At the core of every magnetic material lies a realm where electrons, guided by the principles of quantum mechanics, engage in an invisible dance.
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Similar to miniature atomic tops, the spins of these electrons dictate the magnetic characteristics of the material they occupy. This microscopic ballet serves as the foundation for various magnetic phenomena.
A team of researchers from JILA, led by JILA Fellows and professors at the University of Colorado Boulder, Margaret Murnane, and Henry Kapteyn, has achieved extraordinary precision in controlling these spins. This breakthrough has the potential to reshape the future of electronics and data storage.
In a recent publication in Science Advances, the JILA research team, in association with partners from universities in Sweden, Greece, and Germany, delved into the spin dynamics within a unique material known as a Heusler compound. These compounds are combinations of metals that exhibit characteristics akin to a unified magnetic material.
The researchers focused on a specific compound consisting of cobalt, manganese, and gallium for this study. This compound demonstrated the dual behavior of acting as a conductor for electrons with upward spin alignment and an insulator for electrons with downward spin alignment.
Employing a type of light known as extreme ultraviolet high-harmonic generation (EUV HHG) as a probing mechanism, the researchers can monitor the re-orientations of spins within the compound. This is achieved by exciting the compound with a femtosecond laser, inducing a transformation in its magnetic properties. The crucial aspect for precise interpretation of the spin re-orientations lay in the researchers' ability to adjust the color of the EUV HHG probe light.
In the past, people haven't done this color tuning of HHG. Usually, scientists only measured the signal at a few different colors, maybe one or two per magnetic element at most. In a monumental first, the JILA team tuned their EUV HHG light probe across the magnetic resonances of each element within the compound to track the spin changes with a precision down to femtoseconds (a quadrillionth of a second).
Sinead Ryan, Co-First Author and Graduate student, JILA, University of Colorado Boulder
Ryan also adds, “On top of that, we also changed the laser excitation fluence, so we were changing how much power we used to manipulate the spins.” He highlighted that the step is also experimental for this type of research.
In conjunction with their innovative methodology, the scientists partnered with theorist and co-first author Mohamed Elhanoty from Uppsala University, who visited JILA. Together, they compared theoretical models of spin changes with the experimental data they gathered. The outcomes revealed a robust alignment between the data obtained and the theoretical predictions. “We felt that we'd set a new standard with the agreement between the theory and the experiment,” added Ryan.
Fine Tuning Light Energy
To explore the intricacies of spin dynamics in their Heusler compound, the scientists introduced an innovative tool: extreme ultraviolet high-harmonic probes. These probes are generated by focusing 800 nm laser light into a tube filled with neon gas. In this setup, the laser's electric field drew electrons away from their atoms and then propelled them back.
As these electrons snapped back, akin to the release of stretched rubber bands, they emitted bursts of purple light at a higher frequency and energy than the initiating laser. Ryan fine-tuned these bursts to resonate with the energies of cobalt and manganese within the sample.
This allowed the measurement of element-specific spin dynamics and magnetic behaviors within the material, offering a platform for further manipulation by the scientific team.
A Competition of Spin Effects
In their experiment, the scientist discovered that by adjusting the power of the excitation laser and the color (or photon energy) of their high-harmonic generation (HHG) probe, they could discern the predominant spin effects at different times within their compound.
To validate their findings, they compared their measurements with a sophisticated computational model known as time-dependent density functional theory (TD-DFT). This model is designed to predict the evolution of a cloud of electrons in a material over time when subjected to different inputs.
Utilizing the TD-DFT framework, Elhanoty identified agreement between the model and the experimental data, attributing it to three competing spin effects within the Heusler compound. Rayan says, “What he found in the theory was that the spin flips were quite dominant on early timescales, and then the spin transfers became more dominant. Then, as time progressed, more de-magnetization effects take over, and the sample de-magnetizes.”
Within the sample, the occurrence of spin flips involves a single element as the spins undergo a transition from an upward to a downward orientation and vice versa. In contrast, spin transfers involve multiple elements—in this case, cobalt and manganese—where spins are exchanged between them. This interchange leads to variations in the magnetic properties of each material over time, making them more or less magnetic as the process unfolds.
By discerning the dominant effects at different energy levels and times, the researchers gained a clearer understanding of how spins could be manipulated to enhance the magnetic and electronic properties of materials.
There’s this concept of spintronics, which takes the electronics that we currently have, and instead of using only the electron’s charge, we also use the electron’s spin. So, spintronics also has a magnetic component. The reason to use spin instead of electronic charge is that it could create devices with less resistance and less thermal heating, making devices faster and more efficient.
Sinead Ryan, Co-First Author and Graduate student, JILA, University of Colorado Boulder
Collaborating with Elhanoty and other partners, the JILA team obtained a more profound understanding of spin dynamics within Heusler compounds through their research. Ryan says, “It was really rewarding to see such a good agreement with the theory and experiment when it came from this really close and productive collaboration as well.” The researchers at JILA express optimism about continuing their collaborative efforts to explore additional compounds. This ongoing collaboration aims to gain a deeper understanding of how light can be effectively employed to manipulate spin patterns.
Journal Reference:
Ryan, A, S., et al. (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. doi/10.1126/sciadv.adi1428