Bifurcation analysis of strongly nonlinear injection locked spin torque oscillators

TL;DR

This paper analyzes the bifurcation behavior of strongly nonlinear injection locked spin torque oscillators under various forcing conditions, revealing how parameters influence locking ranges, bifurcations, and hysteresis, with validation against macrospin simulations.

Contribution

It introduces a bifurcation analysis framework for nonlinear spin torque oscillators, linking forcing parameters to dynamic features and comparing predictions with macrospin simulations.

Findings

Locking range asymmetry controlled by parameter derivatives

Occurrence of Taken-Bogdanov bifurcation at power thresholds

Frequency hysteresis related to transient regimes and resonant frequency

Abstract

We investigate the dynamics of an injection locked in-plane uniform spin torque oscillator for several forcing configurations at large driving amplitudes. For the analysis, the spin wave amplitude equation is used to reduce the dynamics to a general oscillator equation in which the forcing is a complex valued function $F(p,{\psi})\propto{\epsilon}_1 (p)cos({\psi})+i{\epsilon}_2 (p)sin({\psi})$. Assuming that the oscillator is strongly nonisochronous and/or forced by a power forcing $(|{\nu}{\epsilon}_1/{\epsilon}_2 |\gg 1)$, we show that the parameters ${\epsilon}_{1,2} (p)$ govern the main bifurcation features of the Arnold tongue diagram : (i) the locking range asymmetry is mainly controlled by $d{\epsilon}_1 (p)/dp$, (ii) the Taken-Bogdanov bifurcation occurs for a power threshold depending on ${\epsilon}_{1,2} (p)$ and (iii) the frequency hysteretic range is related to the transient regime through the resonant frequency at zero mismatch frequency. Then, the model is compared with the macrospin simulation for driving amplitudes as large as $10^0-10^3 A/m$ for the magnetic field and $10^{10}-10^{12} A/m^2$ for the current density. As predicted by the model, the forcing configuration (nature of the driving signal, applied direction, the harmonic orders) affects substantially the oscillator dynamic. However, some discrepancies are observed. In particular, the prediction of the frequency and power locking range boundaries may be misestimated if the hysteretic boundaries are of same magnitude order. Moreover, the misestimation can be of two different types according if the bifurcation is Saddle node or Taken Bogdanov. These effects are a further manifestation of the complexity of the dynamics in nonisochronous auto-oscillators.