Oscillatory transient regime in the forced dynamics of a spin torque nano-oscillator

TL;DR

This paper reveals that the transient response of a spin torque nano-oscillator under RF driving exhibits strong, nonlinear oscillations with frequencies reaching GHz, differing from traditional models and impacting RF applications.

Contribution

It demonstrates the existence of oscillatory transient regimes in STNOs under RF drive, highlighting nonlinear effects not captured by the Adler model.

Findings

Transient oscillations occur when RF current exceeds a critical value.

Oscillation frequency scales with the square root of RF current minus critical current.

Damping rate of oscillations is nearly independent of RF current.

Abstract

We demonstrate that the transient non-autonomous dynamics of a spin torque nano-oscillator (STNO) under a radio-frequency (rf) driving signal is qualitatively different from the dynamics described by the Adler model. If the external rf current $I_{rf}$ is larger than a certain critical value $I_{cr}$ (determined by the STNO bias current and damping) strong oscillations of the STNO power and phase develop in the transient regime. The frequency of these oscillations increases with $I_{rf}$ as $\propto\sqrt{I_{rf} - I_{cr}}$ and can reach several GHz, whereas the damping rate of the oscillations is almost independent of $I_{rf}$. This oscillatory transient dynamics is caused by the strong STNO nonlinearity and should be taken into account in most STNO rf applications.