Deciphering Spin Mysteries Through the Lens of High-Harmonic Probes

In a publication in Science Advances, the JILA research team, in collaboration with partners from universities in Sweden, Greece, and Germany, delved into the intricacies of spin dynamics within a unique material called a Heusler compound. This compound, a blend of metals exhibiting the characteristics of a unified magnetic material, became the focal point of their investigation.

In this investigation, the scientists employed a compound comprising cobalt, manganese, and gallium. This compound exhibited dual behavior, functioning as a conductor for electrons with upward-aligned spins and as an insulator for electrons with downward-aligned spins.

Employing an investigative tool known as extreme ultraviolet high-harmonic generation (EUV HHG), the scientists could monitor the reorientations of spins within the compound. They achieved this by exciting the compound with a femtosecond laser, inducing a shift in its magnetic properties. The crucial aspect for precisely interpreting the spin reorientations lay in their capability to adjust the color of the EUV HHG probe light.

Sinéad Ryan, co-first author and graduate student at JILA, highlighted a departure from conventional approaches, stating that color tuning of high-harmonic generation (HHG) hadn’t been extensively explored in the past. Typically, scientists measured signals at only a few distinct colors, often limited to one or two per magnetic element. In a groundbreaking move, the JILA team introduced a pioneering approach by tuning their extreme ultraviolet high-harmonic generation (EUV HHG) light probe across the magnetic resonances of each element within the compound. This innovative technique enabled them to precisely track spin changes with an unprecedented precision down to femtoseconds, equivalent to a quadrillionth of a second.

Ryan further explained that in addition to tuning the colors of the EUV HHG light probe, they introduced a novel experimental step by altering the laser excitation fluence. This adjustment involved changing the amount of power used to manipulate the spins. This dual approach, combining color tuning and laser excitation fluence modulation, represented an experimental first in this particular realm of research.

In conjunction with their innovative methodology, the researchers partnered with theorist and co-first author Mohamed Elhanoty from Uppsala University. During his visit to JILA, they collaborated to compare theoretical models of spin changes with their experimental data. The outcomes revealed a robust correspondence between the data obtained through experimentation and the theoretical predictions. Sinéad Ryan emphasized that the alignment between theory and experiment set a new benchmark, signifying the success of their approach in advancing the field.

Optimizing light energy for precision

In their exploration of the spin dynamics within the Heusler compound, the researchers introduced a cutting-edge tool—extreme ultraviolet high-harmonic probes. Creating these probes involved directing 800-nanometer laser light into a neon gas-filled tube. Within this environment, the laser’s electric field effectively extracted electrons from their atoms before subsequently propelling them back into place.

Upon recoiling, the electrons mimicked the behavior of released rubber bands, generating bursts of purple light at a frequency (and energy) surpassing that of the initial kicking laser. Sinéad Ryan fine-tuned these bursts to align with the energies specific to cobalt and manganese within the sample. This precision allowed for the measurement of element-specific spin dynamics and magnetic behaviors within the material, offering the team opportunities for further manipulation.

A spin-effect competition

Through their experimentation, the researchers discovered that by adjusting both the power of the excitation laser and the color (or photon energy) of their high-harmonic generation (HHG) probe, they could identify the prevailing spin effects at different intervals within their compound. To validate their findings, they juxtaposed their measurements with a sophisticated computational model known as time-dependent density functional theory (TD-DFT). This model forecasts the evolution of a cloud of electrons in a material over time in response to diverse inputs.

Applying the TD-DFT framework, Elhanoty identified concordance between the theoretical model and the experimental data. This alignment was attributed to the presence of three competing spin effects within the Heusler compound.

Ryan clarified the findings from the theoretical analysis, indicating that in the early stages, spin flips were prominently dominant. Subsequently, as time advanced, the dominance shifted toward spin transfers. Eventually, as time progressed further, the effects of demagnetization became increasingly prevalent, leading to the eventual demagnetization of the sample.

The occurrence of spin flips is characterized by the alteration of spin orientation within a single element of the sample, transitioning from upward to downward and vice versa. In contrast, spin transfers involve multiple elements, specifically cobalt and manganese in this instance, where spins are exchanged between them. This interchange leads to fluctuations in the magnetic properties of each material, causing variations in magnetism over time.

By discerning the dominant effects at various energy levels and timeframes, the researchers gained a more comprehensive understanding of how spins could be manipulated. This knowledge offers insights into enhancing the magnetic and electronic properties of materials, empowering the development of materials with more potent characteristics.

Sinéad Ryan explained the concept of spintronics, which extends beyond conventional electronics by incorporating not only the electron’s charge but also its spin. In spintronics, the magnetic aspect of electron spin is harnessed. The rationale behind opting for spin over electronic charge lies in the potential to develop devices with reduced resistance and thermal heating. This innovation could result in faster and more efficient electronic devices.

Through collaboration with Mohamed Elhanoty and other partners, the JILA team achieved a more profound understanding of spin dynamics in Heusler compounds. Sinéad Ryan expressed satisfaction at the substantial agreement between theoretical predictions and experimental results, highlighting the rewarding outcome of their close and productive collaboration. The JILA researchers aspire to extend this collaborative effort by exploring other compounds, aiming to enhance their comprehension of how light can be employed to manipulate spin patterns.

Resources

1. ^NEWSPAPER University of Colorado at Boulder. (2023, November 10). Unlocking the secrets of spin with high-harmonic probes. Phys.org. [Phys.org]
2. ^JOURNAL Ryan, S. A., Johnsen, P. C., Elhanoty, M. F., Grafov, A., Li, N., Delin, A., Markou, A., Lesne, E., Felser, C., Eriksson, O., Kapteyn, H. C., Grånäs, O., & Murnane, M. M. (2023). Optically controlling the competition between spin flips and intersite spin transfer in a Heusler half-metal on sub–100-fs time scales. Science Advances, 9(45), eadi1428. [Science Advances]