Spin-Torque-driven Terahertz Auto Oscillations in Non-Collinear Coplanar Antiferromagnets

Abstract: We theoretically and numerically study the terahertz auto oscillations in thin-film metallic non-collinear coplanar antiferromagnets (AFMs), such as $\mathrm{Mn_{3}Sn}$ and $\mathrm{Mn_{3}Ir}$, under the effect of anti-damping spin-torque with spin polarization perpendicular to the plane of the film. To obtain the order parameter dynamics in these AFMs, we solve three Landau-Lifshitz-Gilbert equations coupled by exchange interactions assuming both single- and multi-domain (micromagnetics) dynamical processes. In the limit of strong exchange interaction, the oscillatory dynamics of the order parameter in these AFMs, which have opposite chiralities, could be mapped to that of a linear damped-driven pendulum in the case of $\mathrm{Mn_{3}Sn}$, and a non-linear damped-driven pendulum in case of $\mathrm{Mn_{3}Ir}$. The theoretical framework allows us to identify the input current requirements as a function of the material and geometry parameters for exciting an oscillatory response. We also obtain a closed-form approximate solution of the oscillation frequency for large input currents in case of both $\mathrm{Mn_{3}Ir}$ and $\mathrm{Mn_{3}Sn}$. Our analytical predictions of threshold current and oscillation frequency agree well with the numerical results and thus can be used as compact models to design and optimize the auto oscillator. Employing a circuit model, based on the principle of tunnel anisotropy magnetoresistance, we present detailed models of the output power and efficiency versus oscillation frequency of the auto oscillator. Finally, we explore the spiking dynamics of two unidirectional as well as bidirectional coupled AFM oscillators using non-linear damped-driven pendulum equations.