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

TL;DR

This study uses time-dependent density functional theory to analyze how laser intensity influences ultrafast demagnetization in FePt, highlighting the role of electron coherence and nonlinear effects in the process.

Contribution

It provides an ab initio analysis of laser-induced demagnetization, emphasizing the importance of electron coherence and nonlinear effects across different laser intensities.

Findings

Demagnetization scales quadratically with electric field in perturbative limit

Magnetization dynamics occur at even multiples of laser frequency

Electron coherence dominates the ultrafast demagnetization process

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 ${ab~initio}$ 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.