The discovery of graphene spurred research into layered materials, and the subsequent discovery of magnetism in monolayer 2D materials opened new avenues for technological applications. Controlling magnetism in these systems efficiently is crucial. While current-driven domain wall dynamics and gate-controlled anisotropy offer low-power consumption and high-speed device potential, laser-based approaches provide another energy-efficient means to manipulate magnetic properties via demagnetization, spin-reorientation, or modification of magnetic structures at short timescales. Previous ultrafast laser pulse studies on elemental magnets have yielded significant discoveries, including spin switching and all-optical reversal. However, the use of ultrafast laser excitation in 2D vdW magnets, particularly regarding magnetization, magnetic domains, and correlated phenomena, remains largely unexplored. This study uses CrCl₃, a 2D magnetic material with XY ferromagnetic order, as a model system to demonstrate the formation and control of topologically non-trivial vortex quasiparticles using laser radiation. CrCl₃'s inherent ability to host merons and antimerons due to the interplay between in-plane dipole-dipole interactions and weak out-of-plane anisotropy makes it an ideal candidate. The research aims to show how the external torque from ultrafast laser excitation can overcome exchange energy, reverse spin orientations, and generate vortices and antivortices in CrCl₃.

The introduction provides a comprehensive overview of the existing literature on layered materials, 2D magnetism, and ultrafast laser manipulation of magnetic properties. It highlights the advancements in controlling magnetism using current-driven domain wall dynamics and gate-controlled anisotropy. It also reviews previous research on ultrafast laser pulses applied to elemental magnets, focusing on their impact on spin switching, all-optical reversal, and the manipulation of topological properties in magnetic nanostructures. The literature review emphasizes the novelty of applying ultrafast laser excitation to 2D vdW magnets and the gap in knowledge regarding the behavior of fundamental quantities like magnetization and magnetic domains under such conditions. The authors cite numerous relevant studies on 2D magnets and ultrafast laser-matter interaction, establishing the context and importance of their investigation.

The system is modeled using atomistic spin dynamic simulations, with interactions described by a biquadratic spin Hamiltonian incorporating exchange interactions (J), biquadratic exchange interaction (K), uniaxial anisotropy (D), and the dipolar field (Bdp). The exchange interactions for CrCl₃ are parameterized from previous first-principles calculations, considering up to three nearest neighbors. The inclusion of biquadratic exchange and next-nearest neighbor interactions contributes to the stabilization of non-trivial spin structures. The magnetization dynamics is obtained by solving the Landau-Lifshitz-Gilbert (LLG) equation at the atomistic level. The effects of the laser pulse are included using the two-temperature model (2TM), coupling the electronic and phonon baths. The electronic temperature is coupled to the magnetic system through a thermal field in the LLG equation. The laser power density is modeled using a Gaussian function, considering parameters like laser fluence, pulse width, and optical penetration depth. The 2TM includes a heat diffusion term to account for heat dissipation to the substrate. The parameters for the 2TM are determined based on experimental magnetization dynamics of a related compound and existing literature on thermal conductivity and specific heat. The electron-phonon coupling is approximated based on the low Curie temperature of CrCl₃. The topological number, which characterizes the spin structures, is calculated using a surface integral of the magnetization direction vector. The simulations are performed to observe the ultrafast spin dynamics of CrCl₃ upon laser excitation, identifying the formation of merons and antimerons.

The simulations reveal that an 85 fs laser pulse with a fluence of 0.01 mJ cm⁻² causes a rapid reduction of the in-plane magnetization to near zero within 25 ps. The demagnetization process is largely independent of the applied fluence. The system's temperature peaks at 60 K during the laser pulse, exceeding the Curie temperature of CrCl₃. Following the laser pulse, the system undergoes thermal relaxation, and magnetic domains form, leading to a non-zero transversal magnetization. The emergence of small circular areas with non-zero out-of-plane magnetization components indicates the creation of meron and antimeron quasiparticles. These spin textures are characterized using the topological number (N). Merons and antimerons, alongside more complex quasiparticles composed of multiple antimerons, are identified. The formation of these spin textures is attributed to the interplay between laser pulse heating and system thermal equilibration. The long-timescale dynamics of the topological spin structures show collision and annihilation events, often accompanied by spin wave emission. The annihilation process exhibits a 1:1 vortex-antivortex relationship. The lifetime of vortex/antimeron pairs depends on their relative distance. The topological number (N) is used to track the evolution of spin textures over time, revealing fluctuations at early stages and a more stable state at later times. The unit step variations in N correlate with vortex-antivortex pair annihilation events.

The findings demonstrate that ultrafast laser pulses can effectively generate and control topologically non-trivial spin textures in 2D CrCl₃. The laser pulse heating serves as the primary driving force for the formation of merons and antimerons, occurring within experimentally observable timescales. The coupling of the CrCl₃ layer to a heat sink facilitates the stabilization of vortices and antivortices during thermal equilibration, highlighting the importance of substrate selection. The use of appropriate substrates, such as boron nitride (BN), with high thermal conductivity could potentially enhance the observation of these phenomena. The results open up possibilities for tailoring magnetic properties in other 2D vdW materials and pave the way for exploring magneto-optical topological applications.

This study successfully demonstrates the generation and control of laser-induced topological merons and antimerons in 2D CrCl₃. The findings highlight the potential of ultrafast laser excitation for manipulating spin textures in 2D vdW magnets. Future research could explore the application of this technique to other 2D materials and investigate the optimization of laser parameters and substrate selection for enhanced control and stability of topological spin textures. The development of experimental techniques for directly measuring the topological number in these systems would further advance the field.

The study relies on atomistic spin dynamic simulations, which involve approximations and assumptions about the system's parameters and interactions. The two-temperature model simplifies the complex thermal processes during laser excitation. The choice of parameters, such as electron-phonon coupling and heat sink coupling, might influence the simulation results. Furthermore, the experimental verification of the simulation findings remains an important step for future work.