Influence of size and shape on key performance metrics in spin-torque oscillators

Abstract

Spin Torque Oscillators (STOs) are promising solutions in a wide variety of next generation technologies from read-head sensors in high-density magnetic recording arrays [1] to neural oscillator units for neuromorphic computing [2-3]. There are several metrics that can be used to quantify the performance of an STO such as power, quality factor, frequency tunability, etc., most of which are dependent on the design of the STO device itself. Furthermore, determining the most important metric will be contingent on its desired application, meaning that it is crucial to understand how the STOs design parameters influence all aspects of its performance so that its design can be optimized to perform the desired function. Our previous studies have investigated the influence of STO size [4] and shape anisotropy [5] on precession frequency and linewidth. In this work, we expand on our previous research and study the influence of device size and shape on several STO performance metrics and use correlation coefficients to quantify relative magnitude of these effects. This was done by analyzing spin torque oscillations generated from 20 magnetic tunnel junctions with in-plane anisotropy and patterned into elliptical nano-pillars with a wide range of sizes and aspect ratios. For each device, we acquired 20 to 50 data sets at various bias fields and currents and obtained power spectral density (PSD) plots for each set, examples of which are shown in Fig. 1a. Each PSD plot was used to measure frequency and linewidth (as illustrated in Fig. 1b) as well as output power. From these measurements, we then calculated quality factor and power-to-linewidth ratio for each set. We also analyzed each STOs performance in terms of the bias fields and bias currents required to maximize output power and signal quality as well as the frequency tunability with both field and current. All of these performance metrics were compared between all 20 STOs tested.