Charge-to-spin conversion efficiency in ferromagnetic nanowires by spin torque ferromagnetic resonance: Reconciling lineshape and linewidth analysis methods

Abstract

Spin orbit torques (SOTs) are being actively considered for use in the next generation memory devices for magnetization switching, spin oscillators and racetrack memories, including those using magnetic skyrmions. SOTs are fundamentally based on charge-to-spin conversion by the spin-Hall or Rashba effect. Taking advantage of the spin current, the torques can be used to switch magnetic insulators or the free layer of magnetic tunnel junctions with high energy efficiency. Therefore, it is important to accurately measure the charge-to-spin conversion efficiency. SOTs are principally of interest in nanostructured samples, samples with minimum dimension less than a micron. However, the magnitude and form of the torques are most often characterized in micron scale samples, using spin torque ferromagnetic resonance (ST-FMR), a technique that involves analyzing the resonance linewidth [1] or lineshape [2]. On microstructures, these two analysis methods are quite consistent. Here we present ST-FMR results on permalloy Ni80Fe20 nanowires --- with widths varying from 150 to 800 nm --- that show that the standard model used to analyze the resonance linewidth and lineshape give different results [3]. Fig. 1 a-c show examples of ST-FMR spectra. Fig. 1d shows the efficiency of nanowires with different width, which shows greatly enhanced efficiency when the lineshape method is used. We present a ST-FMR model that properly accounts for the sample shape. It shows a much better consistency between the two methods, as illustrated in Fig. 2. Micromagnetic simulations are used to verify the model. These results and the more accurate nanowire model presented are of importance for characterizing and optimizing charge-to-spin conversion efficiencies in nanostructures.