Laser pulse-The researchers used laser pulses to trigger fast electron dynamics that could be converted into attosecond magnetism

Powerful lasers can induce magnetism in solids at the attosecond level - the fastest magnetic response to date. That's the finding of MPSD theorists, who used advanced simulations to study magnetization processes in several 2D and 3D materials. Their calculations show that in structures with heavy atoms, the fast electron dynamics triggered by laser pulses can be converted into attosecond magnetism. The work is in the journal npj Computational Materials.

The team focused on a few benchmark 2D and 3D material systems, but the results apply to all materials that contain heavy atomic components. "Heavy atoms are particularly important because they induce strong spin-orbit interactions," explains lead author Ofer Neufeld. "This interaction is key to converting light-induced electron motion into spin polarization - in other words, into magnetism. Otherwise, light wouldn't interact with the electron's spin at all.

Like tiny compass needles, electrons can also be thought of as having an internal needle that points in some direction in space, such as "up" or "down" - so-called "spins." The spin direction of each electron depends on the chemical environment around it, for example, which atoms it can see and where the other electrons are. In non-magnetic materials, electrons rotate equally in all directions. Instead, when the spins of individual electrons line up with each other to point in the same direction, the material becomes magnetic.

Theorists set out to investigate what magnetic phenomena occur when solids interact with intense linearly polarized laser pulses that typically accelerate electrons inside a substance on very fast time scales. "These conditions are fascinating because when laser pulses are linearly polarized, they are generally not thought to cause any magnetism," Neufeld said.

Unexpectedly, their simulations showed that these particularly powerful lasers do magnetize the material, even if the magnetism is transient - it only lasts until the laser pulse is turned off. The most striking finding, however, has to do with the speed of the process: magnetization evolves over an extremely short time scale, less than 500 attosecs - the fastest magnetic response prediction ever made. An attosecond is one fifth of a second (1 x 10-18 seconds). For scale, a single attosecond equals one second, and one second equals about 32 billion years.

Using advanced simulation tools to explain the underlying mechanism, the team showed that bright lights flip the spin of electrons back and forth. The laser effectively accelerates electrons in circular orbits in the space of a few hundred attoseconds. These powerful spin orbits interact and then align the spin directions. The process can be thought of as a bowling ball sliding across a surface and then starting to roll: in this analogy, light pushes the ball around, and the spin-orbit interaction (the force generated by nearby heavy nuclei as electrons orbit it) makes it roll back and forth, magnetizing it. Two forces work together to make the ball roll.

Neufeld says the results provide fascinating new insights into the fundamentals of magnetization: "We found that this is a highly nonlinear effect that can be adjusted by the properties of the laser. The results imply, though not definitively proved, that the final speed limit for magnetism is tens of attoseconds, as this is the natural speed limit for electron motion."

Understanding these photoinduced magnetization processes at the fundamental level of a range of materials is a key step towards the development of ultra-fast storage devices and changes the current understanding of magnetism.

Source: Laser Net