Ultrafast Laser Pulse Induced Transient Ferrimagnetic State and Spin Relaxation Dynamics in Two-Dimensional Antiferromagnets

We employ real-time time-dependent density functional theory (rt-TDDFT) and ab initio nonadiabatic molecular dynamics (NAMD) to systematically investigate the ultrafast laser pulses induced spin transfer and relaxation dynamics of two-dimensional (2D) antiferromagnetic-ferromagnetic (AFM/FM) MnPS3/MnSe2 van der Waals heterostructures. We demonstrate that laser pulses can induce a ferrimagnetic (FiM) state in the AFM MnPS3 layer within tens of femtoseconds and maintain it for subpicosecond time scale before reverting to the AFM state. We identify the mechanism in which the asymmetric optical intersite spin transfer (OISTR) effect occurring within the sublattices of the AFM and FM layers drives the interlayer spin-selective charge transfer, leading to the transition from AFM to FiM state. Furthermore, the unequal electron–phonon coupling of spin-up and spin-down channels of AFM spin sublattice causes an inequivalent spin relaxation, in turn extending the time scale of the FiM state. These findings are essential for designing novel optical-driven ultrafast 2D magnetic switches.


Computational Methods
All the ground state calculations were implemented with the Vienna Ab initio Simulation Package (VASP). 1,2The exchange-correlation interaction was treated with the Perdew-Burke-Ernzerhof (PBE) functional in the framework of the generalized gradient approximation. 3 The projector-augmented wave method was adopted to describe the electron-ion interaction. 4An energy cutoff of 500 eV and a Monkhorst-Pack 771 k-mesh grid were adopted for geometry optimization and electronic structure calculations.The lattice constants and atomic positions were fully relaxed until the atomic forces were smaller than 0.01 eV Å -1 .The electron relaxation convergence criterion was 10 -5 eV.To describe the Mn-d orbital better, the effective Hubbard U was set as 3 eV. 5The van der Waals weak interaction was considered using the Grimme DFT-D3.To avoid the interaction between two neighboring periodic units, a vacuum region of 15 Å along the out-of-plane direction was used.
To explore the laser pulse induced spin dynamics, we performed rt-TDDFT calculations.The time evolving state functions ( ) were calculated by solving the time dependent Kohn−Sham (KS) equation  as follows: (1) where and σ represent vector potential and Pauli matrices, respectively.The KS effective potential   ()

=
, where and represent the magnetic field of the applied laser pulse (,)   (,) +   (,)     plus an additional magnetic field and XC magnetic field, respectively.The last term in Eq. ( 1) stands for SOC term.During the rt-TDDFT simulations, we froze the motion of nuclei all the time.
The rt-TDDFT simulations of MnPS3/MnSe2 heterostructures were made by using a fully noncollinear version of rt-TDDFT and a full-potential augmented plane-wave ELK code. 6We used a regular mesh in a k-space of 5 × 5 × 1 with a smearing width of 0.027 eV for calculation of spin dynamics.The time step was set for Δt = 0.1 a.u.The laser pulses that were used in the present study were linearly polarized (in-plane polarization) at a selected frequency.All calculations were performed using adiabatic local spin density approximations (ALSDA) 7 with Hubbard U (U = 3 eV) for Mn atoms.
The spin electron relaxation dynamics simulations were implemented with the Hefei-NAMD code (Hefei-NAMD_SOC version). 8,9After the geometry structures were optimized using VASP, the relaxed structures were warmed up to 300 K during 2 ps through repeated velocity rescaling.Then a 3 ps ab initio molecular dynamics (AIMD) trajectory was generated in the microcanonical ensemble with a time step of 1 fs.200 initial configurations were selected randomly in the first 1 ps AIMD trajectory, and 20000 NAMD trajectories were sampled for each chosen initial structure.
Mn atoms projected band structures.

Figure S1 .
Figure S1.Atom-projected band structures.Band structures of (a) Mn1, (b) Mn2 and (c) Mn3, respectively.Red and blue represent the spin up and spin down, respectively.

Figure S2 .
Figure S2.The laser Fluence dependent spin dynamics.Laser pulse-induced dynamics of (a) net magnetic moment and (b) local magnetic moment of Mn1/Mn2 atom at different laser fluences (F) of 24.6 mJ/cm 2 , 55.3 mJ/cm 2 , and 84.1 mJ/cm 2 .Notable ferromagnetic (FM) state transition under highenergy laser power is highlighted.