Measurement of nonlinear frequency shift coefficient in spin-torque oscillators based on MgO tunnel junctions

TL;DR

This paper measures the nonlinear frequency shift coefficient in MgO-based spin-torque oscillators, revealing its small magnitude correlates with narrow linewidths at high bias currents, and confirms theoretical predictions about linewidth behavior.

Contribution

It provides the first experimental measurement of the nonlinear frequency shift coefficient in MgO tunnel junction spin-torque oscillators, validating theoretical linewidth models.

Findings

Linearity in linewidth vs. inverse normalized power plots below and above threshold.

Small nonlinear frequency shift coefficient correlates with narrow linewidths.

Experimental confirmation of theoretical linewidth behavior in spin-torque oscillators.

Abstract

The nonlinear frequency shift coefficient, which represents the strength of the transformation of amplitude fluctuations into phase fluctuations of an oscillator, is measured for MgO-based spin-torque oscillators by analyzing the current dependence of the power spectrum. We have observed that linewidth against inverse normalized power plots show linear behavior below and above the oscillation threshold as predicted by the analytical theories for spin-torque oscillators. The magnitude of the coefficient is determined from the ratio of the linear slopes. Small magnitude of the coefficient has been obtained for the device exhibiting narrow linewidth at high bias current.