Ab initio investigation of laser-induced ultrafast demagnetization of L1${}_0$ FePt: Intensity dependence and importance of electron coherence

Abstract

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 $abinitio$ 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.

arXiv comment12 pages, 10 figures

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