Ultrafast magnetic moment transfer and bandgap renormalization in monolayer FeCl$_2$

TL;DR

This work addresses the microscopic origin of laser-driven ultrafast demagnetization by simulating FeCl2 monolayer with real-time TDDFT and a self-consistent Hubbard correction. It reveals that non-thermal electronic distributions drive ultrafast magnetic moment transfer from Fe to Cl via coupled Fe 3d → LCB-LCB+3 and Cl 3p → HVB charge transfers, with demagnetization peaking at resonant excitation and the bandgap renormalizing by up to $41\%$ on a tens-of-fs timescale. The band structure undergoes rapid reconstruction, including downshifts of higher conduction bands and an upshift of the HVB, linked to evolving $U_{eff}$ and excited-state population. Overall, the study provides a microscopic mechanism for optical control of magnetism in 2D vdW magnets and offers a framework for designing ultrafast spintronic devices.

Abstract

The microscopic origin of laser-induced ultrafast demagnetization remains an open question, to which the non-thermal electronic distribution plays a vital role at the initial stage. Herein, we investigate the connection between the non-thermal electronic distribution and the ultrafast spin dynamics as well as the electronic structure evolution in ferromagnetic FeCl$_2$ monolayer using real-time time-dependent density functional theory (rt-TDDFT) with self-consistent Hubbard $U$ correction. Our simulations reveal that femtosecond laser pulses induce ultrafast magnetic moment transfer from Fe to Cl atoms. More importantly, through a comprehensive analysis of orbital-resolved electronic structure, we elucidate the microscopic origin of this transfer, attributing it to specific intra-atomic and inter-atomic charge transfer pathways driven by non-thermal excitations. The extent of demagnetization of Fe atoms exhibits a non-monotonic dependence on the laser photon energy, reaching a maximum at the resonant excitation. In addition, the dynamical evolution of the band structure was studied based on the eigenstates of the instantaneous Hamiltonian. Under resonant excitation, the bandgap reduction reaches up to $41\%$ within tens of fs. These findings provide fundamental insights into ultrafast spin control and suggest a strategy to optically engineer the magnetism in two-dimensional magnetic materials.

Figures (8)

• Figure 1: (a, b) Crystal structure and magnetic configurations of FeCl2. The arrow's alignment shows the Ferromagnetic (FM) state (a) and antiferromagnetic (AFM) state (b). Golden and green spheres represent Fe and Cl atoms, respectively. (c, d) Electronic structure of the FM configuration: (c) Band structure calculated with Hubbard $U$ correlations and spin-orbit coupling. The valence band maximum (VBM) is set to zero. (d) Partial density of states (PDOS).
• Figure 2: Laser-induced ultrafast spin dynamics in FeCl2. (a) Time-dependent vector potential generating the applied in-plane electric fields. The peak amplitude occurs at $t$ = 15 fs. (b) Time evolution of the magnetic moments on the Fe and Cl atoms under resonant excitation at $1.0 \times E_{\text{g}}$.
• Figure 3: Ultrafast charge transfer pathways in FeCl2. (a) Spin-resolved partial density of states at the ground state. The upward and downward arrows represent the spin-up and spin-down channels, respectively. (b) State-resolved electron transfer from Fe $3d$ orbitals to the four lowest unoccupied bands (LCB-LCB+3). (c) State-resolved electron transfer (hole depletion) from Cl $3p$ orbitals to the HVB. The red, blue, and green curves represent the top three contributing bands, while the gray curves represent bands with minor contributions.
• Figure 4: Ultrafast demagnetization of Fe and electron dynamics. (a) Time evolution of the magnetic moments on Fe atoms under laser excitation with four different photon energies: $\hbar\omega=0.5 \times E_{\text{g}}$, $0.75 \times E_{\text{g}}$, $1.0 \times E_{\text{g}}$, and $1.25 \times E_{\text{g}}$, respectively. (b) Total electron population change, $\Delta n = n(t)-n(0)$, summed over the LCB to LCB+3 bands.
• Figure 5: Ultrafast electronic structure dynamics of FeCl2 under laser excitation. (a) Transient band structure at $t$=33 fs under resonant excitation at $\hbar\omega=1.0 \times E_{\text{g}}$. (b) Transient bandgap evolution at 0, 7.25, 14.5, 21.8, 29, and 33 fs for different excitation photon energies ($\hbar\omega$=0.5, 0.75, 1.0, and 1.25 $\times E_{\text{g}}$). (c) Time-dependent total electron population change, $\Delta n_{\text{total}}$, summed over the HVB, LCB, and LCB+1 bands, corresponding to the photon energies shown in (b).