Precessional spin-torque dynamics in biaxial antiferromagnets

TL;DR

This paper models the precessional spin-torque dynamics in biaxial antiferromagnets, revealing hysteresis, frequency dependence on damping, and proposing a device for electrical control of a tunable terahertz nano-oscillator.

Contribution

It introduces a damped-driven pendulum model for biaxial antiferromagnets and derives a closed-form threshold spin current, advancing understanding of their precessional dynamics.

Findings

Precessional motion exhibits hysteresis dependent on spin current.

Fundamental frequency inversely proportional to damping.

Threshold spin current depends on the minimum supported orbit frequency.

Abstract

The N\'eel order of an antiferromagnet subject to a spin torque can undergo precession in a circular orbit about any chosen axis. To orient and stabilize the motion against the effects of magnetic anisotropy, the spin polarization should have components in-plane and normal to the plane of the orbit, where the latter must exceed a threshold. For biaxial antiferromagnets, the precessional motion is described by the equation for a damped-driven pendulum, which has hysteresis a function of the spin current with a critical value where the period diverges. The fundamental frequency of the motion varies inversely with the damping, and as $(x^p-1)^{1/p}$ with the drive-to-criticality ratio $x$ and the parameter $p>2$. An approximate closed-form result for the threshold spin current is presented, which depends on the minimum cutoff frequency the orbit can support. Precession about the hard axis has zero cutoff frequency and the lowest threshold, while the easy axis has the highest cutoff. A device setup is proposed for electrical control and detection of the dynamics, which is promising to demonstrate a tunable terahertz nano-oscillator.