Ultrafast antiferromagnetic switching of Mn₂Au with laser-induced optical torques

The study addresses fast, deterministic control of the Néel vector in room-temperature metallic antiferromagnets (AFMs) such as Mn₂Au and CuMnAs without relying on heavy-metal spin injection. Traditional control methods effective in ferromagnets (magnetic fields, microwaves) are ineffective for AFMs. The most established AFM control is via spin-orbit torque (SOT), with additional interest in spin-transfer torque (STT) and terahertz (THz) excitations. However, practical limitations remain: SOT switching often requires precise timing or weaker/longer fields to avoid overshoot, or multiple short pulses; STT needs complex heterostructures and still risks overshooting; and THz fields to date have not switched metallic AFMs. Optical AOS has been shown in insulating orthoferrites but at cryogenic temperatures. Recent ab initio work proposed that direct optical excitation can induce staggered fields and torques in metallic AFMs. Experiments have observed related torques in ferromagnets, and theory was extended to AFMs like Mn₂Au, predicting frequency-dependent staggered fields that sum to a net torque. This work investigates in detail whether such laser optical torques (LOTs) at optical frequencies can switch the AFM Néel vector in Mn₂Au, leveraging exchange-enhanced dynamics, and explores deterministic control by exploiting the torque symmetry.

Prior approaches for AFM control include: (i) current-driven SOT switching in Mn₂Au and related AFMs, which can require finely timed pulses or reduced amplitudes to mitigate overshoot; (ii) STT-based switching via femtosecond spin currents in complex heterostructures, still susceptible to overshoot; (iii) THz excitation, which precisely addresses AFM modes but has not yet demonstrated switching in metals due to insufficient field strengths; and (iv) optical switching in insulating orthoferrites at cryogenic temperatures, limiting applicability to room temperature. Theoretical advances using Keldysh non-equilibrium formalism predicted that second-order coupling of optical electric fields can induce staggered fields in AFMs, producing torques on the Néel vector that depend on crystal symmetry, laser polarization, and the Néel vector direction. Experimental evidence for optical torques exists in metallic ferromagnets, and recent theory quantified such effects for Mn₂Au, identifying specific susceptibility tensors yielding significant torques. Compared to SOT, which scales linearly with |E|, the LOT scales with E² and at optical frequencies is decoupled from SOT dynamics, offering a distinct route to AFM switching with potentially reduced overshoot and accessible experimental conditions.

