Within attoseconds, intense lasers magnetize materials.

The fastest magnetic response known to date can be produced by intense laser light in solids on the attosecond scale. Theorists at the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg, Germany, who used cutting-edge simulations to study the magnetization process in a variety of 2D and 3D materials came to that conclusion.

Their calculations demonstrate that the fast electron dynamics started by the laser pulses can be transformed into attosecond magnetism in structures with heavy atoms. The research has been published in npj Computational Materials.

The research team focused on a number of benchmark 2D and 3D material systems, but the findings are applicable to all materials with heavy atomic constituents. Lead author Ofer Neufeld explains that the heavy atoms are particularly significant because they cause a potent spin-orbit interaction. “This interaction is essential for transforming the light-induced electron motion into spin polarization, or magnetism. If not, light simply doesn’t interact with the spin of the electrons. “.

“Heavy atoms are particularly significant because they induce a strong spin-orbit interaction. This interaction is critical for transforming light-induced electron mobility into spin polarization, or magnetism. Otherwise, light simply does not interact with the spin of electrons.”

Lead author Ofer Neufeld.

Similarly to tiny compass needles, electrons can also be thought of as having an internal needle that points in a particular direction in space, e. g. the so-called “spin,” which can be “up” or “down.”. The direction of each electron’s spin depends on its chemical surroundings, including the atoms it can see and the location of other electrons. The electrons spin uniformly in all directions in non-magnetic materials. On the other hand, the material turns magnetic when the spins of the individual electrons line up with one another and point in the same direction.

When intense, linearly polarized laser pulses, which typically accelerate electrons in matter on extremely quick timescales, interact with solids, theorists set out to explore what magnetic phenomena might result. Since linear polarized laser pulses are typically thought not to induce any magnetism, Neufeld says these circumstances are fascinating to investigate.

Surprisingly, their simulations demonstrated that these extremely potent lasers do magnetize materials, even though the magnetism is momentary and only lasts until the laser pulse is turned off. The fastest magnetic response in history is predicted by the magnetization’s ability to evolve on timescales of less than 500 attoseconds, which is the most remarkable discovery. One attosecond is equivalent to about 32 billion years in terms of scale.

The team demonstrated that the powerful light flips the electrons’ spins back and forth to explain the underlying mechanism. Within a few hundred attoseconds, the laser efficiently accelerates the electrons into orbits that resemble circles. In turn, the spin directions are aligned by these robust spin-orbit interactions.

The process can be compared to a bowling ball sliding across a surface before it begins to roll. In this analogy, the light pushes the ball, and the spin-orbit interactions (a force resulting from the nearby heavy nuclei as the electron orbits around them) cause it to roll back and forth, magnetizing it. To start things off, both forces work together.

Neufeld claims that the findings reveal fascinating new information about the fundamentals of magnetization: “We found that it’s a highly nonlinear effect that can be tuned by the laser’s properties. Due to the fact that that is the natural speed limit of electronic motion, the results suggest—but do not definitively prove—that the ultimate speed limit for magnetism is several tens of attoseconds.

It is essential for the creation of ultrafast memory devices to comprehend these light-induced magnetization processes at their fundamental level in a variety of materials, which modifies our understanding of magnetism today.

More information: Ofer Neufeld et al, Attosecond magnetization dynamics in non-magnetic materials driven by intense femtosecond lasers, npj Computational Materials (2023). DOI: 10.1038/s41524-023-00997-7

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