Time domain study of frequency-power correlation in spin-torque oscillators

TL;DR

This study uses micromagnetic simulations to analyze how spectral linewidth broadening in spin-torque oscillators depends on magnetization dynamics, revealing regular and chaotic regimes with distinct linewidth behaviors and underlying mechanisms.

Contribution

It provides a detailed numerical analysis of the origins of linewidth broadening, identifying the transition from regular to chaotic dynamics and the role of frequency jumps in the chaotic regime.

Findings

Regular regime shows linear power-frequency dependence

Linewidth is linear with temperature in regular regime

Chaotic regime exhibits linewidth broadening due to frequency jumps

Abstract

This paper describes a numerical experiment, based on full micromagnetic simulations of current-driven magnetization dynamics in nanoscale spin valves, to identify the origins of spectral linewidth broadening in spin torque oscillators. Our numerical results show two qualitatively different regimes of magnetization dynamics at zero temperature: regular (single-mode precessional dynamics) and chaotic. In the regular regime, the dependence of the oscillator integrated power on frequency is linear, and consequently the dynamics is well described by the analytical theory of current-driven magnetization dynamics for moderate amplitudes of oscillations. We observe that for higher oscillator amplitudes, the functional dependence of the oscillator integrated power as a function of frequency is not a single-valued function and can be described numerically via introduction of nonlinear oscillator power. For a range of currents in the regular regime, the oscillator spectral linewidth is a linear function of temperature. In the chaotic regime found at large current values, the linewidth is not described by the analytical theory. In this regime we observe the oscillator linewidth broadening, which originates from sudden jumps of frequency of the oscillator arising from random domain wall nucleation and propagation through the sample. This intermittent behavior is revealed through a wavelet analysis that gives superior description of the frequency jumps compared to several other techniques.