Atomistic spin dynamics simulations were performed for Mn₂Au using the Landau–Lifshitz–Gilbert (LLG) equation implemented in the VAMPIRE code, augmented with newly implemented laser optical torque (LOT) terms. The typical simulation cell was a cubic crystal of 1600 spins (~3 nm)^3. The effective Heisenberg spin Hamiltonian included ferromagnetic and antiferromagnetic exchange interactions (J1, J2, J3), Au-mediated two-ion anisotropy, fourth-order out-of-plane anisotropy, and fourth-order in-plane rotational magnetocrystalline anisotropy with easy axes along (±110). Magnetization vectors Si are unit vectors (length μ), with polar angle θ and azimuthal angle φ. Anisotropies were implemented via spherical harmonic formulations to ensure accurate four-fold symmetry. Two-ion anisotropy across Au sites was included via vectorial exchange constants. Parameter values (exchange, anisotropy constants, μB=3.72 μB per Mn, TN≈1220 K, lattice constants a=3.328 Å, c=8.539 Å) were taken from literature consistent with experiments. Simulations focused on 90° domain switching without additional in-plane uniaxial strain anisotropy. Laser optical torque modeling: Based on Keldysh formalism, second-order electric-field-induced staggered fields generate a torque τ that depends on the electric field polarization components εj, the Néel vector components Lk, Ll, Lm, Ln, and susceptibilities χjklmn. For Mn₂Au with in-plane magnetization and normal incidence, linearly polarized light with E parallel to the in-plane angle φ yields the largest torque, reducing the tensor set to two dominant components (tensors 4 and 24). The resulting torque exhibits a sin(2p−2δ) dependence (p: laser polarization azimuth; δ: Néel vector azimuth). The torque is represented as an effective field perpendicular to the spin direction: H_LOT ∝ τ(t) sin(2p−2δ) × S / μs. The temporal profile of τ(t) followed a Gaussian with full-width at half-maximum t_p, centered at 1.5 t_p. A representative calibration from ab initio data: for ε ∥ (010), photon energy hν=1.55 eV, and intensity I=1 GW/cm², the LOT magnitude ≈1.1×10^−24 J corresponds to an effective field ≈14.5 mT per moment. The constant broadening Γ=25 meV modeled disorder and finite lifetimes and was kept constant. Simulation protocols: Typically four sequential pulses of duration 400 fs, separated by 8–16 ps, with intensities I in the ~0.3–5 GW/cm² range were applied. Polarization was primarily set parallel to (010) for symmetric toggle switching; rotated polarizations (e.g., φ=3π/8 and 5π/8) were used to generate quadrant-asymmetric torques for preferential (non-toggle) switching. Phase diagrams of switching outcome (90°, 180°, 270° or no switch) were computed versus pulse duration and intensity for different Gilbert damping values (α=0.001 and 0.01). Analytical estimates followed modified AFM dynamics under LOT, defining the critical field H_crit in the long-pulse limit and scaling for short pulses with the exchange frequency ω_ex and anisotropy frequency ω_4. Deterministic control sequences were explored using series of pulses of varying intensities with fixed polarization to combine preferential and toggle regimes. Temperature effects were incorporated by coupling atomistic simulations to a two-temperature (electron-lattice) model for selected intensities/durations. A granular Mn₂Au system (~75×75×10 nm³) was simulated to evaluate switching probabilities under LOT plus laser heating and, for comparison, under heating alone.

- LOT-driven precessional switching in Mn₂Au: Simulations show 90°, 180°, and 270° switching of the Néel vector via exchange-enhanced out-of-plane canting induced by optical-frequency LOT. Toggle switching between orthogonal easy axes is achieved with sequential pulses using the same polarization, as the LOT reverses sign upon 90° rotation of the Néel vector. - Reduced overshoot compared to SOT/STT: The intrinsic symmetry of the LOT term mitigates over-shooting common in SOT/STT precessional switching. Overshoot (180°/270°) occurs primarily at high intensities and short pulses. - Phase diagrams and critical fields: Switching phase diagrams versus pulse duration and intensity reveal windows for 90° switching with sub-ps to few-ps pulses. For α=0.001, absorbed fluence ≈0.5 mJ/cm² (I≈0.3 GW/cm²; H_eff≈6.34 mT) suffices for sub-ps switching. The critical field scales approximately linearly with damping. Analytical long-pulse critical field H_crit = ω_4/(2γ) ≈ 5.16 mT matches simulations for α=0.001. A calibration indicates I=1 GW/cm² yields H_LOT≈14.5 mT. - Representative pulse-driven dynamics: Using 400 fs pulses 8 ps apart, I=2 GW/cm² (H_max≈42 mT) produces successive 90° toggles; I=4 GW/cm² (H_max≈84 mT) can drive 180° toggles. Example traces corroborate out-of-plane canting and exchange-enhanced in-plane rotation. - Preferential (non-toggle) switching via polarization rotation: Rotating laser polarization (e.g., φ=3π/8 or 5π/8) shifts torque maxima away from easy axes, breaking four-fold degeneracy into two “large” and two “small” torque quadrants. This yields handedness preference: φ=3π/8 favors left-handed switching from (1̄10), whereas φ=5π/8 favors right-handed switching from (110), at low fluence. At higher fluence and moderate t_p, toggle switching appears for both starting orientations. - Deterministic control using mixed pulses: With fixed polarization (e.g., φ=5π/8), sequences of pulses with varying intensity can first leverage preferential switching (larger pulse) to move the Néel vector into a maximal-torque quadrant, then use lower-intensity pulses to complete deterministic writing between the two non-equivalent orientations observable via AMR. - Temperature-inclusive results: In a 75×75×10 nm³ grain, laser heating alone (no LOT) yields no switching (no superparamagnetic effect for the considered conditions). With LOT, switching probabilities approach unity over broad ranges of pulse durations. At I=0.6 GW/cm² and φ=3π/8, preferential switching probabilities depend on starting orientation; at I=1.2 GW/cm², shorter pulses give high-probability toggle switching for both orientations, while long pulses show probability saturation consistent with stochastic thermal effects. Transient heating does not quench magnetization or cumulatively raise lattice temperature under the studied conditions. - Energy efficiency and practical metrics: Minimum sub-ps switching estimated at intensity ≈1 GW/cm² and fluence ≈0.65 mJ/cm², lower than reported fluences for GdFeCo AOS (~4.4 mJ/cm²) and Fe-based ultrafast demagnetization used for STT (~6.51 mJ/cm²).

