Efficient and controllable magnetization switching induced by intermixing-enhanced bulk spin-orbit torque in ferromagnetic multilayers

Spin-orbit torque induced ferromagnetic magnetization switching brought by injecting a charge current into strong spin-orbit-coupling materials is an energy-efficient writing method in emerging magnetic memories and spin logic devices. However, because of the short spin coherence length in ferromagnetic layers, the interfacial effective spin-orbit torque typically leads to high critical current density for switching thick ferromagnet, which goes against low-power and high-density requirements. Here, we experimentally demonstrate efficient bulk spin-orbit torque-driven perpendicular magnetization switching under relatively low critical current density in thick Pt/Co multilayers with gradient-induced symmetry breaking. Through tuning the thickness gradient of Pt, the spin-orbit torque efficiency and switching chirality can be highly controlled, which also indicates that net spin current arises from gradient. Meanwhile, x-ray absorption spectroscopy results reveal that the atomic intermixing can significantly enhance the spin-orbit torque efficiency through improving the strength of spin-orbit-coupling of Pt. We also establish a micromagnetic model by taking both gradient-induced and intermixing-enhanced spin-orbit torque into account to well describe the experimental observations. This work would blaze a promising avenue to develop novel spin-orbit torque devices for high-performance spintronic memory and computation systems.