Non-white frequency noise in spin torque oscillators and its effect on spectral linewidth

TL;DR

This study measures frequency fluctuations in spin torque oscillators over various time scales, revealing a dominant 1/f noise component that impacts spectral linewidth and long-term stability, challenging thermal noise-based models.

Contribution

It introduces a comprehensive method to analyze frequency noise spectra in spin torque oscillators, highlighting the significance of 1/f noise on spectral linewidth and stability.

Findings

Frequency noise spectrum is white at high frequencies and 1/f at low frequencies.

Crossover frequency varies from 10^4 Hz to 10^6 Hz.

1/f noise is more prominent in multilayer devices.

Abstract

We measure the power spectral density of frequency fluctuations in nanocontact spin torque oscillators over time scales up to 50 ms. We use a mixer to convert oscillator signals ranging from 10 GHz to 40 GHz into a band near 70 MHz before digitizing the time domain waveform. We analyze the waveform using both zero crossing time stamps and a sliding Fourier transform, discuss the different limitations and advantages of these two methods, and combine them to obtain a frequency noise spectrum spanning more than five decades of Fourier frequency $f$. For devices having a free layer consisting of either a single Ni$_{\text{}80}$Fe$_{\text{}20}$ layer or a Co/Ni multilayer we find a frequency noise spectrum that is white at large $f$ and varies as \emph{$1/f$} at small $f$. The crossover frequency ranges from $\approx\unit[10^{4}]{Hz}$ to $\approx\unit[10^{6}]{Hz}$ and the $1/f$ component is stronger in the multilayer devices. Through actual and simulated spectrum analyzer measurements, we show that $1/f$ frequency noise causes both broadening and a change in shape of the oscillator's spectral line as measurement time increases. Our results indicate that the long term stability of spin torque oscillators cannot be accurately predicted from models based on thermal (white) noise sources.