Reduction of Operating Current by Harnessing the Field- and Damping-Like Torque Ratios in Nonmagnet-Ferromagnet Heterojunctions

With the growing demand for high-speed electronic devices with low energy consumption, spin-orbit torque (SOT) has become a significant focus. SOT can switch the magnetization direction in a material system with broken inversion symmetry, such as a normal metal (NM)/ferromagnet (FM) heterojunction. The SOT consists of two mutually orthogonal vector components along with the injected current direction: the transverse damping-like torque (DLT) and the longitudinal field-like torque (FLT). Numerous studies have mainly centered on the DLT for the SOT switching mechanism. However, DLT and FLT are essential to enhance SOT efficiency because FLT boosts the magnetization precession motion. Herein, heterojunctions consisting of NM 1 (Ta, W, or Pt)/NM 2 (Nb)/FM (CoFeB) are devised to manipulate the FLT-to-DLT ratio (eta) through the change in Nb thickness. Furthermore, experimental confirmation exists for reducing threshold current as eta increases. The SOT devices with substantial eta generate random numbers. The National Institute of Standards and Technology Special Publication 800-90B test verifies randomness and confirms that the SOT devices are beneficial sources for true random number generators (TRNGs). These findings indicate the crucial role of FLT in the SOT switching process and underscore its significance in developing SOT-based TRNG devices. Theoretically, superior field-like torque (FLT) in spin-orbit torque (SOT) devices can reduce switching energy. The study experimentally explores the FLT-to-damping-like torque ratio in normal metal (NM) 1/NM 2/ferromagnet heterojunctions, revealing switching current density behavior. Furthermore, the random bits generated from high FLT-SOT devices exhibit high entropy, showcasing the potential as true random number generators.image (c) 2023 WILEY-VCH GmbH