Ab initio investigation of laser-induced ultrafast demagnetization of L1_0 FePt: Intensity dependence and importance of electron coherence

We theoretically investigate the optically-induced demagnetization of ferromagnetic FePt using the time-dependent density functional theory (TDDFT). We compare the demagnetization mechanism in the perturbative and nonperturbative limits of light-matter interaction and show how the underlying mechanism of the ultrafast demagnetization depends on the driving laser intensity. Our calculations show that the femtosecond demagnetization in TDDFT is a longitudinal magnetization reduction and results from a nonlinear optomagnetic effect, akin to the inverse Faraday effect. The demagnetization scales quadratically with the electric field E in the perturbative limit, i.e., Δ M_z ∝ E^2. Moreover, the magnetization dynamics happens dominantly at even multiples nω_0, (n = 0, 2, ⋯) of the pump-laser frequency ω_0, whereas odd multiples of ω_0 do not contribute. We further investigate the demagnetization in conjunction to the optically-induced change of electron occupations and electron correlations. Correlations within the Kohn-Sham local-density framework are shown to have an appreciable yet distinct effect on the amount of demagnetization depending on the laser intensity. Comparing the ab initio computed demagnetizations with those calculated from spin occupations, we show that electronic coherence plays a dominant role in the demagnetization process, whereas interpretations based on the time-dependent occupation numbers poorly describe the ultrafast demagnetization.