Unification of ultrafast demagnetization and switching*

All-optical spin switching and demagnetization are two sides of the same physics. We find a way to unify
them through the same model that takes into account the Heisenberg exchange between spins and spin-orbit
coupling. We include both the kinetic energy term and potential energy term, so that we can account for the
realistic interaction between the laser and the system. To have efficient spin switching, the electron initial
momentum direction must closely follow the spin's orientation, so that the orbital angular momentum is
transverse to the spin, and consequently the spin-orbit torque lies in the same direction as the spin.
Both left and right circularly polarized light have stronger but different effects in both uniform spin domains and
The demagnetization can be understood from the spin-orbit coupling-mediated spin-wave
excitation. The efficiency of laser-induced demagnetization is very high. We also observe the collapse of the
spin-spin correlation length within 20 fs, consistent with the experimental findings.

[1] G. P. Zhang and M. Murakami, Journal of Physics: Condensed Matter 31, 345802 (2019).
[2] G. P. Zhang , M. Murakami , Y. H. Bai, Thomas F. George, and X. S. Wu, J. Appl. Phys. 126, 103906 (2019).