The potential for fast, deterministic control of metallic antiferromagnets (AFMs) at room temperature without heavy-metal spin injection is highly promising for spintronics. High-Néel temperature AFMs like Mn₂Au and CuMnAs respond asymmetrically to external stimuli, opening possibilities for terahertz (THz) radiation generation and novel AFM spintronics. However, their intrinsic properties, while advantageous, make them insensitive to traditional ferromagnetic control methods (applied fields or microwave excitations). Spin-orbit torque (SOT) is a current method, but limitations like requiring precise timing to avoid over-switching, weaker fields for longer durations, repeated short pulses, or complex heterostructures for spin transfer torques (STTs) pose challenges. THz excitations offer precision but lack sufficient power for metal switching. Previous optical laser switching of insulating AFMs only succeeded at cryogenic temperatures. This research explores a new avenue using laser-induced staggered fields for direct manipulation of metallic AFM order parameters, a concept previously shown in ferromagnetic materials but only recently applied theoretically to AFMs.

Existing research focuses on using spin-orbit torque (SOT), spin-transfer torques (STT), and THz excitations to control antiferromagnetic order. However, each method presents challenges: SOT requires precise timing or complex pulse sequences to prevent over-switching, STT necessitates complex heterostructures, and THz methods haven't demonstrated efficient switching in metallic systems. Prior work on optical laser switching in insulating antiferromagnets has only been successful at cryogenic temperatures due to the limitations of low Néel temperatures in those materials. Recent ab initio work suggests a novel approach using direct optical laser excitation to induce staggered fields and manipulate the order parameter in metallic antiferromagnets. This study extends this theoretical work by applying it to the specific case of Mn₂Au, focusing on the laser optical torque (LOT) as the primary switching mechanism.

Atomistic spin dynamics simulations were performed on a cubic Mn₂Au crystal lattice (1600 spins, (3 nm)³) using the Landau-Lifshitz-Gilbert (LLG) equation and the open-source code VAMPIRE, incorporating new LOT torques. The effective Heisenberg spin Hamiltonian included ferromagnetic (FM) and antiferromagnetic (AFM) exchange interactions, two-ion anisotropy, fourth-order out-of-plane and in-plane anisotropies. The LOT is modeled using the Keldysh non-equilibrium formalism, focusing on the out-of-plane torques to leverage exchange enhancement in antiferromagnetic switching. The laser intensity is modeled with a Gaussian time-dependent profile. The torque tensors depend on the electric field polarization and AFM order parameter components. Simulations varied laser intensity and polarization angle to explore toggle and preferential switching. A two-temperature model was used to simulate laser heating effects on a granular Mn₂Au sample (75 x 75 x 10 nm³). Analytical expressions for the critical switching field were derived based on the AFM exchange frequency, anisotropy frequency, and LOT-induced field.

Simulations showed 90, 180, and 270-degree precessional switching of the Néel vector in Mn₂Au using LOTs. 90-degree toggle switching was achieved with multiple, short laser pulses (400 fs), separated by 8 ps. 180-degree switching occurred with higher intensity pulses. The LOT's unique symmetry allows for both clockwise and counter-clockwise switching using the same laser polarization. Switching phase diagrams showed that ultrafast laser pulses (sub-picosecond to picosecond duration) and low fluences (0.5 mJ/cm²) were sufficient for switching. Rotating the laser polarization created an asymmetric torque profile, enabling preferential, non-toggle switching, controlling the switching direction without changing the polarization angle. Combining toggle and non-toggle switching allowed deterministic control of the Néel vector's two non-equivalent orientations. Temperature simulations revealed that, even with significant ultrafast laser heating, high switching probabilities were maintained over a large range of laser parameters, and the addition of temperature did not significantly alter the switching behavior. The minimum pulse intensity and fluence for sub-picosecond switching using LOT (1 GW/cm² and 0.65 mJ/cm²) were lower than those reported for other all-optical switching methods.

The findings demonstrate the potential of LOTs for ultrafast, deterministic all-optical switching of antiferromagnets at room temperature. The efficiency of LOT surpasses that of other all-optical switching (AOS) methods, offering lower energy requirements and simpler experimental procedures. The ability to achieve toggle and preferential switching, combined with the lower required fluences and pulse durations, suggests a significant advantage over existing techniques. The activation of in-plane AFM THz modes using optical excitation presents a novel opportunity for spintronic device applications.

This study presents a novel method for ultrafast, deterministic all-optical switching of antiferromagnets using laser optical torques (LOTs). The unique symmetry of LOTs allows for both toggle and preferential switching, offering control over the Néel vector's orientation. The method's efficiency and simplicity, demonstrated through simulations, suggests significant potential for room-temperature spintronic devices. Future research should explore the applicability of this method to other antiferromagnetic materials and heterostructures.

The study primarily relies on atomistic spin dynamics simulations. While a two-temperature model was included to account for laser heating effects, a more comprehensive treatment of thermal effects, particularly for larger system sizes, would be beneficial. Experimental verification of these findings is needed to confirm the theoretical predictions. The focus on Mn₂Au limits the generalizability of the findings to other AFM materials. Further investigations into the material-specific parameters influencing LOT-induced switching are required.