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., $\Delta M_z \propto E^{2}$. Moreover, the magnetization dynamics happens dominantly at even multiples $n\omega_0$, ($n = 0, 2, \cdots$) of the pump-laser frequency $\omega_0$, whereas odd multiples of $\omega_0$ 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.