,” Ryan explained. “Then, as time passes, other demagnetization effects take over and the sample becomes demagnetized.”

Spin flip phenomena occur in a sample element when the spins change orientation from top to bottom and vice versa. In contrast, spin transfers occur within multiple elements, in this case cobalt and manganese, as they transfer spins between each other, making each material more or less magnetic over time.

Understanding which effects were dominant, which energy levels and moments gave researchers a better understanding of how spins could be manipulated to give materials more powerful magnetic and electronic properties.

“There is this concept of spintronics, which takes the electronics we have now and instead of just using the charge of the electron, we also use the spin of the electron,” Ryan explained. “So spintronics also has a magnetic component. The reason for using spin instead of electronic charge is that it could create devices with less resistance and less thermal heating, making the devices faster and more efficient .”

Through their work with Elhanoty and their other collaborators, the JILA team gained deeper understanding of the spin dynamics within Heusler compounds.

Ryan said: “It was really gratifying to see such good agreement between theory and experiment, thanks to this very close and productive collaboration.”

JILA researchers hope to continue this collaboration by studying other compounds to better understand how light can be used to manipulate spin patterns.