Spin-Orbit Torque Vector Quantification in Nanoscale Magnetic Tunnel Junctions.

Abstract

Spin-orbit torques (SOT) allow ultrafast, energy-efficient toggling of magnetization state by an in-plane charge current for applications such as magnetic random-access memory (SOT-MRAM). Tailoring the SOT vector comprising of antidamping (<i>T</i>_AD) and fieldlike (<i>T</i>_FL) torques could lead to faster, more reliable, and low-power SOT-MRAM. Here, we establish a method to quantify the longitudinal (<i>T</i>_AD) and transverse (<i>T</i>_FL) components of the SOT vector and its efficiency χ_AD and χ_FL, respectively, in nanoscale three-terminal SOT magnetic tunnel junctions (SOT-MTJ). Modulation of nucleation or switching field (<i>B</i>_SF) for magnetization reversal by SOT effective fields (<i>B</i>_SOT) leads to the modification of SOT-MTJ hysteresis loop behavior from which χ_AD and χ_FL are quantified. Surprisingly, in nanoscale W/CoFeB SOT-MTJ, we find χ_FL to be (i) twice as large as χ_AD and (ii) 6 times as large as χ_FL in micrometer-sized W/CoFeB Hall-bar devices. Our quantification is supported by micromagnetic and macrospin simulations which reproduce experimental SOT-MTJ Stoner-Wohlfarth astroid behavior only for χ_FL > χ_AD. Additionally, from the threshold current for current-induced magnetization switching with a transverse magnetic field, we show that in SOT-MTJ, <i>T</i>_FL plays a more prominent role in magnetization dynamics than <i>T</i>_AD. Due to SOT-MRAM geometry and nanodimensionality, the potential role of nonlocal spin Hall spin current accumulated adjacent to the SOT-MTJ in the mediation of <i>T</i>_FL and χ_FL amplification merits to be explored.