The simulations demonstrate that second-order optical-field-induced torques can directly and efficiently manipulate the Néel vector in a metallic AFM, addressing the challenge of room-temperature all-optical control without heavy-metal current injection. By exploiting exchange enhancement, LOT pulses at optical frequencies trigger precessional dynamics of the in-plane THz AFM mode despite the frequency mismatch, enabling sub- to few-ps switching. The torque’s symmetry naturally yields toggle switching between orthogonal easy axes using identical pulse conditions, eliminating the need to alternate current direction as in SOT-based 90° switching and reducing susceptibility to over-shooting. Rotating the laser polarization imposes a quadrant-asymmetric torque landscape that selects switching handedness, enabling preferential (non-toggle) control. Combining preferential and toggle regimes with tailored pulse sequences allows deterministic writing between the two non-equivalent orientations detectable via AMR, offering a practical control scheme for AFM memory operations. Temperature-inclusive modeling indicates that LOT remains effective under laser-induced heating, with high switching probabilities and no full demagnetization or damaging cumulative heating under the tested conditions. Relative to SOT/STT and THz excitation, LOT offers lower fluence thresholds and more accessible experimental conditions, potentially serving alone or synergistically with other torques to lower energy barriers and suppress overshoot in devices.

This work predicts and analyzes all-optical control of the Néel vector in metallic Mn₂Au driven by laser-induced optical torques. Atomistic simulations reveal sub- to low-picosecond 90°, 180°, and 270° switching via exchange-enhanced precession, with toggle switching emerging intrinsically from the torque symmetry. Rotated polarization introduces preferential, non-toggle switching, and combining pulse intensities enables deterministic control between the two AMR-distinguishable orientations. Temperature-coupled simulations confirm high switching probabilities without deleterious heating. The estimated intensity (~1 GW/cm²) and fluence (~0.65 mJ/cm²) requirements compare favorably to other AOS and STT-based approaches. LOT thus provides a practical, energy-efficient route to activate THz AFM modes with optical pulses for room-temperature spintronics. Future work should explore experimental validation in Mn₂Au and related PT-symmetric AFMs, refine material-specific susceptibilities and disorder parameters, extend to larger, multi-domain systems and device-relevant geometries, and investigate synergistic protocols combining LOT with SOT/STT for enhanced efficiency and reliability.

- Temperature analysis is limited in scope: a full treatment would require larger systems and extensive statistics; only a single granular size (~75×75×10 nm³) and selected pulse conditions were explored. - Material parameters rely on literature values and a constant broadening Γ=25 meV to represent disorder and laser-induced nonequilibrium; real samples may exhibit different Γ, affecting torque magnitude. - Simulations focus on idealized single-domain behavior without in-plane uniaxial strain anisotropy; multi-domain, strain, and defects may modify switching pathways. - The optical torque implementation emphasizes two dominant susceptibility tensors and a specific geometry (normal incidence, in-plane polarization), neglecting smaller tensor contributions and out-of-plane components. - Results are theoretical/simulation-based; experimental switching under identical conditions remains to be demonstrated.