Synchronization and chaos in a spin-torque oscillator with a perpendicularly magnetized free layer

TL;DR

This paper theoretically investigates how alternating current and microwave fields induce synchronization and chaos in a spin-torque oscillator with a perpendicularly magnetized free layer, revealing key nonlinear effects and parameter regions for chaotic behavior.

Contribution

It provides a comprehensive theoretical analysis of synchronization and chaos in a specific spin-torque oscillator, highlighting the roles of nonlinear frequency shift and spin-transfer torque asymmetry.

Findings

Forced synchronization occurs over wide parameter ranges.

Chaos is identified at large alternating current amplitudes.

Microwave field induces narrower chaos regions.

Abstract

Synchronization and chaos caused by alternating current and microwave field in a spin torque oscillator consisting of a perpendicularly magnetized free layer and an in-plane magnetized reference layer is comprehensively studied theoretically. A forced synchronization by the alternating current is observed in numerical simulation over wide ranges of its amplitude and frequency. An analytical theory clarifies that the nonlinear frequency shift, as well as the spin-transfer torque asymmetry, plays a key role in determining locking range and phase difference between the oscillator and current. Chaos caused by the alternating current is identified for a region of large alternating current by evaluating the Lyapunov exponent. Similar results are also obtained for microwave field, although the parameter regions causing chaos are narrower than those by the alternating current.