Intrinsic Phase-Locking Mechanism Enables Terahertz Oscillators in Antiferromagnets

A new theoretical and computational advancement has established the physical foundations for continuous-wave signal generation at terahertz (THz) frequencies using antiferromagnet-based spintronic devices. Through a multiscale framework, researchers modeled the behavior of $Mn_2Au$, a collinear antiferromagnet, demonstrating that its “easy-plane” anisotropy architecture supports an intrinsic phase-locking mechanism capable of stabilizing ultrafast auto-oscillations driven by a spin current.

The Challenge of Synchronization in Antiferromagnetic Oscillators

Historically, spintronics has relied on ferromagnetic materials, which are limited to gigahertz (GHz) frequencies and suffer from energy dissipation and device crosstalk due to massive macroscopic stray fields. Antiferromagnetic (AFM) materials emerge as a superior alternative, as their fully compensated sublattices eliminate stray fields, and the strong exchange interaction pushes intrinsic spin dynamics into the THz regime. However, creating current-driven AFM oscillators faces a critical hurdle: maintaining microscopic phase-locking. In AFMs with perpendicular magnetic anisotropy (PMA), THz oscillations tend to become intrinsically unstable, destabilizing dynamics and collapsing due to the loss of phase synchronization between the precessing sublattices.

Saddle-Node Bifurcation and Threshold-Free Phase-Locking

To solve this impasse and predict the behavior of $Mn_2Au$, the team applied a momentum-dependent canonical transformation to a coupled macrospin Hamiltonian model, validating the findings with large-scale atomistic spin-dynamics (ASD) simulations. Mathematical analysis revealed that the onset of auto-oscillations is governed by a saddle-node bifurcation, whose threshold is determined exclusively by the in-plane fourfold anisotropy barrier. Unlike PMA systems, $Mn_2Au$ exhibits an intrinsic phase-locking mechanism categorized as threshold-free. Once macroscopic auto-oscillations are initiated, the interplay between the strong antiferromagnetic exchange and the easy-plane anisotropy generates a robust restoring force that automatically stabilizes the relative phase of the two sublattices. This stabilization ensures the critical antiparallel alignment is maintained throughout the simulated $q=0$ limit cycle.

“This behavior contrasts sharply with that of antiferromagnets with perpendicular anisotropy, where phase unlocking can destabilize the dynamics.”

Thermal Validation and Practical Implications

The theoretical robustness of the phase-locking was rigorously confirmed through stochastic tests under extreme temperature variations. Independent ASD spot checks performed in a simulated 300 K environment demonstrated that thermal fluctuations in the system remain bounded, presenting relative-phase noise on the milliradian scale. Under these room-temperature conditions, the model maintained a persistent spectral peak at 7.56 THz, without the occurrence of any $2\pi$ phase slips. Furthermore, the controlled injection of a $q=0$ Dzyaloshinskii-Moriya-like perturbation merely resulted in a primary static relative-phase offset, failing to induce the unwanted unlocking of the model. These results conclusively establish $Mn_2Au$ as a highly coherent physical platform for continuous-wave THz generation and provide fundamental general design principles for advancing the next generation of stable antiferromagnetic oscillators.