Quantitative MRFM characterization of the autonomous and forced dynamics in a spin transfer nano-oscillator

TL;DR

This study uses magnetic resonance force microscopy to quantitatively analyze the autonomous and forced dynamics of a spin transfer nano-oscillator, revealing key parameters like threshold current, noise level, and mode symmetry requirements for phase-locking.

Contribution

It provides a quantitative MRFM-based characterization of spin transfer nano-oscillator dynamics, including threshold current, noise, and mode symmetry for phase-locking, which advances understanding of these systems.

Findings

Measured emitted power in autonomous and forced regimes.

Quantified threshold current and noise level.

Demonstrated the importance of mode symmetry for phase-locking.

Abstract

Using a magnetic resonance force microscope (MRFM), the power emitted by a spin transfer nano-oscillator consisting of a normally magnetized Py$|$Cu$|$Py circular nanopillar is measured both in the autonomous and forced regimes. From the power behavior in the subcritical region of the autonomous dynamics, one obtains a quantitative measurement of the threshold current and of the noise level. Their field dependence directly yields both the spin torque efficiency acting on the thin layer and the nature of the mode which first auto-oscillates: the lowest energy, spatially most uniform spin-wave mode. From the MRFM behavior in the forced dynamics, it is then demonstrated that in order to phase-lock this auto-oscillating mode, the external source must have the same spatial symmetry as the mode profile, i.e., a uniform microwave field must be used rather than a microwave current flowing through the nanopillar.