Impact of strain on the SOT-driven dynamics of thin film Mn$_3$Sn

Abstract

Mn$_3$Sn, a metallic antiferromagnet with an anti-chiral 120$^\circ$ spin structure, generates intriguing magneto-transport signatures such as a large anomalous Hall effect, spin-polarized current with novel symmetries, anomalous Nernst effect, and magneto-optic Kerr effect. When grown epitaxially as MgO(110)[001]$\parallel$ Mn$_3$Sn($0\bar{1}\bar{1}0$)[0001], Mn$_3$Sn experiences a uniaxial tensile strain, which changes the bulk six-fold anisotropy landscape to a perpendicular magnetic anisotropy with two stable states. In this work, we investigate the field-assisted spin orbit-torque (SOT)-driven response of the order parameter in single-domain Mn$_3$Sn with uniaxial tensile strain. We find that for a non-zero external magnetic field, the order parameter can be switched between the two stable states if the magnitude of the input current is between two field-dependent critical currents. Below the lower critical current, the order parameter exhibits a stationary state in the vicinity of the initial stable state. On the other hand, above the higher critical current, the order parameter shows oscillatory dynamics which could be tuned from the 100's of megahertz to the gigahertz range. We obtain approximate expressions of the two critical currents and find them to agree very well with the numerical simulations for experimentally relevant magnetic fields. We also obtain unified functional form of the switching time versus the input current for different magnetic fields. Finally, we show that for lower values of Gilbert damping ($\alpha \leq 2\times 10^{-3}$), the critical currents and the final steady states depend significantly on the damping constant. The numerical and analytic results presented in our work can be used by both theorists and experimentalists to understand the SOT-driven order dynamics in PMA Mn$_3$Sn and design future experiments and devices.