Ab initio theory and multiscale modeling of ultrafast laser-induced magnetic processes*

Ultrafast laser-induced demagnetization was discovered two decades ago,¹ however, the underlying fundamental processes of the fast magnetization decay continue to be debated. To elucidate the contributions of the various proposed microscopic mechanisms, we perform ab initio computational modeling, capable of distinguishing these different mechanisms, and compare to recent experiments.
Investigating thin Co films on an insulating substrate we quantify contributions stemming from exchange splitting reduction due to local spin-flip processes as well as spin-wave excitations, and find a surprisingly large magnon contribution to the ultrafast demagnetization.² Conversely, investigating Ni/Au bilayer films we predict a sizable fs spin current injected into the Au layer.³
A different, very interesting process is all-optical helicity-dependent magnetization switching where the magnetization of a material is manipulated with ultrashort circularly polarized laser pulses;⁴ its origin is still poorly understood. We develop materials’ specific ab initio theory for the magnetization induced by circularly polarized light, commonly referred to as inverse Faraday effect, applicable to absorbing materials.⁵ We show that the induced magnetization is strongly materials and frequency dependent and demonstrate a surprising difference between induced spin and orbital magnetizations. Combining effects of thermal laser heating with laser-induced magnetization in a multiscale modeling we predict all-optical switching in FePt nanoparticles with repeated circularly polarized laser pulses.⁶
[1] E. Beaurepaire et al, Phys. Rev. Lett. 76, 4250 (1996). [2] E. Turgut et al, Phys. Rev. B 94, 220408(R) (2016). [3] M. Hofherr et al, Phys. Rev. B 96, 100403(R) (2017) 100403. [4] C.-H. Lambert et al, Science 345, 1337 (2014). [5] M. Berritta et al, Phys. Rev. Lett. 117, 137203 (2016). [6] R. John et al, Sci. Rep. 7, 4114 (2017).