Intense lasers magnetize solids within attoseconds

Ultrafast running of magnetrons in monolayers of BH. a Illustration of the BiH hexagonal lattice and ultrafast actuation of the magnetoresistance – an intense femtosecond laser pulse is irradiated on the material, exciting electronic currents which through spin–orbit interactions induce magnetization and spin flipping. B Band structure of BH with and/without SOC (red and blue bands indicate occupied and unoccupied states, respectively). In the case of SOC, each band is degenerate. c calculated spin expectation value,s_z

Intense laser light can induce magnetism in solids on the attosecond scale – the fastest magnetic response yet. That’s the finding made by theorists at the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg, Germany, who have used advanced simulations to investigate the magnetization process in several two- and three-dimensional materials.

Their calculations show that, in structures with heavy atoms, the fast electron dynamics initiated by the laser pulses can be converted into attosecond magnetism. The work was published in npj arithmetic materials.

The team focused on several standard 2D and 3D material systems, but the results apply to all materials with heavy atomic components. “Heavy atoms are particularly important, because they induce a strong spin-orbital interaction,” explains lead author Ofer Neufeld. “This interaction is the key to converting the light-induced electron motion into x-polarization—in other words, into magnetism. Otherwise, light simply does not interact with the electrons’ spin.”

Just like the needles of a small compass, electrons can also be imagined as having an inner needle pointing in some direction in space, say “up” or “down” – the so-called “spin”. 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 spin equally in all directions. In contrast, when the individual electron spins line up with each other to point in the same direction, the material becomes paramagnetic.

Theorists set out to investigate the magnetic phenomena that can occur when solids interact with highly linearly polarized laser pulses, which typically accelerate electrons on very fast timescales within the material. “These conditions are fascinating to explore, because when laser pulses have linear polarization, they are usually thought to induce no magnetism,” Neufeld says.

Unexpectedly, their simulations showed that these particularly powerful lasers magnetize materials, even though the magnetization is transient — it only lasts until the laser pulse is turned off. However, the most important discovery concerns the speed of this process: magnetization develops over extremely short timescales, less than 500 attoseconds—a prediction of the fastest magnetic response ever recorded. On scale, one attosecond equals one second since one second equals about 32 billion years.

Using advanced simulation tools to explain the underlying mechanism, the team has shown that intense light flips the spin of electrons back and forth. The laser effectively accelerates the electrons into circular-like orbits within a distance of a few hundred attoseconds. These strong spin-orbit interactions then align the spin directions.

The process can be imagined as a bowling ball sliding across a surface and then starting to roll: in this analogy, light pushes the ball around, and spin-orbit interactions (a force created by nearby heavy nuclei as the electron spins around) cause it to roll back and forth, magnetizing. Both forces work together to get the ball rolling.

The results provide fascinating new insights into the fundamentals of magnetization, Neufeld says: “We found that it is a very nonlinear effect that can be tuned by the properties of the laser. The results, although not unequivocally proven, indicate that the maximum magnetic velocity is several tens of attoseconds, because that is the natural speed limit for electronic motion.”

Understanding light-induced magnetization processes at their fundamental level in a range of materials is a crucial step towards developing ultrafast memory devices and changes the current understanding of magnetism.